This paper demonstrates the method used by Lloyd’s Register (LR) to derive an equivalent design wave from a response based analysis (RBA) to represent extreme loads on a weather-vaning FPSO [1] and proceeds to compare the results with that of the industry practice of the response amplitude operator (RAO) based approach. The responses investigated include roll, pitch, vertical wave bending moments, vertical wave shear forces and vertical acceleration. The RBA is based on 3 hourly hindcast metocean data and uses the results of the heading analysis directly, considering the combined effect of wind, wind-sea, current and swell. An equivalent design wave is then derived based on the spectral characteristics of each response instead of the common practice for ship design [2] which uses only the characteristics of the RAOs. For each response the design wave for the RBA and RAO approaches is compared. Deriving equivalent design waves using only the RAO characteristics is found to give some non-conservative and unrealistic equivalent design waves in some cases.

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