Dynamic characteristics of a slender flexible cylinder excited by concomitant vortexinduced vibration and timevarying axial tension
Abstract
Flexible cylinder structures, such as toptensioned risers (TTRs) and tethers of tension leg platforms (TLPs), display complicated dynamic characteristics under the combined excitation of vortexinduced vibration (VIV) and timevarying 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 timevarying tension on the VIV response. Three tension amplitude ratios (T_{v}/T_{c} = 0.1, 0.2 and 0.3, where T_{v} is the amplitude of the varying tension and T_{c} is the constant tension) and six tension frequency ratios (f_{v}/f_{1} = 0.5, 1.0, 1.5, 2.0, 3.0 and 4.0, where f_{v} is the frequency of the varying tension and f_{1} is the 1storder 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, timespacevarying displacements, and motion trajectories. The tension excitation places the vibration into a higherorder mode earlier. For a large tension amplitude ratio (T_{v}/T_{c} = 0.3), the max RMS of the crossflow (CF) displacement remarkably increases because the frequency of the varying tension is equal to the parametric resonance frequency (f_{v}/f_{1} = 2.0). The effect of the tension frequency ratio is enhanced with the increase in the amplitude ratio. For the case of f_{v}/f_{1} = 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 T_{v}/T_{c} = 0.3, indicating that energy transfer is influenced by tension excitation. The inline (IL) vibrations show an alternate conversion between the 5thorder mode vibrations and the analogous 3rdorder mode vibrations.
 Publication:

Journal of Sound Vibration
 Pub Date:
 October 2020
 DOI:
 10.1016/j.jsv.2020.115524
 Bibcode:
 2020JSV...48515524M
 Keywords:

 Vortexinduced vibration;
 Axial tension excitation;
 Flexible cylinder;
 Dynamic characteristics;
 Experimental investigation