This paper describes motion responses of an offshore petroleum floating production storage off loading system (FPSO) in ocean waves. A linearised spectral method is used for calculation of high/low frequency wave forces and surge motion of the FPSO in mooring state. The force between single point mooring (SPK) system and oil-storage tanker and tension of riser are also calculated. The model test of this FPSO is carried out to evaluate the applicability of linearised spectral technique in combined high/low frequency motion responses. It is proceeded in towing tank. It is proved that the results of model test is consistent with that of calculation.

1 Introduction

The single anchor leg storage system (SALS) consists of a mooring chain placed under high tension by a submerged buoyancy chamber, the vessel mooring forces being transmitted from the chain through a welded steel tubular yoke structure attached to the vessel by pivots. A major requirement for a reliable design is to achieve sufficient initial chain tension to prevent it from becoming slack under the action of wave forces, thereby leading to large snatch loads. High tension is also needed to minimise vessel surge motion and the associated angle of rotation of the yoke in order to limit pivot forces. The major advantages of the SALS system is that it is quick to construct, easy to install and is reliable under the most severe environmental conditions in addition to which it can be used in almost any water depth. The aim of the present paper is to give a linearised spectral dynamic analysis for the determination of random motions and pivot forces due to direct wave induced hydrodynamic loads and slowly varying second order wave forces. The spectrum of pivot and riser forces are also important in fatigue analysis.

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