Stick-slip oscillations of drag bits by considering damping of drilling mud and active damping system

摘要

This paper studies self-excited stick-slip oscillations of a rotary drillstring with a drag bit.In previous researches coupling between torsional and axial vibration was studied for a system with two degrees of freedom and without considering the effects of rotary table,damping of drilling mud and active damping system.In this research,for the first time a complete model is considered.This model is a discrete model with two degrees of torsional freedom and one degree of axial freedom.Both frictional contact and cutting process at bit-rock interface have been considered by bit-rock interaction law.There is a coupling between torsional mode and the axial mode through this law.In this paper,the effects of damping,active damping ratios and bit-rock interaction number have been studied on stick-slip motion.The equations of motion have been solved by numerical method.The technique of solution is Euler-forward finite difference technique.This research shows that by increasing the damping of drilling mud,bit parameter and by appropriate selection of active damping ratios,stick-slip oscillation of bit can be avoided and oscillation of rotary table be damped.