Large modern container ships feature an open cross section which results in a low stiffness of the vessel to global hull excitation. The contribution of whipping is a significant portion of the overall acceleration and stress response of such a vessel. Whipping results in an increase of the maximum hull girder bending moment experienced by the vessel as well as an increase in the fatigue accumulation of critical details.The whipping response can be determined using model tests or numerical simulations. In model tests, one can use a flexible backbone model, while different numerical tools exist which can predict whipping in time domain simulations. A hybrid solution comprising rigid body model tests with pressure sensors and a numerical assessment of the structural response based on these pressure loads has also been tested and applied in practice.In order to accurately model the whipping response, hydrodynamic and structural damping of the flexural vibration modes needs to be incorporated. However, damping is a challenging parameter to assess numerically. In this research, we present damping estimates based on in-service measurements from a container ship. An array of accelerometers was used to derive the first three vertical and first twist vibration modes. The flexural vibrations were isolated using two different methods: first, the Enhanced Frequency Domain Decomposition and secondly, the time domain Stochastic Subspace Identification. Both methods were applied to four representative voyages of the vessel covering a variety of environmental and loading conditions.On the two- and three-node vertical vibration modes, consistent results between both methods have been obtained. Overall, the time-domain method provided somewhat smaller damping estimates compared to the frequency-domain method. Both the four-node vertical vibration mode and the twist mode showed larger deviations between the two methods. It should be noted that these modes are less strongly excited and as a result, they are therefore identified less often and the results are associated with larger uncertainty. &nbsp;On the vertical vibration modes, the damping is between 0.5 and 2%. On the twist modes, the damping is significantly larger at 5% and up. The modal frequencies and damping have been correlated with the operational and environmental conditions. A correlation between the combined damping and the significant wave height was observed for the different flexural modes.

Damping Estimation Methods Applied to in Service Measurements of a 350 Metre Container Ship

Paper Proceedings: https://asmedigitalcollection.asme.org/OMAE/OMAE2021/volume/85116

OMAE 2021

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