Diffusion of Hydrogen into Pipeline Steel around an External Weld

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While dedicated hydrogen pipelines have been present on the Gulf Coast of the US for decades, new application opportunities are opening up for transportation of hydrogen as a greener fuel. Some opportunities may be for newly built transportation lines while others may use existing natural gas pipelines that are converted to wholly or partially carry hydrogen. A normal part of operating a pipeline system is reconfiguring the system to add new pipes by making tie-in welds joining the new pipe to the wall of the existing pipe.

Product Number: 51323-19403-SG

Author: William Mohr, Amin Moghaddas

Publication Date: 2023

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Pipelines for carrying gaseous hydrogen are expected to become more common as the use of hydrogen fuel increases. The desire to weld on operating hydrogen pipelines will become of greater interest as more pipelines enter this service. A finite element model approach for hydrogen diffusion from the inside surface has been developed with sequentially coupled assessment of temperature, stress, microstructure, and hydrogen diffusion. The model approach has been evaluated for cases of external bead-on-pipe welds onto X52 pipe carrying 800 psi hydrogen. Hydrogen diffusion into the pipe material increases with increased weld heat input in the external weld. The peak concentration of hydrogen also moves from close to the inside surface to the heat affected zone near midwall as the heat input increases. The peak concentrations are much lower than if the pipe is pre-charged with hydrogen. The weld metal itself is not highly charged with hydrogen compared to the heat affected zone and adjacent pipe.