Development of a Sour HPHT Environment Corrosion-Fatigue S-N Design Curve

In Deepwater Gulf of Mexico (USA), industry and regulatory requirements now require fatigue analysis of in-well barrier equipment for HPHT development wells. The S-N (stress-life) based analysis, specifically utilizing ASME BPVC Sec. VIII Div-2 Part 5 (2013), was the selected analysis methodology for a project. The project required analysis of oilfield grade precipitation hardened nickel-based alloys cyclically loaded in a high temperature environment with sour corrosive produced fluid characterized by significant H2S, CO2, and chlorides, and low pH. As part of the fatigue analysis, a suitable fatigue design curve needed to be selected for the desired metallurgy and environmental exposure. The team initially focused on utilizing an existing industry standard fatigue design curve, but after review, development of a unique corrosion fatigue design curve was required. The targeted initial fatigue design curve was the applicable curve found in ASME BPVC Sec. VIII Div-2 (2013). The team used a proprietary data set, from an environment representative of the expected downhole conditions of the project, collected for a previous project. The data set clearly demonstrated that the applicable ASME BPVC design curve (ASME BPVC Sec. VIII Div-2 2013) did not meet the required level of conservatism in this sour corrosive HPHT environment. The original intent of the data set likely was not to validate or develop a fatigue design curve and the test protocol did yield data that could readily be input into typical design curve analysis. Therefore, some advanced and novel analysis techniques were used to develop a unique S-N fatigue design curve that met the industry standard requirements for conservatively predicting fatigue failure and met the requirements for use within ASME BPVC Sec. VIII Div-2 Part 5 based analysis.