Homopolar pulse welding (HPW) has been proposed as a one- shot, single stage welding process for the joining of pipe in the J-lay configuration. A joint industries program (lIP) has been formed to develop this process for off-shore, deep water applications. Homopolar pulse welding utilizes the high current, low voltage electrical pulse produced by a homopolar generator to rapidly resistance heat the interface between abutting pipe ends, producing a full circumference resistance forge weld requiring no filler metal in only three seconds. This five year program began in February 1993, funded by a consortium of six oil companies (Shell, Exxon, BP, Texaco, Amoco, and Mobil) and one welding contractor (CRC-Evans). The program s ultimate goal is to produce a prototype system suitable for installation on a barge. The goals for the first two years of the program include demonstrating suitability of the process by producing and evaluating welds in different grades and wall thicknesses of three inch nominal diameter API 5L line pipe. Funding for each year of the program is contingent upon meeting program goals for the previous year. In the first five quarters of the program, eighty four welds have been produced, tested and evaluated and a method for non-destructive testing of the production welds has been identified. Motivation for Research J-lay Typically, offshore pipelines are constructed using the S-lay method, which permits the pipe to be welded and inspected in the horizontal position. Multiple welding and inspection stations are possible with this method, being limited only by the deck length of the welding barge. Due to increased buckling stresses, deep water pipelines require the steeper angle of entry into the water presented by the J-lay method. The vertical or near-vertical welding position inherent in J-lay permits only a single welding station, presenting great economic incentive to develop a rapid welding process. Homopolar Off-Shore Pipeline Welding Research Program This joint industry program began in February 1993, being funded by a consortium of six oil companies (Shell, Exxon, BP, Texaco, Amoco, and Mobil) and one welding contractor (CRC-Evans). The five-year program's ultimate goal is to produce a prototype system suitable for installation on a barge. This is to be achieved by building and demonstrating operation of a full scale vertical HPW system capable of producing industry acceptable homopolar pulse welds in 12 inch schedule 60 (0.562 inch wall) carbon steel pipe. The first year of the program has concentrated on producing homopolar pulse welds meeting sponsors' weld quality criteria in 3 inch schedule 80 (0.300 inch wall) X60 ERW and 3 inch schedule 160 (0.438 inch wall) X52 seamless pipe. One of the early requirements of the program was the development of an industrially acceptable basis to evaluate quality of the HPW prototype welds. John Hammond of BP Exploration, London, accumulated and synthesized a list of specifications for weld testing which was examined and agreed to by all member companies. Since that time, weld acceptability has been determined by what is known in the program as the "Hammond Criteria". Homopolar Pulse Welding Process Description Homopolar pulse welding derives its name from the power supply that generates the electric current pulse utilized in the process, the homopolar generator. The first homopolar pulse weld at the Center for Electromechanics was produced 15 years ago, but the homopolar generator was invented by Michael Faraday in the year 1831 and homopolar generators were used as welding power supplies in the first half of this century. They are not in common use because they are inherently high current, low voltage devices unsuitable for power transmission over long distances. P. 563^
Homopolar pulse welding (HPW) is a one-shot resistance welding process being investigated as a method to join API 5L carbon steel line pipe. Homopolar pulse welding utilizes the high current, low voltage electrical pulse produced by a homopolar generator to rapidly resistance heat the interface between abutting pipe ends, producing a full circumference resistance forge weld requiring no filter metal in under three seconds. A five year joint industry program is sponsoring HPW research with the goal of developing the process for deep water offshore pipeline construction utilizing the J-lay method. The jirst two years of the program have concentrated on weld parameter optimization by producing, testing, and evaluating welds in various grades, wall thicknesses, types and compositions of 3 inch nominal (3.5 inch OD) diameter API 5L carbon steel pipe. Mechanical properties of the welds and parent metal were evaluated by tensile testing, impact testing, and hardnesstraverse testing according to guidelines and criteria established by the industrial sponsors. Homopolar pulse welding has demonstrated the capability to produce industrially acceptable full circumference welds in carbon steel line pipe via a rapid, one-shot process. Future work will concentrate on developing the process for commercial field installation, with the programâ??s goal being the demonstration of a prototype system for producing HPW welds in 12 inch diameter pipe in a J-lay configuration. J-LAY FOR DEEP WATER PIPELINES Typically, offshore pipelines are constructed using the S-lay method, which permits the pipe to be welded and inspected in the horizontal position. Multiple welding and inspection stations are possible with this method, being limited only by the deck length of the welding barge. Due to increased buckling stresses, deep water pipelines require the steeper angle of entry into the water presented by the J-lay method. The vertical or near-vertical welding position inherent in J-lay permits a single welding station, presenting great economic incentive to minimize pipe handling and welding cycle times. Springmann and Hebertl described a J-Lay Pipelay System used by McDermott Marine Construction to install deep water portions of Auger 12 inch oil and gas pipelines. This system virtually removed pipe joint handling operations from the weld cycle critical path. Development of a rapid, one-shot welding process would decrease weld cycle times and further increase lay rates. Homopolar Pulse Welding (HPW) has been proposed as a viable one-shot welding process for this application. HOMOPOLAR PULSE WELDING Homopolar pulse welding is properly classified as a resistance forge welding process. The primary components of a HPW system are the homopolar generator and the hydraulic welding fixture. The homopolar generator is a device that stores energy kinetically in a rotating flywheel and converts it to electrical energy in a discharge circuit via electromechanical energy con- Homopolar generators are not in common use because they are inherently high current, low voltage devices unsuitable for power transmission overlong distances. Homopolar generators have been previously used as welding power supplies; Westinghouse2 constructed a homopolar generator resistance welding power supply in the 1930's for pipe seam welding.
The University of Texas at Austin Center for Electromechanics is conducting a research program to study homopolar welding of line pipe for J-Lay applications. In 1995, the third year of the five year research program, process improvements increased Charpy V-Notch impact toughness properties to near parent metal values, while maintaining acceptable strength.After demonstrating repeatable performance, research focused on real world effects including evaluating the effect of poor fit up resulting from misalignment and rough and wavy interfaces. During the final year of the research program, the pipe welding program has scaled up to 12-inch nominal line pipe, a sevenfold increase in cross-sectional area. The paper will cover basics of homopolar welding, mechanical properties, weld upset profile, HPW metallurgy, and the studies of real world effects. Results from homopolar welding of 12-inch pipe will also be presented. HOMOPOLAR OFFSHORE PIPELINE WELDING RESEARCH PROGRAMA consortium of six oil companies (Amoco, BP, Exxon, Mobil, Shell, and Texaco), a welding contractor (CRC-Evans), an equipment contractor (Parker Kinetic Design) and two federal agencies (the Office of Pipeline Safety of the DOT and the Mineral Management Services of the DOI) funded this joint industry program (JIP) to develop homopolar welding for J-lay applications. The research, which started in February 1993, is being conducted at UT-CEM, and has achieved the following major objectives:• optimize weld parameters for homopolar welding 3-inch HSLA API 5L line pipe
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