Drill Pipe Fatigue Analysis: Full Size Apparatus and Coupon Tests

Drill pipe fatigue damage occurs under cyclic loading conditions due to, for instance, rotation in curved sections of the well. Failures caused by crack nucleation and propagation are one of the highest risks to the structural integrity of these pipes. Usually, failure mechanisms develop in the transition region of the tool joint. Several mechanical and metallurgical factors affect the fatigue life of drill pipes. The former are mainly geometric discontinuities such as transition zones, pits and slip marks. The latter are related to the size and distribution of crystalline grains, phases and second phase particles (inclusions). In this study, the roles played by both factors in the fatigue life of drill pipes are studied through an extensive experimental test program. To this end, a fatigue simulator was designed and built to test full-scale drill pipes under rotating cyclic bending and constant tension loading. Additionally, the fundamental fatigue mechanisms are investigated via laboratory tests in small-scale coupons. These tests were performed in an opto-mechanical fatigue apparatus that was specially designed to perform in-situ real time monitoring surface analysis during the experiments.