ABSTRACT

Rapid developments in the field of synthetic fibres have lead to serious alternatives for the traditional catenary mooring systems in several applications. The use of synthetic fibres in mooring systems has received increasing attention, especially for deeper water. However a design based on these fibres requires modification in the analysis methods.

Here the feasibility of mooring systems using a synthetic rope (with Dyneema®) compared to a wire rope is analyzed, for a water depth of 800 m. Differences between the systems are elucidated by showing results of a time domain analysis on a turret moored tanker. Different optimization criteria are considered.

Generally speaking optimum moorings systems based on synthetic rope are stiffer and impose 10wer vertical turret loads than a wire rope mooring at increasing water depth. In return for the smaller tanker excursions, the vertical forces at the seabed are considerable for a synthetic mooring.

INTRODUCTION

Floating production facilities are being used at increasing depths. In deep water conventional mooring systems, based on wire rope and chain, have serious drawbacks and alternative systems (synthetic ropes) become more feasible, see e.g. ref. [11. However the dynamic behaviour of a synthetic mooring system is different from a mooring with wire rope, because of the absence of a catenary and the time-dependent properties of synthetic materials. Thus a conventional design may result in a sub-optimum for a synthetic material.

Dyneema is a High Performance Polyethylene (HPPE) fibre, with a tensile strength similar to steel on a diameter basis, see also [2]. Ropes made of Dyneema are well suited for use in the marine environment under dynamic conditions (see ref. [3] & [4]). On the basis of these successful applications this material has been selected for this study.

A turret moored tanker is a dynamical system subject to excitation due to wind, waves and current, For the design of a mooring system excitation forces and dynamic responses in terms of loading and tanker motion are required. The excitation depends on the environment, the reaction forces are a combination of hydrodynamic forces and restoring characteristics of the mooring system, see ref. [5] for a general discussion.

The excitation forces can be divided in three groups, namely: average load, wave frequency (typical period: 5-20 see) and low frequency. The low frequency excitation is a combination of mean wind, current and wave drift forces, with a typical period of 50-500 sec. The natural frequency of the mooring system is generally in the low frequency range. The wave frequency loads are from the first order wave forces on the tanker and the resulting tanker motions are directly transferred to the mooring system. The average loads are from the steady wind, current and mean wave drift forces.

Here a 200 kDWT tanker has been moored in a water depth of 800 m, and the behaviour of the tanker plus mooring system is studied under survival conditions.

Different optimization criteria are used to evaluate a synthetic mooring system.

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