Advanced Welding Processes for Transmission Pipelines

S. A. Blackmana and D. V. Dorlingb

a. Welding Engineering Research Centre, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, England

b. TransCanada Pipelines Ltd, PO Box 2535, Station M, Calgary, Alberta, Canada, T2P 2N6

The economics of constructing pipelines is dependent on the productivity of welding individual pipes together and the integrity of the pipeline girth welds is critical to the overall integrity and reliability of the pipeline. These demands have led to extensive research and development of advanced welding processes for pipeline applications.

Recent developments in high strength linepipe and new corrosion resistant alloys together with a resurgence of interest in deepwater pipeline installation methods have led clients and contractors to reconsider their construction methods and to look for more economical processes with higher quality weld deposits.

This paper reviews the most recent developments in welding technology suitable for pipeline applications and gives some recommendations for further development.

1.INTRODUCTION

1.1.Background

Pipeline Research Council International, Inc (PRCI) have funded a review of high deposition welding processes to provide their members with information on the processes available and guidance on those processes that may provide a production advantage and would therefore be worthy of further funding. Traditionally, the most suitable high deposition processes have been considered to be single shot welding processes. In this review, a wider view is taken and the potential of advanced arc welding and power beam processes are considered in addition to the single shot welding techniques.

1.2.Objectives

The aim of this paper is to summarise the recent developments in welding technology applicable to the construction of transmission pipelines and to provide recommendations for further development.

1.3.Scope

It is assumed that the reader has some knowledge of the subject area and this paper does not therefore describe the fundamental principles of the processes reviewed.

In this review, emphasis is given to the operational performance of the welding processes, their applicability to pipeline construction and the likelihood of success in developing a robust production system.

The mechanised GMAW process is currently the most widely used welding process for transmission pipelines and there have been a number of recent welding procedure developments that have improved productivity. In addition to extensive use onshore and for S-lay, the CRC-Evans Automatic Welding's GMAW process and Saipem's Passo system have been proven suitable for J-lay installation. The GMAW process therefore represents the current state-of-the-art and represents the benchmark against which other welding processes have been assessed.

2.HISTORICAL DEVELOPMENT OF ADVANCED WELDING PROCESSES FOR TRANSMISSION PIPELINES

One-shot welding is not a new concept in transmission pipelines and automatic flash-butt welding was applied to the first North Sea pipelines. Operation PLUTO (Pipe Lines Under The Ocean) was conducted in 1942-4 to transport petrol from England to the Allied troops after D-Day with 975 miles of 3” pipeline made with a maximum output of 9.6 miles/day. The pipelines were installed across the English Channel using floating reels. Since that time, flash-butt welding has probably been the most actively developed one-shot welding process for pipelines.

In 1969, CRC-Crose International Inc. were the first to apply a mechanised GMAW system for the construction of a transmission pipeline when their internal and external welding equipment was used to construct 5 miles of 762 mm (30”) OD, 7.9 mm (0.312”) wall thickness, 5L X60 pipeline in West Texas [1, 2]. 138 welds were achieved in an 8½ hour working day with a maximum of 25 welds produced in one hour.

Figure 1. CRC-Crose Internal Pipe Welding Unit [1].

The equipment first used by CRC-Crose [1] consisted of a pneumatically operated internal line-up clamp with four welding heads for making an internal root pass (Figure 1) and two welding heads travelled on an external band as shown in Figure 2. Additionally, CRC-Crose developed the now familiar pipe facing machine to allow accurate machining of the weld bevel. Each component was a major technical development but it was the combination of all three parts which made the system successful.

Figure 2. CRC-Crose External Pipe Welding Unit [1].

56 kilometres (35 miles) of pipeline were welded in 1969, 115 kilometres (72 miles) in 1970 and in 1971 6 projects totalling 509 kilometres (318 miles) were completed. There was no doubt that mechanised GMAW had the potential to be more productive in terms of man hours per weld than conventional manual shielded metal arc welding and, as a result of its intrinsically low hydrogen content, gas metal arc welding was essentially immune to cold cracking. However, mechanised welding was an expensive option. Equipment costs were relatively high and economic viability was marginal for long-distance projects and certainly not achievable for short projects. However, since the early 1980s, a number of process and equipment developments have improved the overall productivity and economics of mechanised GMAW. Additionally, the mechanical requirements specified for API 5L X80 pipelines, have effectively made mechanised GMAW the only viable mainline welding process for these high strength materials.

