ABSTRACT
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.
INTRODUCTION
Sulfide stress cracking (SSC) remains as one of the top challenges in H2S sour wells, particularly in high temperature – high pressure (HPHT) sour wells where high strength materials are required. The current basic understanding of SSC occurrence on carbon steel oil country tubular goods (OCTG) is related to the following steps. The hydrogen atoms are generated through the chemical reaction between acidic aqueous components in the sour (H2S-containing) environment and carbon steel at the metal surface. In most corrosive environments, the vast majority of this atomic hydrogen recombines on the metal surface to form hydrogen gas (H2) which harmlessly bubbles off of the metal surface; however, in the presence of environments that contain sulfur species resulting from H2S, the kinetics of recombination can be significantly retarded resulting in increased absorption of the atomic hydrogen by the metal.1 Once the hydrogen atom enters the metal, its diffusion process is controlled by many factors such as hydrogen concentration gradient, hydrostatic stress, microstructural features and defects (such as dislocations, precipitates, segregations and vacancies), etc.
