Underwater Repair Technology

Underwater repair technology John H Nixon

Gulf Publishing Company Houston, Texas

This edition published 2000 by Gulf Publishing Company Book Division -WR O. Box 2608 Houston, Texas 77252-2608 First published 2000, Abington Publishing (UK) © Woodhead Publishing Ltd, 2000 The author has asserted his moral rights All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the publisher. While a great deal of care has been taken to provide accurate and current information, neither the author, nor the publisher, nor anyone else associated with this publication shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. ISBN 0-88415-885-3 Library of Congress Catalog Card Number: 00132-254 Printed in the United Kingdom.

Acknowledgements

Late in 1975,1 was called into the office of the then Professor of Welding Technology, R L (Bob) Apps. He asked me if I would suspend the PhD research programme I was just starting, and spend a few months on a short term contract to investigate underwater welding. Nearly twenty-five years later, I am still carrying out research into underwater welding - and I never did finish the PhD! Welding has always been a subject requiring collaborative research, mainly because of the wide range of skills involved - metallurgists, process engineers, physicists, control engineers, and specialists in statistics, robotics, computer programming and electronics can all be required to contribute to a project. For this reason, the continuing success of the Cranfield activity is dependent on the talents of the staff who have been involved in the research over the years. The Group has been the responsibility of four Professors during its lifetime, Bob Apps, John Rogerson, John Billingham and Tom Stephenson. Major academic members of the team over the years have been Dr Chris Allum, Dr Dave Dorling, Brian Pinfold and currently Dr Ian Richardson. The contribution of the technical staff has also been essential to the success of the Group, and this has included John Bassett, Julian Eley, John Holmes, Bill Lawes, Ray Newman, David Robinson and John Savill. Significant contributions have also been made by other members of staff at Cranfield, and at our industrial partners. It would not be possible to carry out research at Cranfield without financial assistance from a wide variety of sources. The initial research programme was supported by Sub Ocean Services, then a subsidiary of BOC, and British Petroleum, with the guidance of the late Professor Harry Cotton, then Chief Welding Engineer at BP. Major oil companies which have supported subsequent work by the Cranfield team include Amoco, Conoco, Norsk Hydro, Petrobras, Shell, Statoil and Texaco. Offshore engineering companies have also contributed, including Isotek Engineering, Stolt Comex Seaway, Rockwater and Sub Sea Offshore Ltd. Other organisations which have supported work at Cranfield include the

Pipeline Research Committee of the American Gas Association, the National Hyperbaric Centre in Aberdeen and the Offshore Supplies Office. The major supporter of work at Cranfield, for many years, has been the Engineering and Physical Sciences Research Council (EPSRC). The assistance of all these organisations is gratefully acknowledged. I would also like to thank all the people who have provided information for the book, especially C E 'Whitey' Grubbs and Thomas J Reynolds, for their comments on wet welding, and Mike Armstrong of Isotek and Tor Habrekke of SINTEF, for their description of the latest version of the Norwegian Pipeline Repair Spread. Special thanks are due to Ian Richardson, who has supervised most of the research upon which the chapters relating to advanced hyperbaric arc welding processes are based, and has also read the draft manuscript. Any errors in the text are, however, the responsibility of the author. The illustrations for the book have been provided by Hydratight Ltd, of Darlaston, West Midlands (Fig. 3.3, 3.4, 3.5, 9.5 and 9.6), Stolt Comex Seaway, of Aberdeen (Fig. 3.1, 3.2, 4.3, 4.4 and 5.7), Isotek Ltd, of Leeds (Fig. 6.14, 6.15, 6.16 and 6.17), Cranfield Press (Fig. A3.2 and A3.3), the National Hyperbaric Centre, of Aberdeen (Fig. A3.1) and TWI, of Abington (Fig. 6.1,6.4,6.10,6.12,6.13,8.1,8.2,9.1,9.2,9.3 and 9.4). The assistance of all these organisations is gratefully acknowledged. It should be noted that any opinions expressed in the book are solely those of the author, and not of any other person, or of any commercial, governmental or educational organisation. This is particularly true of the final section discussing possible future developments. Finally I would like to thank my wife Audrey for her continuing support, not only during the preparation of this book, but also throughout my career in research.

Contents

Acknowledgements ............................................................

vii

1. Introduction .................................................................

1

2. Offshore Structures, Materials and Standards .........

3

2.1 Gravity Platforms ...............................................................

3

2.2 Jacket Platforms ................................................................

3

2.3 Pipelines ............................................................................

4

2.4 Offshore Engineering Materials ........................................

5

2.5 Standards and Codes .......................................................

6

3. The Joining of Underwater Structures without the Use of Underwater Welding .................................

8

3.1 Lifting .................................................................................

8

3.2 Pipe Connectors ................................................................

9

3.3 Structural Joints .................................................................

15

4. Underwater Engineering Processes Required for Fabrication Operations .........................................

17

4.1 Pipeline Alignment Systems .............................................

19

4.2 Pipe Preparation ...............................................................

21

4.3 Inspection Techniques ......................................................

22

5. Underwater Welding Technology ..............................

25

5.1 Wet Welding ......................................................................

25

5.2 One Atmosphere Welding .................................................

27

5.3 Hyperbaric Welding ...........................................................

29

5.4 Hyperbaric Welding Chambers .........................................

31

This page has been reformatted by Knovel to provide easier navigation.

v

vi

Contents 5.5 Summary ...........................................................................

37

6. Manual Hyperbaric Welding Techniques ..................

38

6.1 Tungsten Inert Gas Welding (TIG) ....................................

38

6.2 Manual Metal Arc Welding (MMA) ....................................

44

6.3 Metal Inert Gas Welding (MIG) .........................................

51

6.4 Flux Cored Arc Welding (FCAW) ......................................

56

6.5 Automated Orbital Hyperbaric Welding ............................

58

6.6 Summary ...........................................................................

62

7. Alternatives to Saturation Diving for Deep Water Applications ......................................................

63

8. Deep Water Arc Welding Processes ..........................

65

8.1 Plasma Welding ................................................................

65

8.2 MIG Welding ......................................................................

70

8.3 Summary ...........................................................................

78

9. Alternatives to Arc Welding for Deep Water Joining Operations .....................................................

80

9.1 Solid Phase Welding Processes .......................................

80

9.2 Mechanical Connectors ....................................................

84

10. Conclusions ................................................................

86

Appendices Appendix 1: Oceanography ......................................................

88

Appendix 2: Diving Technology ................................................

90

Appendix 3: Hyperbaric Welding Research Techniques .........

97

Bibliography ..................................................................... 103 Index .................................................................................. 105 This page has been reformatted by Knovel to provide easier navigation.

1 Introduction

From small beginnings on the West Coast of America prior to World War II, the extraction of hydrocarbons from reserves situated offshore has developed into an industry spanning the entire planet, with activity on every continent. The early small wooden platforms have grown into huge steel and concrete structures standing in 200 metres of water, with more innovative designs, such as tethered platforms capable of operating in deeper waters, and with 1000 metre reserves currently being considered. To achieve this, new grades of stronger, more weldable steel have been formulated, joining and inspection techniques have been greatly developed, and new design concepts have been brought into use. The investment required for these developments has been very high, and as a result the industry has tended to be slightly conservative from an engineering point of view. However, requirements to reduce capital costs and to provide effective techniques for the exploitation of smaller offshore reserves, combined with the necessity to develop joining systems capable of operating without the intervention of divers, have presented the industry with several formidable challenges which must be overcome in the next few years. It is perhaps useful to state the topics which this book is not intended to cover. Offshore materials will not be described in detail, only those characteristics relevant to their joining are discussed. Individual case histories will also not be included, as these are better described by those involved. Most notably, there has been no attempt to include any guide to the cost of offshore operations. These are extremely difficult to quantify, as they are dependent on so many factors - the availability of suitable ships and equipment, the general level of activity in the industry, and the background expertise of the relevant offshore engineering company being the major influences. Publications dealing with these topics can be found in the proceedings of the relevant international conferences listed in the bibliography at the end of the book. Briefly, the book is intended to provide an overview of the techniques available to the offshore industry for the joining and repair of offshore

structures. Joining systems not involving the use of welding are discussed first. The associated engineering systems required for joining procedures are then discussed - pipe handling, hyperbaric chamber design and the like. The principal underwater welding techniques are then described - wet, one atmosphere and hyperbaric welding, followed by a description of the effects of environmental pressure on the various hyperbaric welding processes. To make this section of the book more comprehensible to the general reader, a brief description of each welding process as it is used in general engineering is included before the modifications required for hyperbaric applications are discussed. Finally, those joining systems not currently in use, but which show promise in the laboratory, are described, followed by some speculation relating to how underwater joining technology might develop over the next few years. For general information, appendices describing the relevant diving technology, basic oceanography and the research methods used for the development of hyperbaric welding techniques are included. Although this book describes the technology of underwater repair at all water depths, it should be recognised that the great majority of offshore repair work is carried out at relatively shallow depths, in the region intermittently covered by the water, known as the 'splash zone'. This is because the speed of any object dropped from the platform or an adjacent ship is highest at the water line, while wave loadings, corrosion activity and the growth rate of marine life are all at a maximum just below the surface. In addition, the possibility of collision between shipping and the platform is also maximised at shallow depths. Thus, numerically, the majority of underwater repairs takes place at shallow depths, although the most economically and technologically significant are in deeper water, frequently on pipelines.

2 Offshore structures, materials and standards

Although the size and configuration of offshore structures varies enormously, they can be reduced, for the purposes of a discussion of offshore repair, to three types - gravity platforms, jacket platforms and pipelines which are described in more detail below.

2.1

Gravity platforms

Gravity platforms are enormous structures made from reinforced concrete. They are usually constructed by the normal concrete fabrication process of shuttering and pouring, usually in a deep water site close to a shoreline, frequently on the coast of Norway. As material is added to the structure, it sinks under its own weight and by appropriate ballasting, enabling construction to be carried out at a convenient height. Such structures can weigh several hundred thousand tonnes when complete. They are normally towed to their operating site, at a flotation height controlled by regulating the amount of air in chambers contained in the main structure and, once positioned, this air is released and they sink to the bottom, achieving stability because of their great weight. The internal chambers may then be utilised for oil storage. Considerable care is taken to ensure that the quality of the concrete used in such structures is maintained, regular mechanical and analytical checks being carried out on the material during the fabrication process. Being large, monolithic structures, gravity platforms are not prone to structural damage, but when this does occur, they can be repaired using techniques similar to those employed in harbour works, involving the fitting of shuttering to the main structure by divers and repairing the damaged area with waterproof concrete or grout.

2.2

Jacket platforms

Jacket platforms are equally large but much lighter, usually some tens of thousands of tons, and comprise a large space frame fabricated from low

alloy steel tubes, with complex joints at the intersections, which are termed 'nodes'. They are normally fabricated in graving yards or dry docks, adjacent to the sea. In some cases, removable flotation tanks are added to the structure to allow it to be floated to its operating site, or alternatively a large pontoon is used to carry the structure. Once on site, the jacket is rolled into a vertical position by appropriate ballasting, and is then allowed to sink until resting on the seabed. The weight of a platform is normally insufficient to ensure adequate stability for long term service, so it is fastened to the bottom by long piles, which are slid through pile guide tubes attached to the structure, and then driven into the sea floor. The joint between the pile and the jacket is then made by grouting or swaging. Normally, the main jacket structure is installed first, and then the upperworks, including the production deck and machinery, are brought to the site on a barge, positioned above the jacket, lowered into place by ballasting the barge, and welded into place. Platforms may require modification or repair for a variety of reasons. As the regulations relating to offshore structures are changed, it may be necessary to add strengthening or protective structures. Using an existing platform as a base for additional subsea wells may require the addition of extra mountings for the necessary additional piping and production equipment. As described above, damage may occur due to ship collision or objects dropped from the platform or an adjacent ship, due to unforeseen severe weather, or progressively due to fatigue or corrosion.

2.3

Pipelines

Although other systems exist, such as the transfer of oil into an adjacent tanker via a special mooring system, the majority of hydrocarbon from offshore reserves is currently transported to shore in pipelines, of which some thousands of kilometres have now been laid beneath the North Sea, and tens of thousands internationally. Such pipelines have diameters varying from 300 mm to over one metre, with wall thicknesses up to 30 mm. In order to produce a pipeline with negative buoyancy, which will lie stably on the seabed, the pipeline is often covered with a ballasting layer, normally of concrete, called the weightcoat. Although smaller diameter pipes can be manufactured in long lengths, and then fed on to large coils on specially adapted laying ships, the larger pipelines are normally manufactured from relatively short lengths of pipe. In a fabrication facility onshore, steel plates approximately 12 metres long are rolled to form the pipe, and then longitudinally welded, normally by submerged arc welding. These lengths, covered in weightcoat apart from the ends, which are machined with the required welding preparation, are taken out to the pipelaying barge on a support vessel. It will be apparent from

the manufacturing technique for such pipe sections that the roundness of the pipe can be less than perfect. American Petroleum Institute (API) standards permit tolerances of ±1% on pipe diameter, and 1% ovality. In the worst case therefore, the diameter of a one metre pipe could vary by 30 mm, a typical wall thickness. This is overcome during laying by jigging clamps, but can have significant consequences in later repair procedures. Once aboard the laybarge, the pipe sections are stored until required. They are then fed into the main pipe laying system, or 'firing line', where they are attached to the end of the main pipe. Several layers of weld metal are deposited at a series of stations as the pipe travels along the deck of the vessel, followed by inspection and coating stations before the pipe is fed off the stern of the laybarge, on to a curved 'stinger', a structure designed to support the pipe as it curves to the angle of descent to the seabed. The rate at which pipe can be laid depends on the time from when the pipe is inserted into the firing line, until the root weld is sufficiently strong to permit sufficient tension to be applied to pull the pipe along the line. On larger laybarges, the speed of the pipelaying process is increased by the use of the 'double heading' technique. On a separate welding line, two 12 metre sections of pipe are joined to form 24 metre components, which are then passed to the firing line. However, for this to be possible, the laybarge must be long enough to accommodate the required number of welding, inspection and coating stations spaced 24 metres apart. Pipeline repairs may be required because of a variety of causes, but the majority are necessitated either by welding defects produced during the fabrication procedure and frequently made worse by the stresses imposed during the laying process, or by damage caused by trawling or ship anchors. The incidence of the former can be reduced by close weld procedure control on the laybarge, and the latter by burial or covering of the pipeline in critical regions, close to shore or where fishing activities are known to take place. Part of the design study for a new pipeline will comprise a comparison between the cost of pipe burial or coverage, and the economic consequences of damage to the pipeline.

2.4

Offshore engineering materials

Until very recently, virtually all offshore construction was carried out using high strength, low alloy steels, and these are still used for the majority of applications. Stainless steels, duplex stainless steels, aluminium, Monel, coated materials, and non-metallic materials such as reinforced plastics are beginning to be used in significant quantities. In addition, there is a general trend towards the use of higher strength steels offshore, in order to reduce weight and construction time. In the 1970s

and early '80s, a typical offshore steel had a yield strength of about 350MPa, and was basically a carbon steel with minor additions of other elements such as manganese. More recently, in order to improve weldability, the carbon content of offshore structural steel has been falling, with the required mechanical properties being achieved by careful control of microalloying elements, thermal treatment or mechanical working of the plate material. Current programmes are utilising steels with yield strengths of 450 to 500MPa, while steels of up to 700MPa are being considered for future projects. Achieving comparable weld metal properties, both on the surface and underwater, will require significant development work over the next few years.

2.5

Standards and codes

The construction of offshore structures and pipelines is normally carried out within the framework of design rules and standards laid down by appropriate regulatory bodies, such as the Health and Safety Executive (HSE), or by the various certification authorities, such as Lloyds or Det Norsk Veritas. These specify the means by which the materials and welding electrodes from which structures are constructed are tested to verify that they achieve the mechanical properties required by the design. Qualification procedures for welders, inspection techniques and other aspects of the fabrication process are also specified. However, few offshore codes provide guidance for conducting repairs. Guidance notes were prepared by the HSE in 1990, including sections dealing with welded repairs, friction clamps, grouted sleeves and the filling of members with grout. These notes were based on information gained during a joint industry project undertaken during the mid-1980s, and reported by Wimpey Laboratories in 1988. An ISO standard, dealing with mechanical clamps and grouted connections, was in the course of preparation during 1997, although the general philosophy of the standard was published in 1996 by Harwood et al. The most comprehensive standard relating to underwater welding is that developed by the American Welding Society, of which the latest version was published in 1993 (AWS D3 6-93). This covers wet and hyperbaric welding, and all the conventional arc welding processes are included. Because arc welding is depth sensitive, the standard provides guidance indicating the depth range over which a qualification carried out at a specific depth can be considered valid. Four levels of weld quality, classified by their intended use, are specified with their attendant property requirements. Over the past few years, there has been a significant change in the management of risk offshore, much of it prompted by the lessons learned from the Piper Alpha disaster. Rather than relying on general rules, offshore

operators are required to prepare safety cases for individual structures, indicating possible hazards, the risks involved and the manner in which these are being reduced to a level which is as low as reasonably possible (ALARP). Such techniques would also be applied to underwater operations such as a repair procedure, with risks to divers and support personnel being evaluated, as well as more traditional hazards such as structural stability, fire and explosion. The Design and Construction Regulations published by HSE in 1996 include the requirement to monitor the structural integrity of all underwater structures throughout their lifetime, and to carry out remedial work in the event of any damage or deterioration which might adversely affect its integrity. It introduces the concept of safety critical elements (SCEs) components the failure of which might lead to a major accident, and which are therefore subject to independent verification.

3 The joining of underwater structures without the use of underwater welding

Although underwater welding is used in many repair and fabrication applications, several alternatives exist which may be considered for some situations. Mechanical connectors have been used in land based oil field operations for many years, and a wide range of components, from several manufacturers, is commercially available. Grouting, which has been used for many years for some offshore operations, such as securing piles to platforms, has been utilised for platform repair and modification, and other joining technologies, such as cryogenic connectors, have also been considered for offshore use.

3.1

Lifting

Before considering the techniques listed above, the simplest way to join underwater structures, where it is possible, is to lift them out of the water. In the case of pipelines laid in relatively shallow water, it may be possible to lay two sections with an overlap, and then lift the ends to the surface, to carry out the joining procedure in the dry. Such a technique has the enormous merit that the consumables, welding procedures and personnel used for the original construction of the pipeline can be utilised to make the final joint. Once this has been made, the pipe can be lowered to the seabed, the excess length being allowed to form a loop. This technique is obviously only possible with structures which can be lifted clear of the water, and is thus mainly applicable to pipelines. However, as the water depth increases, the possibility of buckling the pipe during the lifting operation becomes greater, especially when the line is made from large diameter, low wall thickness pipes. Although the tendency to buckle can be reduced by the use of auxiliary buoyancy, the complexity of such operations will probably make alternative joining techniques more attractive.

3.2 Pipe connectors 3.2.1 Mechanical connectors Mechanical connectors have the virtue of a good performance record in land based operations and, being purely mechanical, have little sensitivity to water pressure. They are based on simple engineering principles, consisting of a pair of forgings, hinged to form a split tube, and incorporating mating flanges through which bolts can be passed to clamp the connector around the pipe. The inner surface incorporates seals to eliminate leakage between the pipe and the clamp. Auxiliary buoyancy units may be added to reduce the weight of the clamp underwater. Such connectors are simple, relatively cheap and can be rapidly deployed to support a damaged pipe mechanically or to control leakage. Many companies produce more complex mechanical connectors for the oil industry, which can be adapted for underwater use. They can be slid over the cut end of a pipe, once the weightcoat has been removed, and are held in place by collets - metal wedges which are driven radially into contact with the outer wall of the pipe. Leakage is normally prevented by the incorporation of polymeric seals. The other ends of these couplings usually form flanges, to which other components can be bolted - ball and socket joints to accommodate misalignment, flanged lengths of pipe to bridge gaps, and 'T' or 'Y' pieces. Such systems form a flexible and effective technique for the repair of underwater pipelines. They do, however, suffer from several disadvantages. Mechanical connectors for large pipelines, of the order of a metre diameter, are themselves large, heavy components, and for divers to position them effectively, some form of handling frame is required. Although these are available, the provision of such equipment complicates the basic simplicity of the mechanical connector concept, requiring considerable effort to position it correctly on the seabed. In addition, it is usually necessary to inspect mechanical connectors periodically, to ensure that they remain fully functional and leak free. This represents a cost, and use of diving resources, which will continue for the lifetime of the offshore structure, and must be weighed against the higher initial cost of other repair techniques. More subtly, most offshore structures are subject to design assessment, by the certification authorities, as fully welded structures. If a mechanical connector is used for a repair or modification, it may be necessary to review the certification of the structure, sometimes to the detriment of its performance. In the early 1980s, a new connector concept was marketed by Big Inch Inc, of Houston, and now supplied by Stolt Comex Seaway in Aberdeen. The most important element in the Flexiforge system was an end connector in the form of a tubular forging, as shown in Fig. 3.1. The tubular section on the right of the connector could be slipped over the outside of the pipe,

3.1 The Flexiforge connector.

other mating elements such as the flange shown being welded to the smaller end. Once in place, the pipe-connector joint is made by a mechanical swaging system inserted into the bore of the pipe. This has several rollers mounted on a central mandrel. These are rotated within the pipe, and expanded radially to force the pipe wall outwards into a series of detents machined into the interior surface of the coupling. The pipe is permanently deformed into a series of tight metal-to-metal joints between the pipe and the coupling (Fig. 3.2). The swaging machine can then be removed, and other components bolted to the coupling. A great merit of the Big Inch system is the speed at which joints can be made - 15 minutes has been quoted to install a coupling on a 900 mm pipe.