Despite the success of the mechanised GMAW processes, there was still a desire to further improve welding productivity and particularly for J-lay installation of submarine pipelines. Hence, in the late 70’s and early 80’s, there was a great deal of research investment into the development of novel welding systems for pipeline construction. A number of prototype production systems were developed and some systems were used for limited production tests. However, although several authors proclaimed the success of the processes they developed, none of these welding systems are widely applied today.

Research activity into one-shot welding waned in the late 80’s but, interest was renewed in the mid-90's with talk of major deepwater trunklines, such as the Oman-India and Qatar-Pakistan pipelines, and the demand for major gas transmission pipelines. Unfortunately, economic changes led to most of these major projects being shelved but welding developments are continuing. In selecting suitable welding processes, researchers turned back to the systems previously investigated and with the possible exception of homopolar welding (a variant of resistance welding), no totally new techniques are currently known to be under development. However, technology advances in other industry sectors has meant that some of these processes are more technologically advanced and this has improved the chances of success.

3.REVIEW OF ADVANCED WELDING PROCESS DEVELOPMENTS

3.1.Arc Welding Processes

3.1.1.GMAW

For large diameter transmission pipeline construction, GMAW is the most widely used welding process and all of the major offshore contractors have developed proprietary welding systems. Several companies offer mechanised GMAW for rent or purchase. Although all of these systems are based on the GMAW process, the mechanical design and weld procedure details are very different and this does affect productivity. The use of internal welding machines originally restricted the use of mechanised GMAW to diameters greater than 24” but the development of copper backing systems now permits mechanised GMAW to be used on diameters as small as 8”.

The productivity of mechanised GMAW has increased significantly in recent years but major improvements are still envisaged due to the development of dual torch and twin-wire welding systems. Additionally, several companies are implementating improved control systems with the potential for arc sensing, self-tuning welding conditions and seam tracking. These may improve consistency and allow higher travel speeds to be utilised.

3.1.1.1.Dual Torch Welding Head

Figure 3 shows the Saturnax welding system which is the first dual torch welding system to be widely used for pipeline construction. This welding head deposits two passes simultaneously and has been used extensively onshore and offshore by Serimer Dasa [3].

Over 400 joints/day are known to have been installed by the LB200 on the 42” NorFra pipeline and the DLB1601 is reported to have achieved 492 joints/day on the 24” Interconnector project. In explaining the incentives for McDermott’s development of flash-butt welding, Turner [4] estimated that the LB200 could lay a 36” pipeline at 377 joints/day using flash-butt welding. At this time, 211 joints/day was predicted for GMAW. It can be seen therefore that developments in GMAW have made the use of one-shot systems like flash-butt welding less

attractive and many would not be able to compete with the cycle times now achieved by GMAW.

Figure 3. Saturnax Welding System

In 1999, CRC-Evans introduced their P500 dual torch welding head (Figure 4). This has remote wire feeding and a pendant control and also has the facility for though-arc sensing to guide the welding torch in the weld bevel.

Figure 4. CRC-Evans P500 Dual Torch Welding Head

Saipem and Vermaat have also developed dual-torch GMAW systems.

With dual torches, the deposition rate is not twice that of the single torch systems. In weld procedures conducted by TransCanada, a double torch system was found to have a deposition rate (kg/hr) of 1.4 times that of a single head system. The double torch system had a higher travel speed and required 4 runs to complete an 11.7mm (0.460”) wall thickness whereas the single torch system required 5 runs to complete 12mm (0.472”) wall thickness. As pipeline productivity is often controlled by the root pass production in the first welding station, the main benefit

of these systems is the cost savings that result from the reduced number of fill pass welding stations that are required to keep up with the root welding.

3.1.1.2.Twin-Wire GMAW

Twin-wire GMAW involves the use of two welding wires that are each fed into the same weld pool. This is different to the dual torch welding systems which carry two torches on one welding head but which are spaced some distance apart.

Figure 5. Twin-Wire GMAW

With twin-wire GMAW, the consumables may be passed through a common contact tip or they may be passed through separate contacts tips that each have their own power supply.

Figure 5 shows a twin-wire torch designed by Cranfield University for TransCanada Transmission and fitted to a conventional pipeline welding head. Initial trials have shown that the system can be used in all positions around the pipe and good metallurgical properties have been achieved. The deposition rate is significantly higher than existing single and dual torch GMAW systems for fill pass applications.

