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Ocean Engineering

# Experimental Investigations of the Efficiency of Round-Sectioned Helical Strakes in Suppressing Vortex Induced Vibrations

[+] Author and Article Information
Raed K. Lubbad, Sveinung Løset, Geir Moe

Marine Civil Engineering Group, Norwegian University of Science and Technology (NTNU), Trondheim, Norway

J. Offshore Mech. Arct. Eng 133(4), 041102 (Apr 07, 2011) (10 pages) doi:10.1115/1.4002732 History: Received April 01, 2010; Revised August 12, 2010; Published April 07, 2011; Online April 07, 2011

## Abstract

Vortex induced vibrations (VIVs) may cause a large amount of damage to deep water risers. Helical strakes are used as a mitigating measure to suppress these vibrations. The purpose of this paper is to verify the efficiency of round-sectioned helical strakes in suppressing VIV. It is believed that round-sectioned helical strakes can be more readily mounted on risers for intervention and maintenance compared with sharp-edged strakes that may have to be welded onto the risers. Systematic experimental investigations including 28 configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The experiments were performed in a steady flow flume with an elastically mounted rigid circular cylinder of 500 mm in length and 50 mm in outer diameter. The test cylinder was spring-supported in both the inline and cross-flow directions. The measurements were limited to mapping the displacement of the cylinder. First, the cylinder was tested without strakes as a reference case. The best configuration among the tested round-sectioned helical strake configurations was found to reduce the amplitude of oscillation relative to the bare cylinder case by 96% in the cross-flow direction and by 97% in the inline direction. The main features of this configuration are the number of starts (3), the pitch $(5D)$, and the diameter of the strake $(0.15D)$, where $D$ is the outer diameter of the test cylinder. Additionally, this paper investigates the effects of varying pitch, the effects of surface roughness, and the effects of the ratio between the cross-flow and inline natural frequencies of the test rig on the efficiency of the suggested configuration of round-sectioned helical strakes.

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## Figures

Figure 1

Sketch of the spinning machine that fits ropes helically to the riser during the installation

Figure 2

A schematic presentation of the rig used in the present study: (a) side view and (b) inline view

Figure 3

A picture of the cables that were used as helical strakes during the tests (scale in centimeter)

Figure 4

The locations of the peaks in the time series

Figure 5

The response of bare cylinder versus reduced velocity, (a) CF, and (b) IL

Figure 6

Maximum oscillation amplitude, (A/D)max, versus number of starts at a strake pitch of 5D: (a) CF and (b) IL

Figure 7

Maximum oscillation amplitude, (A/D)max, versus number of starts at a strake pitch of 10D: (a) CF and (b) IL

Figure 8

The relationship between the maximum oscillation amplitude, (A/D)max, and the diameter of the strakes, d, for 3 starts and the 5D pitch configuration: (a) CF and (b) IL

Figure 9

Picture of the test cylinder fitted with the selected configuration of round-sectioned helical strakes

Figure 10

Response of straked cylinder, d/D=0.15, three starts, various pitches: (a) CF and (b) IL.

Figure 11

Strake pitch versus the percentage reduction of the vibration amplitude relative to the bare cylinder

Figure 12

Response of the selected configuration at various frequency ratios: (a) k/D=0.003 and (b) k/D=0.012

Figure 13

Response of the selected configuration at various frequency ratios where the strakes are smooth and the cylinder is rough (k/D=0.0025)

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