Numerical Methods for Modeling the Rock Cutting Process in Deep Sea Mining

Abstract

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.

An Analytical Model for Rock Cutting with a Chisel Pick of the Cutter Suction Dredger

“…Even though dredging operations with a CSD are performed under water pressure, a limited water depth does not cause hyperbaric conditions [39]. In addition, the results in [40,41] show that However, Evans' model [20] and Nishimatsu's model [26] for rock cutting with a chisel pick are developed by two-dimensional analysis. They are only suitable for practical rock cutting in shallow cutting depths (the ratio of the cutting width to cutting depth is very large).…”

“…Even though dredging operations with a CSD are performed under water pressure, a limited water depth does not cause hyperbaric conditions [39]. In addition, the results in [40,41] show that even in deep water conditions, the water pressure has a very small effect on the cutting force in the steady stage of the dredging operation. Therefore, it is acceptable to ignore the effect of the water pressure on the cutting force during rock dredging with a CSD.…”