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

Abstract

Abstract-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 estrategies are evaluated in order to reduce stick-slip oscillations. These estrategies will use 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 supervisory task is made by the driller.

Sliding-mode control of a multi-DOF oilwell drillstring with stick-slip oscillations

“…The model used here considers the drill pipe and drill collars dynamics. It is more general than the torsional lumpedparameter models of one and two DOF [1]- [9], [12] previously proposed. The bit-rock contact is modelled by means of a dry friction combined with an exponential decaying law, which introduces the discontinuity in the open-loop system.…”

“…For example, [4], [7], [10] proposed a vibration absorber at the top of the drillstring (referred to as soft torque rotary system). A classical PID control structure at the surface is used in [6], [11], [12]. More sophisticated techniques are used in [3] and [9] where a linear quadratic regulator and a linear H ∞ control are used, respectively.…”

Suppression of Drill-String Stick-Slip Vibration

“…This class of models has been widely employed in studies focusing in drill-string torsional vibration. The number of disks varies in those studies, for example 2 disks [8], 3 disks [9], 4 disks [10] and even 18 disks [11]. In the current work a 2-disk model has been employed (see Fig.2(b)) to model the CADR experimental drilling rig, following the work presented in [8].…”

Proportional-Derivative Control of Stick-Slip Oscillations in Drill-Strings

“…The slenderness of the drill pipe makes it prone to exhibit undesired oscillations, including bit bouncing, lateral oscillation, torsional oscillation, and whirling motion. In oil and gas drilling, stick-slip behavior exists in 50% of drilling time [1], and the whipping and high rotation of the drill bit may cause bit bouncing and whirling motion [2]. Moreover, formation change is also very complex during drilling procedure, so the interactive model between rock and bit is highly nonlinear [3].…”