In this paper, a series of systematic model tests are performed to estimate the roll damping for a tanker-based FPSO. As the wave damping for roll motion is relatively small compared to other modes of motion, the roll motions around natural periods will be overestimated unless the viscous damping is properly considered. Overestimated roll motions will give too conservative structure scantling for the upper deck structures. A typical midship section of a tanker-based FPSO and a wedge section are selected for tests in a two-dimensional wave tank. The models are made of wood with two different scales in order to account for the scale effect. The effects of bilge keel and bilge radius and forced roll amplitude on damping are also surveyed. The calculations by the potential theory are carried out using commercial computer programs based on the panel method. Appropriate empirical viscous damping factors are determined and provided for future practical use. These results are successfully applied for the FPSO which is converted from a VLCC and a newly built FPSO which has a boxtype hull form. From the studies, the roll damping for FPSOs can be estimated for practical purposes.
As the offshore oil fields march into the deeper ocean, FPSO become a popular solution even in the harsh environment. It has been granted that the turret-moored FPSO of ship type experiences only head seas due to the weather-vaning ability of the turret. However, ship operators report the significance of roll motion, which are aroused from different incident directions of waves from wind and current and the shortcrestedness of ocean waves. Such a roll motion depresses not only processing operations but also safety of tall local structures. They may be a flare boom, a turret swivel stack, a processing plant and their supporting structures.