Coupling of Drill String Whirl and Magnus Effect in Deepwater Drilling

Abstract Deepwater drilling is conducted by applying weight and torque to the drill bit via the drill string from the ship. It is essential to control the behavior of the drill bit appropriately, such as weight on bit and torque on bit, to prevent accidents. Therefore, to understand the drill bit’s behavior, it is necessary to understand the dynamic behavior of the entire drill string. The dynamic behavior of the drill string is mainly expressed in terms of movement in three directions: axial, torsional, and lateral. Various studies have been conducted on these and their coupled dynamics. On the other hand, most of these studies have focused on riser drilling, and the drilling rig is assumed to be fixed to the ground. However, in deepwater drilling, the drill string is affected by the motion of the drillship and sometimes the ocean current during riserless drilling. The authors have previously revealed the self-excited vibration excited by the friction of the heave compensator. In addition, the forced vibration in the torsional direction caused by heave was examined. Thus, in deepwater drilling, new phenomena will likely be revealed in the dynamic behavior of the drill string. This study focuses on the lateral dynamics of the drill string. This study first investigates whirl motion using numerical and small-scale model experiments. Depending on the conditions, backward whirls were observed in small-scale model experiments. In addition, numerical experiments reproduced backward whirls. Next, the coupling of whirl motion and the Magnus effect is investigated to reveal the new phenomenon due to their coupling. Finally, this study investigates and discusses the lateral dynamics in deepwater drilling.