In order to control the longitudinal vibration of the pipe string in the deep ocean mining system, first, the possibility of an active control system was examined, but the active control was found to be hardly " applicable for that purpose, because It required a large amount of power which was difficult to obtain on the ship as well as on the pipe string. Then, the semi-active control was studied, and it was found that the resonance amplitude of the buffer could be reduced to less than the amplitude by the ordinary vibration-absorber attached to the buffer if the spring constant of the absorber was varied with the frequency of the ship motion. Furthermore, the relationships between the spring constant and the frequency have been determined for several cases in order to minimize the resonance amplitude of the buffer. This system needs much less energy to control the vibration, and so it is quite promised for the purpose sought, though a mechanism must be further developed for varying the spring constant of the absorber automatically according to the frequency of the ship's heave-motion.
For mining manganese nodules from deep-sea bottoms, a pipe string is needed to connect the mining ship on the sea surface with the collector on the sea floor. Furthermore, in the pump-lift system as shown in Fig. 1, the string must be equipped with the pump modules and buffer. The pipe string in this system is apt to vibrate longitudinally, laterally and torsionally due to the ship motions. Hence, these vibrations of the string must be controlled for the safe operation of the abovementioned mining system. The behavior of the pipe string with a buffer in the air-lift system has been studied by many researchers.
