Comparing Different Test Procedures to Determine the Coefficients of the Maneuvering Model for an FPSO

Maneuvering models for tanker based FPSOs are somewhat different from the classical maneuvering models. The reasons are zero or low forward speed (current), large mean drift angles, small values of rate of turn and relatively large low frequency (lf) transverse and yaw motions around the mean drift angle. A maneuvering model for a FPSO will be described in the paper. For a FPSO the maneuvering model must comply with both still water and a current field condition. Also the “twilight” zone being defined as the transient from a current field to still water and from still water to a current field (tidal change current) must be considered. In a current field, the coefficients of such a model consist of added mass coefficients, stationary current coefficients and dynamic current coefficients. In still water the coefficients should consist of added mass coefficients and the still water dynamic coefficients. The added mass coefficients $ω→0rad/s$ can be determined by 3-D potential theory. For the stationary current coefficients, classical towing tests for different headings may be carried out. For the determination of the hydrodynamic reaction force coefficients in both still water and in current two methods can be distinguished. With both methods the tanker is connected to the towing carriage by means of the PMM (Planar Motion Mechanism). By running the carriage current can be simulated. The test methods are either the yaw-rotating test or the yaw-oscillatory test. The pure yaw-rotating test is a dynamic test exposing the hull to different low advance velocities while the model rotates with constant rate of turn. In this way the hull will be exposed to the current for the full circle of 360 degrees. The pure yaw-oscillatory test is a dynamic test exposing the hull under a number of headings to different low advance speeds. The model is subjected to a low frequency and a large amplitude yaw motion around the mean yaw heading with regard to the current direction. If the maneuvering model is provided with the dynamic coefficients obtained from either the yaw-rotating tests or the oscillatory tests the results may differ. Model tests have been carried out using both methods. Results will be shown illustrating the difference in the force/moment components of the maneuvering models for a FPSO hull. In this paper the coefficients as used for the maneuvering model are derived from pure yaw-oscillatory tests. To validate the model recently PMM test series were carried for the combined sway and yaw modes of motion. The test series were performed in both still water and forward velocities. The formulation as derived from the pure yaw oscillating tests was applied to the combined yaw-sway motion and the results are presented.