Research Papers: Structures and Safety Reliability

Extreme Wave Condition at Doggerbank

[+] Author and Article Information
Espen Engebretsen

Oslo 0250, Norway
e-mail: e.engebretsen@icloud.com

Sverre K. Haver, Dag Myrhaug

Department of Marine Technology,
Norwegian University of Science
and Technology,
Trondheim 7491, Norway

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received March 23, 2014; final manuscript received April 25, 2016; published online June 2, 2016. Assoc. Editor: Lance Manuel.

J. Offshore Mech. Arct. Eng 138(4), 041601 (Jun 02, 2016) (11 pages) Paper No: OMAE-14-1033; doi: 10.1115/1.4033564 History: Received March 23, 2014; Revised April 25, 2016

In design of offshore wind turbines, extreme wave conditions are of interest. Usually, the design wave condition is taken as the sea state corresponding to an annual exceedance probability of 2 × 10−2, i.e., a return period of 50 years. A possible location for a future wind farm, consisting of bottom fixed wind turbines, is the Doggerbank area. The water depth in this area varies from about 60 m in the north to about 20 m in the south. The hindcast database NORA10 provides sea state characteristics from 1957 to present over a domain covering Doggerbank. Regarding the deeper areas just north of Doggerbank, this hindcast model is found to be of good quality. Larger uncertainties are associated with the hindcast results as we approach shallower water further south. The purpose of the present study is to compare sea state evolution over Doggerbank as reflected by NORA10 with the results of the commonly used shallow water hindcast model SWAN. The adequacy of the default parameters of SWAN for reflecting changes in wave conditions over a sloping bottom is investigated by comparison with model test results. Extreme wave conditions for two locations 102.5 km apart in a north–south direction are established using NORA10. This is done using both, an all sea states approach and a peak over threshold (POT) approach. Assuming the extremes for the northern position to represent good estimates, the wave evolution southward is analyzed using SWAN. The extreme condition obtained from NORA10 in the northern position is used as input to SWAN and the results from the two hindcast models are compared in the southern position. SWAN seems to suggest a somewhat faster decay over Doggerbank compared to NORA10.

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Fig. 1

Map indicating water depth in the North Sea as well as showing the location of Doggerbank

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Fig. 2

Doggerbank bathymetry and two locations where NORA10 data are available

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Fig. 3

Least square fit of Weibull distribution to hindcast data

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Fig. 4

Comparison of exponential and Weibull distribution fit to data points

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Fig. 5

Fitted function to the parameter μ of the conditional distribution of spectral peak period

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Fig. 6

Fitted function to the parameter σ of the conditional distribution of the spectral peak period

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Fig. 7

Fifty-year contour for point A based on hindcast data. Fifty-year sea state, data points, and validity range also indicated.

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Fig. 8

CDF of significant wave height at the northern point (point A) and the southern point (point B)

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Fig. 9

Visualization of grid, input boundary, and output locations used in SWAN analysis of model test results

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Fig. 10

Full-scale significant wave height as function of dimensionless water depth from ten model tests compared to two SWAN simulations

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Fig. 11

Mean measured wave spectrum and two input spectra (theoretical JONSWAP and mean measured) at full-scale depth d = 67.2 m. Note that red and blue line overlap.

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Fig. 12

Mean measured spectrum and calculated spectra from two SWAN simulations at full-scale water depth d = 15 m

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Fig. 13

Wind speed of the storms at point A which, according to NORA10, causes the three largest storms at point B

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Fig. 14

Hm0 for the 50-year sea state at point A and the storms at point A which corresponds to the three largest storms at point B as they propagate toward point B

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Fig. 15

Max significant wave height of the largest storm at point B and corresponding value at point A

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Fig. 16

Hm0 for the largest storm from point A with default settings, without bottom-induced breaking and without wind




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