Bilge keels are commonly installed on commercial ships and FPSO/FLNG units to effectively mitigate roll motions. Accurate predictions of the roll damping and hydrodynamic loads are critical for the structural design of the bilge keels. In this paper, model tests and Computational Fluid Dynamics (CFD) are employed to investigate the nonlinear roll damping and hydrodynamic loads on the bilge keels. Roll decay tests and CFD simulations are carried out for a FPSO vessel to estimate the roll damping and drag coefficients of the bilge keels with extended depths. Also seakeeping model tests and CFD simulations are carried out to estimate the hydrodynamic loads on the bilge keels in regular beam waves. From this study, it is found that the drag coefficients of the Ikeda formula (1976) and Sarpkaya & O'Keefe (1996), originally developed for wall-bounded plates, can substantially under-predict the drag loads on the bilge keels attached to a FPSO vessel without considering a velocity increment factor. This paper presents the first part of the Joint Development Project (JDP) between ABS and Chevron to develop the procedures for the strength and fatigue assessment of bilge keels for FPSO/FLNG units.