Chapter 15 Effect of Friction-Induced Nonlinearity on OMA-Identified Dynamic Characteristics of Offshore Platform Models Tobias Friis, Antonios Orfanos, Evangelos Katsanos, Sandro Amador, and Rune Brincker Abstract The identification of the modal characteristics of engineering systems under operational conditions is commonly conducted with the use of the Operational Modal Analysis (OMA), being a class of useful tools employed within various fields of structural, mechanical as well as marine and naval engineering. The current OMA methods have been advanced on the basis of two fundamental, though, restrictive assumptions: (i) linearity and (ii) stationarity. Nevertheless, there are several applications that are inherently related to various nonlinear mechanisms, which, in turn, violate the two cornerstones of OMA and hence, question its robustness and efficiency. Along these lines, the current study addresses the effect of frictioninduced nonlinearity on OMA-identified dynamic characteristics of an experimental set up consisting of a pair of reduced scale offshore platform models that are connected through a friction-based mechanism. Both time-domain and frequencydomain methods were employed to assess the effect of the varying friction-induced nonlinearity on the OMA-identified modal characteristics. The findings of this study reveal that OMA-based methods provide reasonable identification results implying that nonlinear and nonstationary systems can be described by underlying linear systems, even though, in principles, the basic assumptions of linearity and stationarity are violated. Keywords Friction damping · Random vibrations · Operational modal analysis · Nonlinear systems · Experimental testing 15.1 Introduction The modal properties of engineering systems under operational conditions or ambient vibrations can be readily identified with the use of Operational Modal Analysis (OMA), being a robust framework associated with a class of useful tools employed already within various fields of engineering [1]. Since the 1960s, when the OMA was initially introduced, several advancements have been achieved and sophisticated OMA-based methods have refined the estimation of the modal characteristics and dynamic properties of structures with varying size, geometry, complexity and importance. A pair of fundamental principles regarding the (i) linearity and (ii) stationarity serves the theoretical basis for OMA [1, 2], and therefore, its employment provides reliable identification results for structural systems experiencing stationary response in the linear regime. Nevertheless, plenty of cases can be found, in which, for example, the nonlinear performance of structural systems violates the aforementioned principles of OMA and hence, the reliability of its application is rather questionable. More specifically, the use of friction dampers, which can be quite efficient to mitigate excessive structural vibrations, introduces nonlinearity that may cause the structural system to deviate from its linear response. It is, therefore, rather challenging to investigate the application of contemporary OMA-based methods on frictionally-damped systems and address the effect of the friction-induced nonlinearity on linear invariant systems on the basis of their OMA-identified dynamic characteristics. To the best knowledge of the authors, limited research effort has been already spent to assess the application of OMA for nonlinear systems. Along these lines, Zhang et al. [3] undertook a detailed study to evaluate the identification performance of several OMA methods for a prototype system in the presence of both stiffness- and damping-related nonlinearities. Based on their simulation and experimental study, it was concluded that the linear dynamic characteristics could be extracted from such nonlinear systems. However, the issue of friction-induced nonlinearity needs to be investigated, since this kind of nonlinearity has different characteristics compared to the aforementioned ones. T. Friis ( ) · A. Orfanos · E. Katsanos · S. Amador · R. Brincker Technical University of Denmark, Kgs. Lyngby, Denmark e-mail: tofri@byg.dtu.dk; vakat@byg.dtu.dk; sdio@byg.dtu.dk; runeb@byg.dtu.dk © The Society for Experimental Mechanics, Inc. 2019 D. Di Maio (ed.), Rotating Machinery, Vibro-Acoustics & Laser Vibrometry, Volume 7, Conference Proceedings of the Society for Experimental Mechanics Series, https://doi.org/10.1007/978-3-319-74693-7_15 153
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