Experimental investigation on the VIV of two side-by-side risers fitted with triple helical strakes under coupled interference effect

Triple helical strakes have been proved to be highly effective in suppressing the vortex-induced vibration (VIV) of isolated riser. Nevertheless, when risers work in clusters, interference effect can give rise to more complicated flow-induced vibration (FIV) behaviors, which in turn will largely compromise the suppression efficiency of triple helical strakes. In this paper, an experimental investigation on the VIV of two side-by-side risers in a uniform flow was carried out in a combined wave–current water flume. The riser models, with an effective length of 2.0 m and an aspect ratio of 111.11, were placed in the flume with 40% of their lengths immersed in water. The smooth riser was arranged side by side with the triple helical strakes vibration suppression riser with pitch/height of 17.5D/0.25D. By varying the structure and hydrodynamic parameters such as flow velocity and spacing ratio of the straked riser under interference effect and its dynamic feedbacks to the smooth riser for cases of side-by-side configuration are analyzed base on the observed dimensionless displacement, dominant frequency, and modal weight distribution of the risers. The results show that the interference effect of the straked riser on the smooth riser is significant, and the smooth riser with high velocities and large spacing arrangement is more likely to generate higher-order modes; under the influence of interference effect, the dominant frequency of straked riser is close to that of smooth riser, and this phenomenon disappears at high reduced velocities; the interference effect has a significant effect on the vibration suppression efficiency of the three triple helical strakes vibration suppression device. The maximum reduction of CF direction is 14.64%, and that of IL direction is 35.41%.