Experimental study on vibration responses of flexible riser transporting spiral flow in deep sea mining: Part I - Liquid single-phase transportation

An experimental study is performed for investigating riser responses under flow-induced vibration (FIV) of a flexible riser transporting spiral flow and straight flow in a large-scale simulated transport system for deep-sea mining. A 3D non-contact optical measurement method is employed to examine the characteristics of FIV responses of a 4-m-long flexible riser transporting spiral flow and straight flow in x, y, and z directions. For a flexible riser transporting spiral flow, experimental results show that there is a region of abrupt reduction in amplitude upon reaching the pre-instability region and post-instability region. Also, it is observed that a relatively strong nonlinear disturbance is induced by the circumferential force associated with spiral flow, altering the damping characteristics of riser. When the velocity of circumferential flow of the spiral flow is up to 10 m/s, the amplitude of the flexible riser transporting spiral flow is approximately 58% smaller than that of riser transporting straight flow and also an increase of frequency of about 11% happens. Furthermore, an analysis of vibration stability for flexible riser is made and results show that a significantly converging phase plot is exhibited by the flexible riser transporting spiral flow, resembling elliptical quasi-periodic motion.