Abstract:

A dynamic simulation model of a vertical pipe has been developed for the purpose of investigating three-dimensional dynamic behaviour of a ocean mining pipe. Dynamics of vertical pipe string is modeled by the modified Kirchhoff" s beam theory including large deflections and nonlinear viscous drag forces. A lumped-mass method is developed for numerical analysis of 3-D nonlinear pipe behaviour. 3-D nonlinear simulations of a vertical mining pipe are carried out in time-domain by using the Newmark-β method. Effect of underwater pump modules and buffer on the dynamics of pipe string is investigated.

Introduction

For production of deep seabed mineral resources from 5000m mean water depth, e.g. manganese nodules, a vertical pipe string connecting the surface unit with the collector on the seabed is of great importance. A pipe string lifting up the mineral resources and simultaneously supporting various power lines and signal cables at the outside wall plays the role of the main artery of total mining system. A 5000m pipe string must, with stand the static load due to its own weight and the dynamic load caused by ship motions, wave" s and current. It is pointed out repeatedly in the literature, that the dynamic stresses due to axial oscillation is a critical parameter for design of ocean mining pipe or riser (Chung and Whitney 1981 and 1983, Sparks et. al. 1982, Pakarimen 1988). Its first natural period, about 6 seconds, is in the high energy region of wave spectra, and therefore the resonance in axial vibration mode is a serious problem. A series of investigations for reduction of the axial vibration using vibration absorbers have been conducted by Aso et. al.(1992, 1994 and 1995). Also by the authors the shape-effects of flexible joints on three-dimensional behavior including the effects on the reduction of axial vibrations.

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