Self-Attenuation of Drillstring Torsional Vibrations Using Distributed Dampers

Abstract

Summary During drilling operations, drillstring vibrations cause many downhole dysfunctions, resulting in underperformance, equipment failure, and possibly wellbore damage. Current drillstring vibration mitigation solutions are generally located at a single site, either at the topdrive or close to the bit. However, because the sources of vibration excitation are distributed along the whole drillstring, these single-point vibration damping solutions do not succeed in removing all vibration modes. A distributed vibration damping solution is investigated, wherein damping subs are placed at intervals along the drillstring. The drillpipe rotates on bearings inside the damping subs, thereby reducing mechanical torque. The damping sub has a slightly larger diameter than the neighboring tool joints, lifting the drillstring off the borehole wall and thus eliminating most of the mechanical friction between the damping subs. Viscous friction is introduced in the form of rotary eddy current brakes within the damping subs, which cause the damping of torsional oscillations. The damping sub moves axially on spur wheels supported by bearings, thereby reducing drag forces. The decoupling of rotary and axial movement is a key element for removing a source of excitation. Simulations made with a 4n degree-of-freedom (DOF) transient torque and drag model confirm the expected results. The results of simulations for a use case based on a complex 3D trajectory are presented. With damping subs every 30 m and a damping coefficient of 20 N·s/rd, the topdrive torque is reduced to 40% of that of a plain drillstring, and torsional stability is obtained across the topdrive speed range of 60 to 180 rev/min. By comparison, conventional nonrotating pipe protectors (NRPPs) would have reduced the topdrive torque to 30%, but torsional stability would have not been achieved because the expected damping coefficient was less than 0.02 N·s/rd. The design of the damping sub relies on readily available technological components that can sustain very high pressures and temperatures. The damping subs are therefore compatible with high-pressure/high-temperature applications, including geothermal drilling.