Interaction of wave motion and water hammer on the response of the risers in deep-sea mining

Abstract

The expending global economy is expected to result in a shortage of mineral resources. Deep-sea mining offers a promising solution to this problem by using the hydraulic lifting technique. However, the riser system within this hydraulic system is associated with technical challenges: (a) heave motion induced by ocean waves causes longitudinal stress waves within the riser wall, and (b) blockages at the riser bottom arising from the clogging of mining nodules. The interaction between these phenomena as well as the interaction between the fluid and the riser wall pose technical challenges that remain unaddressed. In response to these challenges, a partitioned shell-based water hammer model with two-way fluid-pipe interaction was developed in this study, aimed at predicting the longitudinal response of water-filled risers. A parametric study is then conducted to investigate the influences of the ocean wave, structural and fluid characteristics on the dynamic response of the riser. The model predicts the spatial and temporal distributions of the pressure and velocity within the fluid inside the riser, in addition to the displacements, velocities, and stresses within the riser wall under both normal operating conditions and end-blocked conditions. The study then provides practical recommendations for engineering hydraulic lifting techniques.