magnatech_ welding the maui a-b pipeline - orbital welding systems

Magnatech: Welding the Maui A-B Pipeline - Orbital Welding Systems

http://www.magnatech-lp.com/articles/mauitext.htm[3/26/2011 9:25:01 PM]

WELDING THE MAUI A-B PIPELINE

An onshore pipe welding setup speeded the production of underwater pipe-laying fora major project off New Zealand

By Peter Butler, J.G. Emmerson, and Rene Van Den Berg Submarine pipelines have been installed all over the world, in diameters ranging from 76mm (3in.) upto 1270mm (50in) or more and in water depths up to 500m (1600ft) using the S-lay method. Thismethod is based on holding the pipe at the end of the pipe lay barge, supporting the over bend by meansof a mechanical structure known as a "stinger," and lowering the pipe to the sea bed at a typicalinclination of 30 deg to horizontal. The pipe is prevented from buckling by tension maintained by the laybarge anchors which are periodically moved forward along the route of the pipeline. On the deck of theS-lay barge, pipe welding, NDT, and pipe joint coating workstations are sequentially positioned to allowuse of standard 12m (39ft) or double-jointed 24m (79ft) pipe lengths. The multiple workstationsoptimize the production rate at the expense of increased vessel length. Most commercially worthwhile oil and gas fields located in shallow waters have already beendeveloped. With the increasing cost of production from marginal shallow water fields, exploration anddevelopment have focused on new sites in increasingly deeper waters. With increasing water depth, thelength and weight of the unsupported pipe span increase and the anchor tension required to preventbuckling also increases. Due to the catenary of the anchoring cables, the ability to provide the tensiondecreases as the water depth increases, until a limit is reached beyond which anchors cannot be used. In the 1950s, the J-lay concept was developed. With the J-lay technique, the pipe is suspended nearvertically from the lay barge, thus reducing the horizontal force required to prevent pipe buckling. TheJ-lay technique has obvious advantages for deep-water applications where it is possible to eliminate theuse of anchors by using dynamic positioning. The main drawback of the J-lay technique is that the nearvertical pipe is difficult to handle and multiple workstations cannot be used. Only one length of pipe canbe welded onto the pipeline at a time and subsequent inspection and coating must be done before thenext weld can be made, resulting in low production rates compared to that achieved by S-lay barges. In 1989, Heerema, a Dutch company, which operates a fleet of semi-submersible crane vessels (SSCV)used for the installation of offshore oil and gas production platforms, decided to diversify into marinepipe laying. Realizing direct competition with established pipe lay contractors using S-lay barges couldnot be commercially competitive, an innovative J-lay system was designed which took advantage of theunique capabilities of the SSCV. Although the J-lay concept was some 30 years old, no functional system had been constructed asthere was no immediate market for it, especially one which could justify inferior productivity whencompared to S-lay barges. Heerema's solution to this limitation was to maximize the length of eachpiece of pipe added to the pipeline. By fabricating the pipe on shore into lengths up to 72m (240ft) andusing the massive crane capacity (4000 tons) of the SSCV to lift each pipe string into position forwelding, they could compensate for a lower productivity rate. J-Lay Project In late 1990, Heerema was awarded a pipe lay contract from Shell Todd Oil Services (STOS) of NewZealand and detail design and construction of the J-lay equipment was begun. New Zealand is supplied with natural gas from the Maui A platform which was installed in the late1970s. To increase available supplies, Shell Todd Oil Services decided to invest in a second platformwhich was to be installed 15km (9 miles) from Maui A. A 20in (51cm) diameter pipeline was planned tocarry untreated gas from this new platform, Maui B, to Maui A for treatment before transport to shorethrough the existing pipelines. The raw gas from the Maui field is extremely corrosive due to the highCO2 content which led to the selection of API 5LX grade X60 pipe internally clad with Type 316Lstainless steel. The 20in diameter seam welded pipe had a nominal 3/4in (19mm) wall of API SLX 60and an internal cladding of Type 316L stainless steel, 1/8in (3mm) thick. It was to be the longest cladsteel pipeline in the world. This was to be no ordinary pipeline project. In addition to the teething problems of a complex andinnovative new technique, the customer required that pipe welds meet a specification far moredemanding than the traditional standard for pipelines (API 1104). In August 1991, a full-scale trial was conducted onboard the SSCV Balder off Vancouver Island,Canada, during which 1km (0.62 miles) of pipe was laid and retrieved. Subsequently, the Maui A-Bpipeline was laid in the Tasman Sea off New Zealand during the period December 1991 to March 1992. The selection of welding equipment was influenced by the requirement for "no defects/no repairs" inthe clad layer and the ability to make an acceptable root pass on pipes exhibiting the maximumpermitted mismatch of 1.6mm (0.062in). Two fundamentally different approaches were considered. The first was the use of an orbital GMAW system using a "narrow gap" bevel geometry. With thisapproach, the root pass was made from the inside, with hot, fill, and capping passes made from theoutside. This approach offered substantial speed due to the low weld volume and high deposition rates. The second alternative was to use orbital GTAW for the root pass, which required a wider groovegeometry and potentially lower deposition rates, but offered a better guarantee of weld metal quality. GTAW or GMAW would be used for fill and capping passes. The restrictive welding specification, the complexities of the clad pipe, combined with the first timeuse of an innovative pipe lay system, led to the decision to place the emphasis on making good weldsthe first time around, and that optimizing the weld deposition rate should be a secondary consideration. Magnatech orbital welding equipment was chosen for a welding development program based on the useof GTAW for the root and hot passes. The purpose of the development program was to establish the root



