ABSTRACT:

In order to reduce the amplitude of longitudinal vibration and the axial stress induced in the pipe string for mining manganese nodules from deep sea-floors, various countermeasures, such 8S the utilization of the shape-effect of the buffer equipped at the lower end of the string, a vibration absorber attached to the buffer, a stepped-pipe configuration with larger diameter in the upper part, an elastic support at the upper part of the string and the combination of the above measures, have been studied by Aso et al. Moreover, the effect of elastic joints built in the pipe string has been studied by Chung et al. This paper reviews and compares these measures, indicating that the stepped pipe string with a vibration absorber and the elastic joints embedded in the string are more promising methods than the others.

INTRODUCTION

For mining manganese nodules from deep-sea floors by a pump-lift system, as shown in Fig. 1, a very long pipe-string is needed to connect the mining ship on the sea surface with a miner on the sea floor. The string, however, is easy to vibrate longitudinally, laterally and torsionally due to the ship motions. Among the vibrations, the longitudinal vibration is found to be the most dangerous (Chung et al.,1981, 1994a, and 1994b; Aso et al.,1991a) and the maximum axial stress induced in the pipe string is the key factor for its design. Thus, the vibration and stress must be reduced as much as possible. Up to the present, various heave compensators have been developed for controlling the vertical motion of the ship. However, These compensators are said not to work perfectly and to leave a small motion of the ship. Thus, it has been said that a passive or semi-active control is more favorable for that purpose.

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