The exclusive economic zone (EEZ) of Japan has a very wide area due to a lot of islands in Japanese Archipelago. As the worldwide supply and demand of natural resources and foods are being tight, the technology for the effective use of the EEZ will become the key for the sustainable development of Japan. From this background, the Ministry of Land, Infrastructure, Transport and Tourism of Japan (MLIT) started an R&D project of floating offshore platform technologies in 2007[1]. In this project an integrated design support tool is developed for evaluating the economical and safety aspects of the offshore platforms. This tool contains the function to estimate an environmental impact of seafloor resource development. We postulated the process of seafloor resource development as follows. The ores mined on seabed are sent up to the platforms as a mixture of the ore particles and water, then the waste water containing inorganic suspended particles is discharged into sea. The particles exhausted in the sea are transported by the advection and diffusion, and sink gradually on seafloor. We developed an analytical code simulating the distribution of discharged particle in both sea and sediments by a simple technique. Mass Consistent Flow Model is used to save the time in the calculation of ocean flow. In addition, the ecosystem model in sediment is developed to estimate the ecological impact of seafloor resource development. It includes benthic organism, zooplankton, bacteria, and particulate organic matter as the components. The ecological impact was assessed based on the recovery period of the biomass. In this paper, the outline of this environmental impact analysis tool and results of the trial calculation for seafloor resource mining are shown.

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