In order to reduce the amplitude of longitudinal vibration as well as the axial stress induced in the pipe string, an active control was applied to the mining system. In the proposed system, the string was hung from the mining ship and supported at the upper part with a linear spring, which was connected to the ship through a linear actuator, providing a control displacement for the system. The active control theory (Optimal Regulator Theory) was applied to the system, and the optimal control force for the actuator to provide the control displacement was determined for various cases by minimizing each quadratic performance index. The results indicate that the active-control system could reduce the resonance amplitude of the pipe string by about 80% as compared with that of the mining system without any controls, and that this reduction rate is about 30% greater than that of the passive control system composed of a vibration absorber. Furthermore, the active control could reduce the resonance axial-stress in the pipe string by about 40% as compared with that of the non-controlled mining system, and this rate is about 23% greater than that of the above-mentioned passive control system.
For mining manganese nodules from deep-sea floor, a pipe string is needed to connect the mining ship on the sea surface with the miner on the sea floor, as shown in Fig. 1. Furthermore, in the pump-liR system, the string must be equipped with one or more pump modules for pumping the nodules up to the ship, and with a buffer for regulating the density of nodule-fluid mixture in the pipe string and for stabilizing the mechamcal behavior of the string. The pipe string in this system is apt to vibrate longitudinally, laterally and torsionally due to the ship motions
