Case Study: Reactive Torque Failure Prevention

摘要

Reactive torque failures occur when an underreamer or hole opener stalls suddenly. The right-handed momentum in the components underneath the stalled tool applies a left-handed torque to the connections. If this left-handed torque is high enough, the connection can back off. In fact, an operator had a reactive torque failure in just this manner. We study that failure as an example in this paper, outlining the way to prevent it in the future. In short, the makeup torque applied to the connection must remain higher than the left-handed torque that the tool might see. While simple enough, the complication comes when trying to calculate the magnitude of left-handed torque under a stalled underreamer. The simplest method is to apply an energy approach when calculating the left-handed torque. This is straightforward mathematically, and in most cases it is conservative due to the lossless assumptions that are used. Unfortunately it is becoming common that something more sophisticated is necessary. This is due to the fact that vibrations are typically ignored in a simple energy model. Since in many holes friction will quickly damp out any vibrations caused, this often does not cause problems. However, if reaming a very big hole— as was the case in our study, and is often true of the larger hole sizes in deepwater operations—the friction may be low enough that torsional vibrations will play a significant role. In order to calculate the reactive torque that might be applied in these latter instances, a numerical approach must be employed that considers the torsional shock wave applied to the components underneath the stalled hole opener. This paper considers these different mathematical approaches, particularly as they were applied to both the reactive torque failure and the resulting preventative design processes.