Observer-based output-feedback control to eliminate torsional drill-string vibrations

Torsional stick-slip vibrations decrease the performance and reliability of drilling systems used for the exploration of energy and mineral resources. In this work, we present the design of a nonlinear observer-based output-feedback control strategy to eliminate these vibrations. We apply the controller to a drill-string model based on a real-life rig. To facilitate the design and implementation of the controller, we employ model reduction to obtain a low-order approximation of this model. Conditions, guaranteeing asymptotic stability of the desired equilibrium, corresponding to nominal drilling operation, are presented. The proposed control strategy has a significant advantage over existing vibration control systems in current drilling rigs as it only requires surface measurements instead of expensive down-hole measurements and can handle multiple modes of torsional vibration. Case study results using the proposed control strategy show that stick-slip oscillations can indeed be eliminated in realistic drilling scenarios.