In order to reduce the axial stress produced in the pipe string for mining manganese nodules, a threestepped pipe string with larger diameter in the upper part was taken into consideration and its longitudinal vibration caused by the ship's heaving motion was analyzed in case of the pump-lift system. The results obtained were compared with those for the uniform pipe string which had the same mass as that of the stepped pipe strings. These results indicate that the natural frequency of the stepped pipe string a little increases as compared with that of the uniform one but the amplitudes of both pipe-strings are little different from each other. Furthermore, it is interesting to note that the maximum axial stress produced in the stepped pipe string is considerably smaller than that of the uniform pipe string, and that there is the optimum configuration of the stepped pipe string for minimizing the maximum axial stress. Then, the method for finding the optimum configuration of the string has been developed, and it is found that the optimum stepped pipe strings for the practical cases considered in this study could reduce the maximum axial stress in the pipe string by about 25% as compared with the maximum stress in the uniform pipe string.
In order to design the deep-ocean mining system for manganese nodules as shown in Fig. I, the maximum axial stress produced in the pipe string is a key factor to be considered (Chung et al.,1981,1994a,1994b, Aso et al.,199Ia), and it must be reduced as much as possible. Up to the present, Aso et al. (1991a, 1993) have studied the 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.
