Vortex-induced vibration experiments of flexible cylinders with floaters: effects of diameters, length, relative distance, total buoyancy and helical strakes

An experimental study was conducted on the vortex-induced vibrations (VIV) on flexible cylinders equipped with floaters (staggered buoyancy elements). This was done to gain a deep understanding of the fluid–structure interactions involved and also to provide a comprehensive set of findings for the validation of theoretical models. The flexible cylinder has a diameter of 21 mm and a total length of 3111 mm, made of a silicon tube filled with steel microspheres. Initially, a parametric modification was carried out that modified the number of floaters, their length, diameter, and relative spacing. Subsequently, an examination was performed taking into account the overall installed buoyancy. Lastly, to comprehend the source or decrease of the VIV, four arrangements of floaters were assessed in the context of helical strakes. Each case was tested in a towing tank at 20 reduced velocities in a range that was sufficient to excite the first three natural modes in the cross-flow direction, \(2.0< Vr < 18.5\) $2.0<Vr<18.5$ , corresponding to Reynolds numbers from \(1 \times 10^3\) $1\times {10}^3$ to \(1 \times 10^4\) $1\times {10}^4$ . Displacements along the cylinder were measured directly through a submerged motion capture system, tracking 22 reflective targets placed along the flexible cylinder. The VIV responses were analysed in terms of amplitudes and frequencies, allowing one to assess the influence of floaters parameters on the global cylinder dynamics. The results indicate that as the length or diameter of the floaters increases, the amplitude responses decrease. VIV appears to also manifest on the floaters when they are clustered around the central length. Conversely, if the floaters are too distant from each other, the VIV arises on the cylinder between them, practically without interference coming from the staggered condition. When comparing arrangements with approximately equal total buoyancy, the cross-flow amplitudes are lower when the arrangement responds mainly on the antisymmetric modal shape (AMS). As expected, strakes placed on the bare cylinder significantly reduce the amplitude of cross-flow responses. However, the effectiveness of the strakes is reduced when the floats are concentrated in the middle of the cylinder length and when the strakes are placed on the floaters. These findings have significant implications for the design of production risers, such as steel lazy-wave risers.