This paper describes a control algorithm for the buffer of a deepsea mining system, in which the buffer is connected to a long slender pipe and then to a surface ship on one end, and to a collector on sea floor through a flexible hose on the other end. A mathematical modeling is established for designing the controller for buffer thrusters, in which the dynamic response of the long pipe is taken into account based on the mode superposition method. The fluid loading acting on the pipe is estimated by using Morison's formula. For simplicity, the surface ship is assumed to be kept stationary, the reaction from the flexible hose is ignored and only the lateral motions are considered. In order to guide the buffer to react only to the low-frequency motion of the surface vessel, the FIR digital filter is introduced to a PID-based controller. It can be shown numerically that the high frequency component of the ship's motion can be effectively filtered out by using the FIR low pass filter.
With the help of modern sea technology, we are now in a position to develop subsea resources in very deep regions. For instance, Fig.1 shows manganese nodules on a deep seabed. (Chung, 1996) The nodules are formed by metallic compounds, which were slowly precipitated out of the ocean water. Among them, cobalt is the most valuable metal, 10 to 30 times more worth than copper and nickel. It is mostly used for super alloys in aircraft industry, but demand is steadily increasing for use in rechargeable batteries for laptop computers and many other applications. Research and development on the deep ocean mining of manganese nodules have been done actively for a long time.
