Chapter 23 Use of Operational Modal Analysis to Identify Systems with Oscillatory Masses Lasse Førde Thunbo, Niklas Carl Ørum-Nielsen, Tobias Friis, Sandro D. R. Amador, Evangelos Katsanos, and Rune Brincker Abstract Structures experience frequently moderate or even excessive structural vibrations due to their exposure to dynamic actions such as wind- and/or wave-induced loads. To mitigate the amplitude of those oscillations, passive damping systems can be used and their efficient performance is dependent on their tuning to the vibration properties of the structure, in which they have been installed. The Tuned Liquid Damper (TLD), being commonly a container including some kind of liquid, has been already used to damp excessive vibrations of different structural systems. However, there are structures that are expected to carry containers of varying size filled with liquids during their lifetime and hence, this inevitable, vibrationwise interaction between the structure and the container may eventually affect, either favorably or adversely, the structural response. The offshore platforms, being usually equipped with a tank at its top for the temporary storage of the extracted oil, constitute an example of structures with oscillatory mass at the top. Hence, it is rather appealing to investigate the dynamic behavior of this coupled system (offshore platform and the oil tank at its deck) and especially, to identify its modal properties. To do so, a thorough study, including both experimental and numerical investigation respectively, is undertaken and the robust Operational Modal Analysis is applied herein to estimate the effect of the oscillatory mass of the TLD on the dynamic properties of the offshore platform. Keywords Operational modal analysis · Random vibrations · Tuned liquid damper · Experimental investigation 23.1 Introduction The trend of using high strength and lightweight materials in structural systems often leads to an increasing flexibility and lack of inherent damping of structures that, in turn, may amplify their susceptibility to various sources of loads like, for example, the wind- and-wave induced excitations. In order to reduce the risk of potential structural failures, external damping devices have been widely used to mitigate excessive vibrations. Along these lines, a simple and economical passive damping device is a Tuned Liquid Damper (TLD), which is known to work effectively for structural systems that are subjected to lateral excitations while responding mainly in the low frequency range. The performance of a TLD, being commonly a container including some kind of liquid, is dependent on how well its own dynamic properties are tuned to the dynamic properties of the structural system per se. In other words, the appropriate interaction between the vibration of the structure and the self-vibration of the TLD defines the effectiveness of this passive damping system to reduce the response of the structure. However, there are structures that are expected to carry containers of varying size filled with liquids during their lifetime. Hence, such an inevitable, vibration-wise interaction between the structures and the containers may eventually affect, either favorably or adversely, the structural response. Along these lines, the offshore platforms are typical examples of structures that carry at their top tanks for the temporary storage of the extracted oil. Hence, it is rather appealing to investigate reliably this vibration-driven interaction between the oil tank and the offshore platform, the latter being continuously exposed to a multi-hazard environment consisting, among others, excessive wind and wave lateral loads. Such an investigation requires to scrutinize the dynamic properties of the coupled system by identifying its modal properties (frequencies, mode shapes and damping ratios). To do so, the robust Operational Modal Analysis (OMA) can be used assuming that vibration responses for both the structure and the tank are available. L. F. Thunbo · N. C. Ørum-Nielsen · T. Friis · S. D. R. Amador · E. Katsanos ( ) · R. Brincker Technical University of Denmark, Kongens Lyngby, Denmark e-mail: s143874@student.dtu.dk; s143857@student.dtu.dk; tofri@byg.dtu.dk; sdio@byg.dtu.dk; vakat@byg.dtu.dk; runeb@byg.dtu.dk © The Society for Experimental Mechanics, Inc. 2021 B. Dilworth (ed.), Topics in Modal Analysis & Testing, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-030-47717-2_23 227
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