For two lifting pipe models which had been in literature proposed for production of manganese nodules out of water depth of 5,000m and 6,000m, respectively, nonlinear dynamic analyses on forced axial vibrations in vertical position and on 3-D tow behaviors are carried out in time domain. A 3-D nonlinear analysis method based on lumped-mass method and incremental-iterative scheme was used. Time integration was conducted by using Newmark method. Underwater pumps and buffer were regarded as a rigid element. Upon the simulations evaluation, two consideration points for lifting pipe design are discussed: structural strength of pipe and total integrated control of mining operation.
In deep seabed mining represented by production of manganese nodules out of 5,000 m water depth, a vertical lifting pipe is the largest part of the production line connecting surface platform with a nodule collector on the seabed via underwater pumping unit. A pipe string transporting the manganese nodules supports various cables al the outside and carries pump modules and buffer during mining operation. Outer diameter of the pipe string of 5,000 m is designed to withstand the static due to its weight in water and the dynamic loads induced by the motions of surface platform and external flows in waves and current, when the inner diameter has been selected for optimal lifting efficiency. It has been pointed out through a number of investigations that the dynamic stresses in axial vibration is the most critical factor in design of lifting pipe, since the fundamental natural period of axial vibration, about 6 seconds, is in the range of high wave energy (Chung and Whitney. 1981 and 1983: Sparks et al., 1982; Pakarinen, 1988). Aso et a1.(1992, 1994, 199:5, 1996) have conducted a series of investigations on reduction of the axial vibration using vibration absorbers with 1-D analysis.