A brief discussion on the choice of a model to simulate drill-string dynamics

Abstract

. A comparison of the dynamics obtained using two different models of a drill-string is made. A structure that is 1200 m long is analysed. In the first model, a two degrees-of-freedom (DOF) approach is used, where 1-axial DOF and 1-torsional DOF are considered. This model has been widely used in the literature. The second model maintains the 1-DOF axial formulation and employs a wave-equation for the torsional part. In both cases, the structural dynamics are coupled with the dynamics of the cutting through the depth-of-cut used in the bit-rock interaction relation. To calculate the previous depth, an advection equation is solved along with the equations of motion. One application case is simulated. The results show that the dynamics differ considerably. This suggests that the second model, with a continuous torsional shaft, could be capturing aspects of the dynamics that are neglected in the 2-DOF approach.