Abstract
Despite a significant research effort to understand and mitigate stick-slip in drill-strings, this problem yet to be solved. In this work, a comprehensive parametric robustness analysis of the sliding mode controller has hitherto been performed. First, a model verification and extensive parametric analysis of the open-loop model is presented. This is followed by a detailed parametric analysis of the sliding-mode controller based closed-loop system for two cases, (i) an ideal actuator with no delay or constraint and (ii) a realistic actuator with delay or/and constraint. It is shown that though the proposed controller works robustly across a wide range of parameters, in the absence of delay, it fails in the presence of a delay, thereby limiting its practical application. Experimental results are included to support these claims. This work underlines the importance of including the inherent system characteristics during the control design process. Furthermore, the parametric analysis presented here is aimed to act as a blue-print for testing the efficacy of relevant control schemes to be proposed in the future.
Highlights
Stick-slip vibration are characterized by phases in which a drill-bit comes to a complete standstill and phases in which a drill-bit rotates with much larger than nominal angular velocity
Due to several desirable qualities such as robustness against unpredicted dynamics, model inaccuracies, significant disturbance rejection, broad scope of parametric tuning which includes controlling the Weight On Bit (WOB) as well as having the knowledge of the bit velocity and a broad control bandwidth, sliding-mode controllers have shown a great promise in mitigating stick-slip issues
Several papers have adopted a similar 2-DOF model resulting in meaningful analysis and results, [14, 43]. This drilling system is driven by an electric motor and can be sectioned into two parts viz: (i) the top drive system modeled by the upper disc and (ii) the drilling pipe to the Bottom Hole Assembly (BHA) and the drill bit modeled by the lower disc
Summary
Stick-slip vibration are characterized by phases in which a drill-bit comes to a complete standstill (stick) and phases in which a drill-bit rotates with much larger than nominal angular velocity (slip). Controllers for drilling systems that are capable of maintaining drill-string rotation at a constant angular velocity and the mitigation of torsional (stick-slip) vibration are of huge interest Due to their simplicity, 1 or 2-DOF pendulum-type models are ideal for controller design [10]. Due to several desirable qualities such as robustness against unpredicted dynamics, model inaccuracies, significant disturbance rejection, broad scope of parametric tuning which includes controlling the Weight On Bit (WOB) as well as having the knowledge of the bit velocity and a broad control bandwidth, sliding-mode controllers have shown a great promise in mitigating stick-slip issues As such, they are some of the most popular control schemes found in literature to address this particular problem, [36, 37]. Details of the drill-string assembly and its derived mathematical model are described in the consequent subsections
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