3.2.2 The Morgrip connector In recent years, the Morgrip range of connectors, manufactured by Hydratight Ltd, has been used for several underwater pipeline repairs. A highly modular system, the common feature of the Morgrip range is the technique by which the connector grips the pipes. This connector is manufactured for standard pipe diameters up to 900 mm (36") and pipe ratings to 25001b ANSI. The couplings have a temperature operating range from -40 0 C to 2500C, but under test they have withstood their maximum rated pressure for extended periods at temperatures above 8000C.

Toggle Mechanism Counter Torque Grips Counter Torque Pins

Before Forging

Forged

End Connector With RTJ Flange

3.2 The Flexiforge installation machine.

In its simplest form, the unit is made up of a central sealing section and two outer, gripper units. When these are drawn together by tightening the longitudinal stud bolts, ball bearings confined in a cage in the bore of the connector are forced inwards, swaging into the pipe to provide the mechanical grip (Fig. 3.3). The same action forces the central seals inwards into close contact with the external surface of the pipe. The only pipe preparation required is to remove any coatings from the outer surface of the pipe, because each ball moves independently, and these can compensate for imperfections in the surface condition and shape of the pipe, as can the specially designed seals. Because the connector grips the outside of the pipe, preparation of the ends of the pipe is not as complex as for welded joints. The unit was originally designed to install flanges on to pipes and although often used to join two pipe lengths, as an alternative to butt welding, it can also be used to attach flanges or blank ends, simply by substituting appropriate components for one of the gripper ends (Fig. 3.4). Because the only action required to install the connector is the tightening of the longitudinal stud bolts, the connector can be fitted into positions where clearance and access are extremely limited, maximising operational flexibility. Where corrosion is likely to be encountered the connector can be made from suitably resistant materials. The pressure rating of the connector can be increased by incorporating several pairs of gripping segments into the connector, and several other options can be incorporated into the connector, including the facility to pressure test the seals after installation, and to inject corrosion

3.3 The Morgrip 1000 series connector.

inhibitor into the coupling for service in harsh environments. The pressure test connections can be seen on Fig. 3.5, between the twin seals on each side of the joint. By pressurising the annulus between the seals to the required hydrostatic test pressure, effective installation of the seals can be confirmed. These connectors have been used for a range of underwater repairs and pipe tie-ins utilising diver installation. A diverless version has been developed and is described later in the text.

3.2.3 Cryogenic couplings One type of pipe joint, normally reserved for small pipe sizes, utilises the properties of an alloy of titanium and nickel, known commercially as 'TineP. It was originally developed for use in the aerospace industry by Raychem. A tubular connector can be machined from such material, in order to connect two lengths of pipe. The internal diameter of the connector is machined to a size which is an interference fit on the pipes to be joined, and often incorporates detents to exert greater, local clamping stress. After machining, the connector is cooled, in a bath of liquid nitrogen, to temperatures below -100 0C. It is then expanded, mechanically or hydraulically, until its internal diameter is a slip fit over the pipes to be joined. It is held at low temperature until it is used, when it is slipped over the joint area, and allowed to warm up. At a critical temperature of approximately -50 0 C, the material 'remembers' its former size, and contracts to grip the pipe ends, forming a metal-to-metal contact joint, in a manner similar to polymeric heat shrinkable sleeving. The high cost of the Tinel, combined with the practical difficulty of handling large, very cold components, has restricted the use of the system to pipes smaller than 200 mm O/D.

Coupling

Flange adaptor

Pipe adaptor

End cap 3.4 Variant forms of the Morgrip connector.

Seal housing Ball gripping segment

Twin seal housing Centre section

External pressure test port

Radial seals Ball cages Studbolts

3.5 The Morgrip 2000 series connector.

3.2.4 Swaged joints Another special purpose mechanical underwater joining system was derived from the technology developed for the underwater explosive welding system described later. A component in that system was a hydraulic mandrel and sleeve used to shape the end of the pipe to a consistent size and circular form. If the inside of a pile guide is machined with appropriate grooves, a similar hydraulic mandrel can be slid inside the pile, and then used to expand the pipe into these detents, rapidly forming a high performance metal-to-metal joint. The pile is initially driven into the seabed to the required depth, after which the driving head is withdrawn. The hydraulic mandrel is then inserted into the pile, and positioned so that the length of pipe opposite the grooves is within the area pressurised by the mandrel. Seals are then expanded to form pressure walls at the end of the mandrel, after which hydraulic pressure is applied within the pile, expanding the pipe out into the pile guide grooves. After depressurisation and retraction of the seals, the unit can be withdrawn, and a mechanical measuring system, mounted on the end of the mandrel, can be used to verify adequate deformation has been achieved. Such a system can be used to produce high performance pile joints rapidly, without the use of grouting techniques.

3.3

Structural joints

The mechanical connectors described above are normally designed to be used for the repair or modification of pipelines. The problems relating to platform jacket operations are somewhat different. The more complex geometries of jacket structures, with their multi-tubular node joints, make the installation of new components more difficult. A large jacket structure, several hundred metres tall, is of necessity made to fairly wide tolerances, up to tens of millimetres in most cases. Physical access to the components inside a structure may prove complicated, as can the problem of lifting or supporting a bulky component within a network of other pipes. If a tubular component is to be removed for replacement, an analysis must be made of the stability of the remaining structure, to determine whether temporary support structures are required to maintain the integrity of the structure.

3.3.1 Mechanical connectors A mechanical connector, which can only transmit load when in close contact with the surrounding structure, represents formidable problems of accurate placement and fitting, in order to avoid high local loading and overstressing of the structure. The facing surfaces may have to be cleaned, and local grinding of the components may be required in order to achieve an acceptable degree of contact between the new structure and the old. In some cases, a thin layer of polymeric material, such as neoprene, has been used to accommodate local surface damage. In addition, the design of mechanical connectors has been greatly improved. Longer bolts minimise the change in tension in the bolt as the structure shifts under load. A frequent failure mode for bolts is fracture at the head to shank joint, or at the base of the nut, due to bending forces generated by tension in the bolt, if the bearing surface is not accurately normal to the bolt hole. To avoid this, washers which have undersides shaped into partially spherical form may be used. Despite these innovations, the fundamental problem of the sensitivity of mechanical connectors to lack of fit remains, and their use to date, for structural repair, has been limited.

3.3.2 Grouted and stressed grouted connections The use of grouted connections underwater has already been mentioned with regard to piling operations, and is a well-developed technique with an established operational record. For structural applications, the joint design

is similar to that for a mechanical connection, except that a gap of some millimetres is left between the components to be joined. After installation, this is filled with grout, which after setting transfers the load through the joint. The performance of such a joint can be described by determining the magnitude of the shear load sustainable by the connection, per unit length, and in many circumstances the length of the joint must be minimised, to reduce weight or to fit between existing parts of the structure. Load is transmitted between the components forming the joint by shear forces within the grout, and at the grout/metal interfaces, and it has been established that this performance is greatly influenced by the surface condition of the metal components. For example, if the surface of the pipe was very smooth, the grouted connection could slide along the pipe at very low levels of applied stress. Improving the performance of such joints involves the use of shear keys on the surface of the pile - usually weld beads perpendicular to the direction of loading. These can be made in several ways, but the most common are wet, hyperbaric or friction welding. Guidelines for the size and spacing of such shear keys were formulated as part of a government- and industry-supported research project in the 1980s, the report on which, published by Wimpey, is included in the bibliography. In some cases, the performance of the joint can be improved still further by a combination of mechanical and grouting techniques, to produce the stressed grouted joint. A grouted joint is made and allowed to cure until the grout possesses appreciable strength. The external sleeve is then tightened by the use of mechanical clamps, consolidating the grout and forcing it into closer contact with the pipe and sleeve. Improved load transfer capability can be achieved in this way, although the design of the joint, and the installation operation, are more complex.

4 Underwater engineering processes required for fabrication operations

Although joining operations, whether by welding or other means, are central to the concept of underwater repair, many other activities are necessary to produce a completed underwater joint. Although proportions vary with the specific situation, joining operations may comprise only 10 to 20% of the total underwater activity involved in the joint production procedure. This section seeks to describe briefly the other technologies involved, although more detail should be sought from specialist organisations. In general, the description will apply to pipeline applications, as these tend to be less varied than jacket operations, but similar techniques are, however, used for platforms. Although offshore structures can be laid on the seabed with increasing precision, as more powerful, dynamically positioned laybarges with satellite positioning systems are becoming more widely used, the site of any underwater repair or joining operation will normally require detailed prior survey. This will determine the precise geometry of the underwater structure, assess seabed conditions at the worksite if this is relevant, and evaluate visibility and tidal conditions. In some cases, the pipeline is buried and must be found using sonar or magnetic sensor equipment. If the pipeline is buried, the seabed must be excavated to gain access to the joint site. This operation is strongly dependent on the condition of the seabed, which may vary from solid rock to near liquid mud. If the surface is mud or small rocks, these may be removed using underwater suction equipment which operates in a manner similar to a large vacuum cleaner. Larger rocks must be removed manually by divers or by hydraulic handling equipment. At the other extreme, it may be necessary to surround the worksite with a cofferdam to prevent the surrounding mud collapsing into the hole being excavated at the worksite. A careful survey of the worksite and planning of these stages of the procedure can save considerable time during the actual offshore operation. Once access to the pipe has been gained, it will be necessary to remove the weightcoat if one is present, in order to gain access to the surface of the

pipe itself. Divers can remove the concrete layer using hydraulically powered grinding equipment and pry bars, but more recently high pressure water jet cutters have been used to reduce the risk of damage to the pipe surface. The pipe must be further cleaned back to bare metal both where the hyperbaric welding chamber seals are to be located, and where the pipe joint is to be made. On all underwater structures it may be necessary to remove protective coatings, marine growth and corrosion to achieve the required smooth, clean surface. Joint preparation practice is similar to that employed in surface based welding, with the difference that there is rarely the manpower available at a hyperbaric repair site which would be used on the surface. Where sophisticated dry transfer pipe welding habitats are being used, as many as four crew members may be in the habitat at once, but this is the exception rather than the rule, and normally only one or two are present. Preparation practice varies considerably depending on whether the repair is to be carried out on a jacket structure or a pipeline. Jacket repairs, which are frequently fatigue cracks, are normally machined out using a combination of tractor mounted milling and grinding tools, with the final finishing being carried out using hand grinding tools. Once non-destructive examination (NDE) has confirmed that the crack has been completely removed, the resultant groove can be filled by welding. Frequently, if the structure has been completely cut through, it will be necessary to close the root aperture using tungsten inert gas welding, because of the higher levels of control available, after which the joint will normally be filled by manual metal arc welding. Because of the irregular shape of the joint, at present manual welding is exclusively used for this type of repair, although robotic systems have been proposed for future development. Similar techniques are used for the replacement or installation of complete platform members. Usually, these will be lowered into place and the fit-up of the weld preparations assessed. It is not unusual for some additional fitting to be required at this stage to produce acceptable weld preparations. Usually, the member will be held in place using temporary clamps until it can be permanently fixed, the workchamber being mounted at each end of the new member in turn. To ensure that leakage does not occur through the new member, this is normally sealed using 'pigs' - spherical balloons made of tough polymer which can be inflated to seal the pipe bore. Although the geometry of pipeline welds is simpler, their operating stresses are frequently higher than those of jacket structure joints. It may also be necessary to control the internal shape of the weld root closely, in order to avoid crack type defects which may promote corrosion if it is known that the pipe may carry sour gases. It is also frequently necessary to limit

the protrusion of root welds into the pipe bore in order to permit the passage of pigs down the pipe for cleaning and other purposes. Such requirements will normally be specified by the pipeline operator during the proposal stage of the procedure, and may influence the choice of welding process.

4.1

Pipeline alignment systems

A major problem with pipelines is to ensure that the two ends of the pipe which are to be joined are maintained in a fixed relationship to each other during the welding procedure. This is normally achieved with the aid of some form of pipe alignment system, a series of hydraulically operated underwater cranes capable of picking up a pipe and manipulating it to the required position and attitude. Such a unit is shown in Fig. 4.3 (pagfe 21) the gripper unit which holds the pipe is in the centre of the picture, mounted on a transverse slide unit. This can be moved vertically by the two side pillars. Two cranes, acting together, are not only capable of moving the pipe vertically and laterally, but also twisting it to achieve angular alignment. Cranes of this type can be incorporated into two basic types of pipe alignment system - those integrated into a single unit and those made up of several components. Single unit systems consist of a large, open based space frame which can be lowered over the pipe from a surface vessel. Integrated into this structure are normally four pipe cranes (Fig. 4.1), each capable of lifting the pipe vertically and moving it laterally, usually with a maximum travel of several Welding chamber shown in raised position Pipe crane 1

Pipe crane 2

Pipe crane 3

Pipe crane 4

Main frame

Pipeline

Seabed 4.1 An integrated pipe alignment system.

Welding chamber shown in raised position Pipe crane 2

Pipe crane 1

Feet to spread load on seabed

Pipe crane 3

Pipe crane 4

Seabed Distance to allow pipe to sag horizontal under own weight

4.2 A three element pipe alignment system.

metres. As described above, it is evident that the angular relationship between the two pipes can also be controlled, ensuring that a square joint between them can be achieved. Some units of this type can also move the two pipes axially, to control the gap between them, but this problem can be dealt with in other ways, as described below. Alignment units of this type tend to be both large - 30 to 50 metres long - and heavy - 40 to 50 tonnes. This means that they require a relatively large diving support vessel to deploy them, and they require a significant amount of space on the seabed, which may be a problem close to other underwater structures. Alternatively, a multi-component alignment system may be used (Fig. 4.2). This consists of a heavy duty pipe crane, capable of lifting a considerable length of pipe. This is deployed some distance from the site of the joint, such that when the pipe is lifted, its own weight will cause it to droop to a horizontal attitude at the position where the joint is to be made, at a convenient distance above the seabed. The distance required can be calculated from the known mechanical properties of the pipeline material. If a similar procedure is carried out on the other pipe, the two ends will be positioned facing each other, and unsupported for some distance back to the pipe cranes. A relatively light duty manipulator can then be used close to the pipe ends, to clamp the pipe ends and to carry out final adjustments to their relative position. Because systems of this type are made up of three ele-

4.3 A Comex pipe crane.

merits rather than one, it will be apparent that the craneage requirements to deploy such a system will be substantially less. However, the heavy duty pipe cranes used in the multi-element system exert significant loads into the seabed, as they lift the lengths of pipe, which means they are only suitable for relatively firm seabed surfaces. In contrast most of the loads in the integrated system are contained internally. A system of this type, also made by Comex, is shown in Fig. 4.3 and 4.4. The pipe crane shown in Fig. 4.3 has been described earlier, and the picture also shows the large supporting feet to increase the area through which the loads are transmitted to the seabed. A second alignment unit can be seen behind the first. Figure 4.4 shows the centre unit from the 'Seahorse' system. The welding chamber incorporates pipe clamps for final alignment of the joint, after which the chamber can be sealed and dewatered.

4.2

Pipe preparation

Once the pipe end has been located and cleaned, it will be picked up by the pipe alignment system, and the end machined with the required weld preparation by means of a hydraulically powered pipe lathe. This process is often carried out in the wet, by divers, but it is also possible to perform it in the dry. Because of the high standard of root welding required, it is rarely possible to bring the raw ends of the pipe together and to carry out a single butt weld. It is more usual practice to position the pipe ends some hundreds

4.4 A Comex 'Seahorse' welding chamber and pipe handling system.

of millimetres apart, and then precisely measure the gap between them at a series of points around the circumference. A small piece of pipe, known as a 'pup', is then machined on the surface to fit precisely into the gap, with root gap tolerances of the order of a few tenths of a millimetre. Offshore pipes made to API standards are allowed a tolerance of ±1% on diameter, and a further 1 % ovality, so in an extreme case, a 1 metre diameter pipe can have a radial misalignment of 30 mm, which is frequently more than the wall thickness. To overcome this, the pipes must be forced to the same shape using similar techniques to those employed in surface based welding, such as an alignment clamp - a circular frame mounted around the pipe, and equipped with a series of radial screw jacks capable of forcing the pipe into the required shape. When acceptable alignment has been achieved, the root weld can be tacked, usually using tungsten inert gas welding, the root weld completed and a second 'hot pass' made to strengthen the root weld. It is not uncommon for the procedure, to this stage, to take as long as the filling of the rest of the weld preparation, using, normally, manual metal arc welding.

4.3

Inspection techniques

At various stages during a welding repair procedure, it will be necessary to carry out some form of inspection to ensure that the weld meets the

required standards. The techniques used are similar to those employed for surface based inspection of welds. By far the most widely used methods employ some form of visual inspection. This may involve cleaning the weld carefully, and then closely inspecting it either by eye or by the use of a closed circuit TV camera for surface breaking defects. Several methods can be used to enhance the contrast between the defect and the weld. For example, a magnetic fluorescent dye can be applied to the region of weld to be inspected. When a magnetic field is applied by means of a permanent or electromagnet, the field will tend to concentrate at the metallic discontinuity created by the defect, and will concentrate the dye to make the crack more conspicuous. Recently attempts have been made to develop techniques which make use of this effect utilising electronic sensors, such as Hall effect probes, which convert magnetic field strength directly into an electrical output. The internal structure of the material can be examined by the use of waterproofed ultrasonic sensing systems. These project a beam of high frequency sound waves into the material, and detect any signal reflected back to the sensor by internal defects. Such equipment, although effective, requires skilful operators to achieve reliable results, and is time consuming to use. Radiography is widely used for underwater NDE, with isotope radiation sources being used rather than the electrical generation of X-rays. The sensitive film is attached to the outside of the member to be inspected, usually by magnets, and the radiation source is placed either inside the member, or on the opposite side. Because of the dangers of radiation exposure to the divers, the source container is opened by remote control or timers, and considerable care is taken in the inspection procedure to ensure that this container is resealed before the divers return to remove the system. These techniques have been well established for many years, however. Several companies, most notably British Gas, have developed 'intelligent pigs' capable of being fed through transmission pipelines and carrying out a range of inspection tasks from the inside of the pipe. This technology is, however, extremely specialised. Other more innovative NDE techniques include the measurement of member wall thickness by the use of specialist ultrasonic probes, and the use of similar equipment to sense whether a member is flooded with water, indicating a weld defect. When a pipeline or jacket member is pressurised relative to the surrounding water, a sensitive microphone may detect the sound of gas or liquid leaking through a weld defect. Microphones may also be used to monitor the sounds emitted by a structure in response to wave loading. This information is then analysed to detect changes in the response of the structure which might indicate structural defects. The high level of

extraneous noise makes this analysis rather complex for practical use at present. Once the weld has been completed and inspected, frequently some form of wrapped coating or anodic protection is applied to the joint area before the pipe is lowered from the alignment system, the welding and alignment equipment then being recovered to the surface.

5 Underwater welding technology

Although alternative joining systems exist, as described in a previous chapter, arc welding is widely used for the repair and modification of underwater structures, and especially for pipelines, with their simpler geometry. The reasons for this are similar to those which led fabricators to utilise welding for the initial construction of these components - welding is a relatively quick and reliable joining technique, capable of producing light and efficient joints. In addition, it has a good track record as an offshore fabrication technique, inspection methods for arc welded joints are well established, and a great deal of data exists relating to the design of welded offshore structures. Three techniques exist for underwater welding - wet welding, one atmosphere welding and hyperbaric welding. They are described in more detail below.

