This paper addresses a wave inference system developed for on-site estimation of directional wave spectra from the measurements of the wave-frequency motions of moored vessels. As a preliminary evaluation of the system’s performance, the results obtained in a 9-month field campaign based on an FPSO operating in Brazil’s Campos Basin are compared to hindcast predictions for the same region displayed by NOAA. The estimation method is based on Bayesian inference algorithms, previously validated by means of numerical and small-scale experimental analysis. A 6-dof inertial measurement unit (IMU) is used for monitoring the motions of the platform. The algorithm also requires the Response Amplitude Operators (RAOs), which in turn depend on the loading conditions of the FPSO. Those are defined before each estimation is performed by means of readings of the tanks’ levels. The accuracy of the predictions also depends on the calibration of two parameters (the so-called hyperparameters of the Bayesian method) and those are specified with respect to the mean period of heave motion. The system is able to identify unimodal or bimodal (cross) spectra using a bimodality criterion combined to an iterative algorithm for separating each component of the global spectrum. As a result, besides the global energy matrix, the statistical parameters required for reproducing the measured directional spectra by means of Jonswap and cosine2s models are provided. The system was commissioned and a 9-month validation campaign was executed. It is shown that the estimated parameters show quite reasonable correlation with satellite-based hindcasts from NOAA for sea states with peak periods larger than 8sec.

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