3.1.2.GTAW

The development of ‘Roboweld’ [5], Saipem’s hot-wire GTAW system, has been abandoned. It was anticipated that Roboweld would be more productive and provide higher quality welds than GMAW systems. However, development of Saipem's Passo system significantly improved the productivity and Roboweld was not a competitive solution.

Heerema used mechanised GTAW for J-lay installation of the Shell Maui pipeline [6] but improvements in pulsed-GMAW now make this a more appropriate choice for this type of application.

3.1.3.Plasma

European patent application EP 0 689 896 A1 and US patent 5599469 cover the keyhole welding of large diameter pipes and it is claimed that a 30” x 19 mm pipe can be welded in four passes at 15, 15, 17, 17 cm/minute respectively. This welding speed is significantly slower than GMAW and it is known from previous research at Cranfield University that orbital plasma welding is extremely sensitive to minor variations in equipment setup. Additionally, due to the large welding torch required, plasma welding would not be very suitable for heavy wall pipes.

3.1.4.FCAW

Flux and metal cored wires have been tested with the mechanised GMAW systems but offer few advantages for normal pipeline applications. However, they are suitable for completion of the fill passes in very heavy wall pipe where the bevel width makes GMAW difficult. Mechanised FCAW is also suitable for fill pass applications where a manual root pass has been used.

In Russia, a self-shielded FCAW process known as ‘Styk’ has been used and it is claimed that 24 mm wall thickness can be welded in four passes. The machine welds vertically up using a shaped shoe to support the weld pool but produces comparatively poor mechanical properties.

3.1.5.Electroslag

Japanese researchers have developed an electroslag process for fixed pipe [7]. The process is similar to the Styk process but a solid wire is used. The electroslag start and stop points must be removed by gouging and rewelded manually. The intended application is water pipes over 2 m diameter and 20 mm wall thickness.

3.2.Power Beam Welding Processes

3.2.1.Electron Beam Welding

Electron beam welding is used in the automotive, aerospace, nuclear and power generation industries and has the capability to produce high quality, single pass welds up to several hundred millimetres thick. Most materials can be welded including steels, stainless steels, nickel alloys and titanium. The main restriction on the use of the process has been the need to operate within a vacuum chamber at vacuum pressures less than 5 x 10-2 mbar and this has so far limited the applications of electron beam welding.

Total first developed electron beam welding of pipelines in the late 1970’s [8] and a prototype J-lay welding system was constructed. Over 500 weld runs were made on 6 different grades of pipe and it was claimed that sound welds could be made in steels up to X100 grade without pre or post heat treatment. However, it was subsequently reported that some welds exhibited poor mechanical properties. It was also found that the high vacuum requirements could not be reliably maintained due to poor sealing and when the demand for J-lay subsided, the research programme was stopped.

Today, advances in other industrial sectors have led to the development of electron beam welding guns that can operate in atmosphere or low vacuum conditions and TWI are using such a gun to develop the process for Saipem [9]. They have successfully developed a prototype system with a single welding head and are producing a production system with two welding heads. The welding head(s) rotate around the pipe which is held in the vertical position and aligned using an internal alignment clamp. A low vacuum of 10-1 – 10 mbar is used and this is quickly achieved with a relatively simple pumping system. The electron beam welding system has a built-in seam tracking system that makes use of back scattered electrons. TWI has demonstrated that, with seam tracking, the process is tolerant to misalignment and that good welds can be made in the 2G position with up to 5 mm of high-low. Unfortunately, CTOD results have not been totally satisfactory and Saipem are not planning to use the system in production in the near future.

For J-lay welding, the electron beam process offers very high penetration at relatively fast welding speeds but in the 5G position used for S-lay, the depth of penetration that can be successfully welded in the overhead position is limited due to problems in maintaining the keyhole. Masuda [10] has reported the development of an internal electron beam welding machine by The Japan Gas Association and Kawasaki Heavy Industries and it is claimed that 30” x 19 mm API 5L X65 pipe was welded in the 5G position. A high vacuum of 3 x 10-4 Torr (4 x 10-4 mbar) was used apparently without filler wire. The weld bead was approximately 2 mm wide and Masuda noted that the maximum tolerable weld gap was 0.2 mm but mismatch up to 2.5 mm could be tolerated. The net welding time was reported to be 7½ minutes with travel speeds of 600, 400, 600, 500 mm/min in the flat, vertical up, vertical down and overhead positions respectively.

