Multiaxial fatigue with parametric uncertainties in axial-torsional drill-string dynamics

Abstract Uncontrolled drill-string vibrations can lead to loss of productivity, with fatigue failure being a major factor that can lead to the rupture of these structures. The main contribution of this paper is to propose a probabilistic model to take into account uncertainties associated with the bit-rock interaction parameters, system parameters, and fatigue properties and evaluate its impact in multiaxial fatigue factors. To test the proposed methodology, stress signals are obtained from a coupled axial-torsional drill-string model, which is calibrated with experimental data. Two multiaxial fatigue models are considered, and, for the configurations analyzed, the model proposed by Findley is more conservative than the one proposed by Papadopoulos. Regarding uncertainties, system parameters have more influence in regions with a greater tendency for occurrence of stick–slip oscillations, and fatigue properties uncertainties are more important in regions with lower oscillations.