Study On Efficient Rock-Breaking Mechanism Of Compound Percussive PDM In Underground Coal Mine

Coal mining is developing more and more deeply,and the problem of water inrushing from floor is becoming more and more serious.Due to the advantages of controllable and adjustable track,far extension of borehole and convenient centralized management,the near-horizontal directional drilling has gradually become an effective way to prevent and control water disasters in coal mines.Due to the needs of water prevention and control of water disasters,directional drilling is usually carried out in hard rocks.At present,the equipment and tools commonly used in underground coal mines can not the requirements of directional drilling in hard rock enough.Specifically,the mud pump capacity is insufficient,the output energy of optional PDM’s is low,and the hard rock layer accelerates the wear of the drill bit,which is prone to "stick-slip" phenomenon resulting in the bit jumping teeth.Although a lot of people has been done on conventional speed-up drilling of hard rock at home and abroad,there are few researchers on nearly horizontal directional speed-up drilling of hard rock in underground coal mine.Aiming at the construction problems of low drilling efficiency in hard rock and serious backing pressure caused by nearly horizontal directional drilling,the author of this paper through the application using the rock mechanics,computational fluid mechanics,mechanics of materials,drilling engineering,engineering machinery,such as the theory,method and technology,developed a compound percussive PDM(impact torque and axial percussive PDM),axial percussive PDM hydraulic characteristics are analyzed,The mechanism of drag reduction and pressurization by axial vibration was explored,the dynamic characteristics of compound percussion were studied,the rock breaking mechanism of compound percussive-rotary was revealed,the purpose of improving the efficiency of directional drilling of hard rock was realized,and the development of underground coal mine drilling technology was promoted.First by analyzing the axial percussion and circumferential twist rushed to speed up rock damage principle,the motion law of the PDM,reveals the working principle of compound percussion acceleration of broken rock,the design is suitable for the coal mine underground directional drilling with compound percussive PDM(torsional impact and axial percussive PDM design),expounds its working principle and hydraulic parameters are computed.Based on the working characteristics of the axial impact screw,the water shock wave equation was derived,and the water shock wave model of axial vibration was established.Combined with axial percussive PDM’s valve port area,mud pump,complex boundary conditions,such as density,flow coefficient,using Matlab programming to solve the results show that the pressure wave within the drill pipes through one-way valve port,the initial reflection,stable superposition state after three achieve stability,Waveforms show positive(cochord)changes;The calculation results of static and transient water hammer pressure difference are compared and analyzed,and the transient calculation model takes into account the influence of pressure wave on flow rate,which has higher accuracy.The fluctuation pressure difference decreases in the form of power function with the increase of valve radius.The fluctuating pressure difference increases linearly with the increase of flow rate.According to the actual working conditions,the optimal value of valve radius and fluctuating pressure difference is determined.Secondly by analyzing the characteristics of compound percussive PDM’s axial vibration compound,under the condition of nearly horizontal drilling tool dynamics model is established,combined with the upper and lower boundary conditions,and the changing rule of the valve port,programming and solving the vibration,the results show that the vibration force produced by vibration sub,the upper tool for traction,the impact of lower composite screw for the axial dynamic pressure,The principle of drag reduction and pressurization by axial vibration is revealed.The results show that there are four stages in the contact between bit and bottom hole: static stage,crawling stage,collision and slow advance stage,and steady pressure drilling stage.The superposition of static load and axial dynamic load in the steady pressure drilling stage is proposed,which is beneficial to improve the drilling efficiency of hard rock.Finally,based on ABAQUS /Explicit display algorithm,a three-dimensional simulation model of compound percussion rock breaking is established.The problem of compound percussion rock breaking simulation is solved by using PDM constant power output as the basis and superposing half sinusoidal impact signal on the output torque result as the idea.The results show that the maximum volume,the maximum stress and the maximum axial displacement of compound percussion cutting are obtained under the optimal parameters,followed by static load + axial percussion,static load + torsional impact,and the minimum value of static load.By comparing and analyzing the calculation results of the optimal and the worst working conditions,it is concluded that appropriate drilling parameters and drilling methods can improve the drilling efficiency of hard rock.Through field tests,the effect is remarkable,which verifies the research results on the efficient rock breaking mechanism of compound percussive PDM in underground coal mine.