Fatigue evaluation of drill pipe by considering ship motions and its application in scientific drilling

Abstract

The fatigue strength of a drill pipe is a critical factor governing the performance of drillings. Substantial research has been conducted related to fatigue evaluation and various factors that have an effect on fatigue. However, earlier studies have expressed concerns over the use of fatigue curves obtained from full scale tests and stress responses obtained via finite element analysis for evaluation: (a) fatigue curves were obtained by utilizing the stress measured at a point far away from the failure area rather than the stress exerted at the failure point, (b) the stress response is not equivalent to the stress expressing the fatigue curve. It is also important to obtain the stress-response amplitude by precisely considering the irregular wave and ship motion response. This study proposes a method to obtain stress and moment responses due to ship motion in irregular waves. Secondly, it proposes a method to obtain the fatigue curve from a full-scale fatigue test suitable for evaluating the cumulative fatigue with stress response and moment response results. Then, the cumulative fatigue is calculated by applying Miner’s rule. This paper also applies the proposed method to a scientific drilling program amid strong ocean currents and uses a special drill pipe. The results by the proposed method are compared with those obtained by previous methods.