Accurate prediction of roll damping is of utmost importance in estimating the roll motion of ships and other ship-shaped floating structures such as floating, production, storage and offloading (FPSO) and floating liquefied natural gas (FLNG) vessels. Roll damping is non-linear in nature and consists of several components including viscous damping which has been found to be highly dependent on flow separation patterns around the vessel. Industry practice of estimating roll damping by means of model testing and theoretical formulae has not always been reliable in estimating viscous damping due to limitations in both model tests and theoretical formulae. It is the objective of this study to use Computational Fluid Dynamics (CFD) methodology to predict roll damping and roll motion accurately for the concept design of a new-build barge-shaped FPSO vessel with bilge keels. In this study, assessment of roll damping is based on prescribed sinusoidal roll motions applied to the vessel in two-dimensional and three-dimensional CFD simulations. Validation of the CFD prediction is performed using published experimental results. From the CFD simulations, roll damping of a barge-shaped FPSO vessel is found to vary considerably with changes in the bilge corner shapes and bilge keel dimensions. Roll damping increases with bilge keel height and roll amplitude. Results from two-dimensional and three-dimensional simulations show differences which may be attributed to the length of the bilge keels and the bow and stern effect. From this study, the optimum bilge keel configuration for application of this new-build barge-shaped FPSO vessel in moderate environmental conditions is identified in the early design stage.