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Research Papers

Kinematics Under Extreme Waves

[+] Author and Article Information
Carl Trygve Stansberg

 MARINTEK, P.O. Box 4125 Valentinlyst, N-7450 Trondheim, Norwaycarltrygve.stansberg@marintek.sintef.no

Ove T. Gudmestad

 Statoil, N-4035 Stavanger, Norwayotg@statoil.com

Sverre K. Haver

 Statoil, N-4035 Stavanger, Norwaysvha@statoil.com

J. Offshore Mech. Arct. Eng 130(2), 021010 (Jun 09, 2008) (7 pages) doi:10.1115/1.2904585 History: Received November 07, 2006; Revised August 22, 2007; Published June 09, 2008

Nonlinear contributions in near-surface particle velocities under extreme crests in random seas can be important in the prediction of wave loads. Four different prediction methods are compared in this paper. The purpose is to observe and evaluate differences in predicted particle velocities under high and extreme crests, and how well they agree with measurements. The study includes linear prediction, a second-order random wave model, Wheeler’s method [1970, “Method for Calculating Forces Produced by Irregular Waves  ,” JPT, J. Pet. Technol., pp. 359–367] and a new method proposed by Grue [2003, “Kinematics of Extreme Waves in Deep Water  ,” Appl. Ocean Res., 25, pp. 355–366]. Comparison to laboratory data is also made. The whole wave-zone range from below still water level up to the free surface is considered. Large nonlinear contributions are identified in the near-surface velocities. The results are interpreted to be correlated with the local steepness kA. Some scatter between the different methods is observed in the results. The comparison to experiments shows that among the methods included, the second-order random wave model works best in the whole range under a steep crest in deep or almost deep water, and is therefore recommended. The method of Grue works reasonably well for z>0, i.e., above the calm water level, while it overpredicts the velocities for z<0. Wheeler’s method, when used with a measured or a second-order input elevation record, predicts velocities fairly well at the free surface z=ηmax, but it underpredicts around z=0 and further below. The relative magnitude of this latter error is slightly smaller than the local steepness kA0 and can be quite significant in extreme waves. If Wheeler’s method is used with a linear input, the same error occurs in the whole range, i.e., also at the free surface.

FIGURES IN THIS ARTICLE
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Copyright © 2008 by American Society of Mechanical Engineers
Topics: Waves , Water , Kinematics
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Figures

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Figure 1

(a) Elevation time series and (b) normalized horizontal velocity profile under crest peak. Regular wave numerical simulation. Moderate steepness k0A0=0.10.

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Figure 2

As Fig. 1, but high steepness k0A0=0.30

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Figure 3

(a) Elevation time series and (b) normalized horizontal velocity profile under extreme crest peak. Bichromatic wave numerical simulation. Local max steepness k0A0=0.40.

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Figure 4

As Fig. 3, but irregular-wave event from numerical simulation. Local max steepness k0A0=0.38.

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Figure 7

As Fig. 6, but another event (Event 2). Local max steepness k0A0=0.39.

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Figure 6

(a) Elevation time series and (b) normalized horizontal velocity profile under extreme crest peak. Irregular-wave event, NHL-LDV experiment (Event 1). Local max steepness k0A0=0.35.

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Figure 5

(a) Elevation time series and (b) normalized horizontal velocity profile under extreme crest peak. Irregular-wave event, ocean basin experiment. Local max steepness k0A0=0.38.

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