ANALYTICAL AND NUMERICAL STUDY ON DRILL-STRING TORSIONAL VIBRATION UNDER WEIGHT-ON-BIT FLUCTUATION

Understanding drill-string dynamics is essential to improving offshore 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 fluctuations due to ship heaving are a major concern that can affect the dynamics in the torsional direction. This paper reports analytical and numerical investigations on the torsional vibration of a drill string under weight-on-bit fluctuation. First, the analytical solution of a single-mass model showed that forced oscillations are caused by WOB fluctuations, which have not been examined extensively in previous studies. In addition, for the same amplitude of WOB variation, vibration at the heave frequency has a greater effect on torsional vibration than at the axial self-excited frequency. Furthermore, large WOB fluctuations increase the risk of stick-slip and reverse rotation of the drill bit. Second, numerical experiments with the multi-mass model yielded results similar to those of the analytical study. These results show the importance of capturing torsional vibrations caused by fluctuations in the WOB and reducing WOB fluctuations.