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TECHNICAL PAPERS

Amplification of Waves by a Concrete Gravity Substructure: Linear Diffraction Analysis and Estimating the Extreme Crest Height

[+] Author and Article Information
E. J. van Iperen

 Shell International Exploration and Production, P.O. Box 60, 2280 AB Rijswijk, The NetherlandsErik.vanIperen@shell.com

G. Z. Forristall

 Shell International Exploration and Production, P.O. Box 60, 2280 AB Rijswijk, The Netherlandsg.forristall@shell.com

J. A. Battjes

Department of Civil Engineering, TU-Delft, P.O. Box 5048, 2600 GA Delft, The Netherlandsj.battjes@ct.tudelft.nl

J. A. Pinkster

Ship Hydromechanics Laboratory, TU-Delft, Mekelweg 2, 2628 CD Delft, The Netherlandsj.a.pinkster@wbmt.tudelft.nl

J. Offshore Mech. Arct. Eng 128(3), 211-223 (Jul 07, 2005) (13 pages) doi:10.1115/1.2199562 History: Received August 24, 2004; Revised July 07, 2005

Diffraction of both regular and irregular waves by a concrete gravity substructure (CGS) was investigated using experimental surface elevation data and computational results of the linear diffraction code DELFRAC . The influence of the box-shaped base that supports the four vertical columns was studied independently from the columns, using data from regular wave model tests of the Malampaya CGS. DELFRAC was shown to give accurate results for the focusing of waves over the submerged structure. Results from regular wave data analysis of model tests of the complete Sakhalin II project Lunskoye CGS were compared to the predictions by the linear diffraction code. For the wave cases tested, the first-order amplitudes were accurately predicted. Diffraction of irregular waves at the Lunskoye CGS was studied in a similar way and linear diffraction theory for random seas gave an excellent prediction of incident wave spectral diffraction, including the peaks in the diffracted spectrum near twice the peak frequency in the input spectrum. The results obtained for the Lunksoye CGS in the present study were consistent with results found in similar studies on less complex structures. An attempt to predict the extreme crest heights from the diffracted spectrum was made using a Weibull distribution, and a second-order expansion of the sea surface that captures the effects of wave steepness, water depth, and directional spreading with no other approximation than the truncation of the expansion at second order. Depth induced breaking appeared to be an important phenomenon limiting the crest heights. The crest heights in a 100-year sea state at the Lunskoye CGS were accurately predicted.

Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 6

Regular wave interaction with LUN-A CGS

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Figure 7

Amplification factor for irregular waves

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Figure 8

Spectra at right rear leg

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Figure 9

Diffracted spectra for two irregular sea-states

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Figure 10

Undisturbed spectrum and probabilities compared to the Rayleigh distribution for test 3

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Figure 11

Undisturbed probabilities compared to the Weibull distribution for tests 3, 4, and 5

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Figure 12

Test 3 undisturbed measurements and second-order simulation

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Figure 13

Test 4 undisturbed measurements and second-order simulation

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Figure 14

Test 3 measurements and simulation

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Figure 15

Test 4 measurements and simulation

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Figure 16

Test 5 measurements and simulation

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Figure 17

Depth limited wave heights during test 3 at right rear leg

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Figure 18

Depth limited wave heights during test 5 at right rear leg

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Figure 19

Comparison of DELFRAC fit to measurements for PA-B and LUN-A

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Figure 1

The Malampaya CGS

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Figure 2

The LUN-A CGS model

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Figure 3

The PA-B CGS model

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Figure 4

Wave interaction with caisson A

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Figure 5

Wave interaction with caisson B

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