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Research Papers

Riser Response Analysis by Modal Phase Reconstruction

[+] Author and Article Information
Didier Lucor

Institut Jean Le Rond D’Alembert,  Université Pierre et Marie Curie, Paris, France

M. S. Triantafyllou

Department of Ocean Engineering,  Massachussetts Institute of Technology, Cambridge, MA 01239

J. Offshore Mech. Arct. Eng 130(1), 011008 (Feb 08, 2008) (9 pages) doi:10.1115/1.2827877 History: Received October 26, 2006; Revised March 08, 2007; Published February 08, 2008

A methodology is presented to identify the vortex-induced vibrational modes of a riser, based on data from computational fluid dynamics coupled to a long beam under tension and placed in cross flow; two inflow velocity profiles are tested: (a) uniform and (b) exponential shear. The modes we estimate are, in fact, nonlinear equilibria between the flow-induced excitation forces and the structural dynamics and are characterized by varying amplitude and phase along the span; these are complex modes, mixtures of traveling and standing waves.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Left: instantaneous riser positions at different time instants; Right: rms response values

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Figure 2

Spanwise distribution of the modal amplitude response ∣ϕn∣

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Figure 3

Span-averaged (black curve) and span-deviation (shaded band) distributions of the modal amplitude response ∣ϕn∣

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Figure 4

Spanwise distribution of the unwrapped phase angle response θn

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Figure 5

Spanwise distribution of wave number kn of the riser at all frequencies (gray scale isocontours) with superimposed modal amplitude response (white isolines)

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Figure 6

(a) Spanwise unwrapped phase angle θn(zj) at two different frequencies. (b) Corresponding spanwise wave number kn.

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Figure 7

Spanwise normalized inflow velocity profile for the exponential case. The maximum current is at the ocean surface.

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Figure 8

Left: instantaneous riser positions at different time instants; Right: rms response values

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Figure 9

Spanwise distribution of the modal amplitude response ∣ϕn∣

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Figure 10

Span-averaged (black curve) and span-deviation (shaded band) distributions of the modal amplitude response ∣ϕn∣

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Figure 11

Spanwise distribution of the unwrapped phase angle response θn

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Figure 12

Spanwise wave number kn of the riser at all frequencies

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Figure 13

(a) Spanwise distribution of (unwrapped) phase angle θn(zj) at three different frequencies. (b) Corresponding spanwise wave number kn.

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Figure 14

Comparison between theoretical and numerical span-averaged phase velocity modal distributions

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