Research On Coupled Vibration And Reducing Vibration Of Drillstring For Rotary Drilling Rig

A drillstring, an important power transferring assembly, secures a crucial component ofrotary drilling rig, and drillstring failure is considered as one of the serious faults. While thedrillstring is working, it is generally associated with excessive vibrations. The continuousvibrations could flaw or even crack the drillstring, and lead to the severe malfunction, sodrillstring vibration is regarded as the principal contributor of the fatigue failures. Therefore,obtaining a better understanding of vibration characteristic and taking strong measures toreduce the vibration have great significance in extending the service time of drill strings,promoting efficiency and enhancing economic performance. And by energy method and thefinite element method, the coupled vibrations of drillstring have been studied suitable for thedrilling and hoisting conditions. Based on the vibration characteristic, a vibration absorber fordrillstring has been designed and tests have been conducted to prove its ability of reducingvibrations. By conducting a research of coupled vibrations of drillstring for rotary drilling rig,the main results are as follows:1. Based on means of Hamilton variation principle as well as the finite element method,the dynamics equation of coupled axial, transverse and torsional vibrations had beenestablished for the drilling condition. Prior to the equation, the influence of the internal andannular drilling mud on the drillstring was analyzed by the boundary layer theory, besides,impacts of drillstring with the borehole wall were also taken into consideration. The dynamicsequation lays the theory foundation for the study of the drillstring vibration characteristic.2. By utilizing the Newmark-β method，the effect rules of the three parameters, includingthe push force generated by a pressurized cylinder, the torque provided by a power head andthe hole depth, were analysed through using MATLAB under drilling condition, and theoptimum rang of the three parameters was determined. The results show that When thedrillstring is totally stretched out and the torque of the power head is100kN m, theamplitude of the axial vibration will turn much greater while the amplitude of its transversalone will decrease as the pull force amplifies, and the optimum rang of the push force is from100kN to150kN; When the drillstring is totally stretched out and the push force is100kN, theamplitude of the transversal vibration will gradually amplify with the torque increasing, andthe optimum rang of the torque is from50kN mto120kN m; When the torque is120kN mand the push force is100kN, the deeper the borehole is, the bigger the amplitudeof the axial and transversal vibration will be, and the drillstring works in a better state withstretching out2or3sections. The results can provide references to the reducing vibration under drilling condition.3. The dynamics equation of coupled axial and transversal vibrations had beenestablished under the hoisting condition. Prior to the equation, the boundary layer theory isemployed to illustrate the influence on the drillstring imposed by the viscosity of the annulardrilling mud and the interaction between the string and borehole walls. Besides, thesimulation analyse on the influence imposed by the hoist force had been conducted, and theoptimum rang of the hoist force was determined. The results show that when the five sectionsof drillstring is hositing, the amplitudes of the drillstring vibrations tend to increase as thehoist force amplifies, and the optimum rang of the hoist force is from50kN to100kN. Theresults can provide references to the reducing vibration under hoisting condition.4. A vibration absorber for the drillstring had been designed, and basis for selecting themain parameters of the absorber was given. The dynamical models of axial vibration andtransverse vibration had been built respectively to carry out simulation analyses. As for theaxial vibraton, the effect of the absorber was maily analyzed in three typical situationsincluding not colliding between the system and the bottom of a borehole, colliding betweenthe system and the bottom of a borehole and subjecting to an external periodic force to thesystem, and the amount of instantaneous energy sink was calculated for the three typicalsituations respectively by Runge-Kuta method. Meanwhile for the transverse vibration, theeffect of the absorber was maily analyzed in the situation of subjecting to a transverse impactto the system and the amount of instantaneous energy sink was calculated. The results showthat the absorber produces good vibration absorption effect both for axial and transversevibration.5. An experiment had been established when layer interation of sandstore and mudstonewas drilling by rotary drilling rig. The acceleration signals were recored before and afterinstalling absorber in four typical conditons. One is when the power head doesn’t convergeand the pressurized cylinder doesn’t generate any pressure and the second condition is whenthe power head doesn’t converge and the pressurized cylinder generates pressure. The thirdcondition is when the power head converges and the pressurized cylinder doesn’t generate anypressure and the forth one is when the power head converges and the pressurized cylindergenerates pressure. And the decreasing rate of amplitude was put forward to describe thedamping effect. The results indicate the the absorber has better vibration damping effect.