3.2.2.Laser Welding

Laser welding does not have the penetration capacity of electron beam welding but it does not require a vacuum and is therefore applied more widely. For many years, laser welding has been used in automotive, aerospace and nuclear applications but with the recent introduction of commercial high power lasers, it is now being considered for heavy engineering applications such as pipe manufacture, structural steelwork, yellow goods and ship-building. There are two types of laser that are suitable for industrial welding applications.

3.2.2.1.CO2 Lasers

Commercially available CO2 lasers are available with output powers up to 25 kW. These lasers have a usable beam power of about 20 kW and can weld up to 28 mm thickness at 1 m/min travel speed. A 10 kW laser would weld 15 mm at the same speed.

CO2 lasers have a wavelength of 10.6 µm and this is not transmittable through glass. Hence, to transfer the beam from the laser source to the workpiece, a series of mirrors must be used. The diameter of an unfocused laser beam increases with the power rating and 25 kW lasers have beams between 45 mm and 70 mm

diameter. As the power increases, heat loss at the mirrors is also increased and it is necessary to design the mirror system with appropriate water cooling to compensate for this. The need to use mirrors and the relatively large beam diameters of high power CO2 lasers makes it difficult to weld complex geometries with CO2 lasers. However, two French companies have developed CO2 laser systems for pipeline welding and one system has been tested offshore by Bouyges Offshore. AXAL is a subsidiary of Interpipe and they have developed a prototype J-lay welding system suitable for lasers up to 20 kW (PCT World Patent Application WO 98/06533). Bouyges Offshore funded ATOL to develop an S-lay system based on a 12 kW laser. This has been used to weld six 0.5 km sections of a 10” x 12.7 mm API 5L X52 pipeline. It is understood that this was a qualification test only and the welds were not part of a service pipeline. All of the equipment is containerised and designed to be easily transported. It is reported that the system was designed for pipe up to 24” diameter with a maximum of 100 mm of concrete. ATOL have subsequently designed a second S-lay system which they are marketing.

CO2 lasers have been used by a number of research institutions and they have also been developed for seam welding of linepipe. TWI have a research project ongoing with the Institute de Soudure and Japanese pipe suppliers and Bouyges offshore are members of this programme. CRC-Evans Automatic Welding have taken patents for internal/external laser welding of pipelines (US Patents 5796068 and 5796069). These cover a concept design and equipment has not been built or tested.

3.2.2.2.Nd:YAG Lasers

The most important aspect of Nd:YAG lasers is the 1.06 µm wavelength of the beam which can be transmitted through a fibre optic cable. This makes it much easier to automate high speed welding of complex geometries. Fibre optic cables can be up to 200 m long with an outside diameter of about 10mm. The fibre optic within the cable is usually 300 or 600 µm diameter. Nd:YAG lasers are not as economical to run as CO2 lasers but, power for power, Nd:YAG lasers usually allow faster welding speeds. Nd:YAG lasers are also solid-state devices and therefore more easily transportable.

Until recently Nd:YAG lasers have been limited in output power and have been restricted to thin sheet applications. In May 1997, Trumpf introduced a 4 kW Nd:YAG laser. This is believed to be the largest commercially available but it is understood manufacturers have the capability to produce a 6 or 8kW Nd:YAG laser if there was a demand. Another method of producing a higher effective beam power at the work piece is to combine the beams from multiple Nd:YAG lasers. TWI have a group sponsored project ongoing and they have coupled three Nd:YAG lasers. Similar multibeam experiments have been conducted by other researchers.

Current Nd:YAG laser systems are lamp-pumped and are only 2-4% efficient. Hence, they require large chillers for cooling and this presents a logistical problem in using them on the right of way. However, diode-pumped lasers are 15-20%

efficient and a 1kw diode-pumped laser will be commercially produced by Trumpf in 2000. Higher power diode-pumped lasers are expected to follow.

