Homopolar Pulse Welding for Offshore Deep Water Pipelines

Abstract

ABSTRACT 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.