5.1

Wet welding

Underwater welding when the arc is operated in direct contact with the water is termed wet welding (Fig. 5.1). It is the oldest underwater welding technique, being used during World War I to seal the heads of rivets which had been strained by mine damage. It is almost entirely carried out using the manual metal arc (MMA) process, and has as its major advantage the absence of a requirement for a welding habitat or chamber. This advantage must be offset against the low toughness and ductility of the deposited weld metal, the low deposition rate possible with wet welding and the high levels of skill required by the welders, although it is usually recognised that wet welding repairs, where appropriate, can be carried out more rapidly and hence more cheaply than equivalent hyperbaric procedures. Widely used in America, it has found little favour in Europe, mainly because of different environmental conditions and design philosophies. Although commonly regarded as a shallow water joining technique, wet welding procedures have been qualified and used at depths of 100 metres.

Arc operating in contact with the surrounding water

5.1 A schematic diagram of wet welding.

During wet welding, because of the proximity of cold sea water to the weld pool, high cooling rates are experienced by the weld metal and its associated heat-affected zone. In addition, dissociation of water within the welding arc ensures the presence of hydrogen in the weld pool. Both of these phenomena adversely affect the final weld. The high cooling rates generated in the HAZ and weld metal, for the types of steel used in offshore construction, generate brittle metallurgical structures of low toughness. These structures are sensitive to hydrogen-induced cold cracking (HICC), a dangerous weld defect which will occur when a weld includes a cracksensitive structure, and is subject to both hydrogen and high levels of stress. In practical structures, the residual stresses generated by the welding process are sufficient to initiate HICC, without additional external loading. In addition, the high cooling rates cause the weld metal to solidify more rapidly than in surface welding, and this may trap slag or gas pores within the weldment or produce weld bead shape defects. Based on reports from diver/welders, it is known that overhead welding is more difficult than welding in other positions, probably due to the disruption of the arc by bubbles of vapour generated by the process. Operationally, various techniques are used to overcome these problems. High heat input welding procedures are used, to combat the enhanced cooling rates, and welders are highly trained to use short arc lengths and carefully regulated electrode weave techniques to minimise hydrogen gen-

eration and weldment shape defects. By the use of such techniques, welded joints can be made which are reasonably strong (perhaps 80% as strong as between similar materials in the dry), but which have lower levels of ductility and toughness than similar welds made in dry environments. It is thought that this is due to the presence of microscopically small cracks in any wet weld, which coalesce to initiate rapid failure when the material is strained. Until recently, wet welding was not recommended for base materials having a higher carbon equivalent (CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15) than 0.4%, because of the risk of HICC. However, recent research by Global Diving and the Colorado School of Mines has developed nickel-based welding electrodes capable of wet welding 0.46% CE steels, in conjunction with improved procedures incorporating the use of multiple temper bead techniques to soften the heat-affected zones of the welds and hence reduce the likelihood of HICC. Once these problems are appreciated, it is possible to design repair procedures to minimise their effects. Weld cross sections are made larger than would be used in dry environments, to reduce weld metal stress levels. The joint is redesigned to incorporate additional stiffening, including saddle plates on the tubular members, and fish plates between members, in order to resist relative movement which would generate strains in the weld metal, causing cracks to propagate. Specially shaped joining pieces can be used to minimise the amount of overhead welding required in a joint. These techniques will add to the complexity of the final joint, and the amount of weld metal that must be deposited during a wet welding repair procedure. In specific cases, it will be necessary to balance this against the effort required to deploy a welding habitat to carry out a hyperbaric repair procedure. Wet welding is widely used in the USA, and the most useful document providing design guidance for wet welding is the current AWS Standard (AWS D3.6). In Europe, with lower temperatures, steels of higher carbon equivalent (CE) and larger structures, it has always been viewed with considerable suspicion. A wet weld was first carried out in the North Sea on a secondary structural member in 1991, and there has been an increased level of interest in the technique recently. However, the widespread utilisation of the technique has not yet developed beyond the attachment of noncritical items such as anode attachments.

5.2

One atmosphere welding

Because a large body of welding technology exists relating to normal atmospheric pressure, a logical approach to underwater welding repair would be to replicate surface welding conditions at the required subsea location. The worksite is surrounded by a chamber constructed as a

pressure vessel, capable of withstanding the water pressure at the depth of the repair location. This pressure increases by approximately 1 bar for every 10 metres of water depth. Once the chamber is in place and sealed to the structure, it is dewatered and the internal pressure can then be reduced to one atmosphere (Fig. 5.2). The repair crew can transfer to the welding chamber in a one atmosphere environment, within a diving bell, to carry out the repair. Despite having the great advantage that the technique can utilise the consumables and welding procedures developed for the surface based construction of the structure under repair, one atmosphere systems have been little used. Their major drawback centres around the problem of sealing the working chamber to a structure or pipeline with a joint capable of ensuring the pressure integrity of the chamber. Operational systems were developed some years ago by the Brazilian oil company Petrobras and Lockheed Petroleum Systems (LPS - a subsidiary of the aircraft company dedicated to offshore engineering systems) for use in the Amazon basin. These were used for tie-in situations, where the end of the pipeline could be fitted with a special coupling for the pressure joint, and where the work chambers were prepositioned on the platform structure. Riser pipes were pre-installed from the welding chamber to the working area of the platform, and the top of the welding chamber incorporated a bell locking ring

Ambient pressure

High pressure seal

One atmosphere pressure

5.2 A schematic diagram of one atmosphere welding.

to enable divers to transfer to the welding chamber at one atmosphere environmental pressure, once the preparatory work was completed. A specially designed pipe end was used, incorporating an attachment point for the cable which pulled the pipe into place, and a spherical fitting which matched a corresponding socket in the chamber wall, forming the required pressure seal. In use, tension on the cable drew the pipe end into the chamber, so that the spherical section came into contact with the socket. The joint could then be completed by inserting and tightening a ring of bolts, clamping the sphere into place and compressing the polymer seals. In its original form, it was intended that divers should perform this task, but current technology would permit the use of an ROV. Once the joint was sealed, the chamber could be dewatered and depressurised, enabling divers to enter, cut the cable fitting off the end of the pipe, and install a length of pipe from the cut end to the pre-installed riser pipe. Although the system would seem feasible for pre-planned operations, the utilisation of such techniques for repair, with the attendant problem of making an effective pressure seal on to a surface from which the weightcoat has been removed, and which may be subject to surface corrosion and the like, would seem difficult. It is very possible that the technology might have been developed further if hyperbaric welding, described below, had not proved so effective operationally. A special case of one atmosphere welding is the use of cofferdams in the splash zone and for shallow depths. Wave forces, tidal action and high levels of volume change make the splash zone an uncomfortable work location, and at least one company has sought to avoid these problems by surrounding the worksite with a pressure resistant chamber connected to the surface by a tube containing an access ladder (Fig. 5.3). The differential pressure in this case is modest, and effective sealing techniques were developed. Considerable effort was expended on ventilation and safety procedures, but the technique has proved practical for certain specialised applications, and has been used for inspection and maintenance procedures, as well as for joining.

5.3

Hyperbaric welding

Hyperbaric welding is the most widely used repair technique for primary structures and pipelines, and represents an engineering compromise between wet and one atmosphere welding, as it removes the major difficulties attendant on both those techniques, the wet environment and the high pressure differential across the chamber. The repair site is enclosed within a working habitat, made of relatively lightweight materials, as it need only resist modest pressure differences. The worksite is dewatered by filling the habitat with gas, which displaces the water (Fig. 5.4). Simple

Support structure

Access ladder

Water surface

One atmosphere pressure

5.3 A schematic d i a g r a m of c o f f e r d a m w e l d i n g .

HP gas supply

Wedge seals

Ambient pressure - gas Ambient pressure - water 5.4 A schematic d i a g r a m of hyperbaric w e l d i n g .

Reinforced chamber

hydrostatics indicates that the gas and water will be at equal pressures at the interface between them, at a point close to the bottom of the chamber, and so the maximum differential will be at the top of the chamber, where the internal pressure will exceed the outside pressure by an amount relating to the height of the chamber, and thus normally by a few tenths of a bar (10s of kPa). This differential pressure is easily resisted by a lightweight steel structure and simple flexible seals, making deployment and sealing of the workchamber operationally feasible. The problem with hyperbaric techniques is that the environmental pressure at which the weld is carried out is essentially that of the worksite. These elevated pressures affect the gas/slag/metal reactions for all welding processes, and the high density gas enhances the rate of heat loss from the weld. The effect of pressure on the various hyperbaric welding processes will be discussed in more detail in the sections on welding technology. Hyperbaric welding research is mainly concerned with ensuring that, for any specific environmental pressure and composition, welding parameters can be specified that will ensure the production of welded joints with properties acceptable to the certification authorities responsible for the structure on which the weld is being made.

5.4

Hyperbaric welding chambers

This section provides a brief description of the design of hyperbaric welding chambers. Because of their radically different design, it is convenient to divide this into two parts - one dealing with chambers for the welding of pipelines, and the other relating to platform structural repairs.

5.4.1 Pipeline welding chambers The geometry of pipeline joints is extremely simple, being a simple length of tube including the pipe ends to be welded. The chamber can be made in the form of a saddle, with open ends so that it can be placed over the pipeline, to which it is secured to counteract the buoyancy forces which occur when the chamber is dewatered. Figure 5.5 shows a typical small chamber design from the 1970s, and indicates how the design of a simple chamber can be modified to permit welding all round a pipe. The chamber is made with two triangular metal ends, which are split to enable them to be removed from the pipe. These are connected to form the frame of the chamber, which is sealed to the pipe using wide strips of neoprene held in place by straps, wrapped around the seal areas indicated in Fig. 5.5. The acrylic sheets slid into seals on the side of the chamber can be raised as required, enabling the diver to reach progressively further around the pipe as the weld proceeds. It quickly became apparent,

Raised acrylic side panel

T clamps Seal area

Seal area

Open base 5.5 A simple triangular welding chamber.

however, that the access problems associated with such small chambers precluded their use except for the simplest of joints, and chambers expanded to provide significant clearance around the pipe, permitting the diver to partially or fully enter the chamber. Large chambers are commonly constructed of sheet steel, 1 to 3 mm thick, on a frame of steel tube or angle material. This type of construction is amply strong enough to resist the modest differential pressures, and can be readily fabricated and modified for specific applications. For pipeline work, the chamber is commonly made as a single unit, with removable seals to fit specific pipe sizes. The chamber has slots in two opposite side walls, enabling it to be positioned over the pipe, after which the seals are clamped around the pipe and tightened to make a waterproof seal. For different sizes of pipe, only the seals need to be changed. Apertures for seals of this type can be seen below the pipe alignment unit in Fig. 4.4. The basic design of the welding chamber can be developed in a number of ways. Usually for pipe joining applications, it is incorporated into some form of pipe handling system (Fig. 4.4). When this is done, the weight and size of the chamber becomes less important, and so the walls of the chamber often incorporate waterproof housings to contain the various items of equipment required for the joining procedure. These not only include the equipment required, such as the welding and pipe preparation systems, but also the electrical and other connections needed to power the equipment. Such housings can be seen as cylindrical projections from the welding chamber in Fig. 4.4. In the case of the more sophisticated dry transfer chambers, a solid floor is often installed once the pipe is in place, with pressure compensation being

carried out by a flexible membrane or piston system, to enable a drier environment to be achieved in the chamber.

5.4.2 Structural welding chambers The supporting structure of steel platforms is a complex space frame, made up of an array of horizontal, vertical and diagonal tubular members, which may be several metres in diameter and with wall thicknesses of several tens of millimetres. These members join at points called 'nodes' - complex structures, which may be cast or fabricated by welding, which transfer loads between members through the structure. Both the nodes and the tubular members are usually fabricated in workshops, but they are assembled in large graving docks close to the sea to facilitate transport of the completed structure. The structure therefore incorporates a large number of welds between the tubular members and the nodes, sufficiently close to the node points that it is not possible to provide adequate clearance within a welding chamber around the member being welded, and it is therefore normally necessary to enclose the complete node within a hyperbaric chamber. Such chambers must be fabricated in several pieces, which are lowered to the worksite and bolted together around the structure. This process is further complicated because the tolerances to which such structures are made mean that the position of any tubular member may be some tens of millimetres different from that specified on the drawing - not surprising in a structure some hundreds of metres tall. This is accommodated by mounting the circular seals for each tubular member on separate plates, which can be moved relative to the main chamber structure, enabling effective sealing to be achieved. It must also be recognised that the chamber will exert loading on the structure, the largest of which is likely to be the positive buoyancy loading when the chamber is dewatered. Each cubic metre of sea water weighs just over one tonne, and while such a load is unlikely to affect the stability of the complete structure, it is necessary to ensure that it is transferred into the main structure effectively and without causing local damage. Figure 5.6 shows a simplified schematic diagram of such a chamber. The main body is constructed from steel sheet and angle, and is designed so that the various sections can be fitted between the structural members before being assembled. They are normally bolted together, neoprene gaskets being used to improve chamber sealing. Once the basic chamber is in place, the individual tubular member sealing plates can be fitted, and then clamped and sealed to the main chamber body. The process of installing such a chamber is a lengthy one - the regions where the seals are to fit must be cleaned of corrosion and marine growth,

Separate plates to seal tubular members

Clamps

Structure

Chamber bolted together along these flanges

5.6 A structural welding chamber.

if present, the chamber assembled, the sealing panels fitted, the chamber dewatered and the seals checked and adjusted. Finally, appropriate supporting structures must be installed so that welders can work within the chamber and the equipment required for the joining procedure can be installed and tested. This process may take longer than the welding procedure itself, and indicates why the industry has always sought to utilise repair techniques on structures which do not require the use of hyperbaric welding chambers. In an attempt to simplify this process, flexible chambers have been developed which can be unrolled around a node structure, and literally sewn together - these can accommodate the changes from design geometry described above much more easily than rigid chambers. However, ancillary equipment cannot be accommodated within the structure of a flexible chamber, staging must be provided for divers to work within the chamber, and the buoyancy loading problem described above must be overcome, usually by the use of a weight around the lower periphery of the chamber. To illustrate the operational flexibility of the hyperbaric chamber concept, the repair carried out in 1991 by Comex on a platform jacket in the Magnus oilfield utilised an ingenious combination of flexible and rigid chamber to overcome access problems at the repair site. During a survey

5.7 A hybrid solid/flexible welding chamber used on the Magnus platform.

of the platform on their Magnus field, BP had discovered a crack in a major structural member. The problem was made more complex because the site of the defect was immediately adjacent to riser pipes, which provided only 700 mm of clearance between the riser and the member to be repaired. It was therefore necessary to design a chamber which would maximise access to the repair site, and provide the facilities required to carry out the repair procedure. The chamber manufactured to overcome this problem largely comprised a conventional rigid structure, within the walls of which waterproof containers held the equipment required for the welding procedure, and incorporating a solid floor to provide a secure footing for the welders (Fig. 5.7). This was clamped to a horizontal member adjacent to the worksite. One wall of the chamber was made of flexible material, so that it could press

against adjacent riser pipes, maximising the working clearance within the chamber. Such developments indicate the manner in which hyperbaric chambers can be modified to incorporate the requirements of specific applications.

5.4.3 Chamber gases Once the welding chamber, of either type, is installed and sealed, it is necessary to displace the water from the chamber. As indicated earlier, this is achieved by admitting gas into the chamber, and when the gas/water interface has stabilised, the pressure of that gas will be approximately that of the surrounding water, rising by about one atmosphere for each ten metres of water depth. Several gases have been used for this purpose, the choice being dependent on a variety of factors. Air has several attractive features as a chamber gas - it is cheap and a large supply is readily available, requiring only compression to the appropriate pressure. However, the high oxygen content of air (-20%) means that at pressures greater than a few bar, the flammability of objects within the chamber is greatly increased, creating a significant safety problem. In addition, in order to maintain the quality of the weld, the weld pool must be protected from both the nitrogen and oxygen in the air, something not easily achieved at high pressures. Air may be a viable chamber gas for splash zone operations, but should not be used at greater depths. Argon is readily available, and has a density similar to that of air. It has a lower thermal conductivity than helium, reducing weld metal cooling rates. However, at high pressures argon can have a narcotic effect on divers, so its use is considered hazardous for manned hyperbaric welding operations. Because of this narcotic effect, when argon is used as a shielding gas, for example for hyperbaric TIG welding, the composition of the chamber gas is periodically analysed, and flushed to clear argon build-up when this becomes excessive. As the problems associated with the use of argon relate to its physiological effects, for diverless welding operations, argon would seem to be the logical chamber gas to use. Helium is significantly more expensive than argon, and is about one-tenth as dense. The major benefit of using helium chamber gas is that it is similar to the gas mixtures breathed by divers when in saturation, at depths greater than 50 metres. A typical diving gas consists of helium, to which a partial pressure of 0.5 bar oxygen is added, irrespective of depth. It is common practice to use such mixtures as a chamber gas for manned welding operations, as the low level of oxygen has little effect on flammability, and the mixture does not represent a hazard to the diving personnel. A significant problem with helium is its high thermal conductivity which increases weld metal cooling rates.

5.5

Summary

It can therefore be seen that, as a result of many years development by the offshore industry, a wide range of welding techniques, and the necessary associated equipment, are available for underwater joining procedures. In many cases, the final decisions relating to the technique employed will require careful evaluation of the problems specific to the individual procedure, particularly those of weld joint size and complexity, depth, tide conditions, access and available space. The availability of equipment, support vessels and appropriately trained and qualified diving teams may also be influential. For these reasons, it is not possible to give guidance which would be universally applicable, but it is hoped that the techniques described above provide ideas relevant to specific situations.

6 Manual hyperbaric welding techniques

Most hyperbaric welding is carried out manually, using techniques closely resembling those used for general fabrication in industry. It was previously stated that in order for hyperbaric welding to be effectively used, the influence of pressure on the welding process must be understood, and procedures must be developed to compensate for these effects. Unless a welding process can be shown to reliably deposit weld metal of acceptable quality, it cannot be accepted for use offshore. The following chapter describes the welding processes which have been used hyperbarically in actual offshore applications, a later chapter being devoted to possible new techniques which may be utilised in the future. For the general reader, the main features of each welding process are described. The influence of environmental pressure, and the changes required to operate the process hyperbarically, are then discussed.

6.1

Tungsten inert gas welding (TIG)

6.1.1 The process Many welding processes are known by several names, usually derived from different parts of the world. In the UK, the term TIG welding is most widely used, but the process is also known as gas tungsten arc welding (GTAW), Argonarc welding (a BOC trade name) and heliarc welding, the latter most commonly used in the USA, where helium is more commonly used as a shielding gas. TIG welding, when carried out manually, requires the use of a welding torch which holds an electrode principally made of tungsten. The forward end of this is sharpened to a point, and an arc is struck between this end and the workpiece (Fig. 6.1). Heat from the arc melts the workpiece, creating a molten pool which will solidify to form the weld deposit. Around the electrode there is a ceramic shroud through which a shielding gas, usually argon or helium, is passed to create an inert atmosphere to protect the elec-

Gas nozzle Filler rod

Non-consumable tungsten electrode Gas shield

Parent metal

Weld pool

Weld metal

6.1 Tungsten inert gas (TIG) welding.

trode and weld pool from atmospheric contamination. Welds can be made using the arc alone, without the addition of further material, such welds being termed autogenous. However, in most cases more material will be required to fill the joint preparation, and this is added separately, in the form of lengths of wire held in the welder's other hand. These features of the TIG process - the independence of the source of heat from the supply of filler material, and the ability to manipulate the torch and filler wire independently - mean that while the technique requires considerable skill and training, it is also considered the most flexible and controllable process available. The arc length - the distance between the end of the electrode and the workpiece - is directly under the control of the welder. To minimise the effect of changes in arc length, the power supply used for TIG welding is of the constant current type, configured so that for the range of output voltages normally employed for welding, the output current does not change significantly. The TIG welding of ferrous materials is normally carried out with the electrode connected to the negative terminal of the welding power supply, the workpiece being connected to the positive terminal. This is because for a constant arc current, approximately twice as much heat is generated at the positive end of the arc as the negative end, and it is therefore possible to operate a specific welding torch at higher currents if negative electrode polarity is used. The basic physics of the TIG arc is the same in all applications. The arc voltage in TIG is made up of three elements - the two 'fall' voltages and the 'column' voltage. The fall voltages are associated with the energy required to displace the electrons carrying the electric current from within

the electrode to the arc, and from the arc into the weld pool. These do not seem to be significantly affected by environmental pressure or composition, and amount to approximately 9 volts in total. The column voltage is proportional to the arc length, and the voltage drop per millimetre of arc length is termed the electric field strength. This is, for argon, approximately 0.8 V/mm, and for helium approximately 1.8V/mm. Thus, a 3 mm long TIG arc, in argon, would have an operating voltage of about 11.4 V, this value being only slightly affected by operating current for currents above about 60 A. The higher electric field strength of a helium shielded arc means that for the same arc length a higher arc voltage, and hence arc power, will be generated. Hence, the melting power and deposition rate of a helium shielded arc will be higher than that of a corresponding argon shielded arc, although the higher price of helium makes this advantage less attractive. Until recently it has been common practice to initiate the TIG arc by a high frequency (HF), high voltage discharge across the arc gap, the discharge being obtained from a special oscillator circuit incorporated into the TIG power supply. The discharge ionises the gas in the arc gap sufficiently to allow the main power supply to pass current through the arc. This technique is losing favour generally, as it is difficult to generate a discharge which can effectively start the arc while complying with recent European legislation on electromagnetic radiation and interference, and HF discharges may damage welding parameter data collection systems unless special precautions are taken. HF initiation is increasingly being supplanted by touch striking - touching the end of the electrode directly on to the weld material itself, as the higher levels of control possible with modern power supplies permit sufficiently close regulation of the short-circuit current to enable this to be carried out without damage to the electrode and consequent tungsten deposits in the weld metal. Modern inverter based TIG welding power supplies, incorporating touch striking facilities, are commercially available and their use is becoming more widespread. TIG electrodes are made by sintering - powdered tungsten metal is mixed with doping materials such as thorium oxide (thoria), which is then heated and pressed to form a solid material. If the powder grain size is wrong, or the constituents are inadequately mixed, 'hot spots', caused by a lack of uniformity in the material, can generate arc instabilities and problems such as the arc not rooting on the tip of the electrode. For critical applications, therefore, only high quality tungsten electrodes should be used.

