Experimental investigation on the effectiveness of serrated helical strakes in suppressing vortex-induced vibrations for a flexible riser

Abstract

The effectiveness of an unconventional configuration of serrated helical strakes (SSs) in alleviating the vortex-induced vibrations (VIVs) of flexible risers was evaluated experimentally in the Reynolds number range of 2600–10,400. Five SSs with varying serration densities and lengths were designed and compared with conventional strakes (CSs) to highlight the unparalleled features of this new geometry. The VIV parameters of the bare and straked risers, including the displacement, frequency, fatigue damage, and suppression efficiency, were systematically investigated. The comprehensive evaluation of the helical strakes was also made considering the factors of amplitude suppression, frequency inhibition, drag force, and cost control. The experimental results confirmed that all the helical strakes were capable of suppressing the VIV responses of flexible riser models. With identical strake areas, the SSs were more effective in inhibiting the VIV amplitude and fatigue damage than the CSs; whereas, the serrated geometry had few effects in improving the reduction of VIV frequency. Moreover, serration density is a critical factor in determining the suppression performance of SSs under the same strake heights and areas. An increase in the strake area enhanced VIV suppression performance; however, excess strakes deteriorated the comprehensive evaluation index due to the increased cost.