Environment Severity Factors and Their Applications in FPSO Hull Strength Assessment

Abstract

Abstract This paper presents an approach to conduct the scantling design and strength assessment for a ship-shaped FPSO to be stationed at a specific site. By introducing the Environment Severity Factors (ESFs), the effects of the site-specific wave conditions and the mooring/risers as well as the topsidestructures on the loads of FPSO hull structure can be considered while the strength analyses and criteria can be kept as simple as those used in the design of ocean-going tankers. The determination of ESFs is based on the direct calculation of wave-induced and motion-induced loads. The determinations and applications of ESFs for two common types of wave conditions are discussed, in which:wave conditions can be represented by a wave spectrum of simple-form or by the long-term wave scatter diagram consisting of wave spectra of simple-form, such as JONSWAP wave spectrum andBretschneider wave spectrum.Wave conditions cannot be represented by a wave spectrum of simple-form, for example, for the cases that both seas (wind or storm generated waves)and swells exist. The latter becomes more complicated if the seas and swells propagate in different directions, which may occur in some offshore areas such as offshore Brazil and West of Africa. For the design and analysis of an FPSO conversion, the ESF concepts can also be used in assessing the fatigue damage accumulated during the past route services as an ocean-going tanker. The proposed approach for the strengthassessments can be applied to both the newly built and the converted FPSOs. This approach can facilitate the hull scantling design and strength assessment. Introduction A Floating Production, Storage and Offloading system (FPSO) is a popular type of vessel used in the offshore oil production. An FPSO may be a newbuild or a conversion from an oil tanker, which has been in service for some years. The discussion of the proposed approach covers both types of FPSOs. In general, FPSO design is more complicated and time consuming than tanker design for the following reasons:FPSO design requires the application of site-specific environmental conditions, while tanker design applies unrestricted-service wave conditions, which are rule-based for all ocean-going tankers. The rule-based design is generally much simpler than the case-by-case design. Besides, FPSO design also needs to consider the specific transit and installation conditions.FPSO design criteria will not be the same as those for tankers. For example, tanker design uses 20-year extreme wave conditions, while FPSO design uses 100-year waves.FPSOs have more complicated structural configurations. The effects of mooring/riser system and topside structures need to be considered in the FPSO hull design. Due to the interaction of these design considerations, the hull structure design may need to go through several iterations.