Techniques to characterize fatigue behaviour of full size drill pipes and small scale samples

Drill pipe fatigue damage occurs under cyclic loading conditions due to, for instance, rotation in curved sections of an oil 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. 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 were investigated via laboratory tests in small-scale samples. These tests were conducted in an opto-mechanical fatigue apparatus that was designed to perform in-situ real time monitoring surface analysis during the experiments, by using differential interference contrast (DIC). Scanning electron microscopy fractography of fatigued samples was also performed. The fatigue-life curve was obtained for the drill pipe API S–135 grade steel from small scale tests. The full size drill pipe test results are coherent with the expected, validating the experimental procedures.