and hot pass parameters and evaluate both GTAW and GMAW techniques for the fill and cap passes. Welding equipment selection, welding procedure development and the training of local welders in NewZealand were conducted concurrently with the required modification to the SSCV for J-lay operations. Equipment Development The welding development program was undertaken at a Heerema associated facility in Zwijndrecht,Holland. Magnatech provided its Series 500 System for the GTAW development. The system consists ofa track-mounted orbital welding head, a constant current power source and controls which integrate theoperation of the welding head and power source. The T Model welding head incorporates torch rotation,filler wire feed, torch oscillation, and arc voltage control functions. The welding head mounts on thepipe using an appropriately sized guide ring, which consists of a metal band that encircles the pipe O.D. The Series 500 controller provides speed regulation and servo control circuitry for the various functionsof the welding head, as well as control of welding current. The controller offers the ability to pulse thewelding current and allows certain weld head functions (wire feed, torch rotation, oscillation and arcvoltage control operation) to be synchronized with the pulsed current. The Cyber-Wave power sourcefrom Hobart Bros., Inc., Troy, Ohio, supplied as part of the system, has sufficient output capability(300A/60% duty cycle) and good arc characteristics required to make welds meeting the demandingspecification. The Series 500 System has an established history of use for marine pipeline projects by a number ofcontractors. This has included the welding of duplex stainless steel pipeline and pipe clad internallywith Inconel 825. The Series 500 system was originally developed for welding pipe work in the steamgeneration and chemical industries. Further refinements were added to optimize it for this pipelineapplication. Magnatech also supplied the orbital GMAW system for welding trials in Holland. This GMAW system issimilar in construction to the Series 500 and consists of a welding head, power source, and controls. The welding head is track mounted on an identical guide ring to the GTAW system. The Pipeliner Modelwelding head also incorporates torch rotation, filler wire feed, and torch oscillation. The power sourceused with the GMAW system was the Transarc 500 model manufactured by Fronius SchweissmaschinenKG, Wels-Thalheim, Austria. This is a pulsed power source which allows synergic control between thewelding head mounted wire feeder and the power source output. The torch oscillation system has theunique capability of a "power boost" mode of operation. The torch oscillation system commands ahigher power output level during the dwell period, when the torch is adjacent to the side walls of thebevel. This feature provides better side wall fusion and has made orbital GMAW welding a viablealternative for many applications. Synergic programming of the Transarc 500 is based on the conceptthat the four basic pulse parameters (voltage, current, time period, and frequency) are factory presetfor a given combination of filler wire size, material and shielding gas. The wire feeder interacts with thefactory preset program which automatically adjusts one or more of the pulse variables to maintainuniform arc energy. The welder sets the pipe base material and wire diameter which leads to theselection of the appropriate pulse relationships stored on a memory chip. Welding Equipment Trials A program of welding trials was conducted using the Magnatech GTAW and GMAW equipment on cladpipe supplied by the customer, STOS. The purpose of these trials was to select the welding process tobe used for the fill and cap passes, and to optimize equipment design for the task. The Maui pipeline was to be welded in the 6G position, with the pipe axis at a nominal 37 deg to thehorizontal, due to the constraints of the J-lay system. For use in the single welding station within thestinger, a double-up welding technique (6 to 12 o'clock) was selected on the basis of maximizing thedeposition rate and minimizing the potential for lack of side wall fusion. The 20in pipe was large enoughto permit the simultaneous use of two welding heads, one traveling clockwise and the other,counterclockwise. All controls required during welding operations are located on the remote pendant, and the operatorhas little need to make any adjustments on the welding head when in motion. The ease of repositioning and resetting the welding heads for subsequent passes was important to theoverall speed of operations given the simultaneous use of two heads on a joint and the double-up weldtechniques. Minor modifications to the equipment were made such as the integration of a clutch in thedrive system to minimize the time required to reposition the weld head. The guide rings which mount the weld head on a pipe are conventionally sized to minimize radialclearance around the pipe joint. There is only minimal clearance between the under surface of the weldhead and the pipe O.D. This created a problem for the Maui application where the pipe lengths weresupplied with a thick coating of insulating material to within 20cm (8in) of each pipe end. The solutionwas to use oversized guide rings equipped with "stand-off legs" to provide rotational clearance of theweld head over the insulation layer. The stand-offs were manufactured of copper and also served as aconvenient work piece "ground" connection. The final modification to the control system was dictated by the "no defects/no repairs" criteria for theclad layer and the considerable impact that resultant cut-outs would have on the pipe lay schedule. Itwas imperative that when welding the clad layer the equipment operated faultlessly or not at all, so withthat in mind a flow switch was added to the shield gas supply. Thus no gas, no arc and, no cut-out. Welding Procedure Development The welding trials used to assess the equipment also provided the basis for welding proceduredevelopment. From the process and consumable variations investigated in these trials the outline



