The Effectiveness of Two- versus Three-start Helical Strakes in Suppressing VIV of Flexible Cylinder

Abstract

The proposed paper investigates experimentally the effectiveness of strakes in suppressing the vortex-induced vibration (VIV) of a long flexible cylinder by varying the pitch, height and the number of helix of the strakes, which differ from the previous studies that only focus on short rigid cylinder. The experiment was conducted in towing tank with constant velocity under subcritical Reynolds number () by using a Poly Vinyl Chloride cylinder with aspect ratio of 162. CCD cameras were installed to capture the amplitude vibration and the frequency responses in both in-line (IL) and cross-flow (CF) directions while the fluctuation of tension was measured by using tension load-cell. The theoretical laminar boundary layer thickness around a circular cylinder was calculated and was used as a benchmark in deciding the height of strakes for the experiment. The purpose of present study is to identify the optimum configuration of helical strakes that can be implemented on a long flexible riser with low mass ratio. The experimental result shows the helical strakes perform well in mitigating the VIV. Significant VIV mitigation is found for the strakes with a certain height which is larger than the laminar boundary layer thickness. However, its effectiveness in suppressing the vibration amplitude of flexible cylinder is far smaller than the rigid cylinder. Varying the pitch of strakes influents the occurrence of lock-in region and prevent the switch of frequency into higher mode. Previous study stated that pitch of 15D is as effective as 5D for a rigid cylinder. However, it does not work on a flexible cylinder. Changing the height of strakes narrows the lock-in region and contributes most in suppressing the vibration of cylinder. Also, it is surprising to find that two-helical strakes perform slightly better than three-helical strakes in lower velocity range. The hydrodynamic forces of cylinders are still in the process of analysing. The information presented in this paper will shed some light on the effectiveness of strakes with different configurations and suggest the appropriate dimension of strakes in term of pitch, height and the number of helix of the strakes as currently the available resources are limited.