Suppression of vortex-induced vibrations using flexible shrouding-An experimental study

A number of offshore compliant structures such as risers, mono-columns, tension leg platforms (TLPs), spars, etc., which are deployed in the offshore field are subjected to vortex-induced vibration (VIV). The harsh environmental conditions often lead to failure of these structures well before their design life period resulting in fatal accidents. To minimize VIV, several investigations were conducted by other researchers. The present study focuses on the experimental investigations of an elastically mounted circular cylinder shrouded with net substructure called as Ventilated Net (VN) to suppress VIV. The VN is an omnidirectional, economical, customizablenet substructure comprising of flexible hollow tubes in a systematic arrangement. This device could be retrofitted to the offshore structures/risers, to attenuate VIV, thereby preventing the structure from fatigue failure. Experimental investigations are conducted to address the reduction of VIV and drag forces acting on the oscillating cylinder shrouded with VN of various configurations. The experiments are performed in towing tank facility in Department of Ocean Engineering, IIT Madras, India. The primary objective of the present investigation is to quantify the VIV amplitudes by employing VN shrouding the cylinder, at high Re regime, ranging from (0.22-2.50) × 105. The effect of spaces between the flexible hollow tubes and shrouding radii of VN around the cylinder are also addressed in this article. From the measurements, it is observed that cylinder with VN of dense mesh at a radial spacing of twice the diameter of the bare cylinder, suppresses VIV by 98% and drag force by 40% at Re of 1.2 × 105.