Experimental investigation of the bending vibrations of a fast vessel

Abstract

Nowadays, the larger flexibility of ship structures, as well as wave-load spectra shifted to higher frequencies due to high-speed operating regimes, allow the onset of significant vibration phenomena. Recently, there has been increasing interest in combining seakeeping with vibration tests that may result in a deeper physical insight into this topic. In fact, experimental investigations on physical scaled models may benefit general purpose techniques for structural dynamic analysis in order to provide an interesting point of view of the hull vibration problem. The application of methods for modal identification, such as the Output-Only techniques, fits the present identification problem, particularly, where the main source of excitation for the structure is the ambient excitation due to the sea waves In this paper, the tests carried out in a towing tank basin with an elastically scaled and segmented model are analysed in order to investigate the bending response, ie. bending vibrations modes (shape, frequency and damping) and whipping oscillations. The modal parameters of the “dry” and “wet” modes of the elastic segmented model are identified with the Output-Only technique, based on the Frequency Domain Decomposition. Then, the ship response is analysed in terms of response amplitude operators and recorded time-histories. The correspondence with the full-scale ship, and the dependence of the aforementioned estimations with respect to the forward speed, are highlighted in the paper.

Keywords:

• Hydroelasticity
• model testing
• structural scaling
• elastic ship response
• equivalent Timoshenko beam
• whipping
• bending modes
• mode identification
• operational modal analysis
• output-only techniques

Additional information

Notes on contributors

D Dessi

Dr Daniele Dessi is a researcher at the Italian Ship Model Basin, Department of Vibration and Noise and a lecturer of numerical analysis at the University of Rome “La Sapienza”. Daniele received his masters degree in 1994 and his PhD in aerospace engineering in 1999. He is a member of the International Ship Structure Committee. His main research activities include hydroelasticity of marine structures, with particular emphasis on slamming related problems and elastic response analysis, and non-linear aeroelasticity. He is co-author of several publications in international journals and conference proceedings concerning fluid-structure interaction problems and non-linear dynamical systems.

R Mariani

Riccardo Mariani is a researcher at the Italian Ship Model Basin, Department of Vibration and Noise. He received his master degree in mechanical engineering in 2001. Riccardo’s main research activities include ship hydroelasticity, focusing in particular on analysis of wave induced vibrations and identification of operational modes. He is co-author of several publications in international journals and conference proceedings.

G Coppotelli

Giuliano Coppotelli is an assistant professor at the University of Rome “La Sapienza” in the Department of Aerospace Engineering and Astronautics. He received his degree in 1993. His main research activities include dynamic characterisation of aerospace structures, with emphasis on system identification, structural damage identification, model updating, aeroelasticity of both fixed and rotating wings, and dynamics of passive piezo-devices. He is co-author of several publications in international journals and conference proceedings.