Use of Operational Modal Analysis in Solving Ship Vibration Issues Anne Boorsma*, E. Peter Carden Lloyd’s Register EMEA, 71 Fenchurch St. London EC3M 4BS, UK *Corresponding Author: anne.boorsma@lr.org Abstract Vibration in ships can cause crew and passenger discomfort and induce structural cracking. The mitigation of excessive vibration requires knowledge of both excitation and response. Observation techniques and advanced Computational Fluid Dynamics have been applied successfully to investigate and characterise propeller excitation. Operational modal analysis (OMA) has proven to be a useful practical technique for investigating and solving vibration issues on ships. The use of these techniques is discussed in the context of two case studies: 1. The investigation of cracking in freshwater tanks due to vibration, and the use of OMA in tracing the transfer of energy from the propeller to the tank panels, are described. Observations of propeller cavitation and measurements of the pressure caused by its development are presented. Solutions to reduce the excitation energy and to shift the natural frequencies of the tank panels are discussed. 2. An example of vibration of a navigation bridge and accommodation block causing crew discomfort is presented. The use of OMA identified several modes that were excited by the main engine and the propeller during normal service conditions. This improved the understanding of vibration of the accommodation whilst reducing the risks associated with the uncertainty of implementing solutions. Introduction Vibration of ship structures is unavoidable because of design constraints such as the relative close proximity of the excitation sources and working and living quarters, the operating environment, construction and strength requirements and the desire to reduce the weight of the vessels and increase the payload. Vibration can cause crew and passenger discomfort and the assessment of vibration for these purposes is described for instance in the ISO6954-2000 standard [1] and in the Lloyd's Register’s Passenger and Crew Accommodation Comfort notation [2]. Higher, and sometimes localized levels of vibration can lead to failure of structural components through fatigue. Vibration levels above which fatigue is likely to occur are given for instance in the Lloyd's Register Ship Vibration and Noise Guidance Notes [3]. Primary sources of vibration excitation on board ships are the propeller, the main engine and auxiliary machinery. In many cases the main and auxiliary engines are reciprocating machines and acceptable levels of vibration for such machinery are set out for example in ISO10816-6 [4]. Levels of acceptable propeller excitation for different ship types are given in [4] Notwithstanding the identification of acceptable levels of excitation and vibration and the computational tools available to predict these in the design stages, ships with unacceptable vibration levels are still being built. This is in part a result of the very nature of shipbuilding, where only a very limited number of ships of one particular design are built, for instance due to different requirements from different owners. This makes it expensive to assess the vibration characteristics of each ship in detail. The prediction of vibration excitation, and in particular the propeller, is sometimes inaccurate. Engine builders in general make relatively large numbers of engines without significant modifications and have detailed guides on expected external engine forces and moments. The same can not be said for propellers that are usually designed specifically for each ship owing to the different hull shapes, shaft speeds, ship speeds, ship powers and operation profiles. Although propeller designers do take into account all these different factors, not all physical phenomena are accurately predicted by state of the art computational tools. This can make it difficult to design propellers with an acceptable pressure signature. T. Proulx (ed.), Modal Analysis Topics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series 6, 281 DOI 10.1007/978-1-4419-9299-4_24, © The Society for Experimental Mechanics, Inc. 2011
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