Stress concentration incorporated fatigue analysis of die-marked drill pipes

Abstract

Fatigue damage of drill pipes occurs in the pipe section at the dogleg region (where the drill pipe changes its drilling direction) due to alternating bending stress during rotation of the pipe. The pipe section may fail due to the cumulative effect of fatigue damage in a single drilling event, or in a number of drilling events. Moreover, dies of gripping systems mark the pipe surface during making and breaking operations (screwed and unscrewed to connect and disconnect two pipes). These marks on the pipe surface cause stress concentration. This paper develops a method for fatigue analysis of drill pipes taking into account the alternating bending stress at the dogleg, the direct axial stress due to hanging weight of drill pipes and the stress concentration due to die-marks on the pipe surface. The application of the method is demonstrated by evaluating cumulative fatigue damage for a number of drilling events and different gripping systems which produce die-marks of different depths. Results have revealed that higher die-mark depths accelerate fatigue failure, which can be predicted more accurately by the method presented in this paper.