Practical approach to modelling and controlling stick-slip oscillations in oilwell drillstrings

Stick-slip friction-induced oscillations appearing in oilwell drillstrings are a source of failures which reduce penetration rates and increase drilling operation costs. For this reason, a major problem to solve is modelling the drillstring torsional behaviour and proposing control solutions to help reduce stick-slip phenomenon. This paper is focused on both problems. On the one hand, it presents dynamic drillstring models in order to reproduce stick-slip vibrations under different operating conditions. The model used for the torque on the bit is the main difference with respect to other models proposed in the literature. On the other hand, some experience-based control strategies are evaluated in order to reduce stick-slip oscillations. These strategies uses the angular velocity at the drillstring upper part, the torque on the bit and the weight on the bit, proved to have a key importance in the reduction of drillstring torsional vibrations. The control approach followed is a decentralized one, i.e., the angular velocity at the top end of the drillstring and the bit velocity are controlled separately. The driller makes the supervisory task.

I. Introduction

Drillstring vibrations are classified depending on the direction they appear. Three main types of vibrations are distinguished: torsional (stick-slip oscillations), axial (bit bouncing phenomenon) and lateral (whirl motion due the out-of-balance of the drillstring). This paper is focused on drillstring stick-slip oscillations. One of the consequences of stick-slip oscillations is that the top of the drillstring rotates with a constant rotary speed, whereas the bit rotary speed varies between zero and up to six times the rotary speed measured at the surface. The mechanism originating this behaviour is now explained.