SYSTEM DESIGN FOR SUSTAINABLE PHOSPHATE MINING OPERATIONS AT THE CHATHAM RISE

摘要

Phosphate nodules are present at 400 m water depth on the Chatham Rise, 240 nautical miles east of the South Island New Zealand, within the New Zealand EEZ. The phosphate occurs as nodules of 1 mm to 150 mm size within a silt/sand layer generally less than 70 cm thick. For the mining operation, a solution oriented system design philosophy has been followed in the mining design studies. These studies have been carried out by a multi-disciplinary team of engineers at Boskalis and draw upon a vast experience in dredging and marine operations. The preferred option for this project, in view of cost efficiency and safety considerations, is to combine the functions of extraction, separation, transport and offloading into a single vessel. Extraction is done by trailer suction dredging, with all the material being brought to the surface. The drag head plus underwater pumps will be suspended from winches, some of which will be motion compensated. The riser system will be a flexible hose. Separation will take place onboard with the silt/sand tailings returned to the seabed with careful control of the return flow of material. Elaborate plume modeling and analysis have been used to design for minimal turbidity and reduced covering of the resources with tailings. An on-board separation plant has been designed which is capable of processing the high production rates and handling the varying composition of the dredging mixture, whilst allowing for operational contingency. Assumptions and interpretations are validated from additional information on the in-situ soil conditions as obtained in 2011 and 2012. The mining vessel will mine for several days recovering up to 50,000 tons of phosphate, then deliver it to a New Zealand port. Annual production of around 1.5 million tons is planned. Current plans are for production mining in 2015. When the Chatham Rise phosphate mining commences it is likely to be the deepest application of trailer suction dredging and the deepest marine production mining to date.