Although these developments in Nd:YAG lasers still do not produce the penetrating power of CO2 lasers, the powers envisaged are close to being suitable for most onshore pipelines (e.g. up to 15mm penetration) and they could also be used for multi-pass welding or as an internal/external combination. Several companies with an interest in pipelines are supporting a group sponsored project at TWI which is investigating high power Nd:YAG welding. PRCI are funding joint research by EWI and Cranfield University to assess Nd:YAG welding for transmission pipelines. EWI and Cranfield have also launched another group sponsored project to specifically develop Nd:YAG and arc-augmented Nd:YAG laser welding for transmission pipelines.

3.2.2.3.Plasma Augmented Laser Welding

A number of researchers have considered arc augmented laser processing using a GTAW, GMAW or plasma arc. Of these, plasma augmented laser welding appears to be the most successful. Plasma-Laser Technologies Ltd of Israel hold US Patent 5 705 785 for a welding head that focuses a laser beam in the centre of a plasma arc. The laser guides and restricts the arc which increases the effective power of the laser and hence improves welding speed and penetration at a relatively low cost. The use of the plasma arc increases the spot size and therefore improves the tolerance of laser welding to root gaps and misalignment.

The system can be used with CO2 or Nd:YAG lasers. For pipeline applications, the use Nd:YAG with a fibre-optic beam delivery system is considered the most appropriate. Plasma-Laser Technologies have conducted trials for Moscow Pipes Assembling Plant. A 630mm diameter 8mm thick pipe was welded with a gap of up to 6mm with a single pass and the use of filler metal. A 1.65kW Nd:YAG laser was used and full penetration was obtained at a processing speed of 30cm/min. A 1420mm diameter 16mm thick pipe was welded in two passes from opposite sides. The speed was 25cm/min and a 2mm overlap was obtained. These results could undoubtedly be improved by using a higher power Nd:YAG laser or multibeam Nd:YAG system.

3.3.Forge Welding Processes

3.3.1.Flash-Butt Welding

Many miles of large diameter pipelines have been installed onshore in the Former Soviet Union. McDermott licensed the process from the E.O. Paton Welding Institute and invested a significant amount of time and capital in developing it for laybarge operation with over 1500 full scale test welds being made [4]. Following trials by McDermott, the process was also accepted for inclusion in API standard 1104. However, a pipeline was never installed. Two reasons given for this are the improvements in conventional welding systems and problems in achieving satisfactory mechanical properties with the available materials. Pipe line-up takes two minutes, the welding cycle takes three minutes and internal flash removal

takes one minute. Flash-butt welding is not entirely a solid-phase process as some material is melted during the flashing operation. The high heat inputs of the process and the relatively long time at high temperature have a degrading effect on TMCP steels and post-weld heat treatment is necessary. According to Sprow (1990), trials for Statoil on 36” pipe required a heating, holding and quenching cycle totalling 6 ½ minutes. Tensile and hardness values were acceptable but scatter and the occasional low fusion line charpy were of concern. This heat treatment cycle is somewhat longer than the three minutes suggested by Turner (1986).

3.3.2.Electric Resistance / High Frequency Induction Welding

Both of these processes are similar in operation and both are used for seam welding pipes. In electric resistance welding, current is applied by direct contact and in high frequency induction welding a non-contacting induction coil is used. The HFI process is now acknowledged to produce high quality weld seams.

In seam welding, the pipe edges are gradually brought together within the electrical field but this is not possible in girth welding where the whole joint surface must be in contact. Due to the ‘skin effect’ obtaining uniform heating can be a problem and this is one of the reasons why homopolar welding has been developed as an alternative process. As in seam welding, flash removal and post-weld heat treatment will be required for girth welds.

US patent 441860 refers to an internal clamping method that can be used with an induction system to align the pipes and apply an axial force to pull the pipes together and create a forged weld. Mannesmann Anlagenbau have published a brochure outlining their proposed J-lay system and induction welding is proposed. They state that successful tests were completed on pipes up to 30” diameter and 40 mm wall thickness and they use the term ‘press butt welding’ to describe the process. The literature survey found one paper published in 1982 and a review paper in 1983 but no recent information.

Although the heating method is different, the overall effect of resistance/induction welding is similar to that of flash-butt welding and it can be expected that there will be similar problems with material properties and post-weld operations.

