Vortex/wake-induced vibrations characteristics of tandem composite risers at moderate Reynolds number

The vortex-induced vibration (VIV) characteristics of a composite riser become complex when subjected to wake interference in a tandem arrangement, resulting in impacts on the typical operation of offshore oil engineering. This study investigates the wake-induced vibration (WIV) characteristics of tandem composite risers with varying spacing ratios at moderate Reynolds numbers (Re = 1600–9600). Experiments were conducted at a uniform flow velocity ranging from 0.1 m/s - 0.6 m/s, with strain signals captured using Fiber Bragg Grating sensors. The experimental results indicate that the single unbonded composite riser exhibits an interlayer slip phenomenon as the flow velocity increases, resulting in reduced bending stiffness and increased displacement compared to the bonded composite riser at equivalent flow velocities. Analysis of the tandem unbonded composite riser reveals a decrease in the displacement of the downstream composite riser as the spacing ratio increases. The displacement interference rate is minimal at a spacing ratio of S/D = 8 in both directions. Furthermore, at moderate Reynolds numbers, the maximum displacement of the downstream riser decreases with an increase in the mass damping ratio for the same spacing ratio, and the velocity threshold with the higher value is more elevated.