The homopolar generator is an inertial energy storage, direct current power supply capable of delivering multi-megawatt power levels for a variety of applications. The research presented here describes the use of the homopolar generator for electrical resistance, upset (forge) butt welding of 3 in. schedule 160, API5L-X52 steel pipe. The motivation for this research is the investigation of homopolar welding as a candidate for single station (“one Shot”) welding of offshore pipeline, as required by the newly developed J- lay pipe laying process.
The homopolar welding process is characterized both experimentally and analytically by developing a three step methodology: tests on a continues pipe, tests with the pipe-to-pipe interface under constant load, and upset weld tests. The continues pipe tests use a single continues pipe as the electrical load for the generator and allow calibration of the system performance by eliminating the dynamic effects of the pipe-to- pipe interface. A simple, closed-form solution for the discharge circuit is used to approximate the electrical lumped-element parameters and a state-variable model is developed and compared to actual system performance. The constant load tests use the same setup as the subsequent upset weld test except the pipes are not forged together. By inserting the interface into the discharge circuit, but eliminating the forging action, the effects of the interface on the process are isolated. A series of upset weld tests was conducted that kept all input parameters constant except initial energy stored in the generator. This allowed comparison of the process output parameters as the energy deposition to the welds increased.
In-process weld parameters were monitored and compared to the destructive evaluation of the welds in order to identify control parameters that could be used for real-time weld quality assurance. Using the upset weld tests in the characterization study in addition to three other series of upset welds, a strong correlation was found between the thermal expansion displacement of the pipes caused during interface heating and the tensile strength of the welds.