| Stick-slip vibration widely exists in deep and ultradeep wells exploration and development.It not only causes mechanical drilling speed lowered and drive power wasted,but also speeds up the aging and failure of drilling tools,which seriously threaten the safety of drilling processes.In recent years,foreign scholars have made breakthrough in the research of drilling stick-slip vibrations,and have developed soft torque control system successful in application.In contrast,domestic research in the field is just at beginning on both theoretical investigation and engineering application.Therefore,stick slip vibration is an imminent problem for our country's oil/gas exploration engineer,and it is urgent to conduct deep investigation on this subject.This dissertation focuses on the stick slip vibration of drilling system.Firstly torsional vibrations nonlinear model of drillstring system is established and nonlinear vibration mechanism is studied deeply.Then the cause analysis of stick slip vibration of the drill string system is carried out,and mechanism of drillstring stick slip vibration induced by whirl motion rubs is investigated.Further,robust control algorithm for inhibiting stick slip vibration is studied in theory.Several control strategies including PID controller based on damping optimum criterion,integral adaptive SMC(Sliding Mode Controller),H? controller and ?controller are investigated and designed in detail,and the optimal control method is found out from these controllers.Finally stick slip vibration control system with independent intellectual property rights is developed in this dissertation.The research work is as follows:1.On the basis of drill string stick-slip vibration model,the formation mechanism of drill string stick-slip vibration is revealed.Two-mass and multi-mass pendulum model of drillstring system are established repectively according to the differences of lumped mass partition granularity.In accordance with different work modes of actual drilling operations,time domain simulation of these two models is carried on.Results show that,drilling system is a nonlinear system with dead-zone characteristics and friction properties.Furter analysis concludes that drilling system is of performance vulnerability and it is necessary to keep the system stable by using controller.The relative torsion angle motion equation of drillstring system is built,and the relationship between system energy and equivalent damping force working under different top drive speed is studied.Equilibrium state differential equation of system simplified model is established,and solution of the equation is obtained by analytical methods under initial conditions.System parameters of most importance include systemdamping ratio,balance-point speed and bit slip and stick friction torque difference etc.Influence of these parameters on stick-slip vibration formation is analyzed in detail.Simulaton results show that negative damping characteristic of equivalent damping force is the cause of producing stick slip vibration.In the case of low damping ratio,low rotation speed and large static-dynamic friction torque gap the drilling system can form stick slip vibration more easily.2.By investigating mechanism of drillstring stick slip vibration induced by lateral vibration rubs deeply,formation mechanism of stick slip vibration while drilling loose stratums is brought to light.Based on D'Alembert's principle,considering various external forces,such as the viscous force,the restoring force,the friction force and the gravity,the dynamic model of stabilized drill collar is established in polar coordinates system.Forward,backward and chaotic whirl motions of the drill collar are analyzed.Emphatically,Rubs of backward whirl is investigated.When drillstring suffers periodic rubs,and satisfies the conditions:(1)rubbing frequency is close to or equal to the natural frequency of torsional vibrations,(2)the system actual damping is less than the critical damping,it will lead to a zero rotation speed of the bit,and stick slip vibration is formed.Further simulation results show that the change of rotary speed and length of drillstring gives rise to change of rubbing frequency and torsional natural frequency,and stick slip vibration can be produced only when the above two conditions are met.3.A study on the control method of drillstring stick slip vibration based on observer is carried out,and a kind of PI(Proportion and Integration)disturbance observer design based on damping optimum criterion is proposed.As for the problem that states of drilling system can not be observed directly,two kinds of PI disturbance observer are designed.Effectiveness and convergence of the observer is verfied by simulation.Based on the observer,coupling-torque feedback PID controller and full states feedback PID controller are designed,and parameters of the controllers are determined by damping optimum criterion.Theoretical analysis and simulation results show that stability can be ensured for both controllers,and stick-slip vibration suppression and steady speed tracking can be realized.And,both controllers have certain robustness.Also based on the observers,two kinds of integral adaptive SMC(Sliding Mode Controller)method is designed: one-order integral adaptive SMC and two-order integral adaptive SMC.Stability of both controllers is verified by using Lyapunov function methods.Effectiveness and convergence of both controllers are proved by theoretical analysis and simulation experiments.These two controllers can guarantee stability of the system,and realize steady speed tracking and inhibition of drillstring stick slip vibration.4.In order to construct a controller without an observer,a robust control strategy based on no observer is designed for various perturbation factors in the drillstring system.Robust stability and robust performance of the proposed control strategy is designed to guarantee the closed-loop system stable operation according to the designed performance.Under comprehensive consideration of many uncertainties of the actual drilling system,augumented model is established.Based on this model,H? controller and ?-synthesis controller are obtained.Performance of the two controllers is simulated and compared.Results show that,?-synthesis controller has better control performance and robust performance while robust stability of both controllers is satisfied.5.On the basis of study on control methods based on observer and no observer above,the optimal control method is found out.By comparing the performance of controllers proposed in this dissertation,time domain simulation is carried out from several aspects including different initial input torque,multiple parameters uncertainties,disturbances and different tracking speeds and so on.Performances,stability and robustness of all controllers are analyzed comparatively.Comprehensive results show that the second-order integral sliding mode controller can restrain stick slip vibration by tracking given speeds,and has good robustness,moderate overshoot and adjusting time.So it can be used as a control strategy in the stick slip vibration control system.6.The overall planning,hardware framework and software design of stick slip vibration control system is studied,and system performance testing has been carried on in the well field.Results show that the system follows the changes of transducer rotary speed and torque,and has good damping effect on stick-slip.Furthermore,the control algorithm can be modified online according to the parameters of real time operating conditions. |
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