Laboratory measurements on the suppression of vortex-induced vibrations and rotation in long-span riserless drill strings

Riserless drilling is a leading offshore technology, but the absence of a riser causes significant vibration during outflow and rotation. However, effective methods for addressing vortex-induced vibration coupling rotation are inadequate. Experiments were conducted on riserless drill strings, and the dynamic responses were analyzed by modal decomposition theory. Results indicate that structures with higher helix numbers and screw heights exit or delay entry to the lock-in region. Triple-helix designs with screw heights of 0.2 Diameter (D) and 0.25D achieve up to 93% suppression efficiency. Increasing helix number and screw height reduces rotating drill string frequencies and delays the occurrence of the dominant frequency jump phenomenon, resulting in enhanced suppression effects. Amplitude suppression primarily occurs in medium and weak regions, while helical strakes significantly reduce amplitudes in lock-in regions. For controlling vibration and minimizing fatigue damage in rotating drill strings, it is recommended to use triple-helix structures with lower screw heights.