Finite element modeling of the hydrogen atom distribution on dented pipeline for hydrogen transport under cyclic loading

Repurposing existing natural gas pipelines for hydrogen transport requires an accurate assessment of the distribution of hydrogen (H) atoms at surface defects such as dents under frequent pressure fluctuations encountered on gas pipelines. In this work, a 3-dimensional finite element-based model was developed to determine the stress/strain and H atom concentrations at an unconstrained dent on an X52 steel pipe experiencing denting, spring-back and cyclic loading processes. As expected, a stress/strain concentration generates at the dent center, while the cyclic loading reduces the stress level and shifts the stress concentration zone from the dent center along the circumferential direction. As the dent depth increases, the maximum H atom concentration is further shifted from the dent center to the side. A coincident relationship between the maximum H atom concentration, von Mises stress, hydrostatic stress and plastic strain does not exist. Pressure fluctuations decrease both the stress and H atom concentrations, providing a beneficial effect on reduced risk of the dented pipelines to hydrogen embrittlement in high-pressure hydrogen gas environments. Further analysis shows that the indenter size has little influence on the H distribution in the dent area.