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Modal Analysis in Ice-Structure Interaction: Deciphering Ice Induced Vibrations, Part 2

Author(s): T. S. Nord; M. Maattanen

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Abstract: An experimental program Deciphering Ice Induced Vibrations (DIIV) was carried out in the Hamburg Ship Model basin (HSVA) through the EU Large-scale facilities program in August 2011. The main objectives were to facilitate understanding and quantify the physical parameters that control the ice induced vibrations. A compliant structure with the ability to change stiffness and mass with a cylindrical indenter at ice-action level was assembled on a carriage and forced through ice sheets. This is the second of three papers describing and analyzing these tests and deal with modal analysis and discusses how to extract ice forces from the measured structural response (displacement and strain-gauge signals). The theory to obtain forces in ice structure interaction, signal processing, indirect load measurements and frequency domain identification form the basis for this paper. A single input-multiple output (SIMO) system describes the experimental set-up, consisting of a load cell, an accelerometer, strain gauges and lasers. Dynamic stress relaxation calibration tests were carried out and the frequency response function (FRF) was established and compared for different output sources. The application of different window functions on the response and excitation signals had no significant improvement on the FRF. True ice force histories obtained from three different sources seem to address the forces correctly, as the histories are almost identical in different crushing regimes. The deviations between FRF components from different signals in frequency domain investigation remain to be identified. Unidentified noise is present in the data so that detecting all natural frequencies and the modal order are not fully achieved. And this is still necessary since the FRF is the link to obtain ice-forces from the measured input signals. The FRF approach offers an efficient method to measure the true ice load in dynamic ice structure interaction.


Year: 2012

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