In order to reduce the longitudinal vibration of the pipe string for mining manganese nodules from deep sea bottoms, the effects of vibration absorbers with linear springs were studied in the previous reports, indicating that the resonance amplitude of the buffer attached to the lower end of the pipe string could be reduced by half as compared with that pertaining to the buffer without any vibration-absorbers. In this study, the vibration absorbers with nonlinear springs, namely softening and hardening springs, were considered and then the nonlinear longitudinal vibrations of the pipe string with the absorbers were theoretically analyzed by the harmonic balance method. The result indicated that the absorber with a softening spring could reduce the amplitude of the longitudinal vibration of the pipe string more than the optimum absorber with a linear spring within the lower range of the excitation frequency of the ship motion (less than about 0.15 Hz), and that beyond this range of frequency the absorber with a hardening spring could further reduce the vibration than the optimum absorber with a linear spring.
For mining manganese nodules from deep-sea bottoms, a pipe string is needed to connect the mining ship on the sea surface with the collector on the sea floor. The behavior of the pipe string with a buffer in the air-lift system has been studied by many researchers (Chung et a1., 1981a,1981b, 1983, 1994a, 1994b, Sparks et a1., 1982, Aso et a1., 1988, Yuan et al.,1990, Mustoe et al.,1992). The longitudinal vibration of the pipe string with pump modules and a buffer has been studied by Aso et al.(1991a, 1993). From.these studies, the longitudinal vibration is found to be more destructive as far as the strength of the pipe string is concerned, and so it must be controlled as much as possible.
