Although FPSO roll damping is well known nonlinear, most of the analysis which depends on platform motion evaluations, as riser, mooring and structural analysis are based on frequency domain approach results and transfer. Due to the nonlinearity, RAOs for roll, for instance, are dependent on FPSO motion amplitude, being different for each sea state of interest. Recent researches, however, have detected a saturation level in roll damping with extended bilge keels, which means a constant damping level for larger rolling amplitudes, where a linear coefficient (viscous based) can be used in platform motion analysis. Based in model tests where this saturation occurs, a nonlinear model was fitted to this damping data in order to predict the roll damping in extreme sea conditions. Those tests have taken into account aspects which have strong influence on viscous damping, loading conditions, hull form and bilge keel characteristics. The nonlinear model implemented could provide a safe level of damping comparing the results with extreme irregular wave model tests, becoming interesting for early design phases of such structures.

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