low high fast slow shaft speed frequency Vibration energy 8x 4x 6x shaft order mode 1 mode 2 mode 3 Fig. 11 A typical tanker deck house of a vessel unrelated to the work described in this paper Fig. 12 Frequency distribution of vibration energy of the navigation bridge during a shaft speed run up The peaks of energy at certain frequencies during the run up were the results of resonance of different modes of vibration with the diesel engine and propeller excitation. An operational modal analysis was performed and the resulting mode shapes, designated mode 1, 2 and 3, are shown in Figure 13. Mode 1 involved the accommodation moving as a cantilever with the bridge wings in phase with the navigation bridge. Mode 2 was a torsional mode of the accommodation with the bridge wings out of phase with the navigation bridge whereas mode 3 involved the accommodation moving as a cantilever with the bridge wings out of phase with the navigation bridge. Mode 1 Mode 2 Mode 3 Fig. 13 Mode shapes of the accommodation Mode 1 and Mode 2 were identified to be within 0.3Hz of each other, however, their character was very different which was seen through examination of their mode shapes. Mode 1 and mode 3 could be described as global modes in that modal ordinates of the accommodation block were of the similar magnitude to those of the bridge wings. In contrast, mode 2 was significantly more localized to the bridge wings. This explained why, during the run up (Figure 12), the maximum response was seen at the frequency corresponding to mode 2 compared to the lower responses seen at the frequencies corresponding to modes 1 and 3. In this case the excitation was not excessive and so possible solutions focused on making structural modifications and to this end an FE model was constructed. It is notable that the closeness of mode 1 and mode 2 would have meant that an operational deflection shape would have been misleading in estimating a mode shape and using it to validate the FE model. The 287
RkJQdWJsaXNoZXIy MTMzNzEzMQ==