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Journal of Offshore Mechanics and Arctic Engineering | Volume 128 | Issue 4 | TECHNICAL PAPERS
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
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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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References

1
Guedes Soares, C., 1991, “Representation of Double-Peaked Sea Wave Spectra,” Ocean Eng.  [CrossRef], 18 (1/2), pp. 167–171.
2
Bitner-Gregersen, E. M., and Haver, S., 1991, “Joint Environmental Model for Reliability Calculations,” in "Proceedings of the 1st Offshore and Polar Engineering Conference"Edinburgh, UK, Vol. I , pp. 246–253.
3
Bitner-Gregersen, E. M., Haver, S., and Løseth, R., 1992, “Ultimate Limit States with Combined Load Processes,” in "Proceedings of the 2nd Offshore and Polar Engineering Conference", Vol. IV , pp. 515–522.
4
Yilmaz, O., and Incecik, A., 1998, “Effect of Double-Peaked Wave Spectra on the Behavior of Moored Semi-Submersibles,” Oceanic Eng. Int., 2 , pp. 54–64.
5
Ewans, K. C., van der Valk, C., Shaw, C., den Haan, J., Tromans, P., and Vanderschuren, L., 2003, “Oceanographic and Motion Response Statistics for the Operation of a Weathervaning LNG FPSO,” in "Proceedings of the 22nd International Conference on Offshore Mechanics and Arctic Engineering (OMAE)"New York: ASME; Paper OMAE2003-37394.
6
Strekalov, S., and Massel, S., 1971, “On the Spectral Analysis of Wind Waves,” "Arch. Hydro.", Gdansk, Poland.
7
Ochi, M. K., and Hubble, E. N., 1976, “Six-Parameter Wave Spectra,” in "Proceedings of the 15th Coastal Engineering Conference", pp. 301–328.
8
Guedes Soares, C., 1984, “Representation of Double-Peaked Sea Wave Spectra,” Ocean Eng., 11 , pp. 185–207.
9
Goda, Y., 1983, “Analysis of Wave Groupling and Spectra of Long-Travelled Swell,” Rep. Port Harbour Res. Inst., Jpn., 22 , pp. 3–41.
10
Torsethaugen, K., 1993, “A Two Peak Wave Spectrum Model,” in "Proceedings of the 12th International Conference on Offshore Mechanics and Arctic Engineering", Vol. 2 , ASME, pp. 175–180.
11
Torsethaugen, K., 1996, “Model for Double Peaked Wave Spectrum,” Report No. STF22 A96204, SINTEF Civil and Environmental Engineering, Trondheim, Norway.
12
Guedes Soares, C., and Nolasco, M. C., 1992, “Spectral Modelling of Sea States with Multiple Wave Systems,” J. Offshore Mech. Arct. Eng., 114 , pp. 278–284.
13
Rodriguez, G. R., and Guedes Soares, C., 1999, “A Criterion for the Automatic Identification of Multimodal Sea Wave Spectra,” Appl. Ocean. Res.  [CrossRef], 21 , pp. 329–333.
14
Hanson, J. L., and Phillips, O. M., 2001, “Automated Analysis of Ocean Surface Directional Wave Spectra,” J. Atmos. Ocean. Technol., 18 , pp. 277–293.
15
Ewans, K. C., 1998, “Observations of the Direction Spectrum in Fetch-Limited Waves,” J. Phys. Oceanogr.  [CrossRef], 28 , pp. 495–512.
16
Guedes Soares, C., and Henriques, A. C., 1998, “Fitting a Double-Peak Spectral Model to Measured Wave Spectra,” in "Proceedings of the 17th International Conference on Offshore Mechanics and Arctic Engineering (OMAE)" New York, ASME Paper No. OMAE-98-1491.
17
Glenn, S., 1992, Confidential Internal Shell Report.

Figures

Grahic Jump Location
+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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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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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.

Grahic Jump Location
+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.
View Large  |  Save Figure  |  Download Slide (.ppt)
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.
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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