D-OSKIL: a New Mechanism for Suppressing Stick-Slip in Oil Well Drillstrings

Limit cycles occurring in oil well drillstrings result from the interaction between the drill bit and the rock during drilling operations. In this paper we propose to use the weight on the bit (WoB) force as an additional control variable to extinguish limit cycles if they occur. In particular we adapt the oscillation killer (OSKIL) mechanism studied in detail in the companion paper [3] to our problem at hand. An approximate analysis based on the bias describing function and completed with some simulations, provides good evidences that the rotational dynamics of the oil well drillstring display such a behavior. Simulations applying the D-OSKIL (D stands for Drilling) mechanism show that the stick-slip oscillations can be eliminated without requiring a re-design of the velocity rotarytable control.