Investigation on Devices for Preventing Parametric Rolling

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Firstly, to understand parametric rolling, an averaging technique was applied to an uncoupled mathematical model of roll and then threshold of parametric rolling and its bifurcation structures were identified. The restoring variation term, which is involved in the mathematical model, was examined with captive model tests for two container ships. As a result, it was shown that a conventional Froude-Krylov component on its own is not sufficient and the wave-making and hydrodynamic lift components are indispensable.Secondly, free-running experiments with a post-Panamax container ship model were conducted in regular, long-crested irregular and short-crested irregular waves. Here parametric rolling with the amplitude of about 20 degrees in head and bow seas was recorded and the effects of operational and environmental aspects were identified.Thirdly, for the trawler that suffered parametric rolling with the amplitude of 21 degrees in head seas without forward speed, the numerical analysis with a 6 degrees-of-freedom model demonstrated that the restoring variation as the Froude-Krylov component is negligibly small and the relationship between natural roll and pitch period is crucial.Finally, based on the obtained knowledge mentioned above, two types of anti-parametric rolling devices were proposed : one is midship sponsons for reducing the restoring variation and the other is an anti-rolling tank for increasing roll damping. These devices were installed in the container ship model and then a free-running experiment was carried out. The results demonstrate that sponsons that is 11.7% of the ship breadth in wide reduce 25% of parametric roll amplitude and the anti-roll tank of that weight is 3.2% of the ship displacement completely exclude parametric rolling. In addition, the numerical study indicated parametric roll of the trawler can be reduced by changing the pitch gyro radius.