6.1.2 Hyperbaric operation For similar reasons to those discussed above, TIG is the most controllable technique available to hyperbaric welding engineers. Because of its sim-

plicity, and because the arc can be operated for long periods without depositing weld metal, it has been widely studied by process physicists, and is better understood than other welding techniques. Hyperbaric manual TIG welding systems are normally identical with surface based equipment, the sole modification being the removal of the operator's control switch on the torch, the power supply being controlled by the surface support crew. Welding is performed in a similar manner to surface practice, the welder/diver holding the torch in one hand and a rod of filler material in the other. Because heat, from the arc, and filler material can be supplied independently, the welder has a high level of control over the size, shape and fusion characteristics of the deposited bead, and can accommodate significant changes in the weld preparation, such as varying root gap and misaligned pipe walls. This has led to its use for critical welding situations such as the root and hot pass welds on pipe joints, and for capping passes and temper beads where the shape and hardness of the material at the toe of a weld must be controlled, normally for fatigue resistance. Many of the procedures and techniques developed for the fabrication of structures and pipelines on the surface can be adapted, with little modification, for hyperbaric operations, provided the effect of pressure on the TIG welding arc is understood. In the enclosed, high pressure environment of a hyperbaric workchamber it was not regarded as safe to use an HF discharge to initiate the arc, and it was common practice to strike the arc by touching the electrode on to a carbon block placed on the workpiece. This avoided the possibility of contamination of the joint with tungsten, if the short-circuit current delivered by the welding power supply proved excessive. Modern power supplies have improved current control, and it should now be possible to initiate the arc by direct contact between the tungsten and the workpiece. The principal effect of pressure on the TIG arc is to increase the arc voltage. The separation of the arc voltage into its 'fall' and 'column' elements has already been discussed, and pressure appears to have little influence on the fall components. However, the electric field strength is affected by pressure, and measurements show that it is proportional to the square root of the absolute pressure, so at a depth of 30 metres, where the gauge pressure is 3 atmospheres (3 bar, 30OkPa), the absolute pressure will be 4 times atmospheric, and the electric field strength will be twice that at sea level. This can be expressed as the equation: V ARC =9 + £/VP where VARC is the arc voltage, E is the electric field strength at one atmosphere in volts per millimetre, P is the absolute pressure in bar, and / is the arc length in millimetres (Fig. 6.2). At the currents normally used

Helium 10bar

Argon 10bar

Arc Voltage (V)

Helium 1bar

Argon 1bar

Arc length (mm) 6.2 The effect of pressure and shielding gas composition on TIG arc voltages.

operationally for TIG welding, arc voltage is only marginally affected by operating current. If a mixture of gases is used, the electric field strength is proportional to the composition, thus a 75% argon, 25% helium mixture would have a field strength of: E = 0.75 x 0.8 + 0.25 x 1.8 = 0.6 + 0.45 = 1.05 V/mm Thus, if one wished to work at a 4 mm arc length, using a 75%Ar/25%He shielding gas, at 80 metres water depth, the required arc voltage would be: yARC = 9 + (V9 x 1.05 x 4) = 21.6 volts If this was provided by a power supply sited on the surface, it would be necessary to add to this figure the additional voltage required because of the resistance of the cables connecting the power supply to the welding habitat. Because of these environmental effects on TIG arc voltage, these factors must be specified before practical voltage operating limits can be established in a welding procedure. This increase in operating voltage is believed to be caused by the enhanced heat extraction capability of the high pressure hyperbaric environment. Because heat is more easily lost from the outer regions of

Process efficiency (%)

Pressure (bar) 6.3 The effect of pressure on TIG process efficiency.

the arc, the arc can minimise heat losses by reducing its diameter. However, this then requires the arc current to be carried by a smaller arc crosssectional area, increasing the arc current density, ionisation level and operating temperature. The additional energy required increases the arc voltage as described above. More detailed descriptions of the physics of high pressure TIG arcs can be found in the bibliography. The efficiency of a welding process is defined as the ratio between the amount of heat generated transferred to the workpiece, and the electrical power of the arc - the arc voltage multiplied by the current. For the TIG process, this falls from approximately 90% at one atmosphere, to about 70% at 6 bar (6 x 102kPa), recovering to about 75% by 8 bar (8 x 102kPa), and remaining constant thereafter (Fig. 6.3). For a constant operating current, the rate of increase of arc voltage with depth is faster than the reduction in process efficiency, and thus the amount of process power available increases with depth. This means that at a depth of approximately 300 metres, TIG can achieve comparable deposition rates with MMA. These deposition rates can be enhanced by the addition of helium to the normally argon rich shielding gas, as in surface based welding practice, but excessive helium additions can cause electrode erosion. It was found in the early 1980s that as environmental pressure is increased, the stability of the TIG arc is reduced. This instability takes the

form of a random oscillation of the arc around its root on the tungsten electrode. Closer analysis of the effect demonstrated that the magnitude of the instability was linked to the mass flow of the shielding gas supply, and it is now believed that it is due to aerodynamic effects associated with the shape of the tip of the tungsten electrode, coupled with buoyancy effects in the outer regions of the arc. In addition, it has been suggested that the velocity of material within the arc falls as pressure is increased, and this would explain the observed increase in the susceptibility of the TIG arc to external magnetic fields. These two effects combine to set the practical depth limit for hyperbaric TIG operations, probably of the order of 500 metres, although this would vary in specific situations, as the stability of the arc is affected by operating current, arc length, weld preparation geometry and the presence of external magnetic fields. In the process description, the effect of poor quality tungsten electrodes on process performance was discussed. Hyperbaric conditions, with reduced arc stability and higher electric field strengths, make this problem worse. It is therefore essential to use only high quality tungsten electrodes for hyperbaric operations, performance testing being undertaken before supplies from a new batch or manufacturer are used operationally. Because of the depth limitations of TIG, it seems likely that it will be supplanted for the proposed diverless welding systems either by plasma welding or some form of MIG welding, both of which will be discussed later. However, because of its relative simplicity and controllability, it will probably remain in use for manual welding and shallow automated applications (less than 300 metres) for the foreseeable future. One significant development in hyperbaric welding is the introduction of a wider range of materials supplanting the traditional low alloy steels. A range of stainless steels, including duplex formulations, aluminium and titanium have been proposed for underwater structures, in order to save weight or enhance corrosion resistance. In many cases, the initial welding procedure development relating to these materials will be undertaken using the TIG process. It seems unlikely that a significant amount of process development will relate to manual TIG, although the introduction of touch striking and arc current pulsing for manual use may follow the availability of suitably equipped welding power supplies. TIG is also used for the automated welding of pipeline joints, and this is discussed in a later section.

6.2

Manual metal arc welding (MMA)

6.2.1 The process Manual metal arc is also known as shielded metal arc welding (SMAW) and 'stick' welding. It utilises the simplest equipment of any arc welding process,

there being no requirement for shielding gas. The consumable used consists of a length of mild or low alloy steel wire of 1.6 to 5 mm diameter, 300 to 450 mm long. The wire is coated with a layer of flux material, which serves several functions. Its primary role is to provide a protective environment for the arc and weld pool. The first is formed by the dissociation, by the arc, of flux components into CO, CO2, or metallic vapour which displace the surrounding air and protect the arc and molten pool from oxidation. Flux components also break down to form a viscous slag, which covers the hot weld bead and protects it as it cools (Fig. 6.4). This slag must be removed before subsequent weld beads can be deposited. The flux can also contain other components such as arc stabilisers, additional metal powder to enhance deposition rate, alloying elements to modify the composition of the weld metal, and exothermic compounds which generate additional heat to improve fusion characteristics. MMA welding electrodes are available in a wide range of weld metal compositions, and with many flux formulations. As supplied, MMA electrodes have the flux removed from one end for about 20 mm, so that they can be inserted into an electrode holder. This normally consists of a simple insulated handle which the welder holds, and which incorporates a clip or screw device to grip the electrode, and to connect it to the welding power supply via a flexible cable. The workpiece is connected to the other output terminal of the power supply. MMA electrodes can be operated using alternating current, or electrode positive or negative, depending on the formulation of the flux coating. The arc is then initiated by rubbing the other end of the electrode against the workpiece until contact is made and current flows, after which the electrode is lifted a few millimetres from the workpiece to form the arc. The heat of the arc

Consumable electrode Flux covering Evolved gas shield Weld metal

Slag

6.4 Manual metal arc (MMA) welding.

Core wire

Arc

Weld pool

Parent metal

both breaks down the flux and melts the metallic core, which transfers to the workpiece. As metal is removed, the welder compensates by moving the electrode closer to the workpiece in order to maintain a constant arc length, at the same time moving the electrode along the joint to form the deposited weld bead. Periodically, the electrode will be consumed to an inconveniently short length, at which point the arc must be interrupted while a new electrode is attached to the holder. The welding process is therefore intermittent. To control the welding current during the arc initiation phase, and for similar reasons to those given for TIG welding, MMA power supplies are of constant current design, to stabilise arc current and electrode burn-off rate. If the output voltage of the power supply is measured when no arc is running, this is termed the 'open circuit voltage', and is normally of the order 60 to 80 V to assist arc initiation, while arc operating voltages are of the order 20 to 25 V. Manual MMA welding is not a difficult process to use, but does demand manipulative skill if acceptable weld bead shapes are to be deposited, and conscientious removal of weld bead slag if defects are to be avoided. Because of the time required to change welding electrodes and to clean the weld bead, the welder will typically only be running the arc for about 30% of the time, limiting the productivity of the process. However, the flexibility of the process, and the simplicity of the equipment used, make it a very attractive process for many industrial situations, particularly for outdoor and site welding applications. Operationally, a major concern relating to MMA welding is the level of hydrogen in the deposited weld metal. If this is excessive, and is combined with a susceptible microstructure and applied stress, it can cause cracking, usually in the heat affected zone of the weld deposit. A common source of hydrogen in MMA welding is moisture absorbed by the flux, which is normally an extruded material held together by organic binding agents. To minimise hydrogen levels, special low-hydrogen electrodes are used, which are either stored in sealed packages, or need to be dried by heating for a specified period prior to use. In either case, the electrodes can only be exposed to the air for a limited amount of time before a further drying operation is required.

6.2.2 Hyperbaric operation Hyperbaric manual metal arc welding (MMA) is carried out using fluxcovered welding electrodes, electrode holders and welding techniques very similar to those used for MMA welding on the surface. Because of the simplicity of the technique and equipment, and the availability of a relatively large number of diver/welders trained in MMA, it is the most widely used

operational repair technique. The principal problem with the technique is its vulnerability to hydrogen induced cold cracking (HICC), which necessitates close control over welding technique, and also over the preparation, storage and handling of the welding electrodes. Hyperbaric MMA has changed relatively little over the past ten years, and very closely resembles surface based MMA welding, although alternating current is not used, as the periodic extinction of the arc reduces the stability of the process. The diver/welder strikes an arc and manipulates the electrode and weld pool in a similar manner to surface based welding. However, certain differences do exist which can create problems. Unlike TIG, MMA arc voltages vary relatively little with pressure as they are governed by the presence of easily ionised material within the arc, derived from the breakdown of compounds contained in the flux. In general, for depths down to approximately 200 metres, the arc voltage will rise from about 20 volts to about 25-27 volts, although this will, of course, vary from operator to operator. However, over the first 60 metres of depth, for a constant current and positive polarity, the electrode burn-off rate increases by about 50%. Similar effects, although not so marked, are observed with electrode negative operation, but the stability of electrode negative operation is significantly worse than electrode positive, so that it is less widely used. When this increase in burn-off occurs, the arc voltage has not increased significantly, and hence the arc energy has not increased, but more of it has been utilised for the melting of the electrode, leaving less available for the melting of the substrate material. Tracings made of sections taken from welds made at the same arc current, but at varying depths (Fig. 6.5) show that whereas on the surface the amount of deposited weld metal and fused parent plate are in the ratio of about 2:1, even by 20 or 30 metres this ratio has increased considerably, to 5 or 6:1. For the same operating conditions, the weld consists of more deposited metal, with less fusion into the weld joint. This causes problems relating to weld pool control in positions other than downhand, which are frequently reported by divers as an excess of

Air 1 bar

Argon 5bar

Argon 16bar

6.5 The effect of pressure on MMA weld bead shapes.

weld metal fluidity, and which are currently overcome by reducing the operating current and electrode size. This effect places an upper limit on the heat input at which it is possible to operate when welding positionally with MMA under hyperbaric conditions. This effect is reinforced by changes in the composition of the deposited weld metal as the operating depth is increased. In a similar manner to surface welding, hyperbaric MMA electrodes protect the weld pool by the addition of metallic carbonates to the flux material. When exposed to the high temperatures of the arc, these dissociate, resulting in the generation of carbon monoxide and dioxide, which protect the weld pool. Under highpressure conditions the balance of these reactions is tipped away from gaseous product (this effect is known as Sievert's Law), and the carbon and oxygen are transferred into the weld pool. The oxygen combines with deoxidants such as silicon and manganese, removing them from the weld metal. Thus the weld metal composition, as depth increases, will progressively change in a manner which tends to make it more hardenable (Fig. 6.6). Attempts have been made to overcome these effects by the compounding of electrodes with reduced carbon levels and alternative shielding systems. These have been only partially successful - the changes with depth remain although the composition can be modified for a specific depth range - and such electrodes are considerably more costly than standard formulations.

Pressure (bar) 6.6 The effect of pressure on MMA weld bead composition.

In addition, the high-pressure gaseous atmosphere within the welding habitat conducts heat much more readily than air at one atmosphere. This effect is particularly noticeable when helium rich gases are used within the chamber, to maintain compatibility with the diver's breathing gas mixture. This high thermal conductivity results in much increased cooling rates, which have two effects on the weld metal. Faster cooling from about 8000C to 5000C creates a weld metal microstructure which is more brittle and sensitive to cracking than if cooled more slowly, while time spent at any temperature above about 1000C allows time for any hydrogen present to diffuse away from the weld zone (Fig. 6.7). The use of relatively high preheat levels can markedly increase the time available for the diffusion of hydrogen away from the weld, but can only have a marginal effect on weld metal hardness, as this is controlled by events at much higher temperatures (Fig. 6.8). In short, elevated environmental pressure restricts the available heat input, while adversely affecting the weld metal chemistry, and causing faster cooling rates which result in a metallurgical structure more sensitive to cracking, while the damp hyperbaric environment can generate higher levels of hydrogen, a major cause of cracking. However, these problems have been overcome by the development of effective welding procedures, and careful working practice in the field. Temperature (degC)

Time

TIOOhyp

6.7 The effect of pressure on weld bead cooling rates.

TIOOamb

Helium 150m Helium 75m HAZ Hardness Hv10

Preheat temperature (degC) 6.8 The effect of pressure and preheat levels on weld metal hardness.

Most welding procedures concentrate on reducing the level of hydrogen available in the arc atmosphere. It has been shown that most of this hydrogen does not come directly from humidity in the habitat atmosphere, but from moisture absorbed as water of crystallisation within the flux material on the electrode when the electrode is exposed to that atmosphere (Fig. 6.9). A combination of housekeeping measures such as careful predrying of the electrodes, combined with storage in sealed, desiccated or heated containers while being transported to the habitat, and restriction of the time an electrode is permitted to be exposed to the habitat atmosphere before use, enable weld metal hydrogen to be maintained at a sufficiently low level to permit the production of satisfactory welded joints. There have been few changes of any significance to the technology of hyperbaric MMA over the past ten years. Its main attractions are the simplicity of the process and the associated equipment and consumables, and the availability of a relatively large pool of qualified diver/welders. Major innovations, such as the use of high performance power supplies, would negate some of these advantages, and to date there seems to be little enthusiasm for them. Recently, MMA consumables have been developed which are more resistant to moist environments than previous types - these were mainly developed for field welding in surface situations - and these are being adopted by some hyperbaric welding contractors.

Hydrogen content (ppm)

Exposure time (hr) 6.9 The effect of exposure to moisture, and pressure, on weld metal hydrogen levels.

Current or proposed automated and diverless welding systems are not based on the MMA process, which would be extremely difficult to adapt to automated or robotic operation. It therefore seems most unlikely that the technique will be utilised for these applications.

6.3

Metal inert gas welding (MIG)

6.3.1 The process Metal inert gas welding was originally developed in the 1940s for the welding of aluminium structures. Its use was extended to steel when it was discovered that it could operate effectively using carbon dioxide as a shielding gas, and it has since been applied to a wide range of other materials. It is also known as gas metal arc welding (GMAW). Strictly, the term MIG should only be used when the shielding gas is fully inert; when active elements such as carbon dioxide or oxygen are added, the process should be called metal active gas (MAG) welding. To further complicate matters, when consumables capable of operating without shielding gas were developed, the term MOG (metal ohne gas - metal without gas) was proposed. However, the term MIG is often used to describe all the process variants.

Gas nozzle Consumable electrode

Contact tube

Gas shield Parent metal

Weld pool

Arc

Weld metal

6.10 Metal inert gas (MIG) welding.

MIG is a consumable electrode process. A hand torch is used, and a continuous wire electrode, between 0.6 and 2 millimetres in diameter, is fed through it (Fig. 6.10). This wire, and the workpiece, are connected to the output poles of the welding power supply, so that when the wire touches the workpiece, an arc is struck which melts the wire and the workpiece to form a molten weld pool. Shielding gas is fed through the torch concentric with the wire in a similar manner to that employed for the TIG process. A wide range of shielding gases is available, normally based on argon, with additions of carbon dioxide or oxygen to improve the wetting action of the weld bead. Helium is sometimes also added to increase arc voltage, which improves fusion levels and deposition rate. Carbon dioxide is not used on its own to shield the welding of materials of typical offshore thicknesses, although it is used in other applications, such as thin sheet steel. In order to allow the welder freedom of movement for the welding torch, it is connected to the wire feed motor and electrode spool by a flexible umbilical. Because of this flexibility, the feed speed of the consumable is subject to random variations, and the process must accommodate both these and the changes in torch-to-workpiece distance caused by movement of the welding torch by the welder, while maintaining a constant arc length. The control mechanism used to achieve this depends on two relationships - that the arc length is proportional to the arc voltage, and that the rate at which the electrode is melted is proportional to the welding current. MIG welding power supplies are designed so that a relatively small change in output voltage causes a large change in current (Fig. 6.11), and are known as constant potential or 'flat' output units. Typically, a fall of 2 or 3 volts will cause the output current to increase by about 100 A, and vice versa.

