Fatigue Assessment of Drill Pipes

Fatigue failures during offshore drilling operations is still a very costly problem. The fatigue behavior of drill pipes is reviewed, and typical failure modes are identified. The effects of drill string curvature during directional drilling on pipe body stress and on the fatigue life is examined. Effects on applied mean stress from drill string weight are discussed. Interaction effects of degradation mechanisms such as fatigue, wear and corrosion are evaluated. Experimental background data and statistical evaluation that form the basis for the current design practice issued by American Petroleum Institute (API) and other guidance in codes and standards is reviewed. Results from several recent testing programs performed under rotating bending of pipes with threaded connections, and tests involving the pipe body under resonance conditions are presented. The tests were made with pipe sizes from 2 7/8 in. to 5 7/8 in. in Grade S-135 pipes. The results are compared with published test data and design guidance such as API Recommended Practice G7 [1]. Recommendations are given for research and testing to improve reliability and the safe operation of drill strings.