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TECHNICAL PAPERS

Dynamic Compression of Rigid and Flexible Risers: Experimental and Numerical Results

[+] Author and Article Information
Alexandre N. Simos

Department of Naval Architecture and Ocean Engineering, University of São Paulo, São Paulo, SP, Brazilalesimos@usp.br

André L. C. Fujarra

Department of Naval Architecture and Ocean Engineering, University of São Paulo, São Paulo, SP, Brazilafujarra@usp.br

It should be noticed that a similar behavior was observed by Aranha and Pinto (2) for the tension amplitude at the TDP of a steel catenary riser, predicted by means of numerical time-domain simulations performed with two different codes.

J. Offshore Mech. Arct. Eng 128(3), 233-240 (Jul 27, 2005) (8 pages) doi:10.1115/1.2199560 History: Received August 25, 2004; Revised July 27, 2005

Dynamic compression and buckling are critical issues in the viability analysis of rigid and flexible risers developed for offshore applications, especially concerning deep-water operations. Those subjects have been addressed both numerically and analytically. However, few experimental data for validation purposes is found in literature. This paper presents a set of experimental results on the dynamic compression of rigid and flexible risers in catenary configurations, obtained by means of towing-tank tests. Two small-scale models have been built, the first one emulating the dynamic behavior of a steel catenary riser (SCR) and the other representing a much more flexible line. Uniform circular motion has been applied to the top of the models, emulating the floating system first-order oscillations. Different amplitudes of top motion have been considered, each one of them imposed with different frequencies of oscillation. Tension has been measured at the top of the models. The influence of current velocity has also been evaluated. Dynamic tension estimations obtained through finite element analysis are compared to the experimental results. Tension amplitude and critical compression load values are evaluated and compared for both, the steel catenary (SCR) and the flexible models. Comparisons show, in general, a fair agreement between simulations and experiments, reassuring the reliability of numerical models. Results also demonstrate that finite element code provides good predictions of maximum tension loads even when the risers are subjected to high levels of dynamic compression and buckle. Nevertheless, it is clearly noted that difficulties arise in the treatment of flexible structures under severe buckling and torsion. The accuracy of analytical methods proposed for the estimation of critical compression loads is also discussed, based on the experimental results.

Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 17

Normalized minimum tension loads. Rigid model. V=0. Experimental: (엯)F=0.75Hz and (◻)F=1.50Hz. Extended numerical results: (●)F=0.75Hz and (∎)F=1.50Hz.

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Figure 18

Excerpt of normalized tension series. Rigid model. A=0.60m; F=0.75Hz; V=0.

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Figure 1

Sketch of the experimental setup at IPT

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Figure 2

Layout of the flexible small-scale model

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Figure 3

Time-series of the top tension on the rigid model. A=150mm, F=1.52Hz, V=0.

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Figure 5

Normalized maximum tension results. Rigid model. V=+0.2m∕s. Experimental: (▴)A=150mm, (∎)A=100mm, (◆)A=75mm. Numerical: (▵)A=150mm, (◻)A=100mm, (◇)A=75mm.

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Figure 4

Normalized maximum tension results. Rigid model. V=0. Experimental: (▴)A=150mm, (∎)A=100mm, (◆)A=75mm. Numerical: (▵)A=150mm, (◻)A=100mm, (◇)A=75mm.

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Figure 9

Normalized maximum tension results. Flexible model. V=0. Experimental: (▴)A=150mm, (∎)A=100mm, (◆)A=75mm, and (×)A=50mm. Numerical: (▵)A=150mm, (◻)A=100mm, (◇)A=75mm, and (+)A=50mm.

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Figure 6

Normalized maximum tension results. Rigid model. V=−0.2m∕s. Experimental: (▴)A=150mm, (∎)A=100mm, (◆)A=75mm. Numerical: (▵)A=150mm, (◻)A=100mm, (◇)A=75mm.

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Figure 7

Time-series of the top tension on the flexible model. A=150mm, F=1.50Hz, V=0.

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Figure 8

Propagating wave arising from buckling at the TDP region of the flexible model

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Figure 10

Normalized maximum tension results. Flexible model. V=+0.2m∕s. Experimental: (▴)A=150mm, (∎)A=100mm, (◆)A=75mm, and (×)A=50mm. Numerical: (▵)A=150mm, (◻)A=100mm, (◇)A=75mm, and (+)A=50mm.

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Figure 11

Normalized maximum tension results. Flexible model. V=−0.2m∕s. Experimental: (▴)A=150mm, (∎)A=100mm, (◆)A=75mm, and (×)A=50mm. Numerical: (▵)A=150mm, (◻)A=100mm, (◇)A=75mm, and (+)A=50mm.

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Figure 12

Excerpt of time-series of tension at the top of rigid model. A=150mm, F=1.00Hz; V=0. (—numerical; —experimental).

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Figure 13

Power Spectra of tension at the top of the rigid model. A=150mm, F=1.00Hz; V=0.

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Figure 14

Excerpt of time-series of tension at the top of flexible model. A=100mm, F=1.00Hz; V=0 (--- numerical; — experimental).

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Figure 15

Power spectra of tension at the top of flexible model. A=0.100m, F=1.00Hz; V=0.

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Figure 16

Experimental values of normalized minimum tension loads. Rigid Model. V=0. (엯)F=0.75Hz, (☆)F=1.00Hz, (*) F=1.125Hz and (◻)F=1.50Hz.

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