Experimental and Computational Studies on a Cylinder with Continuous and Discrete Strakes

Abstract

Cylindrical cross sections are critical components in offshore structures, including jacket platform legs, pipelines, mooring lines, and risers. These cylindrical structures are subjected to vortex-induced vibrations (VIV) due to strong ocean currents, where vortices generated during fluid flow result in significant vibrations in crossflow and in-flow directions. Such vibrations can lead to severe damage to platforms, cables, and riser systems. Consequently, mitigating VIV caused by vortex-induced forces is important. This study investigates the hydrodynamic performance of five strake models relative to a bare cylinder at moderate Reynolds numbers. The models encompass one conventional continuous helical strake (HS) and four helical discrete strake (HDS) with varying segment spacing between the fins. The hydrodynamic performance, specifically lift and drag force coefficients, was computed using a Reynolds averaged Navier–Stokes-based CFD solver and validated with experimental measurements. The conventional HS suppresses 95% of the lift force but increases the drag force by up to a maximum of 48% in measurements. The HDS suppress the lift force by 70%–88% and increase the drag force by 15%–30%, which is less than the increase observed with the HS. Flow visualization showed that HS and HDS cylinders mitigate vortex-induced forces by altering the vortex-shedding pattern along the length of the cylinder. The HDS achieves a reduction in drag compared with the conventional continuous HS. The segment spacing is found to significantly impact the reduction in vortex-induced forces.

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