Analyzing the Stabilizing Tank for the Control of Rolling Motion by Model Testing and the Dynamic Absorber Theory

The FPSOs are a type of offshore platforms that are directional in nature. Since a large ship (presently the use of a VLCC - Very Large Crude Carrier - is very common) is used, there is a great importance the direction the waves hit the hull. Differently from a real ship, the FPSO cannot resort to maneuvering to avoid waves. It has been found that there is a possibility, in certain cases, that a rolling resonant wave may reach some stationary FPSOs and consequently, a very high response may be obtained. Sometimes, it is not possible to use large bilge keel and the alternative is to consider the use of other devices such as stabilizing tanks and U-tubes. Faced with this problem, the present work performed model tests in a deepwater ocean basin showing the effectiveness of the stabilizing tank. On the other hand, it is clear that due to the presence of mass, restoring motion and damping that the design problem may be tackled by the use of classical dynamic absorber theory. For this reason, a simplified problem was formulated by replacing the stabilizing tank by a passive concentrated mass on board. The fully nine dimensional and non-linear model are then recovered. Six degrees of freedom are to describe the ship motion and the renaming three are for the mass on board. Based on these preliminary studies, the work describes the use of tests with reduced models showing the usefulness of the theory in practice. The test results together with 2 and 4 degrees of freedom system addresses the importance of the roll-sway coupling. Subsequently, a careful linearization is made for the purpose of identifying the commanding variables such as the mass, the position above the keel, the damping, the dynamic absorber natural frequencies, etc. After that, several parametric studies have been performed, identifying the range of applicability of the variables. Finally, this theoretical-experimental exercise addresses back the use of the applicability of the stabilizing tank.