In this paper, we experimentally investigate flexible cylinder multimodal responses and their corresponding frequency variations in vortex-induced vibrations. In the experiments, we test the response of a bending-dominated flexible cylinder under uniform flow conditions between the Reynolds numbers of 1600 and 4700 and examine the response for potential hysteretic effects. We study the nonlinear modal interactions by means of analyzing the spatial response using a multivariate analysis technique called generalized smooth orthogonal decomposition (GSOD). Results of this study show that 1:1 (inline to cross-flow) frequency ratios appear in one branch of a hysteresis region and 2:1 frequency ratios appear along a different branch and outside of the hysteresis regime. In addition, GSOD-based modal frequency response curves identify a subcritical Hopf bifurcation in the first two modal oscillations.
Technology, law, and the world's appetite for more energy have pushed oil and gas exploration farther from the shores. This need—moving into deeper waters—has brought extra challenges with it. For example, offshore structures inherently have become more prone to environmental loads that may easily lead to nonlinear responses. Understanding the nonlinear behavior of such structures is therefore critically important to the design and operation of these structures.