The Influence of the Bow Shape of FPSOS on Drift Forces and Green Water

Abstract

Abstract In this paper the influence of the bow shape of FPSOS on drift forces and green water loading is studied. Use is made of the results of model tests and diffraction calculations for three different bow shapes: a traditional tanker bow, a sharp alternative bow without flare and the same alternative bow with a significant flare above the still waterline. For the sharper alternative bows the mean wave drift forces and low frequency drift forces seem to be smaller than for a traditional bow. Due to the fact that the wave frequency motions with the alternative bows are larger, it is not clear yet whether the total mooring line loads for this type of vessels will be lower. For the alternative bows the relative motions around the bow are larger. This results in more water on the deck with the alternative bows than with a traditional bow. Also the impact pressures are significantly higher. The results presented in this paper make clear that it is not sensible to optimize a bow of an FPSO on one aspect only, Introduction With the trend to bring FPSOS to increasingly harsh environments, the problem of solid green water on the bow becomes an important design aspect. This green water cm cause serious damage to the sensitive equipment or superstructure at the bow, as was experienced with operating FPSOS at the North Sea. There are two ways to cope with this problem: protecting critical structures against green water loading or minimizing the occurrence of green water occurrence and its loading. The achieve the last, the bow shape and bow height of the FPSO can be optimized. Especially for new build FPSOS this is an option, but also for existing tankers optimization are possible]. The effect of the bow shape on the occurrence of green water has been a point of discussion for Naval Architects over a large number of years. Some authors report a decrease in deck wetness when a significant flare is applied3 for traditional ships, whereas others find an increase in deck wetness4 with flare. In Ref. 1 the author presented tests with a traditional FPSO tanker with and without bow flare. It was found that for this particular bow the relative motions increase when flare is applied, whereas the water height on the deck decreases slightly. However, it was concluded at that time that this is not per definition a general trend. It should also be noted that it is not sensible to optimize the bow shape of FPSOS solely on green water loading. The bow shape can FPSOs effect other aspects of the design, like the (low frequency) drift forces on the ship and the related mooring forces. A bow optimized on green water loading, can may be increase the mooring forces and vice versa. The effect of the bow shape on the added resistance of sailing ships in waves has been studied by Blok5. Detailed published studies on the subject of drift force dependency on the bow shape of moored FPSOS are not known to the author at present.