The Reduction Of Low-Frequency Motions Of Moored Fpsoâ??S

Abstract

ABSTRACT In this paper the merits of a wave energy conversion device which directly converts wave energy into useful thrust directed into the oncoming waves is investigated both experimentally and by means of computations. The device consists of a horizontal airfoil section located at the keel level at the bow of the storage vessel. In high wave conditions the foil produces mean and low frequency thrust in counter-phase with the mean and low frequency wave drift forces acting on the vessel. The results of detailed experiments using a model of a 65000 dwt tanker in head seas verify that with the foil system a significant reduction in the overall forces on the vessel is obtained. Details are given of the vessel and foil characteristics as well as a review of tests and test results. INTRODUCTION Permanently moored tanker-based storage and production vessels have been in use by the offshore industry for many years. Such low-cost systems have been applied mostly in relatively mild climates due partly to the problems encountered in mooring large vessels in more harsh sea conditions. The reduction of oil prices, the increased knowledge with respect to the design of mooring systems and experience and confidence with use of such systems has, in recent years, led to an increasing number of applications of such vessels in more exposed locations. It is recognized that in a number of cases the economical feasibility of the particular system can be adversely affected by the higher costs of the mooring system which results from the more severe sea conditions for which it is designed. In order to make such systems economically viable disconnectable systems which make it possible to uncouple the vessel from its mooring at times when severe sea conditions prevail, have been designed and built. See reference [1]. In other cases purposely installed thruster systems which are intended to relieve the loads on the mooring system in severe sea conditions have been applied. See reference [2]. If such additional means are not applied the only means available to the designer in order to ensure the integrity of the mooring system and the survivability of the moored vessel is to design heavier mooring systems of which the component parts are capable of accepting higher loads. Another method of approaching the mooring system problem is to optimize the design of the vessel with a view to minimizing the mooring loads. Due to the fact that in general useis made of existing vessels this approach has not been followed systematically up to now. Reduction of the horizontal motions and mooring forces can also be achieved by an increase in the damping of the horizontal motions since the cause of the large amplitude motion and force components is partly due to the fact that the motions are very lightly damped and concentrated around the natural frequency of the moored system. See ref. [3].