Advances in Homopolar Welding of API Linepipe for Deepwater Applications
Authors: Hudson R.S. , Carnes R.W. , Nichols Steven P. 
Published: December 2007

Abstract
The University of Texas at Austin Center for Electromechanics is conducting a research program, funded by six oil companies, two industrial contractors, and two government agencies, to study homopolar welding of line pipe for J-Lay applications. In 1995, the third year of the five year research program, the set of weld parameters, those input variables used to control weld performance and quality, was expanded to include joint geometry, with dramatic improvement in the mechanical properties of two HSLA nominal 3-inch line pipe materials, X60 and X65. These improvements increased the Charpy V-Notch impact toughness properties to near parent metal values, while maintaining acceptable strength. After demonstrating repeatable performance with the new parameters, new research focused on real world effects including tolerancing the parameters and 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, weld parameters and modifications, effects of these changes on real time process data, 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.

Introduction
Homopolar Welding (HPW) is a resistance forge welding process that uses the high amperage direct current discharge from a Homopolar Generator (HPG) to produce full circumferential pipe welds in under five seconds without using filler metal. HPW is being developed as a candidate single station, or "one shot", welding method for J-lay pipe laying.
With the continued discovery of deep water oil fields, the J-Lay method was developed to overcome the problem of buckling, occurring when the pipe enters the water from conventional S-lay barges. J-lay requires single station welding process because the pipe is welded vertically then lowered directly into the water. Besides enabling pipe laying in deep water fields, the reduced cycle time per weld should lower the cost.
Homopolar Offshore Pipeline Welding Research Program. A consortium of six oil companies (Amoco, BP, Exxon, Mobil, Shell, and Texaco) and a welding contractor (CRC-Evans) funded this joint industry program (JIP) at The University of Texas at Austin Center for Electromechanics, beginning in February 1993, to develop homopolar welding for J-Lay applications. Since that time, an additional equipment contractor (Parker Kinetic Design) and two federal agencies (the Office of Pipeline Safety of the DOT and the Mineral Management Services of the DOl) have joined the program. The major objectives of the research program have been achieved. They include:–optimize weld parameters for homopolar welding 3-inch HSLA API 5L line pipe–investigate a range of materials with varying strength, wall thickness, composition, heat treatment and manufacturing method–produce 3-inch welds with acceptable mechanical properties - improve the finished weld profile–design and build a laboratory welding fixture for joining 12-inch Schedule 80 line pipe–demonstrate homopolar weldability of 12-inch pipe–transfer technology in preparation for commercialization of HPW–develop an NDE program
One of the first requirements of the research program was developing an acceptance criteria for the welding program that was compatible with existing welding codes. John Hammond, with BP Exploration of London, developed this document, known as the "Hammond Criteria", which has served to guide the research through the development of a new welding process. P. 953