  We combine welding process modelling with deformation-diffusion-fracture
(embrittlement) simulations to predict failures in hydrogen transport
pipelines. The focus is on the structural integrity of seam welds, as these are
often the locations most susceptible to damage in gas transport infrastructure.
Finite element analyses are conducted to showcase the ability of the model to
predict cracking in pipeline steels exposed to hydrogen-containing
environments. The validated model is then employed to quantify critical H$_2$
fracture pressures. The coupled, phase field-based simulations conducted
provide insight into the role of existing defects, microstructural
heterogeneity, and residual stresses. We find that under a combination of
deleterious yet realistic conditions, the critical pressure at which fracture
takes place can be as low as 15 MPa. These results bring new mechanistic
insight into the viability of using the existing natural gas pipeline network
to transport hydrogen, and the computational framework presented enables
mapping the conditions under which this can be achieved safely.
