Flow-induced vibration fatigue damage of two unequal-diameter flexible cylinders in side-by-side and tandem arrangements

Flow-induced vibrations (FIVs) are the indispensable factor causing the fatigue damage and failure for multi-cylindrical structures in offshore engineering. Mutual interferences between unequal-diameter cylinders complicate the fatigue behaviors. In this paper, an experiment was conducted on two flexible cylinders with a diameter ratio d/D of 0.5 (d and D denote the diameter of small and large cylinders) in uniform flow to investigate their fatigue damage characteristics. Two typical arrangements of side-by-side arrangement and tandem arrangement with a center-to-center spacing ratio P/d = 6.0 (P refers to the center-to-center distance between two cylinders) were examined. The results indicate that the fatigue damage is considerably affected by the arrangement and the diameter ratio. For the side-by-side arrangement, the small cylinder suffers from more serious fatigue damage in the CF direction due to the continuous impingement of the stable biased gap flow. However, the fatigue of the large cylinder is insensitive to the presence of the small cylinder. In a tandem arrangement, the wake shielding effect of the large cylinder located upstream is pronounced, resulting in a significant alleviation in the fatigue damage of the small cylinder. The wake inhibitory effect of the large cylinder is more prominent compared with that of two identical cylinders. The upstream large cylinder behaves analogously to the isolated cylinder. However, its CF fatigue damage is restrained in a few cases of 16.28 ≤ Vr ≤ 18.79 because the vortex shedding frequency is suppressed by the downstream small cylinder. On the other hand, when the small cylinder is located upstream, the wake shielding effect somewhat reduces the IL vibration of the large cylinder. The upstream small cylinder in this unequal-diameter experiment undergoes similar fatigue damage to the cylinder in the equal-diameter experiment.