Output Voltage

Typical 'flat' power supply characteristic

Change in arc voltage

Change in arc current

Output current

6.11 A typical MIG welding power supply characteristic.

If the process is operating in a stable condition, and the torch is suddenly moved away from the workpiece, the arc length will instantaneously increase. This will cause the arc voltage to rise which, because of the design of the power supply, will cause the arc current to fall, which will consequently reduce the electrode burn-off rate. However, the electrode is being fed at a nominally constant rate, and thus momentarily the electrode will be fed faster than it is being consumed, reducing the arc length. This process will continue until the original arc length is restored. A similar sequence of events, but with an increase in current, would take place if the arc length was reduced. This technique, known as self-regulation, has proven to be successful, simple and reliable for surface based GMA welding. However, it does mean that the process is subject to constant random variations in arc voltage and current, which can cause problems with the shape and fusion characteristics of the weld. This short description of self-adjustment has simplified the relationships between the operating parameters, and for a more detailed explanation of the process the reader should consult the bibliography. If it is necessary to transfer metal from the electrode tip to the workpiece without the assistance of gravity - in other words, in positions other than downhand, there are two alternative techniques available using conventional welding equipment. When the arc current is lower than a critical value, known as the spray transition current, molten material forms a

globule on the end of the wire, until its weight overcomes the surface tension forces at the neck of the droplet, when it falls off under the influence of gravity. However, above the transition current, electromagnetic effects project the molten material off the end of the wire in a series of relatively small droplets, independent of gravity, a condition known as spray transfer. Unfortunately, with the ferrous materials used on offshore structures, this spray transition current is relatively high - above 200 amps for a 1.2 mm diameter wire in an argon-rich shielding gas - and the resultant weld pool is too large to be controlled in other than the downhand position. The other transfer mechanism can be initiated by feeding the consumable at a rate which is slightly higher than that which the arc current can burn off. The arc length will steadily reduce, until the arc is extinguished by the wire entering the weld pool. This causes a rapid increase in welding current, until the wire is heated to its melting point and ruptures. The molten material detached from the end of the wire is drawn into the pool by surface tension forces, and the arc is re-established so that the cycle can repeat periodically. This transfer mode is known as 'dip' or 'short arc' welding. The major factor influencing process performance with this technique is control of the rate of rise of welding current, which is normally controlled by the use of variable inductors in the welding circuit. The problem with this technique is that because the arc is periodically extinguished, the heat input to the workpiece is low, and the process is best suited to thin materials. It can thus be seen that the behaviour of MIG welding power supplies is more complex to characterise than that of TIG units. The relationship between output voltage and current is known as the 'static' characteristic, as the conditions it defines do not vary with time. It is often referred to as the 'slope' of the power supply, and is normally close to the 2 or 3 V per 100 A mentioned above. However, in theory, the optimum value of the parameter would vary according to the size and composition of the welding electrode, and some power supplies provided for variation in power supply slope, with the penalty of increased cost and complexity. The rate of current rise referred to in the description of dip welding is known as the 'dynamic' characteristic, and again the optimum value is dependent on the configuration of the welding consumable, with adjustment being provided in many cases. A complicating factor with MIG is that the process requires an electrode feed system complete with variable speed motor, flexible electrode conduit and welding torch incorporating a contact tip where the welding current is transferred to the wire by sliding contact. These components require constant inspection and maintenance, and the vast majority of problems relat-

ing to the operation of MIG welding can be attributed to this part of the welding system. However, very long, continuous, arc times are possible with the process, resulting in high deposition rates, and it is very suitable for operation with automated or robotic manipulation equipment because of the light weight and small size of the welding torch.

6.3.2 Hyperbaric operation Solid wire GMA welding was proposed as an underwater welding technique in the early 1970s. The principal modification to the standard surface based welding system was necessitated by a requirement to keep the welding power supply, and the process controls on the surface, separated from the consumable feed system by a service umbilical some hundreds of metres long. The consumable feed motor and reel of electrode were housed in a watertight container, into which was connected the service umbilical which supplied welding current, power and control signals for the consumable feed motor, and the shielding gas supply. An additional gas supply was fed to a tracking regulator, the function of which was to maintain the internal pressure within the container at a constant level of a few tenths of a bar (some tens of kPa) above that of the surrounding water, to minimise the risk of leakage. The welder's torch was also connected to the container via a short flexible umbilical as in surface based practice. The torch was marinised by waterproofing, and by the removal of the operator's actuating switch, the welding process being controlled from the surface. To keep the torch interior dry while being taken to the welding chamber, shielding gas was constantly passed through it while it was in the water. In early operational use in the mid-1970s, it was found that with the process technology available at the time it was not possible to provide simultaneously an all-positional capability for the process, and to achieve a sufficiently high level of heat input to eliminate the risk of lack of fusion defects within the weld. The elevated electric field strengths observed in hyperbaric TIG welding were also evident in MIG, increasing the magnitude of the arc voltage variations brought about by changes in the electrode-workpiece distance. Because of the characteristics of the welding power supply, these caused larger changes in arc current, which reduced the stability of the process. Typical offshore workpiece configurations, which were normally relatively thick steel sections in close contact with cold sea water, exacerbated the problem by acting as large heat sinks. Because of these problems, the solid wire MIG process was not thought suitable for hyperbaric use, and the FCAW process was developed as a replacement for it.

6.4

Flux cored arc welding (FCAW)

6.4.1 The process The development of flux cored arc welding has closely paralleled that of solid wire MIG, much of the equipment and process technology being identical (Fig. 6.12). Flux cored wires are normally made by a forming and drawing process. A strip of steel is formed into a 'U' shape by a series of rollers. It then passes under a chute where powdered core material is inserted into the groove of the U at a controlled rate. The steel is then formed into a complete circle by further forming, and then reduced to its final size by a series of drawing and annealing operations. Because of the relatively complex manufacturing process, tubular consumables are more expensive than solid wires, but as the formulation of the core material can more easily be changed, they are more readily modified. In addition, a compromise must be struck between the amount of core material present in the final consumable, and the extent to which the diameter of the consumable is reduced, as the sheath and the powder do not respond similarly to the drawing process. It should be noted that Oerlikon do not produce their tubular consumables in this way, but produce a relatively large tube of metal. The central cavity of this is filled with core material, and this is then drawn to produce a seamless tubular consumable. Also, some electrodes have been produced in which the seam in the outer casing has been welded. Tubular electrodes are produced in several varieties, the principal ones being metal cored, rutile and basic fluxed, and self-shielding. Metal cored

Consumable flux cored wire electrode

Contact tube

Evolved gas shield Weld metal

Slag

Arc Weld pool

6.12 Flux cored arc welding (FCAW).

Parent metal

electrodes contain, as their name implies, mainly metallic contents, which can either enhance weld deposition rate, or contain alloying elements to modify the composition of the final weld deposit. Rutile electrodes contain titanium oxide among their flux core constituents, and as this is a good arc stabiliser, they tend to have very good operating characteristics. Basic electrodes are not so easy to operate, but in general are capable of depositing higher quality weld metal. Similar shielding gases are used as for solid wires. A specific group of tubular wires are those designed to operate without shielding gas. These incorporate into the core material deoxidants, nitrogen fixing materials such as aluminium, material which vaporises to form a protective arc atmosphere, and slag forming compounds to protect the cooling weld pool. This combination makes it possible to operate these consumables without additional shielding gas, simplifying the welding equipment considerably. However, their initial cost is higher than that of conventional consumables. In addition, self-shielding consumables are often sensitive to small changes in arc length, and hence voltage, because this directly influences the time the molten droplets are exposed to the atmosphere. Tubular electrodes can be used on most types of conventional MIG welding equipment, although it should be noted that their construction means that they are more prone to crushing than solid wires. Care has, therefore, to be taken to ensure that while sufficient force is applied to the wire to enable it to feed consistently, it must not be deformed by the feed rolls, as this would lead to jamming in the contact tip. Improved feed performance can normally be obtained by the use of four or six roller feed systems, rather than two roll units, and by the use of specially shaped rollers designed specifically for tubular wire operations.

6.4.2 Hyperbaric operation When the limitations of solid wire MIG were recognised in the mid-1970s, the use of tubular consumables was suggested to improve the characteristics of the technique. Tubular consumables could be made in which the flux would contain arc stabilising materials, agents to form slag to stabilise the weld pool and exothermic compounds to add extra heat to the weld pool. A research programme jointly undertaken by Cranfield University and Sub Ocean Services (a division of British Oxygen Company, later transferred to Sub Sea Offshore) started with a standard consumable originally developed for self-shielded surface based operation, and eventually developed specialised consumables capable of all-positional operation at depths of 200 metres, with excellent weldability and weld metal properties. Unfortunately, the relatively high cost of these consumables, and the complexity of the MIG type feeder systems, made the process unattractive to the offshore industry compared with hyperbaric MMA, and little use was made of the technique after 1980.

More recently, a small diameter (lmm) tubular consumable from Oerlikon has been shown to produce excellent welds at a reasonable cost at depths down to 400 metres, and has been used for a limited number of applications. However, for manual welding, the MMA process is still predominant, for the reasons suggested earlier.

6.5

Automated orbital hyperbaric welding

In the early 1980s, the Norwegian offshore hydrocarbons industry faced the problem of conveying product to Norway from their offshore fields. Across the proposed route for their pipelines was a deep region of the North Sea, known as the Norwegian Trench, with a maximum depth approaching 400 metres. There was some concern whether it would be possible for manual divers to weld effectively at these depths, as their physical capabilities became affected by the various problems associated with deep water saturation diving. Because only pipelines, with their relatively simple geometry, were to be sited in the deep water area, the feasibility of using automated welding systems was investigated. Other branches of welding technology have to carry out fabrication operations in areas to which human access must be restricted. The most obvious of these is the nuclear industry, which has developed several remotely controlled, orbital welding systems for use in radiologically contaminated areas. It has proved possible to adapt this technology for hyperbaric use, and several systems have been developed. Figure 6.13 shows an early NEI Weldcontrol orbital welding head, mounted on a length of pipe. To the left of the joint, at the bottom of the picture, the track on which the welding head is mounted can be seen, with the umbilical supplying the unit with power and

6.13 An automated orbital TIG welding system.

gas to the upper left of the picture. On top of the pipe is the main tractor unit, which contains both the traverse motor and the electrode spool and feed motor. To the right of the body is the welding torch manipulator, which can move the torch radially and laterally relative to the pipe. The electrode feed conduit can be seen attached to the torch from the lower left of the picture. Units of this type have a good operational track record and their use has been extended to many industries where high integrity pipe welding is required, and systems exist to weld pipe diameters from a few millimetres to over a metre, from a wide range of suppliers. The machines developed for hyperbaric use are similar in general configuration to that in Fig. 6.13. AU the systems currently in use utilise the TIG process, both because of its controllability and because this was the process used for the earlier nuclear systems. The characteristics of the TIG process at depths to 400 metres were well understood, and the stability problems discussed earlier were shown not to be significant in this depth range. Although there are several systems currently available, their principal features are similar, and the system shown in the following figures is the pipeline repair spread (PRS), developed with the support of a consortium of Norwegian oil companies. All make use of a circular track, which can be split in order to fit around the pipe, as shown in Fig. 6.14. The track is significantly larger

6.14 The mounting ring for the Statoil PRS welding system.

6.15 The welding head of the Statoil PRS welding system.

than the pipe, in order to allow space for electrical preheat mats to be fitted, and is more robust than a conventional welding system track, as in this system it forms the common mounting for a series of modules, which carry out pipe machining, welding and inspection. The complete unit is called the integrated modular tool (IMT) system. The welding head (shown on the right of the ring in Fig. 6.14 and in detail in Fig. 6.15) incorporates a variable-speed travel drive motor, the welding torch and manipulator system, a consumable feed unit, and cameras for viewing the welding arc the two large tubular objects on each side of the welding torch. The silver tube to the lower right is the consumable feed unit, the position of which can be controlled from the surface during welding. Feedback signals from the arc voltage are used to control the arc length, while the lateral axis is used to move the torch in a weaving motion. As a result of extensive welding procedure development, changes to the arc current and electrode feed rate can be synchronised with the torch weave motion in order to maximise fusion at the sides of the weld, while controlling the deposited bead shape. The original PRS system was tested in 1986, and used operationally in 1988. At that time, the majority of the underwater tasks associated with the repair, such as weightcoat removal and the installation of the pipe alignment system, were carried out by divers, only the welding operation being controlled from the surface. Progressive development of the system has reduced the amount of diver involvement, as it is an objective of the Norwegian offshore industry to eliminate all manned diving at depths

6.16 A pipe handling crane from the Statoil PRS welding system.

6.17 The control cabin of the Statoil PRS welding system.

greater than 180 metres as soon as this is technically feasible. This has resulted in the development of remotely controlled pipe alignment frames, one of which can be seen in Fig. 6.16. Similar in construction to the Comex unit shown in Fig. 4.3, it is rather larger, and the ROV interface panel, which enables the unit to be controlled by a remotely operated underwater vehicle, can be seen in the upper right of the picture. Since 1994, all the

'wet phase' operations, which require divers to enter the water, have been carried out under surface control, divers being required only to install the equipment, replace consumables, make repairs and deal with unplanned problems. It is intended that the system will be further developed to be capable of fully diverless operation within the next few years. A typical surface control area can be seen in Fig. 6.17. The area incorporates monitoring and control facilities for the welding system, including the video link described above. For safety reasons and to avoid equipment damage, cameras showing general views of the interior of the welding chamber are also used, and the system controller has an audio link to the diver in the chamber. The facility also includes video and parameter recording systems to enable records to be kept of the welding procedure.

6.6

Summary

To summarise, manual hyperbaric welding is predominantly carried out using TIG for critical situations where a high level of control is required, and MMA for those regions of the weld where high deposition rates are needed. A limited amount of MIG/FCAW has been used, but this is small compared to the other two techniques. These techniques are well established and effective at depths down to 200 or 300 metres. Diver-installed automated TIG systems were developed for use at depths of the order of 400 metres, but their consistency and productivity has led to them being considered for pipeline welding at shallower depths. The Norwegian Government policy of eliminating diver operations at depths greater than 180 metres has led to the design of pipe joining systems which can operate without the assistance of saturation divers, and this development programme continues.

7 Alternatives to saturation diving for deep water applications

As discussed in Appendix 2, it is certain that at some depth, probably in the region of 500 metres but possibly shallower, saturation diving will cease to be viable. Several alternative intervention systems exist to replace the diver, of which the most similar is the atmospheric diving suit (ADS). The ADS is a rigid diving suit designed to resist the pressure of the water, so that a diver within it experiences only normal atmospheric pressure. Fundamental to the operation of such suits is the pressure balanced ball joint, which can withstand high external pressure while retaining sufficient flexibility to enable the diver to move. Originally constructed from light metal alloys, modern ADSs use carbon fibre reinforced polymers for many sections. Because the diver is at one atmosphere, decompression procedures, excessive heat loss and high breathing gas consumption are avoided, enabling most ADSs to have 24 hour survival capability. ADSs have been constructed with legs, to permit mobility on hard surfaces, or with a tubular lower section and thrusters for mid-water operations. The greatest single drawback to the ADS is that because the suits are rather large, they must be heavy in order to achieve hydrostatic balance in water, and they typically weigh of the order of 500 kg. This restricts their dexterity, and particularly means that they cannot operate within a dry welding habitat. For a variety of reasons, the use of ADSs has been relatively restricted, although they have been used to deploy mechanical couplings and other engineering packages. An alternative to the ADS is the use of manned submersibles, from one-man systems such as Mantis to larger multi-crew submarines. Such vessels, equipped with external manipulators, are widely used for scientific work underwater, and for deep water salvage, but they are frequently perceived as too expensive for offshore engineering operations. The explosive system developed by BUPE and described below was developed to be used with the manned 'Pisces' submersible then operated by Vickers Offshore Ltd.

The favoured alternative, for many situations, is the use of remotely operated vehicles (ROVs). These unmanned vehicles, controlled by a surface operator, use electrically or hydraulically powered thrust units for navigation, and mount cameras and other sensor systems to provide feedback on the underwater environment. Originally used for surveying and observation tasks, in recent years they have been developed to perform intervention activities, most notably the planned maintenance of wellheads and other underwater structures. The manipulator systems used on ROVs are not computer controlled, but are technically teleoperator systems, controlled by a human being on the surface, who is reliant on the visual feedback he receives via a video link. Although improvements have been made to manipulator performance in the last decade, they lack the dexterity required to carry out most welding operations directly, and the majority of underwater vision systems would not provide images of sufficient quality to allow effective control of the weld pool. It is therefore unlikely that an ROV would be built capable of carrying out underwater welding itself, but it could be used to deploy a series of engineering packages, each designed to carry out one part of a joining procedure. ROV controlled pipe alignment systems and habitat sealing units already exist, and it is likely that their role will be extended as diverless systems develop. As described earlier, the Norwegians have developed a pipeline repair system which does not require saturation divers to enter the water, all 'wet phase' tasks being carried out by remotely controlled engineering packages with ROV interfaces. A future objective of this programme is to develop a fully diverless pipeline repair system. For specialised repair tasks, it has been suggested that once the hyperbaric habitat has been installed and dewatered, industrial robots could be used within it. These are more versatile and accurate than current generation underwater manipulators, and could utilise the wide range of functional capabilities developed for such systems in general manufacturing industry. Research programmes to evaluate this possibility are currently being undertaken.

8 Deep water arc welding processes

Until the last decade, it was not known whether arc welding processes could operate reliably at depths in excess of about 500 metres. During much of the 1980s, the low price of oil reduced the rate of development of deep water offshore reserves, and the rate of hyperbaric welding process development was slow. However, the 1990s have seen renewed interest in deep water field development, and this has led to renewed interest in joining techniques which can operate at depths of 1000 metres and greater. Although alternatives exist, and will be described in the next chapter, the industry has always maintained an interest in the capabilities of arc welding, for the reasons discussed earlier, that it is a well proven fabrication technique for offshore use. Research programmes at several centres have shown that two arc welding processes, MIG welding and plasma welding, are capable of operating at pressures greater than 100 bar, and could thus be used for deep water joining operations. As in Chapter 6, the description of each process is divided into two sections. Plasma welding has not been previously mentioned, and the main features of the process are described before the modifications required for hyperbaric welding are discussed. In the case of MIG welding, a considerable amount of development relating to the general operation of the process took place between the late 1970s and the mid-1980s, when the process was first considered for deep water operation. These developments are discussed before their application to hyperbaric MIG welding is explained.

8.1

Plasma welding

8.1.1 The process Plasma welding is a development of TIG. A similar non-consumable tungsten electrode is used, and in the most widely used variant of the process,

an arc is struck between this electrode and the workpiece ('transferred arc plasma welding'). The principal difference between the processes is that unlike the free burning TIG arc, the plasma arc is constricted by means of a copper or carbon nozzle a few millimetres in front of the electrode tip (Fig. 8.1). This has the effect of 'squeezing' the arc, reducing its cross-sectional area, and hence, for a similar current, forcing the conductivity of the arc material to higher levels, generating an increase in arc temperature. In addition, because the arc occupies virtually all the orifice area, the flow of gas through the arc is controlled by the rate of supply of gas into the region behind the constriction, known as the plasma gas. If the orifice diameter, arc current and plasma gas flow are in appropriate relationships to each other, as the plasma gas flows through the constriction the layer of gas adjacent to the orifice wall remains relatively cool, forming a thermal barrier which protects the surface of the orifice. A second gas supply, outside the orifice, is provided for shielding purposes in a similar manner to TIG welding. Because the tungsten electrode is recessed within the orifice, it is normal practice to initiate the plasma arc by means of an HF discharge between the electrode and the orifice. A low current auxiliary power supply maintains a 'pilot' arc between these components until the torch is brought within a few millimetres of the workpiece. The plasma gas flow then carries hot gas through the orifice, creating an ionised current path which enables the main arc to be struck. If arc operation is intermittent, the main arc can be extinguished while the pilot arc continues to run, enhancing the relia-

Non-consumable tungsten electrode Orifice gas

Cooling water

Shielding gas

Parent metal

Copper nozzle

Plasma Keyhole

8.1 Plasma arc welding (PAW).

Weld pool

Weld metal

bility of arc re-ignition. Some low current, microplasma systems do not utilise HF, the electrode being spring loaded so that it can be pushed forward to touch the rear of the orifice, the pilot arc being initiated by direct contact. The performance of the plasma process is dependent on a very large number of variables, including arc voltage and current, orifice diameter, shape and position, plasma gas composition and flow, and the internal geometry of the plasma torch. This means that welding conditions developed on a plasma welding system from one manufacturer cannot readily be transferred to a different system, due to the changed internal geometry of the plasma torches. These complications associated with the process have limited its application in industry generally, but the introduction of more sophisticated welding power supplies, and the increased availability of computer-compatible gas flow control systems, together with an improved understanding of the plasma process, are leading to a wider interest in the technique. By changing, principally, the arc current, orifice diameter, plasma gas flow, and electrode-to-orifice position, the plasma arc can generate a wide range of operating characteristics. With a large diameter orifice and low plasma gas flows, the level of constriction applied to the arc is low, and the arc acts in a very similar manner to a TIG arc at a similar operating current. The arc voltage is of the order of 10% greater than the equivalent TIG arc, and the stability is enhanced, but the two are otherwise similar. If the orifice diameter is reduced, the plasma gas flow increased, and the electrode moved upstream from the orifice, the level of arc constriction is increased, enhancing the power density of the arc. Once a critical power density is reached, the arc will penetrate several millimetres of steel, producing an essentially parallel-sided weld in a similar manner to an electron beam or laser weld. This is known as 'keyhole' welding (Fig. 8.2), and can be used to produce high integrity, autogenous butt welds joining square cut edges. Further increases in power density produce an arc which is capable of cutting through a wide range of metals.

