Research On Coupled Vibration And Stability Of Drilling System

As a kind of normal directional drilling equipment for underground coal seam,drilling rig has obvious advantages in drilling construction.However,in actual drilling process,due to the geological characteristics of the soft coal seam and the regenerative cutting action of the drill bit,the drill string system has complex coupled vibrations of axial-torsional-lateral direction.Coupled vibrations will cause the occurrence of stick-slip vibration,which will destroy the quality of the hole and reduce the drilling efficiency,it also can lead to serious accidents such as drills broken,drills stuck and collapsed holes.Moreover,the flow characteristics of the drilling fluid can also cause instability of the drill string system.Therefore,this paper takes the drill string system in coal seam drilling as the research object,studies the characteristics of axial-torsional-lateral coupled vibration,and stability of drilling system,and finds the method of control and avoid self-excited vibration.The main work and conclusion are as below:(1)In this paper,the discontinuous cutting model of the single tooth for drill bit is established by the method of non-uniform toothing along the contour curve of the drill crown.The dynamic equations of the axial-torsional coupled vibration are obtained by refining the force of the single tooth of the drill bit.When the rotation speed of drill string is low and the drill bit is under the high drilling pressure,an abnormal self-excited vibration,like stick-slip vibration,is often generated.By reducing the axial stiffness ratio,axial damping and friction damping,as well as increasing the axial-torsional frequency ratio,torsional damping and process damping,the occurrence of stick-slip vibration can be avoided,thereby improving the stability of the system.(2)If the drill string rotates at a high speed and the bit weight is low,the lateral cutting action of the drill bit causes lateral vibration.At this time,a coupled vibration model considering lateral vibration should be established to analyze the stability of the drill string system.This paper focuses on the lateral force of the bit when the tooth collides with the rock wall,and the dynamics equation in four degrees of freedom are obtained.The stability of the drill string system can be improved by reducing the axial stiffness ratio,the torsional-lateral frequency ratio,the lateral stiffness ratio,the axial damping,the frictional damping,and increasing the torsional-axial frequency ratio,torsional damping,and process damping.And the effect of lateral damping on the stability of the system can be ignored.(3)Considering the flow characteristics of drilling fluid in the drilling string system,a solid-liquid coupled dynamic vibration model is established which contains drilling fluid.The vibration equations of the beam are obtained with the reaction of the damping and buoyancy of the drilling fluid.The nonlinear partial differential equations are solved by Hamilton's principle and finite difference method.The research results show that the drilling fluid has a certain influence on the lateral vibration of the system.Increasing the density and flow rate of the drilling fluid plays a stable role in the drill string system.In this paper,the study on coupled vibration and analysis on system stability of drilling string system can provide reference for the stability prediction of coal drilling and the dynamic optimization design of the drill string structure.In order to suppress the self-excited vibration,the deep hole forming rate of the coal seam is improved,thereby ensuring the safety and high efficiency of coal mining.