Efficiently Predicting Fatigue Life of Drill Collars With Ports Subjected to Variable-Amplitude Bending or Torsional Loads

To enable real-time monitoring of the physical condition of the drilling equipment such as drill collars, a methodology for efficiently predicting the fatigue life of ports subjected to variable-amplitude cyclic bending or torsional loads is needed. In this paper, such a method is reported, which involves several steps. Firstly, elastic finite element analysis (FEA) of a collar port was performed to determine the elastic stress states with unit loads. Secondly, the unit load-based linear elastic solutions with the loading history were superimposed to produce a time history of the stress tensor. Thirdly, the previously established pseudo-elastic stress states were transformed into the true elastoplastic stress and strain states with a cyclic plasticity model and a notch correction rule. Finally, the cumulative fatigue damage was computed with the rainflow counting algorithm and a damage accumulation rule. The resulting fatigue life predictions for the ports were found to agree favorably with the experimental measurements from full-scale fatigue tests of port-containing collar samples with variable-amplitude loads. This newly developed method can be used to predict the remaining useful life of a port in real time with the loads resulting from downhole measurements or a drill string dynamics simulation code.