In-Service Welding Onto Methane/Hydrogen Mixture Pipelines

Abstract

The objective of the recently completed Phase 1 of a joint industry project (JIP) at DNV, the results of which are described in this paper, was to determine if welding onto an in-service pipeline that transports a mixture of methane and hydrogen results in an increased risk of hydrogen cracking and, if so, to develop guidance pertaining to measures that can be taken to mitigate the increased risk. The mechanisms and the extent to which steel line pipe can become charged with hydrogen when transporting methane/hydrogen mixtures were reviewed. An experimental program was undertaken to determine the extent to which elevated weld hydrogen levels can result during welding onto pipe pressurized with blends of hydrogen and methane (Task 2a) and from welding onto steel that has been exposed to high pressure blends for an extended period of time (Task 2b). The results of Task 2a showed various increases in weld hydrogen level, depending on the pipe wall thickness, the weld heat input, and the partial pressure of hydrogen. Reactions between the pipe contents, the line pipe steel base material, and surface oxides were also observed. Task 2b resulted in no significant increase in weld hydrogen level as the result of welding onto steel that had been exposed to a high pressure blend for an extended period of time. Additional work is required to determine threshold conditions beyond which increased weld hydrogen levels do not occur and to quantify increases in weld hydrogen levels for a broader range of conditions, including higher and lower partial pressures of hydrogen.