Modeling of torsional oscillation of drillstring dynamics

Abstract

Drillstring vibration is an important cause of precocious failure of drilling, and torsional vibration is a particular important phenomenon. Stick/slip is a severe type of torsional drillstring oscillation that affects the efficiency of the drilling process and can cause bit damage as well as drillstring failure. In an automatic vertical drilling system (VDS), frictional force between the pads and borehole wall will reduce the drill bit instantaneous rotational speed. The pads of the implementing agencies in VDS constantly pushed against the borehole wall, making bottom hole by a cycle of nonlinear damping force, which lead to chaotic and disorder motion to the bottom drilling tool. Here, the stick–slip vibration is modeled to understand the large amplitude torsional oscillation of the drillstring. Based on the model, the causes for torsional vibrations in VDS and torsional vibrations with and without stick–slip will be evaluated. We show that pushing the borehole wall will cause the drill bit torsional vibration more serious. The results contribute to the better understanding of the dynamics of the push-the-bit VDS. The results obtained can also be applied to the same type of rotary steerable system.