Roll damping is crucial for Floating Storage and Regasification Units (FSRU) design, especially due to operability needs during offloading when the unit is connected to oil/LNG carriers.

In ship-shaped FSRUs, the roll motion is largely determined by damping. Potential roll damping, which can be assessed by well-established calculation methods (strip theory or 3D potential solution), is responsible for a small part of the overall damping. Semi-analytical methods have been developed by Japanese naval architects to model viscous damping and eddy making phenomena, and fit a great amount of experimental data from model and full scale tests.

The bilge keels are an effective device to increase roll damping and reduce roll motion for ships with block coefficient less than 0.8. Practically they contribute to significantly reduce roll motion at resonance. Structural design of bilge keels requires insight studies that include hydrodynamics and forcing regime from fluid that are acting on the plates. Roll damping can be predicted by both calculation and model tests, as briefly presented in this paper, while the forces exerted are difficult to measure at model scale. Careful hydrodynamic studies are required, looking into hydraulic regimes of interaction. In particular, the flow condition around the plates shall be analysed taking into account the low Keulegan-Carpenter number and the fluid volume involved. Computational Fluid Dynamics can help. A correct load history is needed to design bilge keels for strength and fatigue life.

In this paper, the theoretical and experimental aspects of a specific FSRU bilge keels design are presented, with the problems incurring when model scale results are compared with results from literature formulas, and hydrodynamic flow interpretation.