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Research Papers: CFD and VIV

Effects of Strake Coverage and Marine Growth on Flexible Cylinder Vortex-Induced Vibrations1

[+] Author and Article Information
Themistocles L. Resvanis

Mem. ASME
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: resvanis@mit.edu

Zhibiao Rao

Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: zbrao@mit.edu

J. Kim Vandiver

Professor of Mechanical and Ocean Engineering,
Mem. ASME
Department of Mechanical Engineering,
Massachusetts Institute of Technology,
Cambridge, MA 02139
e-mail: kimv@mit.edu

2Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 29, 2015; final manuscript received May 27, 2016; published online June 27, 2016. Assoc. Editor: Celso P. Pesce.

J. Offshore Mech. Arct. Eng 138(5), 051801 (Jun 27, 2016) (11 pages) Paper No: OMAE-15-1075; doi: 10.1115/1.4033821 History: Received July 29, 2015; Revised May 27, 2016

This paper presents some results from the recent SHELL tests at the MARINTEK basin. The tests involved towing densely instrumented flexible cylinders at Reynolds numbers up to 220,000. The main objective is to present the experimental results describing the effectiveness of different amounts of strake coverage and to explore the influence of simulated marine growth. The data are presented in terms of cross-flow (CF) response amplitudes and rainflow-counted damage rates due to the combined CF and in-line (IL) bending stresses. All the results are compared with the bare cylinder cases which will be used as a reference to determine how effective the strakes are in suppressing vortex-induced vibrations (VIV) and how this effectiveness can be affected by marine growth. The results show that even small bare sections (missing strakes) can lead to significant VIV response. Finally, the tests revealed that even moderate amounts of marine growth can quickly negate any suppression coming from the strakes.

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Figures

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Fig. 1

Photograph of straked and bare sections

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Fig. 2

Photograph of strakes with simulated marine growth

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Fig. 3

Cylinder cross section showing the combination of σCF and σIL at some arbitrary angle θi

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Fig. 4

The stress time-histories at four different four different locations around the circumference of the cross section: θ=0 (i.e., CF), θ=30 deg, θ=60 deg, and θ=90 deg (i.e., IL) around the circumference of the cross section at a position x/L∼0.06 along the cylinder length

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Fig. 5

VIV Response under uniform flow conditions of 1.4 m/s (test 4010) for the bare cylinder. Top: CF and IL RMS response amplitude; second from top: CF and IL RMS stresses (MPa) and the largest combination of the signals in both directions; third from top: CF and IL damage rates (1/yr) and the most damaging combination of the signals in both directions; and bottom: angle around the circumference of the cross section where the most damaging combination of CF and IL damage occurs.

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Fig. 6

Maximum damage rate (1/yr) versus towing speed (m/s) in (a) uniform flows and (b) sheared flows. Data shown includes all higher harmonics. CF and IL excited modes listed for a few selected cases.

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Fig. 7

Maximum damage rate (1/yr) versus towing speed (m/s) in (a) uniform flows and (b) sheared flows. Data shown excludes all higher harmonics.

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Fig. 8

VIV Response under uniform flow conditions of 1.5 m/s (test 5021) for the straked cylinder with a 15% long gap in the middle. Top: CF and IL RMS response amplitude; second from top: CF and IL RMS stresses (MPa) and the largest combination of the signals in both directions; third from top: CF and IL damage rates (1/yr) and the most damaging combination of the signals in both directions; and bottom: angle around the circumference of the cross section where the most damaging combination of CF and IL damage occurs.

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Fig. 9

Maximum RMS CF Amplitude at any location along the span versus towing speed (m/s) in uniform flows. The ‘error bars’ indicate the temporal variability of the RMS response computed using a short duration moving window over the entire record length.

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Fig. 10

Maximum damage rate (1/yr) at any location along the span versus towing speed (m/s) in uniform flows

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Fig. 11

Averaged PSD from all CF sensors for uniform flow of 1 m/s, effects of strake coverage

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Fig. 12

Dimensionless response amplitude, Af*, versus dimensionless damping parameter, cf*, computed with SHEAR7

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Fig. 13

Maximum damage rate (1/yr) at any location along the span versus towing speed (m/s) in uniform flows

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Fig. 14

Averaged PSD from all CF sensors for uniform flow of 1 m/s, effects of marine growth

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Fig. 15

Maximum damage rate (1/yr) at any location along the span versus towing speed (m/s) in sheared flows

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