In order to reduce the longitudinal vibration of a pipe string for mining manganese nodules from deep sea floors, the pipe string was assumed to be supported by a linear or nonlinear spring at its upper part. Furthermore. a vibration absorber was attached to the buffer at the lower end of the pipe string as a complementary measure for this purpose. The analyses of the longitudinal vibration and stress produced in the pipe string for this system show that the linear-spring support itself tends to increase the resonance frequency of the pipe string as well as the resonance amplitude of buffer, while reducing the maximum axial stress of the pipe in case of the small spring constant. The hard-spring support strengthens this tendency and the soft-spring support tends to act reversely. When an optimum vibration absorber is attached to the buffer, however, those nonlinear-spring effects on the amplitude and stress become much smaller than the linear-spring ones.
For mining manganese nodules from deep-sea floors, a pipe string is required to connect a mining ship on the sea surface with a miner ox, the sea floor. Furthermore, the string must be equipped with one or more pump modules and a buffer in case of the pump-lift system, as shown in Fig. 1.(refer to the full paper) However. the string is easy to vibrate longitudinally, laterally and torsionally due to the ship motions. Up to the present, it has been found (Chung et al.. 1981. 1994a, and 1994b" Aso et al.,1991a) that the longitudinal vibration is the most dangerous among the vibrations and so it must be reduced as much as possible. For this purpose, Aso et al.(1991a, and 1993) have studied a shape-effect of the buffer on the longitudinal vibration of the pipe string, identifying that the buffer whose shape causes a higher drag force more reduces the vibration.
