Study on S-N Curve and Fatigue Limit of Drill Pipe in Offshore Short-Radius Sidetracking Process

To evaluate the fatigue reliability of different types of drill pipes during an offshore short-radius sidetracking process, the fatigue life and limit of G105, S135, and V150 steel and a new titanium alloy drill pipe were studied in air, high-temperature conditions, drilling fluid, and drilling fluid containing H2S. First, the chemical composition, microstructure, and tensile properties of four kinds of drill pipe materials were tested. Secondly, the fitting effects of different S-N models were evaluated and identified, a fatigue test of four kinds of drill pipe under different environments (air, high temperature, drilling fluid, and H2S drilling fluid) was carried out, and the S-N curves and fatigue limits of different drill pipes under different environments were obtained. Finally, the fatigue sensitivity of drill pipes to different factors was studied, and the potential corrosion fatigue mechanism was explained. The research results show that the fatigue life of a drill pipe in a non-corrosive environment (air and high temperature) is mainly related to steel grade, and the fatigue life of a titanium alloy drill pipe is better than that of a steel drill pipe in a corrosive environment. The dense passivation film on the surface of a titanium alloy drill pipe is an important reason for its better corrosion fatigue life than that of a steel drill pipe. This study provides important data support for selecting drill pipes in offshore short-radius sidetracking.