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Ship dynamic intact stability

By PostDoc Jelena Vidic-Perunovic, the Technical University of Denmark, Department for Mechanical Engineering.



A known physical problem within modern shipping and ship dynamic stability is the parametric roll phenomenon, which represents sudden increase in roll amplitude in longitudinal sea caused by the parametric roll resonance. Due to its violent nature, the large accelerations associated with the onset of parametric roll cause concern for the safety of container ships. Possible consequences include loss of containers, structural damage and even capsize.

Parametric roll is related to the modern ship forms, with a slender hull and a large deck area, which has made these vessels prone to large underwater volumetric changes. This has lead to increased focus on how to counteract parametric roll, either by influencing the design or by changing operation procedures.

In the design phase, international classification societies are working on improving the tools and guidelines. In her project, Jelena has improved the existing model to determine parametric roll by using an accurate roll restoring righting lever and including surge motion of the ship. The physical model is based on a two degree of freedom (DOF) time domain procedure for prediction of parametric roll where roll and surge have been dynamically coupled through the variable encounter frequency.

This combination of motions has been recognized as important since the surge velocity can influence the resonant roll condition. The procedure for roll restoring righting arm calculations accounts for the exact volumetric changes of the submerged hull under the wave. Accurate hull geometry is taken by the method at every time-step, with respect to the instantaneous roll angle and sectional draught (including vertical ship motion and the wave elevation). In general, the finding is that the accuracy of the instantaneous GZ curve calculation is important for the predicted magnitude of the roll response i.e. for deterministic approach since the time-variation in roll restoring represents the main parametric excitation in the system.

Jelena’s model has been validated in an international benchmark study between a large number of different numerical tools. The model has proven to be highly able to predict the likelihood of parametric roll since it captures the resonant roll frequency accurately. 

Furthermore, in order to assess the factors influencing parametric roll, Jelena has applied a probabilistic approach that may be used as a scanning tool in the early ship design. Here an effort was done to find out the most feasible method, given the time frame where the ship master has a possibility to react.





  Papers on the topic:


Ship Dynamic Intact Stability Focus on Parametric Roll


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