Operational modal analysis on a lighthouse structure subjected to ice actions
Torodd S. Nord (Norwegian University of Science and Technology (NTNU))
Knut Andreas Kvåle (Norwegian University of Science and Technology (NTNU))
Oyvind W. Petersen (Norwegian University of Science and Technology (NTNU))
Morten Bjerkås (Norwegian University of Science and Technology (NTNU))
Eliz Mari Lourens (TU Delft - Offshore Engineering)
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Abstract
The sea ice interaction with a structure may cause system changes and affect the feasibility of common approaches to track modal parameters. In this paper, a covariance-driven stochastic subspace identification method is used to identify the natural frequencies from 190 time series of ice actions against a lighthouse structure. The results are sorted into groups defined by the observed ice conditions and governing ice failure mechanisms during the ice-structure interaction. The identified natural frequencies vary substantially within each individual group and between the groups. Recordings with flexural failures and a north-south ice-drift direction consistently rendered the same identified frequencies, whereas crushing seemed to create large amounts of variability in the identified frequencies. The results show the need for more high-fidelity data to assess whether modern system identification methods can be used to monitor system changes and support decision-making for operations of structures in ice-infested waters, such as offshore wind structures.
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