A coupled hydrogen diffusion and material elastic-plastic deformation finite element analysis (FEA) subroutine was developed. The subroutine was used to analyze the hydrogen diffusion process in an oil country tubular goods (OCTG) pipe with a surface flaw under internal pressure loading in a H2S sour environment. It was found that hydrogen diffusion inside the metal is significantly affected by subsurface hydrogen concentration, stresses (including global and local stress such as notch or crack tip stress) and material plasticity. For example, metal subsurface hydrogen concentration normally varies in a sour environment over time due to the chemical reactions between the metal surface and the environment. However, it was found in this study that the subsurface hydrogen concentration variation can lead to a variation of the hydrogen distribution inside the metal and ahead of the flaw tip. Consequently, material cracking resistance is affected by the subsurface hydrogen uptake conditions. The hydrogen concentration variation inside the metal results in significant complexity when attempting the determination of cracking resistance in an environment as a function of hydrogen content. The FEA study demonstrates that a single edge notched tensile (SENT) specimen has a similar hydrogen concentration and evolution process, as well as the crack tip stress state, as an actual pipe with a surface flaw and thus it may be a relevant approach to measure pipe material sulfide stress cracking (SSC) resistance in sour environments. This FEA analysis tool not only can be used to investigate SSC cracking mechanism and material testing methodology of a pipe under various loading conditions and sour environments, but also can be used as a fitness-for-service casing design tool for sour wells.

Subject
Test methods, Materials, Piping, Finite element analysis, Hydrogen concentration, Diffusion, Sour environments, Notches, Stress cracking, Stress, Metals, Hydrogen, Hydrogen stress cracking
Keywords:
Finite element, hydrogen diffusion, SSC
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2019
Association for Materials Protection and Performance (AMPP)
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