Trans RINA, Vol 153, Part A4, Intl J Maritime Eng, Oct-Dec 2011
signals are analyzed using software and a specialized data acquisition system in order to obtain the operational deflection shapes. Figure 7 shows a computer mesh indicating the accelerometer positions, while Figures 8a and 8b show the analysed Operational Deflection Shapes (ODS) and cross spectrum in order to determine the energy content of the frequency.
Figure 6a: Photos of the actual aft structure of the vessel subject to vibrations (helideck).
Figure 8a: Operational Deflection Shape (ODS) of the helideck and aft mooring deck at 21 Hz.
Figure 6b: Photos of the actual aft structure of the vessel subject to vibrations (aft mooring deck).
Figure 8b: Cross power spectrum showing the energy content of each mode, in g2 .
Figure 7: Mesh coordinate display of the positions of the accelerometers in the 2 levels of the aft structure of the vessel (helideck and aft mooring deck.
Twenty accelerometers are then positioned on the helideck surface area as well as in the aft mooring deck surface in order to simultaneously record and capture the actual operational
deflection shape of the structure.
Figures 6a and 6b show the surface areas where the accelerometers are to be positioned. The accelerometers’
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excited by a strong 4th propeller BPF, which due to a low damping of the steel structure, causes a high overall vibration level. The vibration had already caused cracks and failure on the supports of the helipad rails, broken landing lights, cracks on life boat support ladders and discomfort in the accommodation block during the first two years of the operational life of the vessel.
From the cross power spectrum and the animation of the ODS’s, it is possible to observe the natural frequencies and the mode of vibration of the structure. From the modeshape, it is possible to understand the areas of the structure which could potentially need reinforcement. From the cross-spectrum (Figure 8b) at 21 Hz, it appears that there is a strong mode which involves substantial vibrational activity of the helideck and the aft mooring deck. Although 21 Hz is considerably greater than the first BPF of
the vessel, this natural frequency gets
In addition to the hydrodynamic problem requiring further investigation, it is decided to proceed with local
©2011: The Royal Institution of Naval Architects
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