Dynamic Response of Two-Degree-of-Freedom Riserless Drill String for Vortex-Induced Vibration Suppression and Enhancement

The mechanical behavior, dynamic evolution, and flow-field distribution of a two-degree-of-freedom riserless drill string were simulated numerically by using FLUENT fluid simulation software with the user-defined function embedded. The rotation angular velocities before and after the critical rotation angular velocity were used as independent variables, and the reduced velocity range was 3–14. Fluid-structure coupling was realized based on the dynamic overset grid and the SST k-ω turbulence model. Results reveal that the dynamic response of the riserless drill string was considerably affected by rotation and flow velocity, which are coupled with each other. The cross-flow average dimensionless displacement increased with the rotation angular velocity, and rotation considerably enhanced the in-line maximum average dimensionless displacement. However, the cross-flow amplitude caused by vortex-induced vibration was suppressed when the rotation angular velocity reached a certain value. The in-line and cross-flow frequencies were the same, thereby causing the trajectory to deviate from the standard ‘figure-eight’ shape and become a closed circle shape. The vortex did not fall behind the cylinder at low reduced velocity with high-rotation angular velocity, and the structure of the near-wake vortex remained U-shaped. The wake of the cylinder was deflected along the cross-flow direction, thereby leading to vibration asymmetry and resulting in increased vibration instability and disordered vibration trajectories, especially at high-rotation angular velocities.