Abstract
Flexible cylinder structures, such as top-tensioned risers (TTRs) and tethers of tension leg platforms (TLPs), display complicated dynamic characteristics under the combined excitation of vortex-induced vibration (VIV) and time-varying axial tension, which causes serious fatigue damage and is a major concern in ocean engineering fields. In this paper, model tests were conducted on a flexible cylinder with an aspect ratio of 350 and a mass ratio of 1.90 to study the effect of time-varying tension on the VIV response. Three tension amplitude ratios (Tv/Tc = 0.1, 0.2 and 0.3, where Tv is the amplitude of the varying tension and Tc is the constant tension) and six tension frequency ratios (fv/f1 = 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0, where fv is the frequency of the varying tension and f1 is the 1st-order natural structural frequency) were considered. The Reynolds number ranged from approximately 800 to 16,000. The displacements were reconstructed with the measured strains using a model analysis method. The effects of the tension amplitude ratio and tension frequency ratio on the dynamic characteristics were discussed from the maximum root mean square (RMS) of the displacements, power spectral density (PSD) plots, time-space-varying displacements, and motion trajectories. The tension excitation places the vibration into a higher-order mode earlier. For a large tension amplitude ratio (Tv/Tc = 0.3), the max RMS of the cross-flow (CF) displacement remarkably increases because the frequency of the varying tension is equal to the parametric resonance frequency (fv/f1 = 2.0). The effect of the tension frequency ratio is enhanced with the increase in the amplitude ratio. For the case of fv/f1 = 2.0, the sum and difference frequencies disappear, and the bandwidth of the dominant frequency widens. Figures in the shape of the number 8 are not found in the motion trajectory plots for Tv/Tc = 0.3, indicating that energy transfer is influenced by tension excitation. The in-line (IL) vibrations show an alternate conversion between the 5th-order mode vibrations and the analogous 3rd-order mode vibrations.
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