8.1.2 Hyperbaric operation Because, at current operational depths, the TIG process has proven adequate, and the plasma process involves additional system complications (in the form of a more complex torch, a second gas supply and additional welding parameters) to be optimised, the process has not been used on an operational system. Hence, no practical operating data is available. However, many of the systems currently operating with the TIG process could be readily adapted to plasma operation should the requirement arise, as is likely as operational depths increase. At present, the majority of

Gun or torch Cross-section of typical weld

Keyhole

Parent metal

Beam or plasma jet

Close butt preparation Weld pool

8.2 Keyhole welding.

process data relating to hyperbaric plasma welding is derived from laboratory experimental programmes, most notably at SINTEF, in Norway, and at Cranfield University, in the UK. At Cranfield, a modular construction hyperbaric plasma torch has been developed, in conjunction with a specialist welding torch manufacturer, and has been operated successfully at 250 bar, a pressure equivalent to 2500 metres water depth, including initiation of the arc at that pressure. It has been established, however, that the shape of the plenum chamber upstream of the orifice has a significant influence on the operating characteristics of the welding torch, and current research is directed towards optimising the geometry of an operational torch, and reducing its size to improve access into the welding preparation. Unlike other welding processes, equipment designed for surface use cannot be utilised under hyperbaric conditions without modification. The Cranfield torch development programme has evaluated a blade geometry torch which can fit into typical underwater weld preparation geometries, and which has interchangeable components to permit the influence of internal geometry changes to be evaluated, and has a tungsten electrode which can be moved axially through the torch, even when the arc is running. This last feature also permits initiation of the arc without the use of HF. The electrode is connected to the negative output of the power supply, and the workpiece to the positive output, as normal, but a second connection to the positive output is taken, via a relay and high power resistor, to the orifice. To initiate the arc, the power supply is turned on and the electrode moved forward into contact with the orifice. With the relay closed, the current can flow through the orifice and tungsten, the level of which is regulated by the output voltage of the power supply

and the value of the resistor. The electrode is then withdrawn from the orifice, initiating the pilot arc within the torch. To strike the main arc, the torch is brought within a few millimetres of the workpiece, at which point the flow of ionised gas through the orifice forms a lower resistance path than the resistor, and the main arc strikes between the workpiece and the tungsten. The relay can then be opened to prevent any current flow through the orifice. Once the correct value of resistor has been established, the arc can be initiated very rapidly and reliably, at pressures up to 250 bar. Later development torches have modified the blade geometry to enable wire to be fed to the joint, and the electrode movement system has been simplified. Enhanced cooling of the orifice has improved torch durability at high pressures. At present, the process optimisation studies continue, although the enhanced stability of the plasma arc compared with TIG, and its greater resistance to external influences such as magnetic fields, have been confirmed. It is still necessary to carry out research programmes to interface a plasma torch with practical underwater welding systems - diver installed or diverless - to develop acceptable welding procedures, and to confirm the mechanical properties of joints made using the process, although initial results are very promising. One feature of the plasma process is that arc operating voltages may be extremely high, dependent on the mode of arc operation. For lightly constricted arcs the voltage is typically of the order of 10% greater than that of the equivalent TIG arc operating under similar conditions. However, as the degree of constriction increases, the arc voltage rises rapidly and may be several times greater than that of an equivalent unconstricted arc. In addition, the power supply needs to be capable of an output voltage considerably in excess of the mean operating voltage if maximum process stability is to be achieved. This has important safety implications, as it seems unlikely that the process can be used for manual welding, and also because safety systems will be required should the process be used on a diver deployed system, or on diverless systems to protect personnel servicing the equipment. For the Cranfield 250 bar research facility, a plasma welding power supply having a voltage capability approaching 700 V is being used, necessitating stringent safety precautions. Voltage is found to increase with pressure approximately in proportion to the square root of absolute pressure, the fall elements now forming only a small part of the total voltage. However, this behaviour is dependent on the mass flow rate of plasma gas through the constricting orifice, and optimum mass flow rates have been shown to increase approximately with the square root of operating pressure. Generally, voltage is found to increase linearly with plasma gas flow rate and increasing free arc length.

Voltage also increases approximately according to the inverse square of the orifice diameter and exhibits a greater change with welding current than is observed for the free burning gas tungsten arc. The voltage-operating parameter relationship is significantly affected by gas flow behaviour within the constricted portion of the arc, and this behaviour is further complicated by the influence of plasma torch design in the vicinity of the arc generation region and upstream from the orifice. Such factors are highly dependent on gas flow geometry and in general it is not possible to predict plasma arc voltages using simple parametric relationships. Originally, it was not thought that keyhole mode operations would be possible under hyperbaric conditions. This was because early process investigations measured the pressure exerted by the arc on the weld pool, and demonstrated that this fell as pressure increased. As it is this excess pressure which provides the displacing force necessary to open the keyhole, it reduced the potential for keyhole welding at high environmental pressures. However, later work has demonstrated that with optimised plasma gas mixtures and flow rates, keyhole mode operation is possible to at least 100 bar, and possibly higher. Such an operating condition can be used either for autogenous welds replacing conventional root and hot pass procedures, or for arc cutting. Potentially, this capacity for the process to vary its operating characteristics could lead to the development of extremely flexible plasma welding systems within the next two or three years.

8.2

MIG welding

8.2.1 MIG process development Although hyperbaric MIG was rejected in the mid-1970s, it had a series of significant attractions as a process, principally low hydrogen potential, high deposition rates and a high level of compatibility with automated and robotic welding systems. When, in the early 1980s, welding power supplies of higher dynamic response and control flexibility were developed, these were applied to surface based MIG welding, and process variants such as controlled transfer pulse (CTP) MIG and synergic MIG were developed. The following paragraphs briefly summarise the principal developments in the MIG welding process from the early 1980s to the mid1990s, while the next section will describe how this technology has been applied to hyperbaric situations. For a more detailed description of the MIG process developments described below, consult the references in the bibliography. When the first solid-state welding power supplies were developed, they were complex and expensive, but they provided a level of control of welding current, and dynamic performance, previously not achievable with welding

power supplies. Several research programmes were undertaken to investigate the process benefits which could be obtained with this improved performance. An early discovery was that if the arc current was maintained at a level below the spray transition level, and then a current pulse of precisely controlled magnitude and duration applied to the arc, a single droplet of metal could be projected, by electromagnetic forces, across the arc and into the weld pool. It was found that similar metal transfer behaviour was observed at different values of pulse current and duration, and the concept of a detachment parameter (D) was formulated, where D-J2T LJ

-

lp

. i

p

/p being the pulse current, and Tp the pulse duration. For a specified size and composition of consumable, constant values of D will generate similar weld metal transfer behaviour. Figure 8.3 shows the transfer behaviour of a 1.2 mm diameter low alloy steel wire in an argon-rich shielding gas. By applying a series of similar pulses at a frequency such that the rate at which metal was transferred matched the electrode feed rate, a much more stable MIG welding technique became possible, without the random arc voltage and current changes associated with conventional MIG welding. In addition, such techniques allowed open arc, spray transfer MIG welding at

Pulse Current (A)

More than 1 drop per pulse transfer

1 drop per pulse transfer Less than 1 drop per pulse transfer

Pulse duration (msec) 8.3 The effect of pulse current and duration on weld metal transfer.

mean currents low enough to permit all-positional operation, with better fusion characteristics than traditional methods, because of the enhanced control of metal transfer and improved process stability. Conventional MIG welding requires the control of two principal operating parameters - the output voltage of the power supply and the electrode feed speed. Pulse welding increases this number to five - the current level (/p) and duration (Tp) of the pulse, the background current (/b) and time (Tb), and the electrode feed speed. In practical welding operations, welding conditions are required for a range of mean currents, while maintaining control of metal transfer, arc length and process stability. This has led to the development of 'synergic' welding control systems, in which the level of one control input is regulated - usually the consumable feed speed or a notional mean current demand - and the other operating parameters are adjusted to appropriate operating levels. The manner in which each of the dependent parameters varies with respect to the control parameter can be expressed in the form of equations, known as synergic algorithms, the derivation of which occupied several research teams during the early and mid-1980s (Fig. 8.4). The algorithms, once formulated, can be incorporated into the power supply control system. These systems were originally designed as analogue calculation circuits, but are now usually computer

Mean current increasing

Output level

Fixed /p, /b, Tp Vary7b

Time Fixed / p -/ b , 7P Vary lb, Tb

Output level

Time

Fixed/ p , r pJ / b x7 b Vary /b, Tb

Output level

Time 8.4 Some synergic algorithms.

based, enabling a wide range of welding conditions to be available, so that the welder is merely required to indicate the diameter and composition of the welding consumable being used - the system will then automatically provide appropriate operating parameters. Once control systems of this type were available, it became possible to develop more complex control strategies. Many commercial pulse MIG power supplies now operate using a constant current output characteristic during the background phase of the pulse cycle, while becoming constant potential units in the pulse phase. This enables the units to achieve selfregulation to maintain arc length, while still having control of metal transfer. Similar control techniques have been applied to dip welding - rather than using a simple exponential increase in current to fuse the consumable, more complex current waveforms are used, in order to melt the wire with a minimum of spatter and fume generation. Control systems of this type have resulted in significant improvements in the consistency and controllability of the MIG process in recent years, and these developments are still continuing. Systems of the type described above could not effectively control the welding process unless combined with high-performance arc current supply systems. As higher power, higher voltage semi-conductor devices became available during the 1980s, it became possible to apply inverter technology to welding systems. Basically, an inverter system rectifies the incoming electrical supply to provide a DC supply at about 600 V. This is then switched, by a bridge of solid-state devices, to produce a high frequency (20 to 100 kHz) alternating voltage, which is fed to a transformer of appropriate design, with an output voltage of about 50 V. The output is then rectified using high-speed diodes to obtain the required DC output (Fig. 8.5). Regulation of the output level is achieved by control of the proportion of time the switches conduct, a technique known as pulse width modulation (Fig. 8.6). The actual output voltage or current level is compared with that specified by the power supply control system, and should the output be high or low, the 'on time' period is shortened or lengthened respectively. Although this process seems complex, it confers several benefits on the resulting power supply. Operating the main transformer at high frequency enables it to be more efficient, smaller and lighter, with consequent reductions in the weight and cost of the complete power supply. The high switching frequency means that changes to the output level can be made very rapidly, resulting in power supplies having a high level of controllability and dynamic response. Inverter power supplies can be made very electrically efficient, current designs surpassing 95% efficiency over their entire operating range. One consequence of this technique, known as 'switch mode' operation, is that the output of the power supply contains some measure of ripple around the mean, required output level. Although this seems to have

Primary rectifier 600VDC

Secondary rectifier Solid state switches Main transformer (high frequency)

AC Mains

DC Output

Supply

Control signals to solid state switches Output demand

Control system

Measurement of voltage and current output

8.5 Schematic diagram of a primary switched inverter welding power supply.

90% output

Output level

Time

Output level

50% output

Time

10% output

Output level

Time 8.6 The use of pulse width modulation to control mean output power.

little effect on the welding process itself, it may be necessary to provide appropriate signal filtering for process monitoring circuits. To summarise, developments in MIG welding systems over the past decade have resulted in the availability of control systems of very high flexibility, capable of controlling the output characteristics of the power supply so that high levels of process consistency can be achieved, across a wide range of operating parameters. These control systems are complemented by inverter power units of high efficiency and controllability, which are smaller and lighter than traditional welding systems, and which can easily be appropriately housed for surface or underwater operation.

8.2.2 Hyperbaric operation As the technical innovations described above were developed, it was logical to determine whether they would enhance the performance of the MIG welding process in hyperbaric environments. This work has been carried out principally at three organisations - GKSS and the Universitat von Bundeswehr (UvB) in Germany, and Cranfield University in the UK. GKSS have concentrated principally on the dip welding process, while Cranfield has studied mainly the open arc, pulse transfer technique, and the UvB has been concerned mainly with fundamental process research. However, both GKSS and Cranfield have developed similar welding systems - a relatively conventional high-performance welding power supply has been used, controlled by special electronic circuitry developed to provide the operating characteristics required by the hyperbaric MIG arc. Unlike plasma welding, the MIG process is inherently time dependent and dynamic in nature due to the periodic melting and detachment of the consumable electrode. The power requirements of the process are dependent on the prevailing conditions in the arc and are therefore themselves time dependent. The one atmosphere MIG arc can transfer metal under a wide range of conditions because current density falls steadily along its length (Fig. 8.7). As pressure is increased and arc contraction occurs, strong electromagnetically driven plasma jets form at the electrodes which can disrupt metal transfer and destabilise the process (Fig. 8.8). In extreme cases, these opposing plasma jets can expel the metal droplets from the arc (Fig. 8.9), effectively preventing useful metal transfer. However, their influence can be minimised by reducing the length of the arc, inhibiting their development. This behaviour was discovered by the use of very high-speed cinematography of the arc. To control the instability of the hyperbaric MIG arc, the static characteristic must be chosen to optimise the arc current for the required arc length, and is pressure dependent. Current pulsing may also be used to control the mean current and metal transfer behaviour. The dynamic

8.7 One atmosphere MIG metal transfer.

8.8 Deflection of the transferring metal droplets by opposing plasma jet.

8.9 Expulsion of the transferring metal droplets at high pressures.

behaviour of the power supply has a significant effect on process stability. If the response rate is too low, the arc current does not change sufficiently rapidly to maintain equilibrium, and short circuiting or burning back of the electrode to the contact tip occurs. If the response is too fast, the inherent voltage transients cause major oscillations in the welding current, often resulting in arc extinction. To optimise process stability, the static and dynamic characteristics of the power supply must be regulated in a manner which is not only depth dependent, but also responds to changes in arc length and metal transfer behaviour. In order to carry out MIG welding at high environmental pressures, it was therefore necessary to develop a control system of very high flexibility and response. The Cranfield system is a hybrid unit, using a personal computer as a user interface and to define operating conditions, while real-time control is carried out by analogue circuits to optimise response (Fig. 8.10). It is capable of changing the static and dynamic characteristics of the power supply at intervals of 0.1msec. This unit is linked to a dedicated MIG welding power supply capable of delivering 450 A at a maximum of 180 V, with a maximum current slew rate of 500 A/msec. In laboratory trials, control of metal transfer can be maintained to pressures equivalent to a depth of 2500 metres, with no effective process limit

Digital interface board

Digital parameter, control and feedback signals

Analogue computing system

Personal computer

(Generation of real time drive signals for power supply and wire feed system )

(Parameter storage calculation and timing)

Arc

voltage

Drive signals

Keyboard Welding system Data entry and operator interface

Welding torch

Torch umbilical

Power supply and wire feed unit

8.10 Schematic diagram of the Cranfield hyperbaric MIG power supply control system.

as yet encountered. Smaller consumables, usually 0.8 mm diameter, enhance the stability of the process. Although pulsed current solid wire hyperbaric MIG shows promise as a potential repair technique, more work is needed to establish welding procedures to optimise the mechanical and metallurgical properties of the resultant welded joints. If this work proves successful, this form of MIG would be highly compatible with the recently proposed robotic diverless welding systems. Automated and robotic MIG welding systems are already widely used in many industries, and robot performance is maximised by the light weight and small size of the MIG torch. In addition, many of the necessary ancillary systems, such as automated welding torch cleaning and changing units, have already been developed for use in applications such as the automobile industry, and could readily be adapted for hyperbaric operation. When welding systems of this type are operated with tubular cored wires, it is possible to add arc stabilisers to the material within the tube, enhancing the tolerance of the process. For this reason, it is likely that both electronic and chemical stabilisation will be employed for operational deep water MIG/FCAW welding systems. Considerable work is still required, however, to optimise the formulation of tubular consumables, and to ensure that acceptable mechanical and metallurgical characteristics can be guaranteed for the resultant weld deposit.

8.3

Summary

At present, therefore, laboratory trials have demonstrated that two arc welding processes are capable of operating at pressures equivalent to water depths of 1000 metres and greater - plasma welding and MIG welding. Plasma welding is very stable and controllable, having in its less constricted form many of the characteristics of the well-established TIG welding technique. It is also flexible, being capable of power densities which offer the possibility of keyhole welding and possibly cutting. It is, however, electrically very inefficient, with high energy losses from the arc column, and requires very high voltage power supplies for keyhole operations. As it is so similar to TIG welding, it should be possible to equip modified versions of current orbital pipe welding systems with plasma torches, increasing their potential operating depth capability. MIG welding is inherently unstable under hyperbaric conditions, but if appropriate control technology is used, it can produce sound, consistent welds. The process is much more electrically efficient than plasma, but does produce significant amounts of fume, which require the use of a particulate removal and filtration system. Recent work does suggest, however, that this may be less of a problem at depths beyond 1000 m than trials at

lower pressures have indicated. It is likely that the development of dedicated tubular welding electrodes will enhance the stability of the process in its FCAW variant. Automated and robotic MIG welding is widely used in industry generally, and if robotic systems are developed to carry out underwater intervention tasks, the MIG process would be very suitable for their use. Although both processes have been shown to be viable in the laboratory, more development is required before an operational welding system can be specified. In particular, much work remains to be carried out on the mechanical and metallurgical characteristics of the welds deposited by the two processes on typical steels used in the offshore industry, on the formulation of dedicated deep water welding consumables, and the optimisation of operating parameters and welding procedures. However, the possibility of developing systems to enable arc welding to be carried out at depths in excess of 1000 metres currently seems very high, if appropriate industrial support continues to be forthcoming.

9 Alternatives to arc welding for deep water joining operations

The development of diverless underwater repair systems, which cannot utilise the dexterity and fine manipulation skills of divers, is an opportunity to re-evaluate a wide range of joining systems to determine whether any alternative technique is more applicable to these situations than arc welding. Several such techniques are listed below, with a summary of their development to date.

9.1

Solid phase welding processes

9.1.1 Friction welding Friction welding is a joining process in which the source of energy is the heat generated when one component is moved relative to the other - normally being rotated. The two components are pressed together, and when one is rotated, heat is generated which softens the material at the interface, permitting a more intimate contact between them. As the frictioning process continues, the temperature of the interface rises until it is close to the melting point of the material - but it does not actually melt, as molten material forms a lubricant, reducing the frictional force and permitting the temperature to fall below the melting point again. Once the whole interface is heated, the rotational movement is stopped and the pressure is increased to a forging value, which expels the hot material from the interface and forms a bond between the cooler material. Friction welding is fast and readily controllable, and because of its action is insensitive to the surface condition of the components to be joined. It is widely used in many industrial applications, including stud welding, the variant which has been developed for underwater use. TWI, in the mid-1980s, developed a system for the friction welding of studs in an underwater environment. Initially, the process was operated in a similar manner to surface based welding, but it was found that the cooling effect of the surrounding water chilled the pool, forming solid bridges, the

rupture of which greatly increased the torque required for frictioning. This problem was overcome by the use of a polymeric shroud mounted over the stud prior to welding, which both protected the joint area from water ingress and acted as a heat insulator. Figure 9.1 shows a typical stud welded joint, in this case for the attachment of a connecting cable for a sacrificial anode system. The completed joint is at the centre top of the picture, with a stud and polymer sleeve below it. A completed joint which has been bent through more than 90 degrees, to demonstrate the integrity of the joint, is shown to the right. This system has been tested at pressures equivalent to 600 metres of water, and seems unaffected by depth effects. It has been demonstrated that it can be deployed by ROV, and current development programmes are seeking to extend the range of applications for which it is suitable. Although limited to circular or near circular studs of up to 30 mm diameter, the system has found a wide range of applications in the fixing of protective anodes and their connections, and the mounting of shear pins. More recent developments have proposed the use of friction welding to fill or repair cracked components, by the use of friction stitch welding. In this technique, a hole is drilled at one end of the crack which is to be repaired, and this is then filled by friction welding. A cylindrical rod, slightly

9.1 Underwater friction stud welding components and joint.

smaller than the hole, is used to generate a friction weld at the bottom of the hole, which is progressively filled by material removed from the end of the rod. Once the hole is filled, a second hole is drilled, further along the crack but overlapping the first (Fig. 9.2). This is again filled, and the process repeated until the crack is eliminated. This repair technique is currently being evaluated by a European funded research programme. In principle, it would be possible to utilise friction welding to join complete pipe sections, using either radial friction welding, or the use of a pup piece rotating between two stationary pipe sections (Fig. 9.3). Although proposals have been put forward to build a prototype system to evaluate this technique, the high cost of such a system, and the availability of alternative joining systems, has not encouraged this development to date.

