Orbital Welding of Christmas Tree Assemblies for the Terra Nova Project
Orbital Welding of Christmas Tree Assemblies for the Terra Nova Project
Orbital GTA Tube Welding
Development of SOPs for Autogenous Orbital Welding
of 2205 Duplex and 316 Stainless Steel Tubing
by Barbara K. Henon, Ph.D.
Arc Machines, Inc.
Orbital welding is the use of automated or machine welding equipment in which the arc moves circumferentially around the joint to complete the weld while the tube or pipe remains in-place. Autogenous orbital GTA tube welding has become the preferred joining technology in industries such as the semiconductor industry for welding process gas lines, and in the biopharmaceutical industry for process piping. For these industries, where literally millions of orbital welds have been successfully completed, the use of orbital welding technology has become routine. However, in the offshore industry where orbital welding has been applied less frequently, the transition from manual welding technology to orbital welding technology may present a learning curve.
Terra Nova Quality Requirements
When the FMC Corporation Wellhead Equipment Group in Dunfermline, Scotland contracted for the welding of pipework for the supply of 13 sub-sea “Christmas Tree” wellheads for the Terra Nova Capex II Project for Petro-Canada they were faced with meeting very stringent quality requirements. Whereas, on previous manual welding jobs only about 5% of the welds had been subjected to radiography, the Terra Nova project specified 100% visual inspection of the outside (O.D.) of the welds, 100% radiography and 100% dye penetrant inspection. In addition to the requirements for 316 stainless steel, welds on 2205 duplex stainless steel tubing were subjected to ferrite counts, macro and micro examinations, and Vickers hardness testing. Welds on duplex also had to pass the stringent ASTM G-48 test of corrosion resistance and the finished trees were subjected to hydrostatic pressure tests to predict the integrity and performance of the welds in service.
For the Terra Nova project the weld procedure and welders for FMC Kongsberg had to be qualified to FMC’s own weld specification which was based on the requirements of ASME Section IX of the Boiler and Pressure Vessel Code and NORSOK Standard M-601. Whereas qualification of the weld procedure for both manual and orbital welds is done to verify the structural integrity of the weldments, performance qualification for a manual welder means that he has the skill to make an acceptable weld. For an orbital welding operator, performance qualification means that the operator can set up the equipment, establish proper purging and initiate the weld sequence on the machine. If the weld is acceptable, the operator becomes qualified. It is more difficult and time-consuming to acquire the skillset required for a manual welder than to become a certified orbital welding operator. Manual welding skills are increasingly difficult to find which is one reason that the use of orbital welding is increasing. Facing the newly imposed quality requirements and the specification for a 25 year guarantee of the Christmas trees, FMC decided to invest in orbital GTA welding technology for the project.
Christmas tree wellhead in the FMC facility in Dunfermline, Scotland. Photo courtesy of FMC.
Detail of orbitally welded tubing installed on the Christmas tree. Photo courtesy of FMC.
An Arc Machines, Inc. Model 227 orbital welding power supply on a cart in the FMC facility. An AMI Model 9AF-750 weld head is mounted on a tube for orbital welding. Photo courtesy of FMC.
FMC selected a Model 227 microprocessor-based orbital welding power supply and a Model 9AF-750 weld head for autogenous (fusion) welding manufactured by Arc Machines, Inc. in Pacoima, California, USA. With the assistance of Arc Machines UK Ltd., the equipment supplier, FMC was able to qualify their welding procedures and operators. FMC passed all of the requirements for procedure qualification and all of the test welds were found to be acceptable. No deleterious phases such as sigma which could result in embrittlement of duplex materials were observed. Qualification of weld procedure and welding personnel allowed them to begin production welding.
