Suppressing Axial-Torsional Coupled Vibrations in Drillstrings

International audienceIn drilling operation, a wide variety of oscillations causing failures often arise. Torsional vibrations (stick-slip) are originated by the cutting device (bit) motion, these vibrations in turn excite axial oscillations causing a phenomenon known as bit-bouncing. This paper addresses two important challenges: the modeling of the coupled axial and torsional dynamics in a vertical oilwell drilling system and the design of an effective controller to reduce undesirable behaviors. Through the D'Alembert transformation, the distributed parameter model of the drillstring is reduced to a neutral-type time-delay equation which effectively describes the oscillatory behavior of the system. The Lyapunov theory allows to develop an efficient strategy for the control synthesis guaranteeing the elimination of the stick-slip and bit-bounce. This approach leads the " practical " stabilization of the closed loop system. All results can be easily generalized to any time-delay system subject to bounded disturbances. The effectiveness of the strategy is validated through simulations