The Impact of Green Water on FPSO Design

Abstract

ABSTRACT For weather-vaning turret moored FPSO's in survival conditions the bow is always exposed to the wave action. When the waves come above the freeboard, this can result in considerable amounts of green water on the deck. This can cause damage to the sensitive equipment at the bow, like the fluid swivel, piping, turret structureand chemical stores. Taking also into account the tendency in several new designs to place the accommodation superstructure forward of the turret at the bow, the impact of green water on FPSO design will become more important in the future. Based on a series of mode/ tests this paper presents an rientation into the various aspects which play a role in the green water loading on FPSO units. First the occurring phenomena are described. Second the effect of the environmental conditions, such as the wave height, wave period and current velocity, are highlighted. Finally several design aspects are considered. The influence of the bow shape, the position of the equipment on the deck and the shape of breakwaters will be discussed. INTRODUCTION For ships and naval vessels, green water damage to superstructures, deck plating, hatches and topsides equipment is a common occurrence. However, methods for the prediction of green water or deck wetness arelimited in number. Based on model tests with a frigate, the author showed in [1] that these methods are sometimes based on assumptions which can be questioned. It was concluded that the development or improvement of these types of prediction methods is indispensable for the evaluation of the operability and safety of ships in rough seas. For turret moored FPSOs the same conclusion is valid. However, contraty to ships or naval vessels, permanently moored FPSOs cannot change their heading or speed when they come into survival conditions. They should therefore be able to deal with all the loads related to these extreme conditions. In the past the investigation of system safety was generally limited to the mooring loads, the low-frequency motion behavior and related topics. However, in recent years the impact loading due to slamming and green water has become a more important factor in the design of FPSO'S. Due to the fact that the bow of this type of weather-vaning system is always exposed to the wave action, considerable green water loading can occur to the sensitive equipment at the bow. The fluid swivel, piping, turret structure and chemical stores can therefore be damaged. Taking into account the tendency in several new FPSO designs to place the accommodation superstructure forward of the turret at the bow, the impact of green water on FPSO design will become more important in the future. The main objective of the present study was therefore to investigate the phenomena, problems and trends in green water occurrence and loading. This will be done based on an extensive model test series with a typical FPSO. The present paper presents an orientation into the problem. In a later stage, more detailed analysis [2] and computational aspects will be highlighted.