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TECHNICAL PAPERS

Estimation of Wind-Sea and Swell Components in a Bimodal Sea State

[+] Author and Article Information
K. C. Ewans

 Shell International Exploration and Production, Rijswijk, The NetherlandsKevin.ewans@shell.com

E. M. Bitner-Gregersen

 Det Norske Veritas, N-1322 Hovik, NorwayElzbieta.Bitner-Gregersen@dnv.com

C. Guedes Soares

Unit of Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico, 1049-001 Lisboa, Portugalguedess@mar.ist.utl.pt

J. Offshore Mech. Arct. Eng 128(4), 265-270 (Dec 18, 2004) (6 pages) doi:10.1115/1.2166655 History: Received September 24, 2004; Revised December 18, 2004

Methods for separating the spectral components and describing bimodal wave spectra are evaluated with reference to wave spectra from directional wave measurements made at the Maui location off the west coast of New Zealand. Two methods involve partitioning bimodal wave spectra into wind-sea and swell components and then fitting a spectral function to each component, while the third assigns an average spectral shape based on the integrated spectral parameters. The partitioning methods involve separating the wave spectrum into two frequency bands: a low-frequency peak, the swell component, and a high-frequency peak, the wind-sea. One partitioning method uses only the frequency spectrum while the other analyzes the complete frequency-direction spectrum. Comparison of the spectral descriptions and derived parameters against the measured counterparts provides insight into the accuracy of the different approaches to describing actual bimodal sea states.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

Comparison of modeled spectra with actual spectrum for 1 September 1987 1800h. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method.

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Figure 2

Comparison of modeled spectra with actual spectrum for 2 September 1987 0600h. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method.

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Figure 3

Comparison of modeled spectra with actual spectrum for 7 September 1987 0900h. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method.

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Figure 4

Comparison of modeled spectra with actual spectrum for 8 September 1987 1500h. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method.

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Figure 5

Scatter index of spectral models by comparison with the actual spectra. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method. μ and σ are the mean and standard deviations for the respective models, denoted by the subscript.

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Figure 6

rms spectrum and scatter index spectra for three model spectra. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method.

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Figure 7

Comparison of the significant wave height from the model spectra with that from the measured spectra. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method. The mean and standard deviation of the ratio of the modeled to measured significant wave heights are given in the upper left corner.

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Figure 8

Comparison of the mean wave period from the model spectra with that from the measured spectra. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method. The mean and standard deviation of the ratio of the modeled to measured significant wave heights are given in the upper left corner.

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Figure 9

Scatter plot of the significant wave height and mean wave period for modeled wind-sea and swell components. FD refers to the frequency domain model method, TH to the Torsethaugen model, and FDD to the frequency-direction domain method.

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