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.
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. Both the four-node vertical vibration mode and the twist mode showed larger deviations between the two methods. 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. A correlation between the combined damping and the significant wave height was observed for the different flexural modes.