Fatigue damage characteristics of a flexible cylinder under concomitant excitation of time-varying axial tension and VIV

Abstract

Flexible cylindrical structures are susceptible to the concomitant excitation of vortex-induced vibration (VIV) and time-varying axial tension in offshore engineering. The structural instability caused by the simultaneous excitation can lead to severe fatigue damage, threatening the safe application of cylindrical structures. In this paper, experimental research was conducted on a long flexible cylinder with an aspect ratio of L/D = 350 to investigate its fatigue characteristics under the concomitant excitation. The reduced velocity Vr ranged from 1.34 to 26.71. The axial tension excitation was characterized by three axial tension amplitude ratios Tv/Tc = 0.1–0.3 (where Tv is the tension amplitude and Tc is the constant tension) and six axial tension frequency ratios fv/f1 = 0.5–4.0 (where fv is the tension frequency and f1 is the fundamental frequency of the cylinder). The S–N curve method and the linear accumulative damage theory were employed to estimate the fatigue damage. The effects of reduced velocity, excitation amplitude and excitation frequency on the fatigue damage characteristics were discussed in terms of the fatigue damage distribution and maximum fatigue damage. The effect of the tension excitation on the fatigue damage is more pronounced at low reduced velocities (Vr ≤ 5.34) when the VIV is not excited, since the axial tension can excite the mode vibration. The fatigue damage area and maximum fatigue damage are significantly enhanced with the increase of the tension amplitude ratio and frequency ratio due to the occurrence of higher-order mode vibrations. The effect of axial tension excitation on the fatigue damage of the cylinder is more pronounced in the in-line (IL) direction than in the cross-flow (CF) direction. However, for a small amplitude ratio (Tv/Tc = 0.1), the tension excitation is only noticeable with a large frequency ratio (fv/f1 = 4.0). On the other hand, as the reduced velocity increases, the VIV of the cylinder is excited and dominates the fatigue characteristics. The accumulative fatigue damage distribution is aggravated with the increase of reduced velocity. The time-varying axial tension has a negligible effect on the fatigue damage distribution features and the maximum fatigue damage values of the cylinder. Nevertheless, under certain circumstances, the axial tension excitation can slightly alleviate the fatigue damage in some regions along the cylinder.