Orbital Welding of the first two Christmas Trees
FMC subcontracted the welding on the first Christmas tree in 2000, however these welds had an unacceptable 14% reject rate. At this point Arc Machines UK personnel visited the site and worked with FMC on tree number two to improve their operating procedures (SOPs). After changing their approach to incorporate the use of mixed gas containing nitrogen for the duplex welds, the use of pressure balancing to control the weld bead profiles, the use of special alignment clamps and better material control, their weld reject rate fell to a respectable 5% and the time to weld a tree was reduced from 3 weeks to 6 days.
Welding of tree number 3 was begun by a subcontractor and the results were back to being unacceptable. At this point FMC took full control and implemented their own Standard Operating Procedures (SOPs) based on what they had learned from Arc Machines UK Ltd. They put one of their own welders in charge and did the rework of Tree number 3 and all of the remaining trees from 4 through 13 themselves. Once FMC took control of the welding, the reject rates for individual trees were much lower.
The following list of steps was based on information outlined in FMC's Operator Guideline for the Production Welding of Small Bore Tubing:
Step 1. Material control. Material control was found to be essential for achieving repeatable weld results. Batches of material were segregated by material type, i.e., 316 stainless steel or duplex, and further segregated by material grade and finally by heat number. The sulfur content of 316 is known to have a significant effect on weld bead penetration. In fact, the ASME Bioprocessing Equipment (BPE) standard used in the biopharmaceutical industry limits both the upper and lower range of sufur concentration for 316L tubing and weld ends of components installed in that industry in order to minimize welding problems resulting from heat-to-heat variation. Heats very low in sulfur have a weld pool that is wide with respect to the depth of penetration making it more difficult to achieve an acceptable weld. The reason for the heat-to-heat variability in penetration was not immediately apparent to FMC, but once this was understood they took care to join tubing from the same heats together whenever possible. When it became necessary to weld different heats together, they did a minimum of one test joint before proceeding with production welds.
Step 2. Weld head calibration and set-up. The weld head was always calibrated to the power supply on which it was to be used. Since travel speed affects heat input into the weld, this is important for maintaining weld consistency. The weld head must be set up with the correct size of inserts or collets which are used for holding the parts properly in the head for welding. Arc Machines specifies tungsten electrodes of the correct length for each weld head/ tubing size combination in order to set the correct arc gap for the weld. All of these steps must be done in a consistent manner in order to achieve consistent weld results.
Steps 3 and 4. Pipe end preparation. It is very easy to underestimate the importance of proper tube or pipe end preparation for the success of an orbital welding project. Any compromise in this area will affect the quality and repeatability that is expected of orbital welding. FMC machined the tube ends square with a pipe squaring cutting unit without lubrication so as to dress both I.D. and O.D. edges to remove any burrs. This produced very consistent results. Fitters were prohibited from using emery paper or sandpaper for cleaning the tubing since this would leave contaminants that would become entrapped and lead to porosity and weld failure. After the tube ends were de-burred, they were blown out using foam pigs to ensure that there would be no debris or other contaminants left inside the tubes. Prior to welding, tubing ends were cleaned with a solvent cleaner using a clean cloth. Cleaning and fit-up are very critical to the orbital welding process, and this is one area that is particularly difficult to communicate to sub-contractors.
Step 5. De-magnetizing. Parts to be welded were checked for magnetism with the use of a gauss meter. Parts found to be magnetic were de-magnetized by using a de-mag coil or an electromagnet to reduce the magnetism to a maximum of three gauss.
Step 6. Purge and shield gas control. Welding operators were instructed to verify that the correct gas for the material being used was selected. Argon was used as a shielding and purge gas for 316. For 2205 duplex a gas mixture containing 10% helium, 88% argon and 2% nitrogen was used. The helium provides deeper penetration than argon at the same welding current, while nitrogen promotes the formation of austenite enabling them to achieve acceptable ferrite levels of less than 60%. John Mooney and Ian Robertson of FMC report that the ferrite counts were much more consistent with orbital welding than with manual welding allowing them to achieve consistent balanced-phase microstructures with autogenous welding. The non-uniform heat input of manual welding on duplex materials results in localized high ferrite counts that can compromise the mechanical properties and corrosion resistance of the material and thus autogenous manual welds on duplex are not recommended.
