Abstract
Fatigue Crack Growth Rate (FCGR) and fracture toughness behavior of high strength steels used in drilling riser applications was investigated in sour environments. Frequency scan tests were performed were performed on API* 5L X80 (X80), API 5CT C110 (C110), and API 5DP S135 (S135) in moderate to mildly sour environments. Fatigue crack growth rate increased with decreasing frequency and reached a plateau value at low frequencies. The observed behavior is typical for hydrogen enhanced cracking in sour environment. The rate controlling step for FCGR was found to be hydrogen diffusion through the fracture process zone. The critical concentration for crack advance was calculated based on a model based on hydrogen diffusion through the fracture process zone. The critical concentration of hydrogen for S-135 was significantly lower than C-110, and X-80. The critical concentration of hydrogen for crack advance for C-110, and X-80 were similar. Rising-displacement FT tests were performed in a range of environments to determine the resistance (R-curve) behavior. Tests were performed in-situ at a slow K rate of 0.05Nmm-3/2/s over a range of environmental conditions as well as microstructures, namely, S-135, C-110, X-80. The results show that the initiation toughness, and the slope of the R-curve decreased sharply in the sour environments.
