ABSTRACT

A new discrete element method (DEM) is developed for the coupled nonlinear analysis of deep ocean pipes and risers. The DEM pipe formulation is based upon a system of rigid elements connected with concentrated axial and bending stiffnesses, and is somewhat simpler than an implicit finite element approach. Although the DEM employs an explicit time integration scheme which is conditional stable, it does not have the stability problems experienced in previous implicit finite element pipe procedures. Two cantilever problems are presented which validate the nonlinear static and dynamic capabilities of the DEM technique. The two-dimensional DEM model is also used to perform four nonlinear dynamic analyses of a 18,000 n (5,486 m) deep-ocean mining pipe, subject to different boundary conditions. In these calculations the bending arid axial deformations are coupled, and the top of the pipe is hinged at the ship, and its bottom is free. The numerical results of the first two DEM analyses are compared to solutions obtained previously by an implicit finite element method. The third and fourth pipe analyses examine dynamic coupling and shows strong bending-to-axial dynamic coupling, and very weak axial-to-bending coupling effects. It is concluded that the bending-to-axial dynamic coupling can significantly alter the magnitude of the axial pipe oscillations.

INTRODUCTION

A typical deep-ocean mining system consists of a slowly continuous moving ship which maneuvers a deep-ocean pipe, see Fig. 1. The development of this system, which has been identified by the industry as one of the most crucial, involves the analysis and control of the transient behavior of the pipe bottom end caused by the ship's oscillatory and maneuvering motion, hydrodynamic loading due to ocean waves and current. The transient response of the pipe system consists of nonlinear large coupled bending-axial-torsional deformation of the pipe.

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