Bayesian oma of offshore rock lighthouses
Surprises with close modes, symmetry and alignment
James Mark William William Brownjohn (University of Exeter)
A. C. Raby (Plymouth University)
Siu Kui Au (University of Liverpool)
Zuo Zhu (University of Liverpool)
Xinrui Wang (University of Liverpool)
A. Antonini (TU Delft - Coastal Engineering)
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Abstract
A set of seven rock lighthouses around the British Isles was studied by a combination of forced and ambient vibration tests executed with some extreme logistical constraints. Forced vibration testing of the circular section masonry towers combined with experimental modal analysis identified modes with alignment assumed the same as the shaker as well as some interesting effects of helideck retrofit, whereas operational modal analysis revealed the considerable degree of uncertainty in mode shape alignment. Hence Bayesian operational modal analysis was used to characterise the uncertainty and find the best representation of mode shape direction. While perfectly axisymmetric towers would show a single frequency omnidirectional mode, OMA reveals the split modes and allows un unbiased view of directionality. The variability and uncertainty of these mode shape directions are further revealed using Bayesian OMA.