9.1.2 Explosive welding In the late 1970s, a complete pipeline repair system was developed by the then British Underwater Pipeline Engineering Company (BUPE), of Barrow in Furness, under contract to Statoil of Norway. Because it was intended to deploy the system utilising the Pisces manned submersibles operated by Vickers Offshore, in which electrical power was limited, the explosive welding technique was adopted. Explosive welding is a forge welding technique, in which the two components to be joined are positioned lapping over each other, with a slight angle between them. The explosive is coated on to the back of one plate -

9.2 Friction taper stitch welding.

the 'flyer' - and is ignited at one end. As the explosive burns, the flyer is driven forcefully against the 'anvil' plate, and if the surfaces are sufficiently clean, a solid phase bond is formed (Fig. 9.4). The technique is reliable and forms joints rapidly, although confined to lap type geometries. BUPE constructed a series of process modules, which could be transferred to the worksite by Pisces. Consistency of alignment was ensured by the use of a fixture mounted on the pipe, on to which the modules were docked. Weightcoat removal, pipe cutting, cleaning, rounding to size and surface machining could all be carried out by components of the Rotation

Consumable ring

Parent tubes clamped in place

Radial force

Expanding plug 9.3 Radial friction welding.

Explosive

Parent plate

Buffer

Collision front

Flyer plate

Anvil

9.4 Explosive welding.

system, as well as the deployment of the actual explosive joining module. Other applications for the technique were suggested, such as the development of a pipe connector system and a unit to connect to, and thus retrieve, pipe ends. Despite promising initial trials, development was discontinued at the prototype stage. The precise reasons for this are not known, but a level of apprehension concerning the use of explosives within pipelines, and the availability of alternative joining techniques such as the diver assisted TIG systems must have been factors. It would seem that the system was a good idea ahead of its time, and should be re-assessed for deep water applications.

9.2

Mechanical connectors

Mechanical connectors are little affected by depth, but it will be necessary to develop associated pipe handling and connector deployment facilities in diverless form, perhaps by the use of underwater robotic systems. For deep water applications, long term reliability and consistency of performance will be even more necessary than in shallower waters. At least one recently

9.5 The Morgrip 3000 connector.

9.6 The Morgrip 3000R connector.

developed mechanical connector, the Morgrip unit, is currently being evaluated for deep water use. Hydratight have modified the Morgrip connector described earlier by substituting hydraulic actuation for the longitudinal bolts used in the standard connector (Fig. 9.5). Once tensioned using pressure, mechanical latches move into place, locking the coupling to the pipe. The coupling can be removed by applying hydraulic pressure to different ports to unlock the latches. The standard Morgrip features enabling the integrity of the seals to be tested, and to permit the injection of corrosion inhibitor, are also incorporated into the diverless connector. Coupling, pipe connector and blanking variants are available. As part of the evaluation programme for this connector a 16 in (400 mm) example was installed on to a Statoil gas line using modified elements from Statoil's PRS to provide the handling and alignment functions required. The coupling is now in place and tests continue. Although suitable for the smaller sizes of transmission pipeline, the first generation diverless Morgrip joint was rather large and bulky, and a second generation system, to overcome this, has been designed. This reverts to the longitudinal tension bolt design (Fig. 9.6), using a remotely operated multistud tensioner to install the device.

10 Conclusions

The next few years, to the end of the century, are likely to be highly significant in the development of underwater engineering technology. Despite many attempts at diversification, it remains a simple fact that the only organisation with the resources to utilise advanced underwater engineering techniques, and to support their development, is the hydrocarbons industry. Thus, the rate of technical innovation is driven by the requirements of that industry. The offshore industry is conservative, and normally prefers to develop its technology by evolutionary means, an understandable attitude given the environmental and political consequences of a major platform or pipeline failure offshore. However, at some depth in the range between 500 and 750 metres, it must discontinue the use of techniques which have served it well for thirty years, and move to the use of diverless, remotely controlled robotic intervention systems using new joining processes. It has been argued that the engineering solutions required for deep water operations will not require the use of underwater joining. The use of floating production and storage facilities, with product being transferred to land by tankers, and connected to the seabed by flexible riser pipes, eliminates many of the structures currently requiring modification and repair. However, it seems very likely that if exploitation of hydrocarbon reserves in deep water takes place on a significant scale, situations will arise in which the availability of an effective joining or repair technique would be most useful, if not essential. Recent discussions within the industry have suggested that the economic life of current offshore structures may be extended by linking them to new reserves by the use of pipes as much as 150 km long. Not only does this permit the continued use of established facilities, but it also delays any requirement to decommission and remove the structure. This may markedly increase the length of underwater pipeline in operation. While a variety of such techniques exist, the final choice of system, and the timing of such a choice, are questions which are not decided purely by

technical factors. Factors which may influence the rate of development of offshore reserves include the development of techniques to extract more hydrocarbon from existing facilities using new processes such as horizontal drilling, the timing of the return to availability of reserves such as those of Iraq, which are not currently on the market for political reasons, the general level of global industrial activity, the development of more energy efficient industrial and transport technologies, the development of renewable energy sources and the consequences of international legislation to limit levels of greenhouse gases and acid rain. The mounting scale of offshore engineering operations, at depths greater than previously possible, is increasingly being combined with a need to minimise environmental and ecological disturbance by such operations, to reduce capital expenditure and to minimise the time from the start of a project to its generation of revenue. These requirements present a formidable series of challenges to the offshore engineering industry for the next few years. The United Kingdom is a major centre of underwater engineering, and it is hoped that this position can be consolidated as new developments are forthcoming. It has been stated that the amount of money spent to extract the first oil from the North Sea was similar to that spent to put the first man on the Moon, drawing direct comparisons between advanced offshore and aerospace projects. In many ways, the two situations are comparable. In both cases, many engineering studies have been undertaken, but these can never be translated into reality unless they can be justified on political, economic or scientific grounds. When high levels of capital expenditure are involved, technology can never be separated from politics and finance, and a compromise must be reached between those seeking the most technologically advanced solution, and those wishing to achieve the objectives of a project at minimum cost. We have the examples of the 'Challenger' Space Shuttle and Piper Alpha to demonstrate how badly things can sometimes go wrong, but from both of those disasters lessons were learned, and they have influenced the course of future developments. Even if we do not build a tunnel across the Atlantic, as was proposed in one of the few science fiction novels dealing with underwater technology, the next few years seem likely to be exciting and challenging for the offshore engineering industry.

Appendix 1 Oceanography

It is well known that two-thirds of the earth's surface is covered by water. In terms relevant to offshore engineering, these water covered areas can be divided into two regions - the continental shelves and the main oceans. Continental shelves are really extensions of the adjoining land mass, which have normally been eroded by wave action until they are underwater. From the shore line, they increase in depth to about 200 metres, and have a mean width of about 70 km, although both these values can be very variable. For example, most of the North Sea can be considered continental shelf. From the edge of the continental shelf, the sea bottom descends the continental slope, at a gradient of about 4 degrees, until the ocean floor is reached. The slope tends to flatten at a depth of about 2.5 km, and the mean depth of the main oceans is about 6 km, with a maximum of nearly 11km. The depth of the ocean is sometimes reduced at the midpoint between continents, where the volcanic activity associated with the movement of tectonic plates can produce mid-ocean ridges - usually underwater but sometimes, as in the case of the Hawaiian Islands, breaking the surface. There is a complex series of currents within the main oceans, the behaviour of which is still not fully understood, but which seems to be powered by temperature differences across the globe. For this reason, their broad features are stable over long periods of time. A well known example of such a current is the Gulf Stream, which markedly raises the sea temperature around the British Isles, and similar currents exist across the world. In continental shelf waters, sea movement is more likely to be influenced by tidal action and weather - the combined action of high tides and strong onshore winds to produce flooding of coastal regions is a recurring event in many parts of the world. The southern North Sea is markedly tidal, creating currents of several km/hr at full tidal flow, and this can have consequences on diving operations, as an unprotected diver cannot work in currents greater than 3 to 3.5 km/hr. Weather conditions are also the main factor in

the production of waves, which can also greatly influence the feasibility of diving operations, particularly when these are close to the surface. The surface zone of the ocean, a layer 100 to 500 m deep, is similar in temperature to the surface. Below this, there is a layer within which the temperature falls rapidly, called the thermocline. This separates the surface layer from the deep ocean, in which the temperature is broadly constant at about 3.50C. Sea water contains salts and organic materials, and is thus more dense than pure water, although this density varies with both depth and geographical position. For example, the Red Sea, which loses much of its water by evaporation, has a significantly higher density than ordinary sea water while, because of the immense outflow from the Amazon River, the water several hundred kilometres offshore in the Amazon Delta is still fresh. Similar, although less marked, variations take place all round the world. Because the density of seawater is variable, there is no precise correlation between water depth and environmental pressure. This is because the water pressure at any point in the ocean is generated by the weight of water in the column directly above it to the surface (Archimedes' Law), and as the density of this water can vary, so can the pressure. For most practical purposes, however, it is sufficiently accurate to assume that water pressure increases by 1 bar for every ten metres of water depth. A major problem in many offshore engineering operations is the variability of conditions on the seabed. These can vary from solid rock to quicksand, frequently covered with a layer of soft sediment caused by the fall of solid and organic material from the surface layers of the sea. However, this layer may not be present if the sea floor is new material expelled from undersea volcanoes, or if the sea floor is scoured by currents. The contours of the sea floor can also vary widely, from flat plains to jagged rocky regions. Much of this sea floor relief is due to scouring by tidal waters and undersea currents, volcanic eruption and tectonic activity. Increasingly, the ecological sensitivity of many parts of the ocean is being appreciated. The noise, potential pollution and disturbance brought about by offshore engineering operations are factors which authorities approving such activity must weigh against the economic benefit they generate. In particular, areas such as coastlines, islands and reefs are considered particularly sensitive, especially when used as breeding sites by a variety of sea life. Recent offshore engineering projects, most notably the laying of pipelines to onshore processing facilities, have devoted considerable resources to environmental impact studies in order to route such pipes to minimise environmental damage and to restore shorelines once engineering work is complete. It is likely that this aspect of offshore engineering will increase in importance over the next few years.

Appendix 2 Diving technology

Diving, as an activity carried out by unprotected human beings relying on the air in their lungs, is at least 5000 years old and was employed to gather some forms of seafood, and also pearls, from the seabed. Salvage operations using this technique were described by Greek authors, notably Herodotus, during the Classical Period, and the practice continues essentially unchanged to the present day in several parts of the world. The underwater excursions possible by this means are physically very demanding, and are limited to a few minutes at most in time and a few tens of metres water depth. The resurgence of interest in technology which resulted from the Renaissance led to renewed developments in diving equipment. Several systems were described during the Middle Ages, although a lack of appreciation of the effects of water pressure prevented their practical operation. However, a diving bell was successfully used during salvage operations to recover treasure from a Spanish galleon in Tobermory Bay in 1665. Salvage was also the motive behind the development of an early atmospheric diving suit by Lethbridge, a Cornishman, in 1715. The main body of the suit was made of wood, in a manner similar to a barrel, and contained the diver's body. Glass windows were provided for vision and watertight seals surrounded the diver's arms, which were exposed to the sea water. This must have caused considerable discomfort, but the suit was successfully used on several occasions. The first practical diving suit has been ascribed to Augustus Siebe early in the 19th century. This evolved into the standard diving dress, with rigid helmet, sinker weights and a waterproof suit the air content of which could be controlled by the diver, to regulate buoyancy in the water. Air was supplied from the surface via a flexible air line by pumps, initially hand operated but later powered by a variety of techniques. In recent years, the suit has also incorporated diver-to-surface communication systems, via cables attached to the air line. Because a suit of this type affords considerable physical protection to the diver, it is still frequently employed for civil engi-

neering and harbour diving work. Although attempts were made to construct independent diving systems at this time, the requirement to carry sufficient air for a reasonable dive duration in a conveniently sized container necessitated a high-pressure gas storage bottle and it was the 20th century before these became available. During the first half of the 19th century, there was extensive construction of canal and railway systems throughout Western Europe. Frequently, this demanded tunnelling work below rivers or seabeds, or building operations on the seabed to construct foundations for bridges. As more effective pumping systems became available, it was found advantageous to raise the atmospheric pressure within such tunnels or seabed work chambers, called caissons, to levels higher than one atmosphere in order to reduce the rate at which water leaked into them. The result of this was that construction workers were exposed, for extended periods, to higher pressure conditions, and physiological problems were encountered when they were returned to normal atmospheric pressure. These could be disabling or fatal, and were known as caisson disease or the bends, due to the contorted postures adopted by sufferers. As diving equipment developed so that longer dive times became possible, divers developed similar symptoms and it was suggested that environmental pressure change was responsible. Early in the 20th century J S Haldane, an English physiologist, studied various medical problems associated with diving. He concluded that many divers suffered from a build-up of carbon dioxide, exhaled by the diver within the diving helmet, and established standard air supply rates to overcome this problem. He also established formally that decompression sickness was due to the absorption into the blood, while under high pressure conditions, of excess quantities of nitrogen. As environmental pressure was reduced, this nitrogen was forced out of solution in the blood, to form bubbles in the bloodstream. It was the collection of such bubbles at major joints or in the heart which caused the symptoms of caisson disease. The problem can be overcome by controlling the rate at which pressure is reduced, to enable the body to eliminate the excess nitrogen in a controlled manner. Based on these findings, Haldane drew up decompression tables, listing the time required to surface as a function of the depth and duration of a dive if the bends were to be avoided. Although changed in detail, such tables remain the basis of diving decompression techniques to the present day. During the 1920s it was established that as diving depths increased, problems due to the narcotic effects of nitrogen were encountered. Under these conditions, a diver would exhibit disorientated or drunken behaviour patterns and fatal accidents resulted. In order to avoid the problem, air diving operations were limited to depths of less than 100

metres. In modern offshore diving practice, because of the long duration of dives and because divers work every day, an air diving limit of 50 metres is currently enforced. To overcome this problem, in 1927 initial tests were carried out with the diver breathing a mixture of helium and oxygen (heliox) in order to overcome the problems of nitrogen narcosis. It was found that, if the oxygen content of the gas was maintained at approximately 0.5 bar partial pressure, divers could work effectively while breathing such a mixture. Steady development of the technique was carried out during the 1930s. For example, in 1937 a diver in a pressure chamber was compressed to a simulated depth of 152 metres. The use of heliox mixtures is generally known in the offshore industry as 'mixed gas' diving. Because of the limited availability of helium, tests were also carried out using oxygen/hydrogen mixtures, which are not explosive when the oxygen content is less than 4%. This technique was successfully developed in Sweden in the 1940s and 1950s, but safety concerns relating to hydrogen leakage from gas stores into the surrounding atmosphere, and a generally favourable helium supply position, restricted the application of such mixtures. During the Second World War, the high-pressure gas bottles required for independent systems became available, and self-contained underwater breathing apparatus (SCUBA) units were developed for a variety of military purposes. Their exploitation for scientific, technical and sport activities occurred in the post-war period, with Jacques Cousteau being a pioneer in this field. Although divers could work effectively at depths greater than 100 metres using heliox mixtures, the decompression times required from these depths made diving operations inefficient, and during the 1950s the saturation diving technique was developed, as distinct from the previous 'bounce' diving practice. This involves the use of pressure chambers on board the vessel from which diving operations are carried out, which are large enough for divers to live in for an extended period - several weeks in many cases. At the start of diving operations, the diving team enters these chambers, and is compressed, or 'blown down', to a pressure approximating to that at the relevant operational depth. To reach the worksite, the divers enter a small diving bell which is detachable from the main chamber and can be lowered over the side of the vessel. At the operational depth, the pressures within and outside the diving bell are the same, and the divers can open the bell door and proceed to the worksite. The operation is reversed to recover the divers. In order to maintain continuous diving activity, it is common for the dive team to consist of two or three work crews, so that as one crew is recovered, fresh divers can be deployed. A specialist team of diving supervisors is required to oversee diving opera-

tions and to control environmental conditions within the living chambers and the diving bell. This involves the removal of carbon dioxide, the addition of oxygen to compensate for that consumed by the divers and the maintenance of viable levels of temperature and humidity. At the end of diving operations, the entire dive team can be housed within the living chambers while decompression is carried out at a controlled rate. This is a very lengthy process, several days being required to decompress from 250 or 300 metres. During such diving operations, the diver wears an insulated, waterproof 'dry' suit, frequently capable of being heated to counteract the low water temperatures. The collar of such a suit incorporates a locking ring, to which a rigid helmet, similar in size and shape to a motorcycle crash helmet, can be attached. The diver is supplied with breathing gas, suit heat and communications via an umbilical connecting him to the diving bell and carries a reserve supply of gas in a back-mounted bottle, in case of umbilical failure. Safety regulations normally require that one member of the dive team remains in the diving bell at all times, to act as back-up in case of an accident or equipment failure affecting one of the other divers. This duty is normally rotated among all the dive team members during a bell excursion. The usual sport diving practice of using a separate mouthpiece and goggles is not common among industrial divers, as it affords less physical and thermal protection, and reduces the quality of diver communications. The advantage of such a practice, which enables the diver to change breathing equipment underwater or share a gas supply with another diver is of less significance for a surface-supplied, industrial diving situation. The utilisation of mixed gas and saturation diving techniques has made possible the development of the offshore engineering industry as it presently exists, particularly with regard to underwater intervention tasks such as inspection and repair. The problems associated with mixed gas diving are discussed below in the context of diver support of underwater engineering activities. Diving physiology and medicine is a specialised discipline, and the advice of qualified experts in that field should be sought if a more detailed discussion of such problems is required. Perhaps surprisingly, the human body is generally little influenced by environmental pressure, at least for some tens of bar increase. Problems will rapidly arise due to pressure change, however, if it is not possible to equalise the pressure within the various parts of the body. This is most frequently a problem when the Eustachian tube, between the mouth and the ears, becomes blocked, and it is not possible to equalise pressures on both sides of the eardrum. This effect is similar to that felt by many people during ascent or descent in an aircraft, but on a greater scale, and unless care is

taken, it is possible to burst eardrums due to unbalanced pressures. More serious, but less frequent, problems can occur with the lungs. Although control of the rate of depressurisation is an effective preventative measure for decompression sickness, it should be remembered that standard diving tables are drawn up to apply to average human beings. The rate at which nitrogen is expelled from the body is influenced by many factors, most notably the amount of fat present. Monitoring is required to ensure that susceptible individuals do not suffer decompression sickness during depressurisation, and if symptoms occur, the sufferer is repressurised and decompression resumed at a lower rate. One obvious effect of breathing heliox mixture is the distortion of the voice. The speed of sound in heliox is greater than that in air, and this results in a change in the frequencies produced by the larynx or voice box, creating the well known 'Donald Duck' speech characteristic. Although of no physical significance, this effect leads to communication difficulties, which can have psychological and safety implications. The pattern of voice distortion is complex, and although electronic helium descramblers have been developed, many offshore personnel prefer not to use them, once they have accustomed themselves to the effects of voice distortion. High pressure heliox conducts heat much more readily than normal pressure air, resulting in problems relating to temperature control and heat loss for divers. In order to reduce heat loss to acceptable levels to avoid hypothermia, the temperature within the living chamber must be held close to 300C. While diving, the diver's breathing gas must be warmed, and for long excursions a heated suit is used. If a diving bell suffers an accident, and the umbilical supplying it with power is severed, the most immediate problem for the divers within is to conserve heat, and special survival suits have been developed for this purpose. Even using these, the time for which divers can survive without external heat supplies is severely limited. Diving is a physically demanding and skilled activity, and before a diver can be employed in the offshore industry, a course of instruction licensed by the Health and Safety Executive must be successfully completed. Initially, the diver will be qualified for air diving operations, and after a period of experience, can opt to undertake further instruction in order to carry out mixed gas diving. In addition, divers must undergo periodic medical examinations throughout their active diving lives and the stringency of these has been progressively increased over the past few years. One result of the increased medical supervision is that some long term medical effects of deep diving have been established. The most well known of these is long-bone necrosis - the death of marrow within long bones, which can result in lesions and joint problems in the long term. It is believed

that this is due to restriction of the blood circulation within the marrow. For this reason, during periodic medical examinations the long bones are X-rayed to detect this problem at an early stage. Reservations relating to the long term medical effects of deep diving have led the Norwegians, in the early 1990s, to declare diving operations beyond 180 metres an enhanced hazard activity. The long term objective of Norwegian industry is to eliminate all manned diving operations as soon as this is technically feasible, and they have funded several research projects which would contribute to this objective. Even without these concerns, other short term effects of deep diving can be observed. At depths greater than 200 to 250 metres, a series of physiological effects, collectively known as high pressure nervous syndrome (HPNS) can be observed. These can comprise disorientation, nausea, tremors in the hands, loss of concentration and similar effects. HPNS represents a practical problem because the susceptibility of individuals is extremely varied and it particularly affects activities such as welding, which demand high levels of mental concentration and manipulative skill for extended periods. It has been suggested that HPNS is caused by signals leaking out of nerves at the wrong location because of the thinning of the insulation around the nerves due to pressure effects and this can be counteracted by the use of an anaesthetic gas - often nitrogen - in the breathing mixture which has the effect of thickening the nerve insulation. This has led to the development of helium/oxygen/nitrogen breathing gases generically called Trimix, and some success has been claimed for this approach. It has also been observed that a reduction in the rate of pressure increase during the compression process, or carrying out the compression in a series of stages, has a palliative effect on HPNS. The use of hydrogen/oxygen mixtures for very deep water diving has also been investigated. Using these various techniques, divers have been successfully pressurised to the equivalent of approximately 700 metres under test conditions. However, it is apparent that at some depth, saturation diving will cease to be viable. The progressive reduction in diver capability, the increased effort required even to breathe the dense gas, increased decompression times and physiological complications will all combine to steadily increase the cost of manned diving operations with depth, and a point must come at which they cease to be viable. This will not be a rigid limit - there will always be circumstances where a diver can be deployed to unusual depths for a specific, short term or valuable activity - but it will depend on the circumstances of each specific intervention operation. Unless more data unfavourable to deep diving becomes available, which will raise the depth limit, the author's own guess is that such a limit will be found at about 500 to 600 metres. Alternatives to manned diving operations have been discussed earlier.