3.3.3.Homopolar Pulse Welding

Homopolar pulse welding [11] is a resistance process operating on the same principle as electric resistance and induction welding. In this case however, a homopolar generator is used to deliver a single very high current DC pulse and the weld operation takes only 2-3 seconds. Due to the short welding cycle, a narrow heat affected zone is formed. It is claimed that required mechanical properties can be obtained without post weld heat treatment

Until recently, work was ongoing at the University of Texas at Austin. Parker Kinetic Designs specialises in the design and manufacture of homopolar systems for a range of industrial applications and they hoped to market homopolar pulse welding for pipelines. Most work had been completed on 3” steel pipe but some

12” diameter pipe welds had been made. Theoretically, the process can be scaled up to cover any size pipe with the required power being proportional to the cross-sectional area of the pipe. However, the research programme at the University of Texas was suspended early as it became clear that a very high capital investment would be required to make the process commercial.

Figure 6. Homopolar Welding

3.3.4.Shielded Active Gas-Forge Welding

The SAG-forge process was developed by AMR Engineering. The two main differences between this and the other forge welding processes are the use of an active reducing gas to surround the weld zone and the inventor’s claim that flash removal is not required. The welding time is in the order of 2-3 minutes. Difficulties have been experienced in welding steel pipe and a post-weld induction heat treatment is required. AMR Engineering have three patents covering various aspects of the process.

Statoil investigated this system for the Zeepipe project and welding trials were conducted on board the Allseas laybarge, Lorelay. Since that time, developments have mainly considered non-ferrous pipe materials.

3.3.5.Magnetically Impelled Arc Butt Welding

Developed for the joining of thin-wall steel tubing, MIAB welding is a high-temperature forge-welding process, which, in general, exhibits weld characteristics similar to the other hot forge techniques. The essential difference is in the way in which heat is generated prior to the application of the forging force. With the MIAB welding process, the square-edged pipe faces to be joined are separated by a small gap and a welding arc is established across the gap. A static, radial magnetic field is superimposed in the gap which causes the arc to move around the pipe ends as a result of the interaction of the arc current and the magnetic field. The speed of the arc is very high (150 m/s or greater) and results in uniform heating of both pipe faces. After sufficient heating time, the pipes are

rapidly forged together to produce a solid phase bond with a characteristic flash or upset.

The process has been exploited mainly in the European automobile industry to weld a range of carbon and low-alloy steel components such as car and truck axles, drive shafts, shock absorbers and gas-filled struts. All of the commercially-available machines are stationary, floor- or bench-mounted units into which the parts to be joined are fed. A low-noise portable system has also been developed by NKK Corporation of Japan for welding of low pressure gas distribution pipelines in densely populated urban areas where excavation and construction can only be carried out at night and the excavations reinstated before dawn. The system was not designed for high production applications. An average of 60 metres of pipe per day was welded.

In order to determine the feasibility of the MIAB welding process for cross-country construction of small-diameter gas transmission pipelines, The Welding Institute was contracted to work with TransCanada Transmission in establishing the optimum welding conditions and resultant weld quality for the joining of NPS 4, 3.2 mm W.T., Grade 290 MPa pipe. A prototype system was designed and built by TWI and delivered to TransCanada where it was commissioned in 1992. Two welding head assemblies were produced. The first head is designed for Nominal Pipe Sizes (NPS) 3, 4 and 6 with a maximum forge force of 300 kN. The second head has jaw insert and magnet assemblies for NPS 6, 8, 10, and 12 pipe and a maximum forge force of 600 KN (Figure 7).

Figure 7. MIAB NPS 12 Welding Head

The development of the MIAB system has currently been suspended. The process was proved suitable for small diameter thin wall pipes. A major limitation of the current process is the relatively thin wall thickness that can be welded because the rotating welding arc tends to move around the pipe diameter and does not heat the full wall thickness.

3.4.Friction Welding Processes

3.4.1.Radial Friction Welding

TWI developed radial friction welding specifically for pipe welding. It overcomes some of the handling problems of rotary, orbital and linear friction welding by rotating a radial compression ring between two stationary pipes. The ring is rotated whilst under compression such that heat is generated and a friction weld is created between the ring and the two pipes. Monitoring of machine parameters during the weld cycle provides the main quality control check. An internal mandrel is used to prevent an internal flash from forming but the external surface must be machined after welding.

The system was originally developed for Norwegian contractor, Ugland, but is now being developed by Stolt Comex Seaway (SCS) [12]. SCS have a prototype machine suitable for welding up to 6” diameter and are currently developing a production machine capable of joining pipes between 6” and 12” diameter. It is intended that this system will be installed on the Seaway Falcon.

