Experiment on the vortex-induced vibration of an S-shaped flexible riser coupled with an oscillating platform

This paper reports the experimental results of the vortex-induced vibration (VIV) of an S-shaped flexible riser hanged underneath a two-degree-of-freedom (2-DOF) oscillating cylindrical platform. A buoyancy module of approximately triple the riser diameter was mounted on the one-third of the riser length measured from its base to produce the S-shaped configuration. The VIV response of the flexible riser filled with two typical fluid media, including water and helium, was tested in the depth-averaged reduced velocity (U¯r) range of 9.32–26.41. The experimental results indicate that the change of internal fluid medium alters the spatial distribution of dominant frequency and excited modal order. The mode competition is enhanced when the internal fluid shifts from water to helium, and the critical U¯r for mode transition becomes higher. Based on the coincidence of dominant frequency, four types of coupling response are identified, and two characteristic lengths are quantified. The dominant role of the vibration system is determined by the occurrence time percentages of coupling lengths and dominant frequencies. The vibration of water-filled riser is dominated by itself, and the influence of buoyancy module is negligible. In contrast, the in-plane response of helium-filled riser is significantly affected by the buoyancy module, and the influence is amplified with the increase in U¯r.