Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
Figure 4b: Frequency Response Spectrum measured in the aft area of the vessel (helideck).
sixth. In some instances, the 3rd and the 4th Blade Passing Frequency
amplitude than the first BPF. The diagnosis of this frequency spectrum pattern is unusual
(BPF) components show much higher in vessels with
non-cavitating propeller operation and this pattern points to a hydrodynamic type of vibration-related problem. In a typical frequency spectrum pattern of a non-cavitating propeller, one would observe that only the first blade passing frequency is of significant amplitude compared to its higher multiples, which demonstrate much lower insignificant amplitudes.
In order to obtain a better understanding of the nature of the problem, a frequency sweep is
requested to be
performed, which, for a ship in operation, requires an acceleration from zero to MCR rpm and from MCR to 0 rpm. This would involve vessel’s acceleration from 0 rpm to MCR rpm and from MCR rpm to 0 rpm, under different
operating conditions (ballasted and fully
loaded). A cascade plot of Frequency Response Spectra, usually called “spectrogram”, is then recorded and analyzed in order to reveal possible resonances excited by the multitude of the propeller BPF’s. Potential resonances are revealed as horizontal or vertical lines of increasing brightness when intersected by a harmonic “order”, such as a Blade Passing Frequency component.
Frequency components intersecting with potential natural frequencies of the structure (faint yellow vertical lines can be seen in Figure 5). While the situation identifies a hydrodynamic problem stemming from the propeller, the natural frequencies of the affected structures still need to be determined. As it is not practical to perform modal tests through shaker testing on a vessel structure during a chartered voyage, it
is decided instead to check the
Operational Deflection Shapes (ODS) of the affected structures through vibration monitoring in order to get a better understanding of the modeshapes for potential reinforcement.
Figure 5: Spectrogram taken through run-up and run- down from the centre of the vessel’s helideck at the aft area of the vessel.
©2011: The Royal Institution of Naval Architects
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