welding procedure was established. This was then refined during the period of training andqualification of welding operators in New Zealand. The process options conducted were: 1) GTAW for the root and hot pass using Type 309MoL stainless steel filler wire which was selected toprovide the required root bead chemistry. 2) GTAW fill and cap using stainless steel filler wires. 3) GMAW fill and cap using solid stainless steel wire in the "short arc" mode. 4) GMAW fill and cap using flux cored stainless steel filler wires in the pulsed spray mode. The GMAW welds were made with a variety of shielding gases and in both uphill and downhillprogressions. Also considered for the fill and cap was GMAW with carbon steel over pure iron buffer layer. This isknown to have been used successfully on a previous occasion on a pipeline of similar metal. Theprocedure was not actively pursued because adequate properties were obtained from the stainless steelfiller wires. Also, it would have required an increase in the amount of equipment to be installed in thealready congested stinger welding station and have increased the risk of contamination of the stainlesssteel. No significant problems were experienced in producing consistent root and hot passes once theparameters were established. The ability to deposit consistent root passes was very dependent uponthe consistency of the weld preparation and fitup. The addition of an internal line-up clamp during thetrials influenced the established root parameters which had to be modified to compensate for theincreased heat sink. The hot pass(es) was developed to provide an adequate barrier to"overpenetration" by the GMAW and to fill the groove to a point where there was adequate access forthe GMAW nozzle. The groove geometry, 6G weld position, and the stainless filler metal all created difficulties during theevaluation of the GMAW process. All personnel, regardless of experience, had difficulties with the GMAWprocess throughout the trials. Constant observations and adjustment of the welding head were requiredduring its progression around the pipe, making it hard to establish a set of parameters in which therewas sufficient confidence. Bead placement was found to be critical as the stainless pool exhibited poor fluidity and did notreadily wet the carbon steel pipe. There was a tendency for the pool to become too large and cease tosupport itself. A narrower weld preparation may have been of assistance but had to be sufficiently wideto provide access for the GTAW root. The included angle of the joint was reduced to get better supportfor the weld pool and reduce the overall weld volume, but this made steering of the torch within thegroove much more critical to avoid collisions with the bevel side wall. In order to further reduce thewidth of the narrow groove, substantially extended contact tips were evaluated, but this approach wasrejected because of unacceptable tip life. Gas Metal Arc Welding The GMAW development program identified a number of problems with often conflicting solutions. Solid wire made satisfactory caps when used in the uphill progression, downhill capping was susceptibleto cracking. The 0.9mm flux cored wire was more user friendly than the solid wire and was also foundmore forgiving with respect to bead placement. Qualitatively flux cored wire was preferred, both for operability and freedom from defects. Quantitatively solid wire produced better mechanical properties. GMAW promised improved production rates over GTAW, but the larger bead size increased the size ofthe heat-affected zone, typified by coarse-grained structure, high hardness values and low impactenergies. It was concluded that the GMAW equipment was being operated close to the limit of its capabilities. The combination of welding wire, shielding gas, power supply programming, feed/travel rates and torchsettings which would provide repeated clean welds was not found. The only technique found to makedefinitively acceptable welds, both quantitatively and qualitatively was GTAW. To develop and qualify areliable GMAW fill and cap procedure and then to train and qualify the operators would not be possiblewithin the time allowed by the project schedule. Consequently, GTAW was selected for the completeweld. A final set of GTAW welds was made with different wire alloys evaluated for the fill and cap. Thesetrials showed comparable performance for both alloys of wire, the Type 309MoL stainless wire beingpreferred by the welders because of weld pool behavior but the 309L wire giving superior mechanicalproperties. As both wire grades had properties well in excess of requirements, 309MoL stainless waschosen for all welding. The GTAW equipment and 309MoL stainless welding wires were delivered to New Zealand to start fourmonths of procedure development and operator training. At this stage it was necessary to select thegroove geometry to be used, as this was a critical factor in the development of the ultrasonic testequipment and testing procedures. Procedure development led to the selection of 25 deg includedangle. Although not the minimum used during the trials, the benefits of reducing the angle furtherbecame less significant with respect to volume. Conversely, the reduction of angle increases theprobability of lack of side wall fusion and cause large movements of the arc voltage control where theweld preparation is irregular. The most effective way to reduce weld volume is to reduce the root nose, which was possible becauseof the use of a ceramic "chisel" shaped gas cup. Root pass geometry is a critical factor, however; itaffects the ability to deposit an acceptable root bead. The length and thickness of the nose dictate thesuccess of root penetration. In production, the nose thickness was controlled within a tolerance of0.15mm (0.0006in). If the length of the nose is too short, penetration is not assured; if too long, "suckback " is probable at the 6 o'clock position and overpenetration, or even blow through, at the 12 o'clock



