Abstract
To quantify the fatigue strength of bottom hole assembly (BHA) components used in high-bending drilling conditions, a multi-scale approach for fatigue analysis of BHAs was developed, which entails calculations at the BHA scale, the component scale, and the feature scale. At the BHA level, a beam-type model was constructed, with the BHA information and drilling parameters as the input. The calculated shear force, contact force, and bending moment were then used as the loading conditions for the finite element models at the component level, which involves drill collars and threaded connections. A local strain based fatigue damage criterion was thereafter employed to compute the fatigue life at the critical features. Part of the outcome from this multi-scale model is a relation between bending moment and fatigue life, which is inherent for each collar or each connection. This relation can then be used to design a BHA, track the fatigue damage of a BHA during operation, and perform root cause analyses if a fatigue related failure has taken place. The methodology was validated with lab experiments and field practices.
