Torque Feedback Used to Cure Slip-Stick Motion

Abstract

Abstract A novel idea to control the rotary speed incorporates the torque signal, in addition to the speed signal, to produce a more uniform motion of the drillstring. This produce a more uniform motion of the drillstring. This method is easily incorporated into existing equipment used to control the rotary speed. We have tested this control system on a full-scale research drilling rig and have demonstrated that such a system can stop slip-stick motion and prevent it from starting. As an additional benefit, such a control system leads to smoother rotation of the bit which can lead to a reduction in axial and lateral vibrations of the drillstring. In this paper we present the theoretical basis behind such a system and give an example of the experimental results. Introduction Torsional drillstring vibrations, in particular slip-stick oscillations, have been studied intensively in the last few years. Frequency analysis of the driving torque has shown that a large number of peaks in the torque spectrum can be identified as torsional drillstring resonances. The sharpness of the resonance features indicates that there is little damping of such vibrations. Slip-stick motion of the bottom hole assembly can be regarded as extreme, self-sustained oscillations of the lowest torsional mode, called the pendulum mode. Such a motion is characterized by finite time intervals during which the bit is non-rotating and the drillpipe section is twisted by the rotary table. When the drillstring torque reaches a certain level (determined by the static friction resistance of the bottom hole assembly), the bottom hole assembly breaks free and speeds up to more than twice the nominal speed before it slows down and again comes to a complete stop. It is obvious that such motion represents a large cyclic stress in the drillpipe that can lead to fatigue problems. In addition, the high bit speed level in the problems. In addition, the high bit speed level in the slip phase can induce severe axial and lateral vibrations in the bottom hole assembly which can be damaging to the connections. Finally, it is likely that drilling with slip-stick motion leads to excessive bit wear and also a reduction in the penetration rate. It is therefore desirable to cure, or at least to reduce, the torsional vibrations in the drillstring. P. 277