Although the topic of roll damping of vessels at sea is already brought to the attention of naval architects by Froude more than 100 years ago, the physics of it remain intriguing, even today. An accurate prediction of the motions of offshore structures in harsh environments, designed for 25 years continuous operation, is the topic of this paper. Model test experiments for two FPSO’s developed by SBM for Petrobras are discussed. It is shown that the FPSO submerged riser balcony on one side of the vessel contributes to the roll damping through similar physics as the bilge keel does.Flow memory effects are discussed in detail since these are shown to have a noticeable effect on the roll damping coefficients. The paper further employs 3D CFD simulations to enhance the understanding of the fluid behaviour around the FPSO appendages, necessary to construct a rational and accurate roll damping model in the future.