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Offshore and Structural Mechanics

An Analytical/Computational Approach in Assessing Vortex-Induced Vibration of a Variable Tension Riser

[+] Author and Article Information
Per M. Josefsson

Department of Mechanical Engineering, University of Houston, Houston, TX 77204-4006

Charles Dalton1

Department of Mechanical Engineering, University of Houston, Houston, TX 77204-4006dalton@uh.edu

1

Corresponding author.

J. Offshore Mech. Arct. Eng 132(3), 031302 (Mar 17, 2010) (7 pages) doi:10.1115/1.4000500 History: Received September 11, 2008; Revised September 16, 2009; Published March 17, 2010; Online March 17, 2010

The transverse vibratory response of a long, slender vertical top tension riser, subject to an ocean current, is studied. The problem is treated as a coupled fluid flow/vibration problem, which is solved numerically. The fluid flow part is represented by the 2D Navier–Stokes equations, with large-eddy simulation turbulence modeling and strip theory, which are solved numerically to obtain the flow field and determine the vortex-shedding behavior in the flow. The approach flow is a shear flow ranging in Reynolds number from 8000 to 10,000. Given the flow field and vortex-shedding behavior, the transverse fluid forcing function can be determined at a given instant, which becomes the input to the Euler–Bernoulli beam equation to calculate the displacement of the riser, using a technique that involves the Wentzel–Kramers–Brillouin (WKB) method and modal decomposition. The boundary conditions for the fluid flow equations are updated each time step as the cylinder moves. The natural frequency of the riser is tension dominated, not bending-stiffness dominated. With the decrease in tension with increasing depth, the natural frequency is affected. Therefore, the solution will be influenced by the depth-dependent tension. This study has indicated some interesting features regarding the vortex-induced vibration of a variable-tension riser. The vibrational response is greater for a variable-tension riser than for a constant-tension riser, when the variable-tension riser is assumed to have the same top tension as the constant-tension riser. Thus, this is one reason why it is important to take into account the variable tension when estimating fatigue failures of marine risers.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

The riser as represented in strip-theory formulation

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Figure 2

The rms displacements of a steel riser with constant tension and variable tension

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Figure 3

The maximum displacements of a steel riser with constant tension and variable tension

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Figure 4

Instantaneous displacement of an empty riser

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Figure 5

The rms displacements of a mud-filled riser

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Figure 6

The maximum displacements of a mud-filled riser

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Figure 7

Instantaneous displacement of a mud-filled riser

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Figure 8

Spectral distributions for plane 45 for the empty riser

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Figure 9

Spectral distributions for plane 45 for the mud-filled riser

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