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FPSO design is highly wave condition dependent. Both extreme response and fatigue life can be
significantly affected by site-specific wave environments. Collecting accurate wave data is an
important part of the design.
Wave spectral shapes have significant effects on fatigue life. Choosing the best suitable spectrum
based on the associated fetch and duration is required.
The bandwidth parameter E of responses is dependent on spectral (peak) period only. The effects of
H, on E can be ignored. The value of E can easily approach 0.6 for waves, 0.4 for stress responses.
In the calculation, E should not be simply ignored; Otherwise, an additional error of 5% to 10%
could be introduced.
In predicting extreme responses, the long-term approach is preferred because it has less uncertainly.
However, the authors recommend using the long-term approach together with the Short-term
approach for obtaining a conservative result.
The short-term extreme approach depends on long-term prediction of extreme wave spectra and
proper application of the derived wave spectral family. It is not necessarily simpler than the long-
term approach.
In the examples, probable extreme values predicted by long-term approach are larger than that by
short-term approach up to 9% (when a=]).
The proposed method for extreme response and fatigue assessment is reliable and applicable to any
type of FPSO, including FPSO conversions.
ACKNOWLEDGEMENTS

The authors wish to thank Dr. D. Liu, Senior Vice President, for his encouragement and support. The
views expressed in this paper are those of the authors and are not necessarily those of ABS.

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