Both hindcast and measured wave data were used to establish various descriptions of the swell spectra off West Africa in the West Africa Swell JIP (WASP). The effect on system responses of the various data types and swell spectral descriptions has been estimated.

The response of single degree of freedom oscillators to wave spectra can be used to produce maximum responses as a function of natural period and damping, analogous to the approach used in earthquake engineering. The results can be used to gain insight into the sensitivity of floating systems to the swell characteristics of the location and also to the differences between hindcast and measured data sets. Extreme values of system response can be calculated and used to estimate design spectra, by scaling the measured or hindcast spectrum that produced the largest response.

The sensitivity of the responses of floating systems to various simplifications of the hindcast spectra was evaluated by calculating the responses of an FPSO and CALM buoy for a location off Nigeria and southern Angola. It was found that there is little loss of accuracy if the hindcast spectra are decomposed into a maximum of two partitions, one of which may be a wind-sea, and the swell partitions are described with a lognormal function, the wind-sea partition with a JONSWAP-Glenn frequency function, and the directional spreading with a wrapped normal function, together with parametric forms for the widths of the lognormal and wrapped normal functions. As a consequence it is concluded that the parametric forms for the swell frequency and direction spreading can be used in design.

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