Abstract

This paper presents the results of both theoretical and experimental studies on the dynamic tensions and motions of the multi-component mooring lines such as chain with clump weights and/or spring buoys. Especially, the author's attention is paid to analysis of the dynamic behavior of a mooring line under the excitation caused by the motion of floating platform.

Appearance of new types of multi-component mooring lines demands the development of numerical methods which are able to be applied for the analysis of dynamic behavior of various types of mooring lines. In this paper a new method is proposed by the authors, which is motivated by the lumped mass method originally developed by Walton and Polachek (1959). The present method, however, is somewhat modified to be applied to the analysis of the multi-component mooring system and extended to be able to include the elastic deformation of the mooring line. The time histories of dynamic tension predicted by the present method are compared with the experimental ones with excellent agreement.

Introduction

In recent years, ocean platforms have become more and more complex and at the same time the requirement of the mooring systems used for those platforms becomes more severe. For use as these mooring systems, the mooring lines with a combination of chains and wire ropes, and those connected with buoys and/or clump weights have come to be used. Since the dynamic behaviors of those mooring lines are complicated and somewhat different from those of conventional single lines, the dynamic analysis of those lines become more important for investigating the feasibility and safety of mooring of the floating platforms.

For this study, a new method of the non-linear dynamic analysis of multi-component mooring lines is developed to obtain a better understanding of the dynamic behavior of multi-component mooring lines. In the present method, the continuous distribution of the mooring line's mass replaced by a discrete distribution of lumped masses at a finite number of points on the line. This replacement amounts to idealizing the system as a set of point masses and non-mass linear springs. At the present analysis, non-linearity?s of viscous damping acting on the mooring line are considered.

The present method has a great potential of application for engineers as it does not require lengthy procedure of numerical calculation and it can save a good deal of computing time. One average run over 4 cycles of harmonic motion may (240 time steps) require approximately 10 seconds in the case of the mooring line model of 9 segments by using IBM 3033. For example;

Governing Equations Of Motion

First, a mooring line is represented by a set of discrete masses interconnected by springs as illustrated in Fig. 1. The external forces acting on a mooring line are gravity, hydrodynamic forces and line tension.

The governing equations of motion of j-th lumped mass are as follows;

  • (MATHEMATICAL EQUATION AVAILABLE IN FULL PAPER)

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