Faced with the complexity of the oil and gas wells drilled today, the fatigue phenomenon is the most significant cause of drill string failure. Different sources of cyclic loadings acting on drill string can cause the fatigue phenomenon during well operations: rotation of equipment in a curved portion of the well, rotation of a buckled structure, and vibrations in the drill string…
We developed a methodology and numerical models to evaluate the fatigue of equipment during drilling operations of wells with complex trajectories. A step-by-step algorithm, coupled with a 3D mechanical behavior model of a structure in well, is used to implement the fatigue approach in each calculation increment.
The first model is based on the cumulative fatigue concept using S-N curves of drill-pipes and a fatigue accumulation process by applying the Miner damage law. This model requires the knowledge of S-N curve of equipment which can be determined from full-size drill-pipe tests which are very costly, or by using the empirical method based on Moore’s machine fatigue test (small scale).
The second model is based on the crack propagation theory. This model requires the existence of an initial crack in the equipment, and the coefficients of the crack propagation law in the material which can be determined from standardized tests on small samples.