Analysis Of Laws Of Drill String Stick-slip Vibration And Soft-torque Control

In the actual drilling process, the interaction of the drill string and borehole wall or thebottom will trigger a series of unpredictable vibration problems, result in the bit sticking withthe stick-slip，affect the stability of the wall seriously, threaten the safety of undergroundoperations, lead to failure or even break drill, bring direct economic losses. As the drill stringlength increases, torsional stiffness will be lowered, in addition the underground frictiontorque, makes it very difficult to turn drill underground, thereby creating a stick slipphenomena. As the drill string increases, the torsional stiffness will be reduced and the dragtorque will be grated, these factors will make the underground drill string rotation verydifficult and cause the phenomenon of stick-slip. Stick-slip phenomenon is a movementbetween complete adhesion contact and sliding contact (ultimate rubbing contact). Displayingin the drill string drilling process, this phenomenon is a stick-slip vibration. This vibrationcan be explained that the bit is motionless for some time, and when the torque on drill stringis large enough, the bit will rotate at a high-speed. The stick-slip vibration of the drill string isan intensive low-frequency vibration, and the instantaneous speed of bit is high during thevibration, which maximum value can be even more than2times of the turntable. During thedrill string stick-slip vibration, the bit speed can instantly achieve to a great value, which islikely to cause the drill string damage. In addition, the torque fluctuates very greatly in thisprocess, which not only affects the efficiency of drilling operation, but also threats thedrilling safety, and the too large actual torque may exceed the limits of the device, resultingin the drilling operation not be carried out.For elimination of stick-slip vibration in the drilling process of drilling performance ofthe entire drilling system, many experts and scholars have done much research, addressingprimarily the stick-slip vibration in drilling systems modeling and control issues. The paperbuilds mathematical model of drill string stick-slip vibration, derives motion equation of thedrill string, sums general rules of stick-slip vibration of drill, furthermore proposes thecontrol program of drill soft-torque control system. According to the current state of thedrilling system, soft-torque control system automatically improves drilling conditions.The purpose is to curb the harmful vibrations in the drilling process and inhibit sliding drilling,sticking, thereby to protect the drill and improve drilling efficiency.Through detecting the torque and speed of the motor in real time, soft-torque controlsystem forecasts running trend of underground parts in advance, and determines whether itneeds compensation and the size of compensation through the predicted trend of thesystem.Compensation depends on the current drill, drill bit, bottom hole assembly and therotary table/top drive and other data, and forms a speed compensation signal as a systemfeedback compensation. Under the action of the feedback compensation, motor torque getschanged. Improved torque makes the bit rate stay away from stick-slip region, therebyeliminating the drill bit sticking and inhibiting bit stick-slip phenomenon. On the controlmethods to overcome the technology difficult to achieve detection of drill status, throughexisting data and to detect data estimate bit status, incorporate drill into the scope ofcontrolled object indirectly, achieve closed-loop control of the realization of the drill bit.After experimental study on contrast, this paper proposes soft torque control methods that areused to change the actual changes in speed with torque motors, thus reducing motor torquefluctuations, eliminates stick-slip vibration effects, enabling soft torque control of drill pipe...