Margins of safety in FPSO hull strength

FULL REPORT

The important factors that should be considered when carrying out ultimate strength analysis are identified and discussed. Specific simplified procedures for determining ultimate strength are reviewed and compared against finite element approaches and probabilistic methods.
Sources of reserve and residual strength in FPSOs are identified and compared to those in discrete structural systems (e.g. offshore jacket structures). Performance indicators that are used for measuring the reserve and residual strength of structures are discussed and appropriate measures are selected for use in this study.
The report then draws back together the results from published investigations. There appears to be a trend that the reliability (i.e. factor of safety) in sagging is lower than that for hogging.
However, there is considerable variability in this, and so each hull arrangement needs to be considered on it’s own merits. Differences in reliability for sagging and hogging should ideally be minimized at the design stage for new installations, although there may be operational measures that can be adopted for existing installations.
Results from published investigations are then reinterpreted using the newly developed performance measures. Using simplified procedures for determining ultimate strength, five FPSOs representative of the North Sea fleet are also analysed. It is shown that hull configurations and relative member properties are important influences. Differences in the definitions of strength measures are noted, underlining difficulties in drawing comparisons between various FPSOs. However, it is clear that the margin (usually referred to as reserve strength) between the load corresponding to first component failure and the load corresponding to ultimate strength can vary considerably. It is also noted that the margin (usually referred to as the residual strength) between the load corresponding to ultimate strength and the load corresponding to some pre-defined displacement beyond the ultimate strength can vary considerably. It is concluded that there is no measure that can be used on its own to describe the performance of FPSO structures under extreme loading.

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