INVESTIGATION OF DRAG FORCES FOR FLEXIBLE RISERSUNDERGOING VORTEX-INDUCED VIBRATION IN SHEARED FLOW

The drag forces of a flexible riser undergoing vortex-induced vibration(VIV) in sheared flow were investigated using a scale model test.The mean drag forces were determined based on beam theory using the strains measured by the fiber bragg grating strain sensors of the riser.The distributions of the drag coefficients along the riser and Reynolds numbers(Re), and the VIV amplification of the drag coefficient, were studied, and a new empirical model to estimate the drag coefficient on a flexible riser undergoing VIV was proposed.The results show that VIV can amplify the drag coefficient, resulting in the local drag coefficient of the riser up to 3.2.For Re values from 1.0×10⁴~1.2×10⁵, the mean drag coefficient value was between 1.3 and 2.0, and decreased as Re increased.Furthermore, the proposed empirical prediction model, which accounts for the effects of current, the VIV dominant modal number and the frequency in the cross-flow direction, can predict riser drag coefficients under VIV accurately at high Re values.