Modeling and experimental identification of drill string torsional dynamics under uncertainties

Abstract

This D.Sc. thesis proposes new perspectives for modeling drill string torsional dynamics under uncertainties. This work develops a novel stochastic hysteretic (nonreversible) bit-rock interaction model. Firstly, a new nominal interaction model, which depends not only on the bit speed, but also on the bit acceleration is developed. Then, a new stochastic model for the bit-rock interaction, taking into account the inherent fluctuations during the drilling, is also proposed. Furthermore, a new test-rig is proposed to analyze drill string dynamics and bit-rock interaction, which is able to reproduce stick-slip phenomena while drilling a rock sample using standard masonry bits, as well as to validate bit-rock interaction models. An original strategy for modeling uncertainties globally, based on terms of the nonparametric probabilistic approach, considering a simple torsional model for a drill string, is also proposed. This strategy allows to control the dispersion level of each interior and interface DOFs of each drill string substructure independently, which can provide more information to improve the operational safety.