The Characteristics And Stability Analysis Of Self-excited Vibration Of Drilling System Of Coal Seam Rig

Coal seam rig is widely used in coal mine gas extraction due to its unique advantages.In actual drilling,the drilling system will generate self-excited vibration due to the characteristics of the soft coal seam and the regeneration cutting of the drill bit.Self-excited vibration includes axial vibration,lateral vibration and torsional vibration which can destabilize the drilling system,reduce the hole quality and drilling efficiency,and lead to serious accidents,such as the crushing of the drilling tool and the drill and the collapse of the hole.Therefore the drilling system of coal seam rig is taken as the research object to study the characteristics and linear stability of self-excited vibration in this paper,find the control method and avoid the self-excited vibration.The main work and conclusions are as follows:(1)The axial-torsional coupling model of drill string system is established.Considering the regenerative cutting of drill bit,two delay models-state dependent delay(SDD)and normal delay(CD)are established.First,the stick-slip vibration characteristics of SDD model at low rotational speed(0?10rad/s)are analyzed.The model shows a typical stick-slip vibration at ?rot=5 rad/s,manifested as the accumulation of torque during the viscous period and the sudden change of velocity during the slip phase.As the system input speed increases from 5rad/s to 10rad/s,the stick-slip vibration will gradually disappear.Comparison of SDD model and CD model at 50rad/s,the characteristics of the SDD model are more accurate than the CD model.At the same time,different input speed of the system will have a great impact on the model.(2)Linear stability analysis of the coupled axial-torsional model was carried out.In the case of small control parameter ?,there is not much difference between the CD model and the SDD model,but with the increase of the control parameter p,the stability interval of SDD model gradually increases and is obviously higher than that of the CD model.The pure axial vibration,pure torsional vibration and axial torsional coupling vibration of the system were analyzed respectively.The results show that the axial vibration plays the role of stabilizing the torsional vibration.In the process of linear stability between the control parameter p and the input speed ?,it is found that the low-speed instability mode of the system is stick-slip vibration.The high-speed instability mode of the system is bit bounce.(3)Hopf bifurcation of axial-torsional coupling model considering SDD and CD were analyzed using multi-scale method.Under the CD model,there only exists subcritical Hopf bifurcation in the system.Under the SDD model,the Hopf bifurcation of the system depends on the control parameter ?,with the increase of p,the subcritical Hopf bifurcation point on the right side of the stability lobe map will be gradually transformed into the supercritical Hopf bifurcation point.Through the study of the instability amplitude of SDD model,it is found that the amplitude of stick-slip vibration of drilling system increases with the increase of cutting depth.Under the same parameters,higher speed drilling systems require greater depth of cut for destabilization.(4)The lateral vibration of the drilling system is analyzed,and the whirl model of the drill string and the axial-torsional-lateral coupling model considering the lateral cutting of the drill bit are respectively established.It is found that there are mainly three stages in the whirl of drill string.The influence of the fluid force and contact force on equilibrium of the drill string was analyzed.The results show that the discontinuous normal contact force and tangential contact force between the drill string and the hole lead to the occurrence of saddle-knot bifurcation in the system equilibrium point.The fluid force is the main reason for whirl.By contrasting the axial-torsional-lateral coupling model and the axial-torsional coupling model,it is found that the dominant factor in the cutting force and frequency components of the model is still the axial vibration.However lateral vibrations with lower amplitude and frequency than axial vibrations will make the system extremely unstable.The self-excited vibration characteristics and stability are studied in this paper,it can provide reference for the prediction of drilling stability and the dynamic optimization design of drill string structure.From the perspective of suppressing self-excited vibration to improve the deep hole porosity,and then ensure the safety and high efficiency.of coal mining.