Orbital Welding Used on Hondo's Firewater System
Orbital Welding Used on Hondo's Firewater System
Replacement of the firewater system on the Hondo Platform in the Santa Barbara Channel off the coast of southern California required fabrication of a piping system incorporating the latest concepts in material selection, metal fabrication, and joining technology. Previous success with use of thin-walled, copper-nickel pipe provided superior corrosion resistance in a marine environment compared to carbon steel.
Orbital welding was evaluated as an alternate joining method. Coupons of 6-in. and 3-in. copper-nickel pipe were welded using orbital welding equipment. Examination and testing showed the welds to be of excellent quality. Copper-nickel is a difficult material to weld by hand. Results of initial qualification welding tests showed only two of the eight manual welding candidates passed. This was an additional incentive to use the orbital welding equipment.
The firewater system fabricated is the seawater distribution system supplying deluge spray nozzles at various points on the platform. The complete firewater system consists of the firewater manifold system of 10-in. pipe that distributes water to the smaller diameter pipe and ultimately to the sprinkler heads located in a total of 11 zones or areas.
The project required 9,000 ft of pipe of all sizes up to 10 in. and would need 3,140 welds. Piping of two in. and under was to be joined by threaded fittings. While most of the butt welds could be done with the orbital equipment, some welds, such as welding the weld-o-lets and thread-o-lets to the pipe, were weld joints that were not practical to weld by machine. These were done manually.
Harmony Construction networked with other companies in order to find skilled manual welders that could do the required manual welds on copper-nickel, and who would be capable of learning to operate the orbital pipe welding equipment within the specified time.
Equipment had to be obtained and the operators trained in its use. A Model 215 microprocessor-controlled pipe welder power supply and a Model 15 weld head were obtained from Arc Machines. Six days were spent training the experienced hand welders on the use of the orbital equipment.
Preliminary weld program development was done so the operators could become familiar with the travel speed, welding currents, arc voltage control, pulsation rate, wire feed rate, and oscillation of the torch across the weld joint. Operators are able to make adjustments in these parameters within prespecified limits during welding and to make minor adjustments in steering the torch as required.
The weld programs, with all of the parameters specified for a particular size of pipe, are entered into the power supply memory via the Model 215 Program Operator Pendant. In order to weld that size again, the program number is recalled and the exact same parameters are executed with allowances for overrides as specified.
When the operators were comfortable in setting up and operating the equipment, the task of developing welding schedules for production welding of each size of pipe began. The welding crew started with the 6-in. pipe with a 0.134-in. wall, followed by smaller sizes. The training and the procedures for every size except the 10-in. pipe were completed. The 10-in. pipe, which was the most challenging, was done last.
The 10-in. pipe was for the firewater manifold system. It had a wall thickness of 0.187-in. and required the same J-bevel preparation used on the smaller pipe. Welders found difficulty in obtaining sufficient penetration for a consistent root pass. The land was extended by 0.020-in. and this allowed a good root pass. While the smaller size pipes could be completed in just two passes (root pass and cap pass), the 10-in. pipe required three passes.
The specifications required that the welds conform to ANSI B31.3. Qualifying welds were subjected to tensile tests, side, root, and face bend tests, and radiography. It was important to get a good mixture of the filler wire, which was Monel 67 (70% copper and 30% nickel), into the root pass in order to prevent the formation of pinholes which would disqualify the weld. The base metal contained 88.6% copper, 10% nickel, and 1.4% iron. The greater nickel content of the filler wire provided an increase in the tensile strength of the weld to about 48,000 psi, compared to 44,000 psi for the base metal.
Copper-nickel fabrication must be protected from any contact with carbon steel, iron, or oxides that can initiate corrosion of the copper-nickel. To meet specifications, special clean areas were built within the shop, but isolated from the main shop areas and dedicated to copper-nickel fabrication. These areas were walled off with plastic panels and the air was circulated and filtered much like cleanrooms used for high-purity industries.
Fittings, such as elbows, tees, and flanges were arranged by type and stored in a clean storage area. Prior to welding, the oxide layer on the outside of the pipe was removed for a distance of 2 in. from the weld joint. Special tools such as aluminum oxide grinding wheels and stainless steel brushes were marked with yellow paint to indicate their use on copper-nickel exclusively. Work tables in these areas were stainless steel and all surfaces (vice jaws, pipe stands) that might come into contact with the copper-nickel were covered to protect the pipe from contamination.
