Influences of the helical strake cross-section shape on vortex-induced vibrations suppression for a long flexible cylinder

Wan-Hai Xu,Li-Qin Liu,Ying-Xiang Wu,Ying-Sen Luan

doi:10.1007/s13344-017-0050-1

Abstract

An experimental study on a bare flexible cylinder as well as cylinders fitted with two types of cross-sectioned helical strakes was carried out in a towing tank. The main purpose of this paper is to investigate the effects of strakes’ crosssection on the vortex-induced vibrations (VIV) suppression of a flexible cylinder. The square-sectioned and roundsectioned helical strakes were selected in the experimental tests. The uniform current was generated by towing the cylinder models along the tank using a towing carriage. The Reynolds number was in the range of 800–16000. The strain responses were measured by the strain gages in cross-flow (CF) and in-line (IL) directions. A modal analysis method was adopted to obtain the displacement responses using the strain signals in different measurement positions. The comparison of the experimental results among the bare cylinder, square-sectioned straked cylinder and roundsectioned straked cylinder was performed. The helical strakes can effectively reduce the strain amplitude, displacement amplitude, response frequencies and dominant modes of a flexible cylinder excited by VIV. And the mean drag coefficients of straked cylinders were approximately consistent with each other. In addition, the squaresectioned and round-sectioned strakes nearly share the similar VIV reduction behaviors. Sometimes, the strakes with round-section represent more excellent effects on the VIV suppression of response frequency than those with squaresection.

Highlights

Slender marine structures, such as drilling and production risers, tendons and mooring lines, and free spanning pipelines, could be subjected to vortex-induced vibrations (VIV) when exposed to ocean current flows
Much research work has been done on this typical fluid-structure interaction phenomenon throughout centuries, both numerically and experimentally, toward the understanding of the dynamic response features of VIV (Naudascher, 1987; Sarpkaya, 2004; Gabbai and Benaroya, 2005; Williamson and Govardhan, 2008; Wu et al, 2012b; Fan et al, 2015) and the VIV suppression (Zdravkovich, 1981; Bearman and Owen, 1998; Galvao et al, 2008; Assi et al, 2009; Wu et al, 2012a; Huera-Huarte, 2014; Gao et al, 2015a, 2015b, 2016)
4 Conclusions The VIV reduction of a flexible cylinder fitted with two types of cross-sectioned strakes were experimentally investigated in the towing tank

Summary

Introduction

Slender marine structures, such as drilling and production risers, tendons and mooring lines, and free spanning pipelines, could be subjected to vortex-induced vibrations (VIV) when exposed to ocean current flows. Among the various geometrical forms of devices for passive VIV suppression, helical strakes are the most common in both air and water, as can be seen in chimneys and subsea tubulars, such as risers, tendons, jumpers, and horizontal pipeline spans (Zdravkovich, 1981). Strakes can prevent the shedding from becoming correlated along the span, and destroy regular vortex shedding. This is why the efficiency of the VIV suppression is very well by means of helical strakes

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Conclusion