ABSTRACT:

This paper presents a detailed description of a modelling strategy suitable for investigating the non-linear dynamic response characteristics of Single-Point-Mooring (SPM) systems in both two and three-dimensional ocean wave environments. A finite element formulation which not only accounts for non-linearities associated with the large displacement response of the cable itself but also considers the influence of structure fluid interaction in the resultant dynamic response, has been especially developed for modelling the cable elements of such a system. The hydrodynamic loading imposed by the ocean environment in the absence of lift is based upon the relative velocity formulation of Morison's equation whilst the kinematics necessary to this modelling are simulated for irregular twodimensional (uni-directional) and three-dimensional (multi-directional) sea states using a numerically efficient spectral-based algorithm. An example of an SPM system (a sub-surface buoy with 3 symmetrically disposed cable restraints) previously chosen from the literature has been respectively subjected (numerically) to the actions of a single Airy wave, a uni-directional irregular wave and multi-directional waves with different levels of directional spread by way of illustration of the performance of the model.

INTRODUCTION

Cable elements are widely used in various offshore applications that include: mooring lines for ships; the vertical restraints of a tension leg platform; the inclined restraints of a guyed offshore tower and of Single-Point-Mooring (SPM) systems amongst several others. Although the modelling of the large displacement non-linear restraint characteristics of cable elements has been of engineering interest for many years it is only in recent times with advances resulting in developments and extensions of the finite element methodology can it be claimed that this complex modelling has attained a sufficient degree of realism in the case of dynamically responding structural systems that incorporate cable elements in their design whether on land or at sea.

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