Experimental study on vibration responses of flexible riser transporting spiral flow in deep sea mining: Part II - solid-liquid two-phase transportation

An experimental study is carried out to investigate the responses of flow-induced vibration (FIV) on a flexible riser within a large-scale particle mixing transport device utilized in deep-sea mining, in which both spiral and straight solid-liquid flows are examined. The three-dimensional displacement field of the flexible riser is measured using a non-contact optical measurement system in time domain. The influence of fluid dynamic effects on the FIV responses of the riser, including velocity of straight flow, circumferential velocity of spiral flow, average concentration of solid-liquid two-phase flow, and internal flow density waves are investigated. For the mode of straight flow transport, the presence of internal flow density wave leads to the variation of dynamic load inside the riser, manifesting as periodic, significantly large-amplitude flutter of the riser, and noticeable bending deformation of the riser. Conversely, the flow pattern of the spiral flow results in a relatively uniform distribution of dynamic pressure, suppressing riser bending deformation, reducing the pulsation effects induced by internal flow density fluctuations. It is observed in frequency domain analysis that the riser vibration system transporting spiral flow exhibits multiple vibration modes, with higher-order frequencies being dominated and strong nonlinear characteristics.