In-service welding allows for safe, cost effective installation of full-encirclement repair sleeves and hot tap branch connections while a pipeline remains in service. Upon the wide-scale use of hydrogen and blended gas pipelines, in-service welding will continue to be an important aspect of pipeline operations. When steel is exposed to hydrogen gas, atomic hydrogen can be adsorbed at the surface, which may occur rapidly at elevated temperatures, such as those encountered during in-service welding. While the primary defense against hydrogen cracking during in-service welding is to strictly limit the introduction of hydrogen into these welds, the ability to safely weld onto an in-service pipeline in hydrogen or blended gas service could be adversely affected by the uptake of hydrogen.

The objective of an on-going joint industry project (JIP) at DNV is to experimentally determine if welding onto an in-service pipeline that contains hydrogen or blends of natural gas and hydrogen results in an increased risk of hydrogen cracking and, if so, to develop guidance for measures that can be taken to mitigate the increased risk. The results have shown that various increases in weld hydrogen level can occur depending on the pipe wall thickness, the weld heat input, and the partial pressure of hydrogen. Burnthrough experiments were also conducted using pipe sections pressurized with hydrogen and blended gas. The results indicate that ignition is more likely with blended gas than with natural gas alone, and that ignition is highly likely with pure hydrogen.

The results of this project will allow in-service welds to be safely made onto pipelines that transport hydrogen and blended gas so that full-encirclement repair sleeves and hot tap branch connections can be installed while the pipeline remains in service.