1) Azipod® is a registered trademark of ABB Modal Testing and FE-model Validation of Azimuthing Thruster Vesa Nieminen, Research Scientist, VTT Technical Research Centre of Finland, Vuorimiehentie 5, Espoo, P.O.Box 1000, FI-02044 VTT, Finland Matti Tervonen, Principal Lecturer, D. Sc. (Tech.), Seinäjoki University of Applied Sciences, Kampusranta 9A, FI-60320 Seinäjoki, Finland (previously in ABB Marine Oy) ABSTRACT Vibratory behavior of an azimuthing thruster was studied with FE-models and the results were verified by full-scale experiments. Studied thruster systems are used both for main propulsion and for steering of vessels. Modeling techniques were developed to take into account the most significant factors and phenomena affecting on the vibration behavior of the structure in real operation conditions. Modeling of structural properties such as bearings, hydraulic steering system, electro-mechanical interaction and rotor dynamics were investigated. The FE-model included also a part of the ship structure. Influence of the surrounding water on the vibration behavior was also studied. A combined FE-model for the structure and surrounding water were constructed and natural frequencies and modes were calculated. Vibration measurements were conducted in dry dock as well as during normal ship operations. Modal parameters in air and in water during operations were determined experimentally. In water the excitation came from ice block impacts and the modal parameters were estimated by Operational Modal Analysis. Operational Deflection Shape analysis was also utilized. Differences between the calculated and experimentally determined modal parameters in air were found to be small. The calculated global natural frequencies in water also corresponded reasonably well with the measured ones. Introduction Operating conditions and vessel installations have great effects on the dynamic behavior of marine structures. This is one of the main challenges met when reliable simulation and dimensioning models were developed for Azipod® 1) azimuthing thrusters, Fig. 1. So, the developed large and complicated FE-models had to be tested and verified in several stages and conditions. The main goals of this and the larger background study have been: • to find out all the phenomena that have significant effects on the dynamic behavior of the structure in real operating conditions • to develop modeling principles and techniques to take into account the phenomena affecting considerably on the vibration behavior of the structure in real operation conditions • to develop reliable measuring system for use in real severe operating conditions • to make the simulation models more accurate by developing them according to the measurement results • behavior of the system Full-scale dynamic tests with different analyses, i.e. conventional modal analysis, operating deflections shape analysis (ODS) and operational modal analysis (OMA), were performed both in dry-dock (thruster installed to the vessel but not in the water) and in totally real operation conditions. Modeling techniques and significance of the following topics for the vibration behavior were studied, see also Fig. 1: • main supporting bearing, i.e. slewing bearing • hydraulic steering system • surrounding ship structure • fluid-structure interaction 1 to find out explicit numerical values for the effects and significance of the real operating conditions on the dynamic T. Proulx (ed.), Linking Models and Experiments, Volume 2, Conference Proceedings of the Society for Experimental Mechanics Series 5, DOI 10.1007/978-1-4419-9305-2_1, © The Society for Experimental Mechanics, Inc. 2011
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