Abstract
Understanding drill string dynamics is essential to improving drilling efficiency and preventing accidents. Previous studies have mainly focused on self-excited oscillations due to stick–slip and bit-bounce. However, in deepwater drilling, large weight-on-bit (WOB) fluctuations due to heave motion are a significant concern that can affect torsional dynamics. This article reports analytical, numerical, and field data investigations on torsional vibrations of a drill string in deepwater drilling. First, we analyzed the field data from a deep-sea scientific drilling vessel Chikyu. The field data showed that heave, WOB, and torque oscillated at similar frequencies. This result indicated that the drill string could have forced torsional vibrations due to heave instead of self-excited vibrations due to stick–slip. Second, we analytically and numerically investigated drill string torsional dynamics. The torsional vibration of the drill string can be described only by self-excited vibration due to stick–slip if WOB is constant. However, when the WOB fluctuates, the forced vibration of the WOB must be considered. The results showed that vibration at the heave frequency had a more significant effect on torsional vibration than the axial self-excited frequency for the same amplitude of WOB variation. In addition, large WOB fluctuations increased the risk of stick–slip and reverse rotation of the drill bit. Numerical experiments with the field data showed that the forced torsional vibration caused by heave motion was predominant in deepwater drilling. These results show the importance of capturing forced torsional vibrations and reducing WOB fluctuations.
