Abstract

Helical strakes are widely used to suppress VIV of slender structures. In this study, the effects of the pitch and height of helical strakes on suppression of VIV were studied experimentally. Experimental results show that the ratio of strake pitch and height could be an indicator to discuss the suppressing efficiency of helical strakes.

Introduction

In the field of ocean industries, there are many cylindrical structures. Marine risers are one of them. These structures receive various external forces such as hydrodynamic force by waves and currents. Meanwhile, vortices shed from the structures by the current or the motion of structures. They cause a vibration of structures. This phenomenon is called Vortex-Induced Vibration (VIV). VIV is not only to the cross flow direction. It contains a high frequency component to the main flow (In-line) direction. This in-line oscillation is also important as well as transvers one. These vibrations cause fatigue damage because VIV gives structures cyclic load. A response of a cylinder with two degrees of freedom had been reported (N. Jauvtis, 2003). It is also a problem that VIV causes an increase of drag force. Therefore, VIV of structures should be suppressed. Blevins and Coughran (2009) proposed a guide lines for avoidance of VIV using two parameters, which are mass and damping of bare cylinder.

There has been a great deal of work concerned with VIV suppressing device. It is revealed that the appendages such as splitter plates and fairings are effective to suppress VIV of the structures, riser pipes, and cables. If some appendages are attached to the structures, the frequency and amplitude of VIV will change. It is also noted that such appendages affect on the torsional force acting on the structures. For example, Chung and Whitney (1993) examined the effects of straight cables on VIV and the torsion of the pipes or cables under various flow angles of attack. The parameters in their study were the diameter and the number of straight and helical cables attached on the bare cylinder.

In late years, helical strakes are used widely because it is easy to handle. There are many researches to investigate the strake effect on suppressing VIV (Trim, 2005; Gao, 2015). Even though the effects of strake height to suppress VIV was also reported in the paper described above (Blevins and Coughran, 2009), there is some research on the effects of both the strake height and pitch to suppress VIV.

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