This paper details an experimental study on bilge keels of up to 20% of half beam of a FPSO with rectangular geometry. Both free decay and forced oscillation tests were conducted on a range of bilge keel widths at different amplitudes and frequencies. The results show that, for a given amplitude of roll motion, the damping coefficient increases with increasing bilge keel size up to a certain point and then declines. Numerical simulations using the free surface random vortex method were performed on rectangular cross-sections with bilge keels which show good agreement with the experimental results. Simulations of the effect of even larger keels indicates that the same trend for damping coefficient as a function of keel size is found. An examination of the simulation results suggests a likely explanation for this behaviour.
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