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ABSTRACT
This paper aims to present the vibration characteristics and vibration control of complex ship structures. It is shown that input mobility of a ship structure at engine supports, due to out-of-plane force or bending moment excitations, is governed by the flexural stiffness of the engine supports. The frequency averaged input mobility of the ship structure, due to such excitations, can be represented by those of the corresponding infinite beam. The torsional moment input mobility at the engine support can be estimated from the torsional response of the engine bed section under direct excitation. It is found that the inclusion of ship hull and deck plates in the ship structure model has little effect on the frequency-averaged response of the ship structure. This study also shows that vibration propagation in complex ship structures at low frequencies can be attenuated by imposing irregularities to the ring frame locations in ships. The structural modifications of the local supporting structures such as engine beds in ships can control the vibration responses of ship structures due to machinery excitations at higher frequencies.
Keywords: Hull structures, vibration response, FEA analysis, wave propagation.
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1. INTRODUCTION
An unwanted side effect of building faster and lighter ships is the increasing noise and vibration in ships. In order to retain the full benefit of building faster ships without compromising the ride comfort and safety, effective noise and vibration control needs to be implemented to ship structures. Nonetheless, due to the complexity of ship structures and the coupling of different wave types in the structure, control of wave propagation in ship structures by active control methods is expensive and ineffective, while traditional passive vibration control methods such as adding damping materials is only effective at higher frequencies. Most severe damage to ship structures, however, is caused by large deformation and high dynamic stress concentration from low frequency vibration. The low frequency noise and vibrations also contribute most to discomfort onboard ships. Consequently, alternative methods are sought in this paper to control ship structural vibration in the low frequency range.
Ship hull vibration can be generally classified into two categories, global and local vibrations. For global vibration, the whole hull girder of a ship is vibrating in response to the...