The roll damping of a FPSO assessment is a different subject than the ship case. The fact that the FPSO is not moving changes the flow hydrodynamics in such a way that the well established understanding is no longer applied. This is so at least for certain particularities such as flat bottom, no lift effect due to zero velocity, and so on. Recent researches have proven the strong effect of the vortex shedding on the roll damping of a FPSO mainly when large width bilge keel are present. Although these effects are known by a long time for ships, the increase of the vortex magnitude due the large width bilge keels on a FPSO has let to uncertainties about the behavior of the structures and the situation is challenging. It has been understood that the vortex can modify deeply the pressure distribution along the FPSO hull in such way that the final roll dissipation is higher. Surprisingly, under certain conditions the memory effects are small. The use of visualization techniques allied to the analysis of several decay tests for the same hull can help the understanding of several aspects such as the uncertainty in the measurements and the vortex behavior.

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