Control of Distributed Damping Subs for Torsional Vibration Mitigation in Drilling

Abstract

Drill string vibrations are an important cause for extended costs in drilling. The field of vibration mitigation therefore has been very active. Two main approaches exist, either surface or downhole mitigation methods. This thesis focuses on the control of a downhole vibration mitigation tool. The viscous damper is based on eddy current braking. A magnet is fixed to a nonrotating sleeve around the drill pipe, covered in a non-magnetic, conductive material. The relative angular movement between those two induces damping forces. The damping coefficient can be actively controlled by changing the position of the permanent magnet. Under certain circumstances, the sleeves can slip, possibly harming the borehole wall, casing, or the sleeve itself. Several damping subs can be used together, distributed along the drill string. This work is based on simulations with an axial-torsional drill string simulator. In the first contribution, we evaluate if active sleeves - as opposed to passive sleeves (where the magnet is fixed to one position) - are worth further investigation. For this, the controller is introduced and stability maps are created covering a broad range of operating conditions (feed rate and RPM). We found that active sleeves are superior to passive ones if only a few sleeves are used. Building on this, some simulation assumptions are relaxed to make it more realistic. Without communication with the surface, the setpoint is unknown to the sleeves. A downhole setpoint estimation is added to cope with this. Also, measurement errors are introduced. A parameter study on the measurement errors showed the effects of its different components on the controller. We showed that the controller can reduce the vibrations also with reasonable measurement errors being present. From this point, two different ways were chosen. Noticing frequent slipping of sleeves, we introduce an anti-slip logic. This works as an adaptive saturation on the controller output, reducing the applied damping force, and with this the harmful slipping motion of the sleeves, while the performance in dampening pipe vibrations is kept up. To adapt to different friction forces along the wellbore, an exploration strategy is needed. The results show that the controller performance can be kept at a similar level, while possible damages due to slipping can be reduced with the anti-slip logic. The second path led to evaluating if the sleeves can also be beneficial when there is already a stick-slip mitigation system in place at the top drive. For this, the stiff top drive controller is replaced by a tuned PI controller. Here the effects of both methods are investigated at different bit depths. We figured that a combination of both mitigation methods can be beneficial in wells with high side forces.