A welding head is shown installed on the copper-nickel pipe. The flange is sealed to purge the pipe internally.
The pipe end preparation was crucial to the success of any orbital welding project. Thin walled copper-nickel pipe is prone to becoming eggshaped in storage, and it is vital that the ends be round for facing and for welding. The pipe was cut to length with a bandsaw and a Tri-Tool 206B Bevelmaster was used for the end-preparation of the two 6-in. diameter piping. A Tri-Tool 212B was used to prepare the bevels on piping from 6-12 in. diameters. The tools feature a mandrel with a Teflon pad which fits inside the pipe to make it round for machining.
The recommended end preparation for orbital welding of the pipe was a
"J" formation with a 25° bevel and a 3/32 radius, a 0.050-in. land
with a 0.030-in. extension held to very tight tolerances. In order to achieve a
consistently perfect end preparation each time, a sample coupon with an ideal
bevel for each pipe size was prepared. This model coupon was used to set the
machine tool to the bevel.
In addition, the end-preparation equipment was mounted on an adjustable stand with casters so it could be moved into position to prep the end of the pipe without bending or stressing. This assured an unvarying bevel and uniform preparation necessary for repeatable, high-quality orbital welds.
Fitting end-preparation required the fitting mandrel attachment, which fit inside the 90's and allowed it to center so the tool could make the bevel. The large clamshell elbows had weld seams on either side which had to be ground down on the inside (ID) so the mandrel could center. Clearly, the preparation must be better and more consistent for orbital welding than for manual welding, but the extra time spent in preparation is recovered during welding, and the quality of the automatic orbital welds is far superior to manual welds.
For the copper-nickel pipe, pure argon was used for the shield gas. A mixture of 75% argon and 25% helium was used for purging the inside of the pipe to protect it from oxidation during welding. The gas was specially mixed in a mixing manifold from helium stored in cylinders and the argon from the cryogenic source.
The gas mixer is economical, and the mixed gas gives better temperature control on the root pass, and produces welds of the highest quality. To save on gas, special purge plugs were inserted into the pipe on either side of the weld joint so that only a limited volume of purge gas was required to cover the weld joint. The welders devised a system with a toy car with four-wheel drive to pull the purge plug to the desired location inside the pipe. When the weld was done, they pulled it out by the attached string.
Progress was slow at first, with over two hours required to complete one diameter in. of weld. There was gradual improvement over the first month so that, after 30 days, it took about 0.07 hours for an inch of weld. The average manual welder does about 22 diameter in. per shift, while the average machine operator does 40-50 diameter in.
The most efficient use of the orbital welding machine was achieved when it could be operated continuously. Back-up personnel were made available for end-preparation and pretacking of the joints to be welded orbitally so that the welding machine operator could concentrate on the welding. Two welders operating the orbital equipment showed far greater productivity than any of the four manual welders. The best orbital welder completed about three times as many diameter in. of weld in the same time period as the best manual welder. If preparation time is factored in, it took about four hours to do a 6-in. pipe weld manually compared to about an hour for the same weld done orbitally. The arc time for the orbital weld was about 10-15 minutes.
Low reject rate
The use of orbital welding on copper-nickel pipe dramatically reduced the reject rate compared to manual welding. The reject rate on the orbital welds was about 5%, compared with 15% rejection of the copper-nickel welds done manualIy. With the orbital equipment, the quality was obtained on the first pass, and the welds no longer had to be cut apart and rewelded. An on-site inspector reviewed the X-rays of the finished welded assemblies and visually inspected the welds.
The piping was fabricated for one zone at a time and installed on the platform. Stub-in flanges were welded onto the pipes in the shop and the flanges were bolted together in the field to hold the system together, so no field welds were required. Monel and grafoil, an oil-impregnated graphite material, were used in the flange gaskets to prevent leaks.
Copper-nickel piping is part of a trend towards the use of more corrosion resistant materials. This installation demonstrates the successful partnering between the consumer, the contractor, and the equipment supplier.
By Alex Guiscardo, Exxon USA, Ed Dumas, Harmony Construction
and Barbara K. Henon, Ph.D., Arc Machines, Inc.
Alex Guiscardo is the operations surveillance supervisor for Exxon in the Santa Ynez Unit. He holds a BSME from the University of Texas.
Ed Dumas is project manager for Harmony Construction, Santa Paula, California.
Barbara Henon is a technical instructor of orbital tube welding for Arc Machines. She holds a PhD in biological sciences and is vice chair of the ASME Bioprocess Engineering Subdivision.