position. The joint fitup also affected the weld penetration. Pipe alignment had to be tightly maintained in anycase to provide continuity of the clad layer. The welding procedures were finally tested and qualified in New Zealand and the pipe lay vessel, SSCVBalder, readied for production welding. Production A local New Zealand company was awarded the contract to make the 72m (240ft) pipe strings. Thiswas done at a dockside facility also using GTAW for the entire weld. A production line was set up usingeight Magnatech Series 500 systems to complete some 1200 welds. The pipe strings were placed in fivespecially constructed racks, each containing 42 strings, which were loaded onto a cargo barge fortransport to the SSCV Balder. The racks were lifted onto the SSCV Balder in sheltered waters beforesailing to the Maui field. Pipe laying operations were begun in December 1991. Initial progress, hampered by weather,equipment teething troubles, and the learning curve of a New Zealand crew that had been trained fromscratch, basically wrote off the first month. In early January 1992, production settled into a routinewhich began to achieve optimum cycle times. This is not to say that no further problems occurred. Inthe early stages of production a number of weld repairs or cutouts were made due to equipmentmalfunctions. These incidents were shared among the welding equipment, the internal tools stringwhich lined up the joint and provided the argon purge, and the overall pipe lay system. Toward the endof the pipeline weld discontinuities were attributable to welding operator errors. Most typically thesewere short lengths of lack of interpass fusion and tungsten inclusions. Tungsten inclusion occurredfrom stub outs not completely ground out and from the use of wire cutters to grip the tungstenelectrode (the fine nicks causing flakes of tungsten becoming detached during welding). The workstation was enclosed to prevent disruption of torch gas shielding by wind. Preparation ofpipe ends for welding was done on the deck using regular pipeline equipment. this equipment could notbe used in the welding station due to its size and the 6G pipe orientation. The internally clad pipe couldnot be cut by thermal methods, so any cutout in the welding station had to be done by a machine tool. The tool selected was of the rigid bracelet type with separate form tools for parting and beveling. During welding trials and procedure development high levels of residual magnetism wereexperienced. At these times a degaussing unit was used which generally prevented the simultaneoususe of two welding heads and thus halved the production rate. From the onshore experience it wasanticipated that the level of residual magnetism would increase over the length of the pipeline. Thedegaussing unit was ready to be used at all times during welding. However, in production, whileindividual joints exhibited residual magnetism, there was no general trend of increasing magnetic field. This is thought to be attributable to the austenitic filler metal breaking the magnetic continuity of thepipeline. When magnetism was encountered the influence on welding in the clad layer was minimalcompared with the effect on the fill passes. A further precaution against magnetism and arc blow wasthe use of multiple welding current return cables attached to the feet of both guide rings. Internal Tool String During J-lay operations the open end of the pipe was some 240ft from the welding station and therewas no access to the open end when the pipe ramp was raised. Thus, any equipment required inside thepipe had to be suspended from the open end. To overcome this problem an integrated tool string wasdeveloped to combine a joint alignment clamp, complete with argon purge system and an x-ray system. A buckle detector could also have been added if required. These items were linked by a single umbilicalcable to a winch at the top end of the pipe ramp. The umbilical cable provided all services to the toolstring, with a tension member included to support the weight of the tools string and the resistance of abuckle detector. The layout of the tool string kept all hydraulic units below the weld zone, thus avoiding risk ofcontamination in the event of oil leakage. The line-up clamp was a commercially available item to whichsome modifications were made in order to integrate it with the other components and remote controlsystem. The x-ray system was based on "crawler" technology, with the tube powered and switched viathe umbilical cable. A positioning system was designed to align the x-ray tube with the finished weld. Afail-safe clamp was placed at the bottom end to prevent accidental loss of the tool string. The x-raytube was mounted between the line-up and fail-safe clamps which, when both were energized, held thex-ray tube centrally within the pipe for good panoramic exposure. All instrumentation and controls for the tools string were located in the welding station. Nondestructive Testing The Maui project specification required both radiographic and ultrasonic inspection to very highstandards. All NDT was conducted in the welding station and it was essential to minimize both test timeand interpretation. Radiography had to be performed by the panoramic x-ray technique due to both technical and safetyrequirements. Radiation safety in the welding area was a major concern because the stinger controlstation was located in the same area. Only essential personnel remained in the welding station whenthe x-ray tube was in use, and were monitored for radiation dosage. In practice, radiation levels werevery low as a lead shield was deployed over the weld area during x-ray exposure. Most conventional pipeline radiography is done by battery-powered crawlers which have a limitedamount of power available due to the capacity of the batteries. Because of the need to conserve powerand reduce downtime due to battery replacement, rapid cycle radiographic film is normally used. The



