Fatigue Crack Growth Rate and Fracture Toughness of Non-Sour Service Grade X65 and X80 in Sour Environments

Abstract

Offshore flowlines are subject to large thermal transients particularly during shut down. These thermal transients tend to vary from about 200°F to ambient temperature and place a large fatigue demand on the pipelines. The transients typically are very low frequency events with high amplitudes, which can lead to lateral buckling. The loading rates associated with the transients are very slow, hence the toughness properties of the material in environments at low loading rates is important. The pipelines are also subject to high stress and thus need to have good toughness properties. Riser materials are also subject to significant fatigue loading, though under conditions different from flow lines. They are typically pre-loaded and operate under smaller amplitudes of loading and at higher frequencies associated with wave motion. There has been some work performed to understand the performance of carbon steels in dynamic applications like risers and flow lines in sour service. This work is focused on exploring a range of environmental and loading conditions that are relevant to offshore production to evaluate the non-sour service grades X65 and X80 to gain greater insight into the role of chemistry and thermo mechanical processing on the sour service fatigue and fracture behavior of line pipe steels. Two different grades of non-sour service grade seamless pipe X65 and X80 were investigated in a range of sour environments. The concentration of diffusible hydrogen in the range of environments to relate the environment assisted cracking properties to the concentration of hydrogen. Fracture toughness measurements indicated that crack initiation and tearing modulus were dependent on the concentration of diffusible hydrogen. The lowest toughness properties was obtained in pH = 5, 4.6psia H2S. FCGR exhibited a1/√f dependence in all conditions suggesting that hydrogen diffusion was likely the rate limiting step before reaching a plateau. X65 and X80 exhibited the highest FCGR in pH = 5, 4.6psia H2S. The FCGR of X80 was lower than X65 under the same environmental conditions in all cases while the plateau frequency was very similar for both X65 and X80. Paris law measurements performed at constant Kmax and decreasing ΔK at plateau frequencies exhibited elevated FCGR levels above the in-air values consistent with the frequency scan data.