Preliminary Design of a Trench Cutter System for Deep-Sea Mining Applications Under Hyperbaric Conditions

Seafloor massive sulfides (SMSs) are formed as precipitation products from hot hydrothermal fluids as a result of mixing with cold seawater and are mostly found at depths between 1500 and 3600 m. These deposits are formed in tectonically active zones of oceans (midocean ridges and "back-arc" spreading systems) and are the result of the global heat transfer from the mantle in the oceanic crust. As these fluids mix with the cold surrounding seawater, metal sulfides in the water are precipitated on or in the nearby seabed. The appearance of the solid mineral occurs as cylindrical chimney structures: the so-called black and white smoker (caused mainly by the presence of iron, copper, zinc, and sulphur). Larger sulfide occurrences are mostly originated in several generations of hydrothermal cycles, and they form deposits that can range from several thousands to about million tons. SMS contains pyrite (iron), galena (lead), sphalerite (zinc), and chalcopyrite (copper). Deep-sea mining is concentrating now to exploit such deposits. So far, the only known commercial project is the one developed by Nautilus Minerals, which is based on a horizontal system. This paper describes a preliminary design of a novel cutting tool developed for a vertical mining approach. The vertical mining method is preferred when rough terrain is expected and the device is easy to relocate. Using an atmospheric and hyperbaric cutting model, the cutting energy for two selected SMS deposits has been estimated and validated with real onshore excavation sites performed with a cutting tool normally used for diaphragm wall installation (i.e., the trench cutter technology). Preliminary results suggest that the estimated cutting energy is 2.9 times higher with respect to the measured one in atmospheric conditions. This could be because the model considers the worst case scenario, i.e., higher energy due to the maximum cutting forces assumed. This factor has been used to design a cutting tool which is able to work up 2000-m water depth to mine SMS deposits.