3.5.Other Welding Processes

3.5.1.Explosive Welding

Two explosive welding processes have been developed for pipeline construction and both have been field tested. ‘High Impact’ welding has been applied to 16”, 42” and 48” pipelines by TransCanada [13]. A bell and spigot joint was used and a 5 minute post-heating cycle was required to reduce bond line hardness.

The ‘VONO’ system was developed by International Technologies A/S and was demonstrated to a group of pipeline contractors on a landline in Sweden in 1991. The system uses a shaped internal ring with an explosive cartridge assembled behind it. A coil of sheet steel is wrapped around the joint to contain the blast. The demonstration was performed on 16” x 7 mm pipe. This system is no longer being promoted for pipeline welding. With both of these systems, the time for setting up the joint is a significant factor and the welding of heavy wall pipe is not considered feasible.

3.5.2.Thermit Welding

For pipeline applications, thermit welding is currently limited to the welding of anode bonding cables but it is used for the welding of railway tracks. In 1992, Stuart Gibson [14] investigated the possibility of using thermit welding for 2G and 5G welding of pipelines. Internal and external pre-formed sand moulds were used to contain the molten material generated by the aluminothermic reaction and the joint was formed as a casting. The pipe was 8” x 15.87 mm. Weld defects similar to those found in castings were observed. The weld metal had a carbon content of 0.46% and showed a high hardness although a tensile failed at only 374 MPa. The welding time was only 60 seconds but it was necessary to wait one hour before the weld metal had cooled and the mould could be removed.

3.5.3.Diffusion Bonding

Sumitomo Metals (European Patent Application 769 344 A1) have developed a high speed diffusion bonding system for joining small and medium diameter tubulars using an amorphous metal insert. A one minute welding time is quoted for a 60.5 mm x 3.8 mm carbon steel tubular. Stainless steel tubulars were also welded. European Patent Application 803 313 A2 also relates to diffusion bonding of pipelines.

3.5.4.Wide Gap brazing

Lugscheider et al. [15] have developed a brazing technique for large diameter components that can tolerate joint clearances of up to 2 mm. Although it is claimed that large parts can be welded, the test results quoted relate to pipe of 70 mm x 7 mm. The time for heating up and brazing was less than three minutes but a 30 minute drying procedure and 20 minute cooling time were used.

4.CONCLUSIONS

1) Based upon the above review, three processes are considered to offer the most potential in the short/medium term. These are:

a) advanced GMAW systems,

b) laser welding and,

c) electron beam welding.

2) For many pipeline applications, mechanised GMAW is still considered the most suitable process. The use of dual-torch or twin-wire GMAW provides increased productivity at low development risk and welding equipment is readily available. Additionally, the process control systems used in pipeline GMAW are currently very basic or non-existent and improvements here could improve reliability and reduce repair rate. In particular, the use of self-tuning welding parameters and seam tracking could allow higher travel speeds.

3) Historically, most interest has been taken in the one-shot welding systems and one of the main reasons given is increased welding productivity. However, increased productivity is not easily related to the total welding time. For example, in the 5G position, the current GMAW systems complete the root and root/hot pass at up to 1.2 m/minute and multiple welding heads operate simultaneously. Hence, each head must cover only one segment of the pipe and the actual welding time for the root and root/hot pass is very short. Once the root or root/hot pass are completed the pipe can be moved and the fill and cap passes welded in subsequent welding stations. Therefore, the critical path is either the completion of the root or root/hot pass or the total welding cycle time averaged over the number of welding stations. Although the one-shot welding systems offer a total welding time much faster than the total welding time for GMAW systems, they must be completed in one welding station and the critical path operations can be much longer due to the slower setup and the need for post-weld operations such as flash removal. However, the power-

beam processes do not suffer from the same problems as multi-pass and multi-head welding is possible with laser and electron beam welding. These processes may also be used to partially complete a weld and an alternative process used for fill passes. Therefore, the power-beam processes are currently being more actively developed than the one-shot processes.

4) Laser welding does not require a vacuum and technology is relatively accessible. CO2 lasers or Nd:YAG lasers could be used but the ability to use fibre-optic beam delivery systems makes Nd:YAG more attractive. The development of pumped-diode Nd:YAG lasers will improve the feasibility of using lasers on the right-of-way. The use of plasma augmented laser welding improves the penetration capability close to that suitable for onshore pipelines and also improves the tolerance to pipe misalignment at relatively low additional cost. For heavy wall pipe, multibeam Nd:YAG welding is a suitable option but requires a high capital investment. Alternatively, a laser could be used to complete a high speed root pass and the joint filled with an advanced GMAW process. The use of internal and external laser welding is also possible.

