Modeling and Analysis of Non-Rotating Damping Subs for Removing Torsional Vibrations in Drilling

Abstract Drill string vibrations are a frequent cause of reduced drilling rates as well as sustained damage to bottom-hole assembly (BHA) elements and downhole tools during drilling operations. Stick-slip oscillations, in particular, have been observed to occur in field operations, not only while drilling but also while rotating or reaming with the bit off bottom. Several solutions for mitigating drill string vibrations through active control of the top drive speed or torque have been developed over the past decades. These have been successful to a certain extent but have a limited impact in longer wells where one faces large side forces. Another existing solution uses non-rotating drill pipe protectors to reduce the mechanical friction between the drill string and the formation. Hence, the excitation source for the torsional vibrations is reduced, but, at the same time, larger axial drag forces are induced. The distributed nature of the sources of excitations and negative damping also requires that the drill string vibration problem is addressed in a distributed fashion. On the one hand, it is desirable to reduce the impact of those sources of excitation and negative damping. On the other hand, increasing viscous damping can naturally damp out vibrations at their inception. Based on this principle, a passive sub using a non-rotating sleeve and introducing viscous friction has been designed. At the local level of the damping sub, axial and torsional motions are decoupled in order to attenuate vibration excitations caused by the transfer of mechanical friction between axial and tangential sliding. The damping sub outer diameter is slightly larger than the outer diameter of the surrounding tool joints to support the local weight on an element that presents lower mechanical friction forces than the neighboring contact points. Finally, the damping sub introduces decoupled viscous friction forces in both the axial and rotational directions so that incipient vibrations get quickly damped out. Several damping subs can be spread along the drill string to augment the damping effect. This novel concept is investigated through simulations with a transient axial-torsional drill string model with distributed interaction with the wellbore modelled using Coulomb friction. The damping subs are included in the model at selected locations along the drill string. The model is completed by boundary conditions represented by the top drive dynamics and the bit-rock interaction. Case studies representative of field operations are simulated with different damping sub positions along the string to evaluate the resulting reduction in torsional drill string vibrations and mechanical friction during combined axial and rotational motion. The simulation results show that the viscous damping provided by 2–4 damping subs, placed at the locations where high side forces are expected, can effectively remove torsional oscillations and reduce the top drive torque. These results indicate that use of the proposed damping subs can reduce damage to downhole tools, improve drilling performance and also achieve lower energy consumption by the drilling rigs.