Maui specification did not permit the use of rapid cycle film because of the required image quality. Finegrain, high contrast film was used to provide the clarity needed to detect discontinuities in the cladlayer. The longer exposure times were made possible by supplying the power through the umbilicalcable to the x-ray system. It was decided to use mechanical ultrasonic inspection because of the speed and assured inspectioncoverage. Welds from the trials were made available to two companies offering different ultrasonicinspection systems. A choice had to be made between the traditional pulse-echo technique and thenewer time-of-flight technique. It was decided to use the pulse-echo technique because it wasimperative for all parties involved in the acceptance of the welds to be able to readily understand theoutput and seemed less susceptible to misinterpretation. An order was place with Rontgen Technische Dienst b.v. (RTD), a Dutch company, for the supply oftheir Rotoscan ultrasonic testing system and all NDT technicians required for the work offshore. Weldsmade during the offshore trials were used by RTD to make calibration blocks and to assess the signal-to-noise ratio of the stainless steel weldment. It was found that the signal-to-noise ratio was well withinthe limit set by the project specification and that there was significant variation around the weld. Careful selection of probes and probe arrangement eliminated most of the spurious signals resultingfrom the carbon steel/stainless steel interface which has been recognized to be the cause of difficultiesexperienced previously. The Rotoscan system produces a printout which gives an immediate impression of the weldacceptability. The scan is made in zones, each of which is aligned on the printout with the other zones. The printout was used directly for general acceptance purposes, with further investigation manuallyused to define areas for repairs. Closer analysis of the traces for zones at the clad layer interface wasrequired to substantiate whether any indications actually extended into the clad layer, thus requiring theweld to be cut out. Welds from the offshore trial were sectioned at apparent defects identified by NDT, and themacrosections were compared with the radiographs and Rotoscan printout to provide comparative datawith respect to the interpretation of poorly defined indications. It was found that the stainless steelfiller metal in carbon steel pipe, plus the influence of the clad layer, gave rise to signals which requiredmore sophisticated interpretation techniques to be developed. This work enabled the test procedures tobe refined prior to the start of production. A dossier of radiographs, photomacrographs and Rotoscanprintouts was prepared for use during production to assist with the interpretation of radiographs. Thisavoided weld cutouts due to overcautious interpretation in the early stages of production. Conclusion The pipeline was successfully welded and inspected to a high standard using sophisticated equipmentand techniques in an aggressive marine environment for which they were not originally intended. Muchof the work done during the project has gone toward the overall improvement of the equipment usedand contributed to the understanding of welding and inspection of clad pipe. The SSCV Balder was able to continue laying pipe in the severe weather conditions of the Tasman Sea,where waves of 23ft (7m) did not stop production. In laying the Maui A-B pipeline, Heerema proved theoperability of an innovative pipelay technique and in so doing has provided the pipeline designers themeans of specifying pipelines in previously inaccessible locations. Not only does the J-lay enable thelaying of pipelines in deep waters but also, due to the low horizontal component of tension in thepipeline, across sea bed of uneven topography. For the development of oil and gas resources in remoteareas the SSCV Balder represents a single economic solution to both the platform installation and thepipelaying.

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