This paper reports experimental tests on vortex-induced vibration (VIV) of a long flexible riser towed horizontally in a wave basin. The riser model has an external diameter of 16 mm and a total length of 28.04 m, giving an aspect ratio of about 1750. Reynolds numbers are within the range from 103 to 104. The mass ratio of the riser model is 1.0 Fiber optic grating strain gauges are used to measure the dynamic response of the model. The VIV response strain, vibration frequency and amplitude, displacement, and displacement standard deviation of the flexible riser model were examined in both cross-flow and in-line directions. The experimental results showed that the cross-flow dominant mode may reach up to 6 and the in-line dominant mode over 12 with dominant frequencies given by a Strouhal number of about 0.18. Higher harmonics such as the third, fourth and fifth vibration components were also observed.
Vortex-induced vibration (VIV) of offshore structures subjected to ocean currents is a widely occurring phenomenon in offshore engineering, which may cause large fatigue damage. Despite the large number of documents dedicated to the problem of elastically mounted rigid cylinders with one or two degrees of freedom, there are only a few experimental studies on VIV of a long flexible pipe, especially those involving high mode numbers. Some studies concentrate on VIV of a flexible cylinder like Vandiver (1993), Lie et al. (1998), Park et al. (2004), and Song et al. (2009). However, the aspect ratios in these studies are not large. Several investigators such as Huse et al. (1998), Trim et al. (2005), Vandiver et al. (2005), and Vandiver et al. (2006), have conducted experiments with aspect ratios of more than 1000 to study VIV of long flexible cylinders.