identification of the mode shapes and their characterizations which was provided by OMA was extremely valuable in developing palliative measures. The identified mode shapes were useful in deciding the extent of the ship structure to include in the FE model. Conclusions Despite advances in computational techniques vibration problems still occur on board ships. For some problems it is possible to reduce the excitation energy as was shown for an LNG carrier (case study 1) where the location of a vortex generator could be optimized using Computational Fluid Dynamics. In cases where it is more difficult to alter the excitation (case study 2), operational modal analysis has proven to be useful for the detailed characterization of modes (natural frequencies and mode shapes), giving a better understanding of the problem and helping to reduce the risks associated with the implementation of structural modifications. Vibration problems onboard ships are most easily solved by modifying the excitation characteristics. However, if this is not possible, structural solutions are favored. References 1. Mechanical vibration - Guidelines for the measurement, reporting and evaluation of vibration with regard to habitability on passenger and merchant ships, ISO 6954:2000(E) 2. Passenger Crew and Accommodation Comfort Notation, 2007, Lloyd’s Register 3. Lloyd’s Register Ship Vibration and Noise Guidance Notes, 2006, Lloyd's Register 4. Mechanical vibration – Evaluations of machine vibration by measurement on non-rotating parts – part 6: Reciprocating machines with power ratings above 100kW, ISO 10816-6:1995 5. Van Overschee P, De Moor B, Subspace Identification for Linear Systems, 1996 Kluwer Academic Publishers, USA, pp254 6. Peeters B, De Roeck G, Reference-Based Stochastic Subspace Identification For Output-Only Modal Analysis, Mechanical Systems and Signal Processing (1999) 13(6), 855-878 7. Van der Auweraer H, Peeters B, Discriminating Physical Poles in High Order Systems: Use and Automation of the Stabilization Diagram, Instrumentation and Measurement Technology Conference, Como, Italy, 18-20 May 2004 288
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