Investigation on the effectiveness of helical strakes in suppressing VIV of flexible riser

•The mean drag coefficient of strakes is surprisingly lower than a bare cylinder.•However, large drag penalty is found in particular case at high reduced velocity.•Strakes’ pitch affects the occurrence of lock-in and prevents higher mode frequency.•Strakes’ height plays the major role in suppressing the vibration of riser.

The effectiveness of the strakes in suppressing the vortex-induced vibration (VIV) of a long flexible cylinder is investigated by varying the pitch (p) and height (h) of the strakes. Measurements of cross-flow (CF) vibration amplitude, in-line (IL) and CF frequency responses, and hydrodynamic forces are presented for low mass flexible cylinders with helical strakes. Cylinders with aspect ratio of 162 were applied with constant pre-tension to restrict the vibration in IL and CF directions. Six cases with about 180 runs were executed under subcritical Reynolds number (Re = 0.144to 1.384) of uniform flow. Calculation on the laminar boundary layer thickness around a circular cylinder was performed as a benchmark in deciding the height of strakes. Present study shows that at certain strakes’ height which is larger than the laminar boundary layer thickness, significant VIV mitigation is found. However, the suppression effectiveness of helical strakes on flexible cylinder is less notable compared to a rigid cylinder. The change on pitch of strakes affects the occurrence of lock-in region and prevents the frequency to switch into higher mode. On the other hand, increase on the height of strakes narrows the lock-in region and plays the most major role in suppressing the vibration of a cylinder. The hydrodynamics forces indicate an unexpected phenomenon where the bare cylinder possesses higher force coefficients compared to cylinders with helical strakes. However, large drag penalty is still found in a particular case at higher reduced velocity. The most effective configuration of strakes in terms of the dynamic responses is p = 10D and h = 0.15D model. However, model of p = 10D and h = 0.10D performs better in reducing the hydrodynamic forces.