This paper addresses the development, installation and initial tests of a system for wave spectra estimation from the measurements of the first order motions of a moored FPSO located in Campos Basin, Brazil. The estimation 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, and this information is sent to a remote data-base, also accessed by the wave-estimation system. The algorithm also requires the Response Amplitude Operators (RAOs), and they depend on the loading conditions of the FPSO.
A previous analysis considering typical loading configurations of the tanks showed that the wave estimation is mainly dependent on the total displacement of the vessel, and not on the load distribution among the tanks. Hence, the RAOs for the full-range of drafts (or total displacements) were numerically generated, considering a uniform distribution of the load among the tanks. Since the draft of the platform was not directly measured, the loading levels of the tanks are obtained from the automation system of the platform, and the draft is then estimated. Finally, the heading is measured by a gyrocompass, and it is necessary for the definition of the global wave direction. The Bayesian estimation is executed at time-spans of 30min. A parametric optimization algorithm is then applied for the calculation of the wave spectrum parameters from the raw-spectrum obtained by the Bayesian estimation.
A user-friendly interface was also developed, with on-line information about platform motions, estimated wave spectrum, peak statistics and data history. Since all information is accessed by network, the wave system can be installed either on-board or in the on-shore monitoring center.
The system was commissioned and a partial 3-month validation campaign was executed. The spectrum results were compared to NOAA estimates. As expected, low-period wave components (smaller than 8s) could not be estimated with accuracy, since the FPSO presents small motion response for these components. Swell and high-period wave components estimates presented good qualitative and quantitative agreement with satellite prediction.