In order to analyse the longitudinal vibration of the pipe-string in the deep sea, first, the drag and added mass coefficients of various buffer-models vibrating axially in water were evaluated by the method developed by the authors. Then, the forced longitudinal vibration of the pipe-string equipped with a pump-module and a buffer was analysed theoretically by introducing the fluid forces evaluated with the above-obtained coefficients. Furthermore, the axial stress induced in the pipestring was calculated. The results indicate that the buffer whose shape causes a higher drag force is more useful for reducing the amplitude of the vibration and the axial stress in the pipe-string, and that the highest-drag buffer used in this study causes a half amplitude of the vibration and about 63% axial stress at the first resonance as compared with those produced by the lowest-drag one.
For mining mineral resources from deep-sea bottoms, a pipe-string is needed to connect the mining ship on the sea surface with the collector or miner on the seafloor. Furthermore, in the pump-lift system of ore, the string must be equipped with the pump-modules for pumping the mineral resources up to the ship and with the buffer which regulates the slurry-density of ore-fluid mixture in the pipe-string and plays a role of the weight to stabilize the mechanical behavior of the string. Hence, the longitudinal and lateral vibrations of the string, which are caused or initiated by the ship motions, must be analysed for the design of the above mining system. Besides, to analyse these vibrations the fluid forces acting on the pump-modules and buffer must be evaluated, and to calculate these forces by the Morison" s formula, the drag and added-mass coefficients of the pump-modules and buffer vibrating in water must be known in advance.