Performance of helical strakes in suppressing the FIV fatigue damage of two long flexible cylinders in a tandem configuration

Abstract

Helical strakes are the most common vibration suppression devices for cylindrical structures undergoing vortex-shedding. Many problems with the performance of helical strakes on flow-induced vibration (FIV) fatigue damage suppression of two tandem cylinders have not been solved yet. The effectiveness of three-strand helical strakes with a pitch of 17.5D and a height of 0.25D (where D is the structural diameter) in suppressing fatigue damage of two tandem long flexible cylinders undergoing FIV was investigated in this paper. A constant spacing ratio (center-to-center distance to cylinder diameter) of 8.0 was selected. Four cases of the two cylinders, i.e., naked-naked, naked-straked, straked-naked and straked-straked, were executed under subcritical Reynolds number (Re ≤ 1.6 × 104). Based on the strain responses from the experimental tests, fatigue damage was estimated by using S–N curves and Miner's rule. To highlight the role of helical strakes in FIV fatigue damage suppression, an isolated naked cylinder was considered as a benchmark. It is shown that helical strakes exert substantially the same effect on FIV fatigue suppression of the upstream cylinder as they do on an isolated cylinder. The upstream cylinder significantly affects the performance of helical strakes on FIV fatigue suppression of the downstream cylinder. Both vortex breaking and prevention of interaction of two shear layers predominate the primary mechanism of the downstream straked cylinder when the upstream one is naked, which reinforces its cross-flow fatigue damage and hereby lowers the performance of helical strakes. In contrast, due to wake-induced vibration (WIV), the downstream straked cylinder laid behind a straked one might encounter much more serious fatigue damages in both the cross-flow and in-line directions than the case when the upstream cylinder is naked.