Step 7. Pressure balancing. On some materials and with some wall thicknesses when welding in the 2G position, there is a tendency for the weld bead to become concave at the 12:00 o'clock position on the outside (O.D.) of the weld. The technique of pressure balancing to prevent this concavity has been widely used in the semiconductor industry (SEMI F79- 0703). By equalizing the pressure on the liquid weld pool during welding it is possible to achieve an optimized weld bead profile that is neither concave nor convex. At FMC this was done by inserting a tee at the weld joint connected to a Magnehelic® pressure gauge with one end of the pipe connected to the purge and the other end capped off. The I.D. purge gas flowrate was increased in order to build up a pressure of 3 inches of water. The purge was turned off and the pressure was held for 60 seconds to identify any leaks, and then the end cap was replaced with a reducer. With the shielding gas running on manual purge the I.D. purge gas flowrate was slowly increased until the correct pressure was shown on the Magnehelic®. Then, with the shield and purge gases running, the Magnehelic® was replaced by the weld head and, after a suitable prepurge, the arc was initiated.
Step 8. Alignment. Proper consistent alignment of parts and electrode location with respect to the joint is essential for weld repeatability and this was one of the main difficulties at the beginning of the project. Any deviation, horizontal or vertical, of the electrode position with respect to the joint could result in incomplete penetration or missing the weld joint. To overcome this problem a pipe alignment clamp was designed to assist with the alignment of the pipe joint prior to the commencement of the weld. If, for any reason, the alignment clamp could not be used, welding could not proceed without approval of the supervisor. In either case, a secondary inspection was required to ensure that the tungsten electrode was properly lined up to the weld joint and that the arc gap between the tungsten tip and the weld joint was correct.
Step 9. Correct weld program. Weld parameters for each size of tubing or pipe vary according to diameter and wall thicknesses. Weld current (amperage) increases in proportion to the wall thickness and electrode rotational speed (RPM) is inversely correlated to the pipe diameter. Weld programs or schedules for each size of tubing or pipe are stored in the memory of the microprocessor-controlled orbital welding power supplies. Prior to welding the operator calls up the program he wishes to use from the power supply memory. The weld schedule for FMC's 3/8 inch pipe was a STEP procedure with zero R.P.M. during the primary pulse time and a travel speed of about 4 inches per minute during the background pulse time. Fairly long pulse times were used for optimal penetration and to make an attractive weld bead. The time per weld was 1 minute and 13 seconds including a total pre-purge and post-purge time of 25 seconds exclusive of fit-up time.
Step 10. Coupon welds and Quality Assurance. FMC welding operators were required to make two test welds or coupons at the beginning of each shift before they were allowed to make production welds. One of the test pieces was subjected to a visual inspection, and a dye penetrant inspection as well as radiography. The second test piece was either split through the weld for visual inspection or inspected on the I.D. with a borescope. The test pieces sent for outside inspection were returned to FMC and had to have been found acceptable before production welding could begin. All test results were recorded in a Coupon Logbook, and all of the NDT results of production welds were recorded in the Production Orbital Logbook along with each weld procedure that was used during the shift.
Step 11. Labelling of production welds. All of the production welds were identified with a label at the time of welding and recorded in a log for the radiographer to work from. The welds being radiographed had to be loosened from their fixings to allow 360° access around the weld for gamma and X-ray inspection.
Steps 12 to 16. Radiography. These steps provided for FMC or a subcontractor to make arrangements between the various parties for organizing the radiography operation. This involved clearing the work zone of working personnel to provide a safe working environment, and adhering to all aspects of FMC Radioactive Hazard Management. The radiographic subcontractor had to be given notice regarding the time and location and number of welds so they could give a 7 day notice to FMC Hazard Management. In cases of restricted access to weld joints, the radiographer would have had to visit the site to view the restricted access. Once the radiographs were completed, FMC required the results with 12 hours (maximum) after the last weld was radiographed. At that time, all the radiographed welds were checked by the plumbing contractor as either accepted or rejected. Rejected welds were cut from the hydraulic line, then re-welded, re-tagged and new weld identification applied. The radiographed welds were then checked against the weld log and checked whether other NDT was required. When this information had been completed, it was handed over to the FMC Quality Assurance Department or FMC NDT/Welding Inspection.
