The heave, roll and pitch damping of a CALM buoy contains linear contributions (wave radiation) and quadratic contributions (drag loads). The linear damping can be determined by diffraction-radiation calculations, but for the quadratic contributions model test data are required. In this paper a semi-empirical model is formulated for the calculation of the quadratic heave, pitch and roll damping.
Forced oscillation tests were carried out using a CALM buoy model with several different skirts. Different oscillation amplitudes and frequencies were considered. From the test results added mass and damping coefficients were determined. These results were used to validate the semi-empirical model.
Large deep water CALM buoys are becoming a common option for transfer of oil from a FPSO to a shuttle tanker, for example at a number of oil fields West of Africa. Although the environmental conditions are often relatively mild, an accurate prediction of the CALM buoy motions and the loads in the mooring lines and export risers is still important, for example for fatigue analysis. In Cozijn (2004) it was shown that for the accurate simulation of the behaviour of a moored CALM buoy in waves the use of a fully dynamic coupled mooring analysis method is essential. In this type of time-domain simulations the dynamic coupling effects between the CALM buoy and its mooring system are taken into account. However, as shown in Bunnik (2002) and Cozijn (2004), non-linearities in the wave loading and the hydrodynamic reaction forces remain complicating factors, which are not easy to model.
The CALM buoy heave, pitch and roll damping contain both linear and quadratic contributions. The quadratic contributions are of viscous origin (drag) and are mainly a result of eddies separating from the sharp edge of the buoy skirt.