A Prediction Method for Horizontal Plane Behavior of FPSO Under the Single Point Mooring

Abstract

It is well known that in single point mooring or anchoring the slowly varying oscillation of a ship is caused by action of current and wind. During the slowly varying oscillation, extraordinary tension occurs in the mooring line when the ship’s yaw angle becomes nearly maximum, and incurs, as the case may be, in breakdown of mooring lines or unforeseen drift of anchors. Floating Production, Storage and Offloading (FPSO) systems are often moored as Single Point Mooring (SPM) systems. SPM systems can be Catenary Anchor Leg Mooring (CALM) systems or Single Anchor Leg Mooring (SALM) systems. It has been required to predict and evaluate performance of horizontal plane behavior of FPSO in current, wind and waves, since the workability and safety of FPSO become important from the stand point of the Life Cycle Engineering. Numerical simulation is one of the practical methods for prediction of FPSO performance and it needs quite accurate values of hydrodynamic coefficients in the mathematical model. Recently some attempts on improvement of accuracy in prediction of the hydrodynamic coefficients were made and approximate formulae for hydrodynamic derivatives including the interaction effect of main hull form and appendages were also proposed. Recently extensive studies for numerical models which describe components of hull, propeller, rudder, thruster, wind and waves separately, and these interactions have been made successively. In this paper, first, the basic equations of maneuvering motion are explained. And, an estimation method of slender body theory for hydrodynamic force acting on the hull is outlined. The authors explain numerical models to obtain FPSO coefficients for the horizontal plane behavior from mathematical model of ship maneuverability. And, numerical test of FPSO under the slowly varying oscillation is carried out. Finally, a new mathematical model is proposed to describe the current forces acting on FPSO under the slowly varying oscillation.