| Research Abstract |
For mining mineral resources from the deep-sea floor by pump-lift systems, a pipe-string is needed to connect a mining ship on the sea surface with a collector on the sea floor, and the string must be equipped with a buffer at the lower end and pupm nodules in between. However, the pipe string is apt to vibrate longitudinally, laterally and torsionally by the ship motions. Among the vibrations, the longitudinal vibration is more destructive and must be controlled as much as possible for the safe operation of the above systems.
In 1992, the passive control methods by the shape of the buffer and the dynamic vibration-absorber attached to the buffer were studied in order to reduce the longitudinal vibration. It was found that the longitudinal vibration of the pipe string could be reduced by about 50% when the methods were applied.
In 1993, active control methods were studied for reducing the vibration and proved to be inapplicable to the mining systems because of difficulties of securing a large input control-power and the position of installing an actuator for the power. Then, a semi-active control method was studied by changing the stiffness of the spring in the vibration absorber according to the excitation frequency of the mining ship. This control promised to reduce the vibration more than the passive control.
In the final fiscal year (1994) of this research, the results obtained by the above control methods were compared with each other from the view points of controllability and economy, and the passive control methods were found to be advantageous over the other methods. Then, the optimum shape of the buffer was studied and the result indicated that the cylindrical buffer with disks was best. Moreover, the optimum conditions for the last buffer were clarified by finding the relationship among the aspect ratio, the diameter ratio of the disk to the buffer, the interval between disks and Keulegan-Carpenter number.
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