5) Electron beam welding is particularly attractive for J-lay welding as the full wall thickness can easily be completed in one single pass at a travel speed of up to 1 m/min. Multiple welding heads can also be used to improve the cycle time. The use of an internal electron beam welding head for 5G welding avoids the problems in achieving a vacuum with an external welding gun. Although penetration in the 5G position is limited, this system offers the potential to deposit a high speed internal root pass with fill and cap passes produced by advanced GMAW.

REFERENCES

[1] Dye, S. A., “New automatic machine for pipe welding”, Metal Construction and British Welding Journal, 2, (3), March, 1970: pp 111-114.

[2] Randall, M., “25 years of automatic welding”, Pipeline Digest, April 1994.

[3] Hansen, E. and N. Poirier, “Offshore welding technology applied to a major onshore pipeline construction project: Maghreb to Europe 48” gasline in Morocco”, Pipeline Technology, Proceedings of 2nd international conference, Ostend, Belgium, 11-14 September, 1995: Vol III, pp 119-137.

[4] Turner, D.L., "Flash Butt Welding of Large Diameter Oil and Gas Pipelines", Proceedings, Pipeline Engineering Symposium, New Orleans, USA, 23-27 February, 1986. pp 15-20.

[5] Belloni, A. and M. Celant, "Development of an Advanced [Automatic Hot Wire Tig Welding] System to Weld Corrosion Resistant Alloys and

Clad Pipes", OMAE 1993, Proceedings 12th International Conference, Glasgow, UK, 20-24 June, 1993. Vol 3a pp 83-90.

[6] Butler, P., J.G. Emmerson and R. Van Den Berg, "Welding the Maui A-B Pipeline", Welding Journal, Vol 72, No. 11, November 1993. pp31-38.

[7] Fujimoto, T., K. Akahide and E. Nagano, "Field Automatic Girth Electroslag Welding Method of Large Diameter and Thick Wall Pipes", Welding and Performance of Pipelines, Proceedings of 3rd International Conference, London, UK, 18-21 November 1986. pp187-196.

[8] Sivry, B. de, "J-Configuration Laying - Electron Beam Welding", New Technologies for The Exploration and Exploitation of Oil and Gas Resources, Proceedings, 2nd EC Symposium, Luxembourg, 5-7 December 1984. pp 921-930.

[9] Punshon, C.S. and A. Belloni, "Reduced Pressure EB Welding for Offshore Pipelines", Joining and Welding for the Oil and Gas Industry, Proceedings, International Conference, London, 30-31 October 1997.

[10] Masuda, H., "New Technologies for Transmission Pipelines", IIW Document X11-1488-97, 1997.

[11] Carnes, R.W., R.S. Hudson and S.P. Nichols, " Advances in homopolar welding of API linepipe for deepwater applications", Joining and Welding for the Oil and Gas Industry, Proceedings, International Conference, London, 30-31 October 1997.

[12] Hutt, G., "Radial friction welding of offshore pipelines", Duplex Stainless Steels '97 Proceedings, 5th World Conference, Maastricht, Netherlands, 21-23 October 1997. Vol 2, pp837-843.

[13] Basaraba, R. H. and E. Karpiel, "High Impact Welding TransCanada Pipelines' Eaglehead Loop", Proceedings International Gas Research Conference, Toronto, Canada, 8-11 September 1985. Session 2, pp 408-417.

[14] Gibson, S.W., "Thermit Fusion Welding of Pipes", MSc Thesis, University of Manchester Institute of Science and Technology, Manchester, UK, 1992.

[15] Lugscheider, E., T. Schittny and E Halmoy, "Wide Gap Brazing of Pipeline Systems", Pipeline Technology Conference, Proceedings, Oostende, Belgium, 15-18 October 1990. Vol A, pp 8.21-8.28.

ACKNOWLEDGEMENTS

In compiling the report for PRCI, an extensive list of publications was reviewed and discussions held with many people involved in the welding industry. These are too numerous to reference here and the authors wish to acknowledge the support received.