Model tests were conducted to assess the effects of roll center on the roll damping of ship and FPSO sections. The damping was very sensitive to the position of the roll center and then can not be neglected for roll motion prediction. Also the optimum location of roll center can be suggested in the sense of reducing the roll motion for actual application. Typical sections of box type and bilge type were selected. The forced roll tests were carried out at a twodimensional wave flume. The vertical locations of roll center were shifted from near-bottom to above main deck level of the section. Instead of free-roll decay test, the location of roll center can be adjusted easily by adapting the forced roll test The roll damping for different positions of the roll center was measured and compared.
The roll motion responses of ship and offshore structures have been studied by many ship researchers. At an early stage, the seaworthiness of the crews and passengers was the most important factor. While, nowadays, the structural safety is an important issue for huge floating structures such as very large bulk carriers, mega-floats and FPSOs for their roll-induced torsional moment in oblique waves. Also many FPSO operators report that roll motion of the FPSO is important for satbty of production facilities (Park et. at., 1999). The roll motion depends much upon the roll damping at resonance. The roll damping moment is determined by section shape, roll frequency and the position of the roll center. The roll damping itself is very small. In this case of ships with zero forward speed, roll damping moment is caused by the radiation of surface waves, the skin friction and the eddy making. The last two components come from viscosity.
