Modal testing and FE-model validation of azimuthing thruster

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.