Three-Dimensional Coupled Responses of a Vertical Deep-Ocean Pipe:Part I. Excitation At Pipe Ends And External Torsion

Three-dimensional (3-D), nonlinear, coupled, axial, bending and torsional responses of an 18,OOO-ft pipe system are studied with the new nonlinear finite element method (FEM) code presented in Part 1 with an example of the recovery of manganese nodules in the Pacific Ocean. For this Part 11, the pipe top is pinned to a ship in waves, and its bottom end is attached with equipment (e.g., buffer) and is free and independent of the self-propelled seafloor nodule miner. The pipe system is subject to a vertically varying, current flow when establishing the static equilibrium configuration. For dynamic analysis, the pipe top is excited by periodic large-amplitude horizontal, as well as vertical, motions, the internal slurry flow, and the external hydrodynamic forces. For torsional coupling, a consistent mass-matrix formulation is used. The external torsional moments induce biaxial bending deflection and vibration in response to a unidirectional ocean current and cause a large pipe twist. The axial-to-torsion and axial-to-bending couplings are found to be strong. Response periods to large-amplitude excitations vary from the top to bottom of the pipe. The upward internal slurry flow reduces the axial stress and increases the horizontal displacements. Numerical stability of the solution is sensitive to the specific sequence of load steps, large flowinduced torsional moment, excitation frequencies, and excessive axial excitation amplitudes. The present response characteristics are quite different in many aspects from those examples in Part 1 with both ends of the pipe restrained.