Subsea Lifting System for Deep-Seabed Rare-Earth Rich Mud

Deep-seabed mud containing a high concentration of rare-earth elements, including yttrium, has been discovered in the western North Pacific Ocean near Minami-Torishima Island, Japan. However, production of the rare-earth rich mud is challenging because of its location at water depths of over 6000 m. We propose a new subsea lifting system for deep-seabed rare-earth rich mud. The lifting system consists of a small diameter marine riser and an inner work string. At the lower end of the work string, a hydraulic jet pump is equipped so that rare-earth rich mud slurry can be easily sucked from a sea-bottom mud collecting device and lifted through the riser annulus. The jet pump is driven with power fluid pumped from a floating mining vessel. To evaluate the suction performance of the jet pump and the flow assurance in the annulus, numerical simulations were performed for various kinds of power fluid rates and jet pump configurations. The simulation results suggested that the proposed lifting system could, in principle, lift slurry containing rare-earth rich mud continuously to a surface floating vessel. Also, the hydraulic jet pump mechanism could be optimized to maximize the suction caused by the Venturi depressurization effect and to achieve a commercially feasible mud lifting rate of 3500 ton/day. For a pump configuration with three pairs of diffusers and suction lines, a drive fluid flow rate of 700 gal/min was found to be sufficient to meet the economic production criteria.