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Recent developments for deep-sea mining have shown multiple scenarios of gaining mineral deposits of Seafloor Massive Sulfides (SMS). One of the problems for these scenarios is the overall large energy consumption of processing rock material which are a technological challenge and are increasing production costs. This paper compares two methods for deep-sea rock excavation on their energy consumption, based on rudimentary calculations. The best known scenario for gaining mineral deposits from the seabed is to excavate rock materials with a crown or drum cutter and pump the fluidized crushed materials to the vessel at the surface. This process requires high cutting forces deep-sea due to the hyperbaric effect at large water depths, when cutting with full cavitation. This high energy consuming process therefore requires a considerable amount of subsea installed power. An alternative scenario is to use a hydraulic grab for excavating mineral deposits and not crush all the materials entirely subsea. Using a grab would be very beneficial in rough terrains and unstable seafloor conditions, compared to track-driven vehicles typically used for crown or drum cutters. Also specific cutting forces are much lower when using a grab, because it is not cutting at full cavitation in hyperbaric conditions. However the main advantage is to keep most of the rock intact which allows the material to be crushed at the surface. Mechanically uplifting large pieces of rock therefore could have the advantage that most of the required power can be installed at the surface, rather than subsea for the traditionally proposed hydraulic pumping systems. The rock can then be further crushed under atmospheric pressure at the surface, avoiding the hyperbaric effect. The combination of using a grab and further crushing at atmospheric conditions is more energy efficient and therefore requires substantially less installed subsea power. Using rudimentary calculations, a great reduction of energy consumption is found for using a grab compared to typically used crown or drum cutters. Substantially less subsea installed power is required for excavating the mineral deposits with a grab. Although additional crushing needs to be done at the surface, the overall required installed power for using a grab still can be much less than fully subsea excavating and crushing.

The increasing demand on precious metals has motivated the development of a promising industry, deep sea mining. Currently major technical challenges exist in the development of this new industry, such as the vertical transportation, the seabed excavation process and the stability of the riser system. This paper will focus on the excavation process on the seabed. Considering the fact that the deep sea mining excavation process may occur at 3000∼6000 meters water depth, the hyperbaric pressure applied by the sea water will greatly influence the cutting process. Especially when the cutting speed of the cutter is very high, the so called “dilatancy hardening effect” (Brace and Martin, 1968) may make the seabed rock very difficult to excavate. These factors will make the rock excavation in deep sea much different from shallow water, which is the case in a normal dredging project. In this paper, the physics of the hyperbaric excavation process will first be described into detail. Because the hyperbaric rock cutting experiments are expensive, it is more feasible to make a numerical model to simulate the process, which eventually can replace the experiments. The main difficulties are to model the failure of rock and the interaction between the rock and the pore water. Considering the scale of the problem and the characteristics of the material, it is concluded that the discrete element method (DEM) will be the best tool to simulate the rock behavior. On the other hand, to describe the influence from the hyperbaric pressure which is induced by the sea water, governing equations for the fluid phase are derived and the finite volume method (FVM) is chosen to solve the equations. This paper will give a detailed description about the numerical methods and their interactions regarding this specific problem and show some preliminary tests on clay-like material cutting process.

In marine environment, floating photovoltaic (FPV) plants are subjected to wind, wave and current loadings. Waves are the primary source of fatigue damage for FPVs. The climate change may accumulatively affect the wave conditions, which may result in the overestimation or underestimation of fatigue damage. This paper aims to present a projection method to evaluate the climate change impact on fatigue damage of offshore FPVs in the future. Firstly, climate scenarios are selected to project the global radiative forcing level over decadal or century time scales. Secondly, global climate models are coupled to wind driven wave models to project the long-term sea states in the future. At last, fatigue assessment is conducted to evaluate the impact of climate change on fatigue damage of FPVs. A case study is demonstrated in the North Sea. A global-local method of fatigue calculation is utilized to calculate the annual fatigue damage on the FPVs’ joints. The conclusions indicate that there are decreasing trends of significant wave height and annual fatigue damage in the North Sea with the high emission of greenhouse gases. The fatigue design of FPVs based on the current wave scatter diagrams may be conservative in the future. The manufacture cost of FPVs can be reduced to some extent, which is beneficial to the FPV manufacturers.