Experimental investigation on vortex-induced vibration of a flexible pipe in combined uniform and oscillatory flow

Vortex-induced vibrations (VIV) induced by ocean currents can cause severe fatigue damage to marine risers. The background current and oscillatory flow caused by the motion of the floating platform form a combined flow field, which complicates the VIV of risers. In this paper, a series of model tests were conducted in a towing tank to investigate the VIV characteristics of a flexible pipe in combined uniform and oscillatory flow. Effects of oscillating amplitude, oscillating period and uniform flow velocity on VIV of the pipe were investigated and discussed based on the experimental results. VIV fatigue damage of the pipe was also calculated. Intermittent vibration, hysteresis and mode transition were observed in combined flow. Large oscillating amplitude and short oscillating period tend to enlarge the fatigue damage. But when the uniform flow velocity is too high, small oscillating amplitude and long oscillating period tend to enlarge the fatigue damage. Critical short period and combined ratio threshold are discovered in the VIV of the pipe in combined flow. VIV characteristics in different types of flows are also compared. VIV in combined flow may cause severe fatigue damage, which should be given enough attention.