A Numerical Study of Hydrogen Diffusion in a Part-size Sleeve-on-Pipe Mock-up Exposed to Two Different Levels of Post Weld Heat Treatment

Deeper waters impose considerable challenges for eventual remote pipeline repair and remote hot-tapping. The purposes of this study were (1) to replicate the thermal history of a laboratory-produced part-size sleeve-on-pipe mock-up, which was designed to replicate the geometry and highest restraint levels of a remote pipeline repair scenario that was exposed to pre- and post-heating; and (2) to examine the decay of hydrogen in and adjacent to a test weld, which closed the gap between the sleeve and the pipe. Measured temperatures and Δt_8/5 weld metal cooling rates were reproduced satisfactorily by the computer model. Two test cases were selected for modeling. One created a crack-free weld, while the other did not. The main difference was the pre- and post-weld heating temperature. Two approaches for the modeling of hydrogen diffusion were applied, one based on an apparent diffusion coefficient and another based on the calculation of trapping. With respect to the incidence of cracking and the level of hydrogen obtained after the post-weld heating period, both approaches for the modeling of hydrogen diffusion support the experimental findings.