Role of Hydrogen in Fatigue Crack Growth Rate (FCGR) of X65 alloys: Modeling study

摘要

Low alloy steels such as X65 are common choices for offshore flowlines and risers in sour service applications. These alloys become susceptible to corrosion fatigue because of large thermal transients and wave motion that lead to fatigue loading. Hydrogen diffusion to the regions with high tensile stress is an accepted mechanism of failure under these conditions. However, not much has been explained with respect to the variables that are ΔK, frequency, pH, and pressure of H_2S that affect the Fatigue Crack Growth Rate (FCGR). It is known that FCGR increases with the decrease in frequency until it reaches a plateau. The plateau frequency changes with the changing load. In this work, a finite element modeling study was carried out to explain the trends in FCGR upon changing frequency and the load. The diffusion of hydrogen was modeled at various conditions. The simulation results were compared to the experimental results. We propose that the plateau in the FCGR occurs when the frequency is reduced below a threshold value because the hydrogen concentration reaches a maximum steady state value in a time period. The shift in the plateau frequency occurs with load because the time to steady state increases with the increase in the load.