Configuration and Performance Analysis of Deep Ocean Mining Flexible Riser

Abstract

Deep-sea minerals such as polymetallic nodule, hydrothermal sulphides and ferro-manganese crusts have for long attracted attention as an alternative source of metals to terrestrial deposits. To bring these minerals up to the land, flexible risers are needed. As the mining industry developing towards deep sea area, the conveying system is usually designed as a combination of steel riser and flexible riser. According to different transport requirements, various flexible riser configurations, such as steel catenary riser, lazy-wave riser and saddle-shaped riser, have been proposed. During mining operation, the riser bears gravity, buoyancy, wave and current force, therefore the assessment of structural safety and reliability is quite challenging. In addition, the riser response caused by the mining vehicle motion during working process in a large area should also be considered. To guarantee a safe operation and service life of the riser, it is necessary to carefully design its configuration and to analyze its performance. In this study, taking the saddle-shaped riser as our model, the influences of main design parameters on the riser configuration, tension and stress are examined. These parameters include the installation position of buoyancy modules, the buoyancy ratio and motion of mining vehicle. Firstly, the analysis model of the riser response is established based on FEM in which the nonlinear large displacement and deformation of the structure are considered. Secondly, through our FEM simulation, the distribution and variation of tension and stress along the axial length of risers with different configurations are presented. Finally, the impacts of the mining vehicle motion on riser response are discussed. Our numerical results show that a small change of the buoyancy position and buoyancy ratio may lead to a significant change of the riser configuration, but a little change of riser tension/stress. And the saddle-shaped riser has a good tolerance performance to the bottom-end excitation.