Research On The Stress Corrosion Analysis And Mechanism For The BOP Steel In The Wet Environments Of H₂S

Blowout preventers are very important large, thick-walled pressure equipment used for oil and gas production. The working reliability of a blowout preventer depends on its proper reliability and system reliability. Preventer failures may result from multiple causes, any of which may trigger enormous blowout risks and lead to inestimable losses. As oil and gas exploration and exploitation develops, the oil & gas production industry has to face more and more rugged production environments. The increasingly serious problem of stress corrosion of blowout preventers by CO2, H2S and salts in oil and gas wells has attracted increasing attention to the problem, especially in the case of wet H2S environments. Both domestic and foreign scholars have been carrying out large-scale research on the problem of stress corrosion cracking in wet H2S environments, and up to now, certain achievements have been made. As factors of stress corrosion are diversified and there are certain limits to researches on the cracking corrosion mechanism and the testing technology adopted, there are still many unknown fields requiring further research and approach, especially the research on the stress corrosion mechanism of steels used for preventer construction.Based on an analysis and presentation of the status quo of and existing problems with the relevant research both at home and abroad, this article analyzes the main factors affecting the corrosion-resistant performance of steels (ZG35CrMo steel and ZG25CrNiMo steel) used for preventers for wet H2S applications from the angle of universal application. These factors include defective structure, environment (H2S concentration, pH value, Cl- concentration and stress level) and residual stress within materials. Also, a preventer stress distribution model is established to make the following important research achievements:1) The results of the finite element analysis and the actual stress tests show that the stress in a ram preventer is most concentrated where the geometrical shape changes, i.e. the place of the preventer shell and the flange hub, the rounded corner where the vertical through-hole in the shell internal surface joins the ram chamber hole. These are places where the sectional dimensions of the preventer are likely to change suddenly and cause local increases/concentrated stress. Concentrated stress in these structural parts tends to deteriorate the strength of the preventer shell and reduce pressure-resistance capacity. When the pressure load increases suddenly, the parts where the stress is the most concentrated will be the first to reach the yielding limit, and local plastic deformations will occur within a small scope. This type of repeated local plastic deformation will lead to cracks that will ultimately spread over time and damage the integral structure of the preventer shell.2) In the wet H2S environment designed for the experiment, the stepwise regression analysis of the stress corrosion susceptibility index F(A) of ZG35CrMo steel shows that H2S concentration affects the stress corrosion susceptibility index F(A) of ZG35CrMo steel most remarkably; Cl- concentration has no significant correlation with; pH value and H2S concentration will have an interaction on F(A); as H2S concentration increases, the stress corrosion susceptibility index of ZG35CrMo steel gradually steps up; under the same H2S concentration, the stress corrosion susceptibility index of ZG35CrMo steel gradually steps downs as pH value increases.3) In the wet H2S environment designed for the experiment, the stepwise regression analysis of the stress corrosion susceptibility index F(A) of ZG25CrNiMo steel shows that H2S concentration affects the stress corrosion susceptibility index F(A) of ZG35CrMo steel most remarkably; Cl- concentration has no significant correlation with F(A); pH value and H2S concentration will have an interaction on F(A); as H2S concentration increases, the stress corrosion susceptibility index of ZG25CrNiMo steel gradually steps up; under the same H2S concentration, the stress corrosion susceptibility index of ZG25CrNiMo steel gradually steps downs as pH value increases.4) In the wet H2S environment designed for the experiment, the stepwise regression analysis of ZG35CrMo steel shows that Cl- concentration affects the corrosion rate most remarkably; pH value has no significant correlation with the corrosion rate; Cl- concentration, H2S concentration and stress level will have an interaction on the corrosion rate; under the same H2S concentration and stress level, as Cl- concentration increases, the corrosion rate of ZG35CrMo steel steps up gradually; under the same Cl- concentration and H2S concentration, the corrosion rate of ZG35CrMo steel gradually steps up as stress level increases; under the same stress level and Cl- concentration, as H2S concentration increases, the corrosion rate of ZG35CrMo steel steps up gradually.5) In the wet H2S environment designed for the experiment, the stepwise regression analysis of ZG25CrNiMo steel shows that Cl- concentration affects the corrosion rate most remarkably; pH value has no significant correlation with the corrosion rate; Cl- concentration, H2S concentration and stress level will have an interaction on the corrosion rate; under the same H2S concentration and Cl- concentration, as stress value increases, the corrosion rate of ZG25CrNiMo steel steps up gradually; under the same stress level and Cl- concentration, as H2S concentration increases, the corrosion rate of ZG25CrNiMo steel gradually steps up as stress level increases; under the same stress level and H2S concentration, as Cl- concentration increases, the corrosion rate of ZG25CrNiMo steel steps up gradually.6) The comparison of slow strain rate testing (SSRT) and weight loss testing results shows that both the stress corrosion susceptibility index and the corrosion rate of ZG35CrMo steel are higher than those of ZG25CrNiMo steel, due to the fact that the strength and hardness of ZG35CrMo steel are higher than those of ZG25CrNiMo steel. The higher the strength of a steel is, the more susceptible to stress corrosion the steel is.7) ZG25CrNiMo steel with a defective structure has a stress corrosion susceptibility index and a corrosion rate higher than those of ZG25CrNiMo steel with a normal structure. This may be attributed to the following reasons:Troostite and upper bainite have a relatively big internal stress. Thermodynamically, their crystalline lattices are in a state of unbalance and are therefore are susceptible to wet H2S stress corrosion. Meanwhile, the presence of numerous defects in troostite. and upper bainite, such as coarse and densely distributed microscopic impurities and loose substances, will accelerate the diffusion of hydrogen atoms and wet H2S stress corrosion.8) A comparison of the electrochemical testing results of parts where there are residual compressive strength and residual tensile stress shows that the residual residua stress may increase the corrosion rate. This is because the testing sample develops cold deformation caused by bending. Therefore, the microscopic structure of the material has changed, i.e. increases in slip step, vacancy density and dislocation density. From the angle of energy, these defects are in a state of unbalance and are located where hydrogen tends to accumulate and there is high energy. Moreover, the residual tensile stress contributes to the likeliness of hydrogen diffusion.