Other radical techniques for deep diving exist, such as flooding the diver's lungs with fluid, but these are at an early experimental stage. To summarise, diving operations at depths less than 50 metres are normally carried out using the bounce diving technique, breathing air. At greater depths, heliox breathing mixtures are employed and are combined with saturation diving techniques to permit effective diver deployment. At depths greater than about 250 metres, some reduction in diver capability due to a variety of causes is observed, becoming progressively more marked until diving operations cease to be viable.

Appendix 3 Hyperbaric welding research techniques

Before a complex and costly underwater procedure is undertaken, it is apparent that the personnel required to carry it out must be appropriately trained and the equipment and joining processes used must be shown to be capable of performing consistently and to the required standard. This appendix will concentrate on those aspects of training relevant to the development of the joining process. In the initial phases of the development of underwater welding technology, when operations were only conducted at relatively shallow depths, underwater welding procedure development could be undertaken in diving tanks or shallow lakes, using the same equipment and divers equivalent to those employed for actual offshore operations. Such trials are still useful for splash zone procedures and to evaluate equipment, such as welding habitats, before they are used for deeper water operations. Some open water trials have been carried out at considerable depth. For example, in the early 1980s, Sub Sea Offshore carried out welding trials for British Petroleum utilising the facilities of the Underwater Trials Centre, at Fort William, at a depth of 160 metres in Loch Linnhe. These involved the deployment of the Centre's diving support vessel and an additional pontoon to carry the welding and chamber support systems. It is apparent that the cost of such operations will be considerable, and although Loch Linnhe is relatively sheltered, they will be subject to external influences such as weather conditions. To avoid weather and transport problems, several offshore companies invested in hyperbaric facilities within their onshore bases, within which divers could be trained and procedures evaluated. Figure A3.1 shows such a system at the National Hyperbaric Centre in Aberdeen. A conventional diving tank can be seen in the lower right of the picture, while in the centre is the main equipment access door to the facility. This leads to a workshop area, with adjacent machine shop and electronics workshop, and which is adjacent to the end of the main working chamber, which has a full section door which can be raised to provide unrestricted access. The

98 A3.1 The National Hyperbaric Centre, Aberdeen.

accommodation chambers are situated on an upper floor, with the control room behind them. In this case, two or three work teams can be accommodated at different depths to maximise utilisation of the facility. To the left of the picture is the hyperbaric medical facility operated by the local Health Authority, incorporating a complete hyperbaric operating theatre. The Centre also includes seminar rooms and presentation areas. The system is permitted to operate at pressures equivalent to 300 metres water depth for manned operations and 500 metres for unmanned experiments. Using facilities of this type the development welds required to evaluate welding electrodes or equipment were added to training or qualification 'dives'. Although useful, such activities meant that welds were carried out by several divers of differing abilities, often as the final phase of strenuous qualification dives, and the variation due to human factors was considerable. In addition, such diving activity was expensive and consumed human resources better used for productive activity. The requirement for more cost effective research systems, capable of carrying out welding tests in a consistent manner, led to the development of unmanned hyperbaric welding research facilities in the mid-1970s. Although less flexible than systems using human welders, the daily operating cost of unmanned facilities is, on average, about 10% that of manned diving systems and trials carried out using automated equipment are far more repeatable. Several centres for unmanned hyperbaric welding research have been formed, in various countries. GKSS and the Universitat von Bundeswehr in Germany, SINTEF in Norway, Cranfield University in England, and South East Research in the USA, are the principal centres for such activity in the 1990s. Since the mid-1980s, the principal activity of these centres has been to develop systems and procedures for automated welding at depths of 300 to 500 metres using GTAW, or to examine the feasibility of alternative welding techniques for the depth range 500 to 1000 metres. Figure A3.2 shows the internal welding system used in Cranfield's 100 bar system, commissioned in 1987. A simple 3-axis manipulator is capable of the same range of movements as the orbital welding systems described in the main text. The torch manipulator is seen to the right centre of the picture, in this case mounting a GMAW torch. This manipulator is capable of vertical and lateral movements, while the workpiece moves under the torch mounted on a traversing table. All the axes are under computer control and use DC servo motors with speed and position feedback. To the left of the picture the electrode feed system can be seen. The arc can be viewed from several angles through acrylic windows, the internal light and particulate filtration system aiding visibility. Although their configurations vary considerably, chambers of similar specification exist in several research centres throughout the world, princi-

A3.2 The welding system, Cranfield 100 bar vessel.

pally in France, Germany, Norway and the USA. Most of them have facilities capable of operating to pressures of the order of 100 bar, equivalent to a water depth of 1000 metres. These were adequate when only North Sea locations, with a maximum depth of 400 metres, were their likely market. However, since the erly 1990s, oil companies have started to move into progressively deeper waters. The first company to carry out significant deep water operations was probably Petrobras, the national oil company of Brazil, operating in the Amazon Basin to depths approaching 1000 metres. More recently, operations have begun to recover oil reserves west of the Shetlands, at similar depths. Currently, the techniques used are derived from deep water drilling operations and can be carried out without manned intervention. However, if more complex structures are required, or some unforeseen circumstance occurs, repair and modification techniques will be required. The consensus of the oil companies is that arc welding will be used, if it can be shown to be consistent enough and to produce welds of acceptable quality. For this reason, Cranfield University has designed and commissioned a new 250 bar hyperbaric welding facility, HyperWeld 250, which became operational in 1996, and is currently engaged in arc welding feasibility studies in the pressure range 100 to 250bar (Fig. A3.3). This system has a maximum operating pressure of 250 bar, equivalent to a water depth of 2.5 km, and is believed to be the highest-pressure hyperbaric welding research facility in the world. A more detailed description of the Hyperweld facility can be found in publications cited in the bibliography.

101

A3.3 The Cranfield 250 bar vessel.

Although interesting, research facilities are not ends in themselves. To conduct a successful underwater welding or joining operation, a company needs two things. An operational system is required - equipment and consumables which can be shown to have the capability to perform the required task, together with appropriately trained and qualified personnel. The second requirement is a suitable welding procedure, which has been shown to produce joints to a standard approved by the customer and certification authorities. A challenge for the project engineer is to produce a specific proposal for a specific offshore task incorporating these using the minimum of time and cost. Previous operational experience, internal expertise, development specific to the application and the results of strategic research must be combined in the most cost effective way for the proposal to be successful.

Bibliography

The offshore industry does not have a long tradition of publishing scientific papers, mainly because much of the technology has commercial sensitivity. However, there is a wide range of publications available, provided one accesses the correct sources. Many of the papers listed below can be found within the TWI WELDASEARCH database. Other useful sources are the proceedings of many of the annual conferences relevant to the industry. These include the Offshore Mechanics and Arctic Engineering (OMAE) series, the International Conferences of the International Society of Offshore and Polar Engineering (ISOPE) and the International Conferences on the Behaviour of Offshore Structures (BOSS). There have also been a series of International Pipeline Conferences which include useful material. The Journal of the Society for Underwater Technology, and the various journals serving the welding industry have also published relevant articles. Useful material may also be acquired from the offshore engineering industry itself, and its suppliers. The list of papers below is not exhaustive, but does include material which will expand the concepts put forward in this book and add detail to the overall picture. Their reference lists will point the interested reader towards further publications. Allum C J and Quintino L, 'Pulsed GMAW: interactions between process parameters, Parts 1 and T Welding and Metal Fabrication, March and April 1984. Billingham J, Nixon J H, Richardson I M and Sharp J, 'A review of the technology of deepwater repairs for offshore structures' 8th International Conference on the Behaviour of Offshore Structures (BOSS 97), Delft, July 1997. Billingham J and Nixon J H, 'A survey of underwater welding techniques' Endeavour, third quarter, 1987. Christiensen N, 'The metallurgy of underwater welding' International Institute of Welding International Conference, Trondheim, June 1983. Craig E, 'The plasma arc process - a review' The Welding Journal, February 1988. Dijk O and den Ouden G, 'The effect of pressure on the TIG welding process' International Institute of Welding International Conference, Trondheim, June 1983. Dos Santos J F, Szelagowski P, Schatstall H G and Dobernowsky A, 'Preliminary investigations of the effect of short circuit variables on metal transfer above 60bar abs' 22nd Offshore Technology Conference, Houston, May 1990. Grubbs C E, 'Underwater wet welding (a state of the art report)' 12th International Conference on Offshore Mechanics and Arctic Engineering, Glasgow, June 1993.

Habrekke T, Armstrong M and Berge J O, 'Deep water pipeline welding and repairs using modern computer technology to create a diverless future for Statoil' International Conference on Computers in Welding, San Francisco, 1997. Huissmann G, Hoffmeister H and Knagenhjelm HO, 'Effects of TIG electrode properties on wear behaviour under hyperbaric conditions 37 to 40 bar' International Institute of Welding International Conference on Welding under Extreme Conditions, Helsinki, September 1989. Ibarra S, Olsen D L and Grubbs C E 'Underwater welding of higher strength offshore steels' 21st Annual Offshore Technology Conference, Houston, 1989. Lucas W, 'TIG and plasma process fundamentals' TWI Seminar on Exploiting TIG and Plasma Developments, Leicester, June 1981. Malone R B and Ralston J, 'Hyperbaric welding of exotic steel pipeline' 11th International Conference on Offshore Mechanics and Arctic Engineering (OMAE92), Calgary, June 1992. Matsuda F, Ushio M and Tanaka A Y, 'Metal transfer characteristics in pulsed GMA welding' Transactions of the Japanese Welding Research Institute, January 1983. McGlone J C, 'Repair and maintenance in a hazardous environment - the North Sea' International Journal for the Joining of Materials, January 1989. Nixon J H and Richardson I M, 'Open arc pulsed current GMAW - application to hyperbaric operations.' The American Society for Metals International Welding Congress, Toronto, Canada, October 1985. Nixon J H and Richardson, I M, 'The design and construction of a 250 bar hyperbaric welding research facility' 14th Conference on Offshore Materials and Arctic Engineering (OMAE95), Copenhagen, June 1995. Sharp J V, 'Strengthening and repair of ageing North Sea platforms - a review' 12th International Conference on Offshore Mechanics and Arctic Engineering, Glasgow, June 1993. Sharp J V, Kam J C and Birkinshaw M, 'Review of criteria for inspection and maintenance of North Sea structures' 12th International Conference on Offshore Mechanics and Arctic Engineering, Glasgow, June 1993. Wimpey Laboratories, 'Grouted and mechanical strengthening and repair of tubular steel offshore structures' OTH 88283, HSE Books, 1988.

Index

Index terms

Links

A acoustic monitoring of offshore structures

23

alternatives to arc welding for deep water joining operations

80

to saturation diving for deep water applications

63

alignment clamp, pipe

22

American Welding Society underwater welding standard

6

27

American Petroleum Institute (API) pipe tolerances

5

22

anodic protection of underwater structures

24

Archimedes’ Law

89

argonarc welding, see tungsten inert gas welding atmospheric diving suit (ADS)

63

autogenous TIG welding

39

automated orbital hyperbaric welding

58

B bends, the bibliography Big Inch, Inc., USA

91 103 9

bounce diving

92

British Petroleum, UK

35

57

British Underwater Pipeline Engineering, UK

62

82

This page has been reformatted by Knovel to provide easier navigation.

83

105

106

Index terms

Links

C caisson disease

91

carbon block arc striking, TIG welding

41

carbon equivalent of steels for wet welding

27

‘Challenger’ space shuttle

87

characteristics (static and dynamic) of MIG power supplies

54

cofferdam welding

29

column voltage, TIG welding

39

composition of hyperbaric MMA welds, effect of pressure on

48

concrete repair

3

continental shelf

89

controlled dip MIG welding

73

controlled transfer pulse (CTP) MIG welding

70

cooling rates in hyperbaric MMA welding

49

Cousteau, J

92

cracks, structural, repair of

18

Cranfield plasma arc initiation procedure

68

Cranfield plasma welding torch

68

Cranfield University, UK

57

68

99

100

MIG welding control system cryogenic pipe couplings

77 12

D decompression tables

91

deep water arc welding processes

65

mechanical connectors

84

This page has been reformatted by Knovel to provide easier navigation.

75

107

Index terms detachment parameter, MIG welding Det Norsk Veritas design rules

Links 71 6

diving technology

90

dip transfer in MIG welding

54

‘double heading’, pipelaying

5

dye penetrant inspection of underwater welds

23

E electrode burn off rate, hyperbaric MMA

47

electric field strength effect of pressure on

41

TIG welding

40

environmental impact of offshore engineering operations

87

explosive welding

82

89

F fall voltages, TIG welding

39

‘firing line’, pipelaying

5

Flexiforge mechanical connector

9

floating production and storage systems

86

flooded member detection

23

flux cored arc welding (FCAW) hyperbaric operation

57

process description

56

free diving

90

friction welding

80

friction stitch welding

81

friction stud welding, underwater

80

This page has been reformatted by Knovel to provide easier navigation.

108

Index terms

Links

G gas tungsten arc welding, see tungsten inert gas welding GKSS, Germany

75

grouted connections, structural

15

99

H Haldane, J S Health And Safety Executive (HSE) guidance notes

91 6

heliarc welding, see tungsten inert gas welding heliox diving gas mixtures

92

high energy bonding, see explosive welding high frequency (HF) arc initiation, TIG welding

40

high pressure nervous syndrome (HPNS)

95

HSE Design and Construction Regulations

41

7

HSE regulation of commercial diving

94

Hydratight Ltd, UK

10

85

hydrogen control in hyperbaric MMA

50

induced cold cracking (HICC)

26

levels in MMA welding

46

hyperbaric plasma arc voltages

69

hyperbaric welding

29

chamber, flexible

34

chamber gases

36

chamber, hybrid

34

chambers, pipeline

31

chambers, structures

33

research techniques

97

techniques, manual

38

Hyperweld 250, Cranfield, UK

100

This page has been reformatted by Knovel to provide easier navigation.

109

Index terms

Links

I industrial robots for hyperbaric welding

64

initiation of plasma arc

66

inspection techniques, underwater

22

integrated modular tool system (IMT)

60

inverter welding power supplies

73

ISO standard on mechanical joints

6

J joint area cleaning

18

K keyhole plasma welding

67

keyhole hyperbaric plasma welding

70

L Lethbridge diving suit

90

lifting of underwater structures

8

Lloyds design rules

6

Lockheed Petroleum Services, USA

28

long bone necrosis, diving associated disease

94

M Magnus oilfield repair

34

‘Mantis’ submersible

63

manual metal arc welding electrode description

45

hyperbaric operation

46

process description

44

productivity

46

This page has been reformatted by Knovel to provide easier navigation.

110

Index terms

Links

mechanical pipe connectors diver installed diverless installation

9 84

metal active gas welding (MAG), see metal inert gas welding metal inert gas (MIG) welding advanced hyperbaric operation

75

advanced process development

70

hyperbaric operation

56

process description

51

metal ohne gas welding (MOG), see metal inert gas welding metal transfer in MIG welding

53

MIG welding shielding gases

52

mixed gas diving

92

Morgrip mechanical connector

10

diverless

84

N National Hyperbaric Centre, Aberdeen, UK

97

NEI orbital welding system

58

nitrogen, narcotic effects of when diving

91

nodes, platform

4

North Sea tides

88

Norwegian government diver depth limit

62

Norwegian Trench

58

33

O oceanography

88

ocean currents

88

Oerlikon hyperbaric FCAW consumable

56

58

This page has been reformatted by Knovel to provide easier navigation.

111

Index terms

Links

offshore engineering costs

1

offshore materials

1

one atmosphere welding

27

open circuit voltage, MMA welding

46

oxy/hydrogen diving gas mixtures

92

5

P Petrobras, Brazil

28

‘pigs’ - pipe seals

18

piling of jacket platforms

4

pilot arc, plasma welding

66

100

pipe alignment clamp

22

crane

19

machining, underwater

18

pipelaying pipeline alignment systems

5 19

integrated

19

multi element

20

pipeline repair spread (PRS), Statoil

21

59

pipelines coiled

4

fabrication

4

underwater

4

Piper Alpha

6

87

‘Pisces’ submersible

63

82

plasma constrictor orifice

66

plasma jets, effect on hyperbaric MIG metal transfer

75

plasma welding, hyperbaric operation

67

plasma welding, process description

65

This page has been reformatted by Knovel to provide easier navigation.

83

112

Index terms

Links

platforms gravity

3

jacket

3

wooden

1

preheat, effect of in hyperbaric MMA

49

pressure and depth, relationship between

89

process efficiency, hyperbaric TIG welding

43

process variables affecting plasma welding

67

protective wrapping of underwater structures

24

PRS pipe crane

61

pipeline repair spread (PRS), see Statoil PRS pulse metal transfer, MIG welding

71

pulse width modulation control of power supplies

73

pup piece

22

R radial friction welding

82

radiography of underwater welds

23

Raychem, USA

12

remotely operated vehicle (ROV)

61

robotic MIG welding

78

64

S safety critical elements (SCEs)

7

saturation diving

92

scuba diving systems

92

seabed excavation

17

seabed, variability of

17

‘seahorse’ pipe alignment system

21

seawater, variable density of

89

89

This page has been reformatted by Knovel to provide easier navigation.

81

113

Index terms

Links

self regulation in MIG welding

52

self shielding FCAW consumables

57

shear keys for grouted connections

16

shielded metal arc welding, see manual metal arc welding Siebe, Augustus

90

Sievert’s Law

48

Sintef, Norway

68

South East Research, USA

99

splash zone

2

spray transfer in MIG welding

54

standard diving dress

90

standards and codes

6

Statoil pipeline repair spread (PRS)

99 29

59

‘stick’ welding, see manual metal arc welding stinger, pipelaying

5

Stolt Comex Seaway, Aberdeen

9

21

61 structural crack repair

18

structural grouted connectors

15

structural stressed grouted connections

16

structural joining systems

15

structural mechanical connectors

15

sub ocean services, UK

57

Sub Sea Offshore, UK

57

summary of deep water arc welding process characteristics

78

survey, underwater

17

swaged pipeline joints

14

synergic algorithms

72

synergic MIG welding

70

This page has been reformatted by Knovel to provide easier navigation.

34

114

Index terms

Links

T tank diving trials tanker loading system

97 4

thermocline

89

TIG arc stability, effect of pressure on

43

TIG welding depth limitation

44

Tinel’ alloy

12

touch strike arc initiation, TIG welding

40

tubular MIG welding consumables, manufacture

56

tungsten electrode materials

40

44

tungsten inert gas (TIG) welding hyperbaric operation

40

process description

38

TWI, UK

80

U ultrasonic inspection of underwater welds

23

underwater engineering processes

17

trials centre, Fort William, UK

97

welding inspection techniques

22

welding technology

25

Universitat von Bundeswehr (UvB), Hamburg

75

99

unmanned hyperbaric welding research facilities

99

V visual inspection of underwater welds

23

voice distortion in heliox

94

This page has been reformatted by Knovel to provide easier navigation.

115

Index terms

Links

W wall thickness measurement

23

weightcoat

4

removal

17

weld metal composition changes in hyperbaric MMA

48

wet welding

25

Wimpey Laboratories report on grouting

6

16

This page has been reformatted by Knovel to provide easier navigation.