Step 17. FMC re-check. After all of the radiography and dye-penetration information was handed over to FMC, FMC personnel reviewed the information.
Final Orbital Welding Results
FMC completed a total of 1893 orbital welds for the entire project with 1779 radiographs found to be acceptable and 123 rejected to end up with a 7% reject rate overall with much lower reject rates on individual trees (See Table 1.). The last tree was completed in 2003. The average number of welds per tree was 160 with 42% of the welds of 316 stainless steel welded to itself (78% tube-to-fitting and 22% tube-to-tube), 11% of the welds were 2205 duplex stainless steel (UNS S31803) to 316, and 47% of the welds were 2205 duplex welded to itself. Tubing size was 0.375 inch diameter for 316 with wall thicknesses of 0.049 and 0.065 inch while the duplex 3/8 inch high-pressure pipe had a wall thickness of 0.071 inch.
Once FMC took an active role in developing new weld procedures they were able to meet the updated more stringent quality requirements for welding on the Terra Nova project. At the beginning of the project FMC's reject rate had been quite high due to human error and access-related problems. They were successful in their use of autogenous orbital welding because, although orbital welding power supplies are very consistent from weld to weld, other parameters such as pipe end-preparation, cleaning, purge gas composition and quality, gas flow rates, I.D. weld pressurization, alignment of weld components and arc gap must also be controlled to tight tolerances to realize consistent weld results. FMC developed written procedures (SOPs) that all of the welding personnel were trained to carry out. Welding supervisors and inspectors watched for any deviations in the procedures that could result in an unsatisfactory weld and finally were able to correct potential problems at the earliest possible stage. On orbital welding applications such as this, when procedures are written and everyone follows them, proper procedures become routine and the results are invariably positive.
Similar positive experience with orbital welding has been obtained at FMC's Houston operations; in fact, in the case of one sub-sea tree, all 150 welds were completed with zero rejects. For this particular case the different alloys, 316, 2507, and Inco 825, were welded to themselves and to each other with autogenous orbital GTAW with a mixed gas. FMC in Dunfermline has also had excellent results with Arc Machines' Model 79 weld head for multi-pass welds with filler wire of duplex flanges to 3 and 6 inch pipe with 0.750 inch wall thickness and they have other applications for orbital welding in mind as well.
FMC guarantees the Christmas Tree wellheads for 25 years. If any problems arise during the warranty period, FMC is obligated to pull the tree to make repairs. This is very costly, so sound corrosion-resistant welds that perform well in service are essential. Orbital welding has many advantages over manual welding, particularly for higher alloyed materials such as duplex. For example, the uniform heat input of orbital welding makes it possible to achieve more predictable ferrite numbers and a reduced incidence of deleterious phases. The use of orbital welding results in a cleaner shop environment than manual welding since no grinding is required. FMC intends to continue with their orbital welding program in FMC plants worldwide which includes branches in Singapore, California, and Houston as well as Scotland. As the bar is raised with quality assurance and weld inspection standards in various industries, the demand for orbital welding will follow.
The author would like to acknowledge the following people who contributed their expertise to this article: John Morris, Technical Sales Engineer, AMI UK Ltd.; Ian Robertson, Manufacturing Manager, FMC; John Mooney, Welding and Fabrication Supervisor, FMC
SEMI F79-0703 Practice for gas tungsten arc (GTA) welding of fluid distribution systems in semiconductor manufacturing applications.
ASME Bioprocessing Equipment Standard (BPE) 2002