In order to reduce the longitudinal vibration of pipe string for mining manganese nodules by the pumplift system, first, various buffers were designed to cause higher drag-forces. Then, the forced longitudinal vibration of the pipe string equipped with a buffer and two pump-modules was analyzed theoretically by introducing the ship's heave-motion. The result indicated that the buffers having larger cross-sectional area normal to the direction of its vibration were more useful for reducing the resonance amplitude of the vibration of the pipe string if their volume and mass were kept constant. A buffer which satisfied the above condition was found to be the cylindrical one with larger disks. This kind of buffer reduced the resonance amplitude of the vibration by about two fifth as compared with that of the cylindrical buffer without any disks. However, the diameter of the disk was limited because the dynamic behavior of buffer became unstable when it increased more than the height of the buffer. Furthermore, there was the optimum interval between the disks, depending on the aspect ratio of the buffer and Keulegan-Carpenter number.
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, the string must be equipped with the pump-modules for pumping the nodules up to the ship and with the buffer which regulates the density of nodule-fluid mixture in the pipe string and plays a role of the weight to stabilize the mechanical behavior of the string. 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 above-mentioned mining system.
