The responses of floating systems are sensitive to the detailed shape of the swell portion of the wave spectrum. Knowledge of swell is particularly important for sites off West Africa. The West Africa Swell Project (WASP JIP) was formed to analyze the available data on West African swell. Measurements were obtained from Shell, Ifremer, Chevron and Marathon. Hindcast data came from Oceanweather and the NOAA Wavewatch model. Sites ranging from Nigeria to Namibia were considered.
Modeling the dispersion of swell over long distances indicates that the resulting power spectrum will have a triangular or lognormal shape. Sampling variability makes it difficult to distinguish between those shapes or Jonswap or Gaussian forms, but lognormal spectra generally provided good fits. The models also indicate that the width of the spectrum in both frequency and direction should be inversely related to the peak frequency. Fits to the measurements established detailed relationships for each location.
Calculating the response of single degree of freedom oscillators to the measured and hindcast spectra produces response spectra which give the maximum response as a function of natural period and damping. Extreme values of system response can be calculated from the response spectra. The largest responses come from uni-modal spectra, and design spectra can be estimated by inflating them.