Research On Dynamics And Optimization Method Of Mechanical Stable Platform In Automatic Vertical Drilling Tool

Boreholes,with high vertical degree,can be drilled with automatic vertical drilling tools,by which the disadvantages of traditional deviation prevention techniques are solved and the demands of modern vertical drilling are satisfied.In these vertical drilling tools,without any electronic parts and components,the deviation control of fully mechanical vertical drilling tools(MVDT)is relied on their mechanical stable platform,in which an eccentric block can be deflected and held on the lower side of borehole under the effect of gravity.Owning to the advantages of lower cost,simple structure,high temperature resistance,high reliability and high suitability,MVDT has become one of the most important research hotpot and development direction in the field of vertical&deep well drilling.However,compared with electric control vertical drilling tools,the precision and limitation of deviation control of MVDT is relatively low.Therefore;the control performance of mechanical stable platform of MVDT need to be improved urgently.In this paper,two working modes of MVDT,dynamic push mode and static push mode,are analyzed in detail.During the control process,the friction torque of dynamic push mode is relatively small in contrast,which is benefit to design a mechanical stable platform with better control performance and deviation precision.According to the characteristics of dynamic push mode,the well straightening process of MVDT is carried out by periodically pushing the higher side of the borehole wall,the push force during one period can be simplified as equivalent pushing force.With continuous beam column theory and formation-bit action model proposed by H-S.Ho,the analytical method of reliability and well trend of dynamic push mode are established respectively by using pushing force and equivalent pushing force.The results show that,the mechanical strength of MVDT is satisfied with drilling requirements,during inclination decreasing process,the MVDT should be mounted with a drill collar and a bit with higher side cutting ability to improve its deviation-drop ability,but during straightening drilling process,a bit with lower side cutting ability should be seleceted to obtain higher verticality.As the most critical component of MVDT,the design of mechanical stable platform is directly determined the performance of the drilling tool.The working mechanism of the mechanical stable platform is that,the eccentric torque of eccentric block can overcome the resistant torque of plate friction and bearing friction,then the eccentric block can rotate to the lower side of borehole and hold the open of plate valve on the higher side of borehole.According to statistic mechanical analysis,the cross section of eccentric block can be design as half round shape to obtain larger eccentric torque,due to the effect of friction,a critical deflection angle is exist between the stable position of eccentric block and the theoretical lower side of borehole.Compared with the friction of bearings,the friction of plate valve,induced by the pressure different of drill fluid,presents far more influence on the deflection behavior of eccentric block.When the torsional vibration occurs,the rotation speed of bottom tools changes with time,the friction coefficient of plate valve will be changed between dynamic and static,as well as the power of friction changes.As a result,the dynamic response of mechanical stable platform will presents complicate behavior.The dynamic response of mechanical stable platform can be analysis by lagrangian approach,the kinetic parameters of torsional vibration is fixed by self-excited vibration model and field drilling data.The results show that,the deflection behavior of eccentric block is a kind of single attenuation motion without torsional vibration,if the torsional vibration occurs,the behavior of eccentric block can be change to a kind of pendulum-like motion.The more the amplitude of torsional vibration,the more the amplitude of the swing motion of eccentric block,and with the period of torsional vibration increases,a subordinate period gradually appears in one period of the pendulum-like motion.Four typical motion of eccentric block are extracted to describe the deflection behaviors,under the effect of torsional vibration,the motion of eccentric block will be change between these four typical behaviors.When the maximum eccentric torque of eccentric block is small,under the torsional vibration with large amplitude and short period,the motion of the deflection would be chaos.The control performance of mechanical stable platform can be detrimentally influenced by torsional vibration with large amplitude and high frequency.Based on the effect of torsional vibration above,just considering the constant speed of BHA is not enough during design,which can be optimized by using lead heavy eccentric block,convex plate valve and unidirectional hydraulic turbine.The results show that,the lead heavy eccentric block can meliorate the flow field of inner tool and decrease the resistance force of rotation.The convex plate valve can effectively decrease the friction torque of plate valve under the permission of small leakage,and the height of convex must be less than 0.5mm to ensure the leakage is small enough.The torque of unidirectional hydraulic turbine under the flushing of drilling fluid can partially or entirely counteract the friction,by which the critical angle between the stable position of eccentric block and the lower side of borehole can be substantially decreased.After optimization,the dynamic response of mechanical stable platform is significantly improved.For obtaining the optimum value of hydraulic turbine torque,a series of rig text of dynamic friction torque of bearings is carried out.With considering the optimal torque of plate valve,a unidirectional hydraulic turbine,with the torque is 110 N·mm under the rate of flow is 30L/s,should be selected as optimal component.Through texting the pushing response of MVDT in laboratory,the optimizing effect is evaluated.The results show that,after optimizing,the control precise of mechanical stable platform is improved from 8° to 2.5°,when the rotation speed is low,the control precise could reach to 1.6°,which is an ideal optimizing effect.This means that the MVDT is satisfied with the demand of field application,which creates a good foundation for downhole testing.