Assessment of oscillation patterns of rigidly mounted prime movers through modal analysis (IGF-No. 13072 BR)

Responsible research centres: Rostock University (Prof. Prescher), Motoren- und Energietechnik GmbH (Prof. Bludszuweit)

Time period: 01.10.2001 - 31.08.2003

Final report: CMT 2/2004


Project summary:

 

The aim of this project is the identification of natural frequencies, mode shapes and modal damping ratios of big two-stroke engines in container vessels. The measurement results of a 5500 TEU vessel with 10RTA96C engine and two 2500 TEU vessels with 7L70MC and 7RTA72UB engines, that represent the state of the art, are presented. These big engines are coupled with the ship structure by different kind of top bracings (hydraulic and mechanical side stays). For all engines the lower modes (H-Mode and X-Mode) are identified, while for the bigger ten cylinder engine also a C-Mode is identified. It is confirmed, that the technique is fully developed to be used for measurements at shipyards because the measurements occurred at different conditions (in the harbour during charging and discharging, at the shipyard during construction period and by night).

It is shown that the top bracings cause an increase of the damping coefficients, especially for the hydraulic bracing. A large number of mechanical side stays can also cause a increase of damping ratio. The detuning of the system caused by the top bracings is characterised by a shift on the frequency axis of the resonance peak with maximum amplitude and higher damping of engine modes. Comparisons with FEcalculations indicate that the detuning causes a small change of natural frequencies. But the modal coupling causes that global modes get higher amplitudes than the modes without bracings so that the modal damping increases for engine modes.

The FE-calculations also indicate that implementation of modal damping ratios in numerical calculations increase the accuracy of resonant amplitudes. Best concordance of measured frequency response functions and FE-calculations is reached after a model updating.

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