Torque Feedback Used to Cure Slip-Stick Motion

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

Controlling torsional vibrations of drill strings via decomposition of traveling waves

“…In order to eliminate vibration energy from the string, the reflection coefficient of the top drive should be small for the relevant stick-slip frequencies. In [5,6,18], control concepts are presented, which achieve a low reflection coefficient for a certain frequency band.…”

A Systematic Approach to Performance Drilling in Hard Rock Environments

“…STRS technology modifies the top drive operating characteristics by introducing compliance (k f ) and damping (c f ), as shown in Figure 15. As torsional waves propagate along the drillstring, the 'soft' operating characteristic absorbs the energy and prevents the development of stick-slip (Halsey, et al, 1988;Sananikone, et al, 1992;Jansen, et al, 1995). The first STRS systems were introduced decades ago, and while some notable successes were achieved (Javanmardi, et al, 1992), the advent of alternating current (AC) top drives made the original implementations less effective.…”

A Field Method for Controlling Drillstring Torsional Vibrations

“…The stick A FIELD METHOD FOR CONTROLLING DRILLSTRING IADCISPE 23891 TORSIONAL VIBRATIONS phase of a stick-slip oscillations increases the probability of having a stuck pipe or a twistoff; while the slip phase is damaging to the bit because the maximum bit speed can reach as much as two to ten times the speed at the surface (1,2,3). Moreover, stick-slip oscillations can cause the bit to spin backwards (3,4) and; i destroy a PDC bit 1 unscrew the bottom hole assembly (BHA) downhole induce lateral and axial vibrations which are harmful to the BHA and the downhole equipment Clearly, it is very desirable to eliminate or at least reduce the DTV's; furthermore, eliminating DTV's also eliminate stick-slip oscillations which results in a smoother rotation of the bit (1).…”

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Torsional vibrations are detrimental to the life of drillstring components (1,2,3,4). Although methods of controlling them have been proposed (1,3,4), their general dependence on strain gauges to measure drillstring torque make them unsuitable for the drilling environment.

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