Investigation on the spoiler vibration suppression mechanism of discrete helical strakes of deep-sea riser undergoing vortex-induced vibration

Vortex-induced vibration (VIV) is an important factor that causes fatigue damage of high aspect ratio structures under the action of the fluid. In this paper, a type of discrete helical strakes vibration suppression device was designed. The vortex-induced vibration suppression experiment of the discrete helical strakes of the deep-sea riser under uniform flow in the wave-current combined water flume was carried out. The riser model adopted a transparent flexible plexiglass tube with a length of 2.0 m, an outer diameter of 18 mm and an aspect ratio of 111.11. The helical strakes were made of a hexahedral artificial rubber material. According to varying the equidimensional discrete spacings of the discrete helical strakes, varying the hydrodynamic and structural parameters such as the outflow velocity and the number of helixes, the physical mechanism of spoiler of the vibration suppression device was revealed. The suppression efficiency of the device on the vortex-induced vibration of the riser and the influence on the dynamic response of the riser were explored. The results show that the discrete helical strake can release the vortex behind the riser to multiple directions, destroy the spatial correlation of the vortex from multiple dimensions, and change the distribution of the original pressure field. The discrete single-helical strakes have low vibration suppression efficiency in the lock-in region and the dominant frequency of the riser doesn't change significantly compared with the counterweight bare riser. The number of diversions of the discrete double- and triple-helical strakes increases, and the disturbance to the incoming flow enhances, which makes excellent vibration suppression efficiency in the whole velocity range. And the dominant frequency is reduced, the vibration intensity is significantly reduced and the fatigue life is improved. With the increase of the discrete spacings, the vortex separation release rate increases slightly, but it doesn't affect the change of the diversion angle along the longitudinal direction of the riser. It can still significantly disturb the spatial correlation of the vortex, disperse the vortex and maintain high suppression efficiency. In the equidimensional discrete spacings, the dominant frequency, vibration suppression efficiency and fatigue damage are less sensitive to discrete spacings.