Previously, a very long vertical pipe was modeled and used for the simulation of its dynamic behavior for lifting manganese nodules or crusts from 800 to 6,00G-m deep seafloor. Recently, a pipe with elastic (flexible) joints was introduced (Cheng and Chung, 1995; Chung, Cheng and Zheng, 1995) with the merit of changing eigenfrequencies and changing 3-D response characteristics, including torsion. Also, it was shown that the mass or buffer at the bottom end of the pipe (Chung and Whitney, 1981) and the axial vibration absorbers along the pipe with no joints (Aso, 1992 and 1994) can also effectively reduce the axial stress near its resonance, through uncoupled axial pipe response analysis. In the present paper, deep-ocean mining pipe systems of 4,000 and 18,000 ft in length are reviewed, and the 3-D dynamic coupled behaviors of 4 pipe systems of 18,000 ft in length are compared through the 3-D nonlinear coupled analyses, using a nonlinear code, 3DNLPIPE, analyzing the effects of axial dampers along the pipe (Cheng and Chung, 1996), both with and without elastic joints, and the axial, as well as bending, stiffness of the elastic joints. It is found that the axial dampers on the pipe with the elastic joints, when the axial damping is large, give very favorable dynamic responses near the axial resonance. The axial damping does not make the biaxial and torsional vibrations reach steady state, as the clastic joints do, and furthermore greatly reduces the axial, as well as the bending, stresses. The axial dampers slightly decrease the pipe deflection, while the elastic joints increase it slightly. The ship motion excites the horizontal (x-) and vertical (z-) motions concurrently of the pipe in its initial static equilibrium configuration, which is deflected by the ocean current along the pipe and torsional moments.

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