Drillpipe fatigue damage occurs under cyclic loading conditions due to, for instance, rotation in a dogleg region. Usually, failure mechanisms develop in the transition region of the tool joints and the die-marks due to gripping systems intensify it. In this paper two approaches are presented to evaluate damage in drillpipes; FEM and Cox Regression Model. First, Finite Element Method is used to evaluate cumulative effects of fatigue damage in a number of drilling events with respect to rotating cyclic bending and constant tension and internal pressure in a G-105 drillpipe. The results show that how die-marks or other surface crushes can reduce the fatigue life of the pipe. The presented graphs can be easily used to determine the allowable length of a G-105 drillpipe below dogleg that consumes the fatigue life of the pipe section. In the second approach, as a case study, the Cox Regression Model, a broadly applicable and the most widely used method of survival analysis is used to evaluate the distribution of survival times for the failure data of the southern oilfields of Iran. The resultant cumulative survival and hazard functions can reliably predict the time of failure and assist the engineers to evaluate cumulative damage.

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