Sea state 1

Sea state

NOAA ship Delaware II in foul weather onGeorges Bank.

In oceanography, a sea state is the general condition of the free surfaceon a large body of water—with respect to wind waves and swell—at acertain location and moment. A sea state is characterized by statistics,including the wave height, period, and power spectrum. The sea statevaries with time, as the wind conditions or swell conditions change.The sea state can either be assessed by an experienced observer, like atrained mariner, or through instruments like weather buoys, wave radaror remote sensing satellites.

In case of buoy measurements, the statistics are determined for a timeinterval in which the sea state can be considered to be constant. Thisduration has to be much longer than the individual wave period, butsmaller than the period in which the wind and swell conditions varysignificantly. Typically, records of one hundred to thousandwave-periods are used to determine the wave statistics.The large number of variables involved in creating the sea state cannotbe quickly and easily summarized, so simpler scales are used to give anapproximate but concise description of conditions for reporting in aship's log or similar record.

World Meteorological Organization sea state codeThe WMO sea state code largely adopts the 'wind sea' definition of the Douglas Sea Scale.

WMO Sea State Code Wave Height (meters) Characteristics

0 0 Calm (glassy)

1 0 to 0.1 Calm (rippled)

2 0.1 to 0.5 Smooth (wavelets)

3 0.5 to 1.25 Slight

4 1.25 to 2.5 Moderate

5 2.5 to 4 Rough

6 4 to 6 Very rough

7 6 to 9 High

8 9 to 14 Very high

9 Over 14 Phenomenal

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Sea state 2

Character of the sea swell

0. None

Low 1. Short oraverage2. Long

Moderate 3. Short4. Average5. Long

Heavy 6. Short7. Average8. Long

9. Confused

Direction from which swell is coming should be recorded.Confused swell should be recorded as "confused northeast," if coming from the direction of northeast.

Sea states in marine engineeringIn engineering applications, sea states are often characterized by the following two parameters:• The significant wave height H1/3 — the mean wave height of the one third highest waves.• The mean wave period, T1.The sea state is in addition to these two parameters (or variation of the two) also described by the wave spectrum

which is the product of a wave height spectrum and a wave direction spectrum . Somewave height spectra are listed below. The dimension of the wave spectrum is , andmany interesting properties about the sea state can be found from the spectrum.The relationship between the spectrum and the wave amplitude for a wave component is:

• ITTC[1] recommended spectrum for fully developed sea (ISSC[2] spectrum/modified Pierson-Moskowitzspectrum):[3]

• ITTC recommended spectrum for limited fetch (JONSWAP spectrum)

where

and

An example function might be:

Sea state 3

Thus the sea state is fully determined and can be recreated by the following function where is the wave elevation,is uniformly distributed between 0 and , and is randomly drawn from the directional distribution

function [4]

In addition to the short term wave statistics presented above, long term sea state statistics are often given as a jointfrequency table of the significant wave height and the mean wave period. From the long and short term statisticaldistributions it is possible to find the extreme values expected in the operating life of a ship. A ship designer can findthe most extreme sea states (extreme values of H1/3 and T1) from the joint frequency table, and from the wavespectrum the designer can find the most likely highest wave elevation in the most extreme sea states and predict themost likely highest loads on individual parts of the ship from the response amplitude operators of the ship. Survivingthe once in 100 years or once in 1000 years sea state is a normal demand for design of ships and offshore structures.

Footnotes[1] International Towing Tank Conference (ITTC) (http:/ / ittc. sname. org/ ), , retrieved 11 November 2010[2] International Ship and Offshore Structures Congress[3] Pierson, W. J.; Moscowitz, L. (1964), "A proposed spectral form for fully developed wind seas based on the similarity theory of S A

Kitaigorodskii", Journal of Geophysical Research 69 (24): 5181–5190, Bibcode 1964JGR....69.5181P, doi:10.1029/JZ069i024p05181[4] Jefferys, E. R. (1987), "Directional seas should be ergodic", Applied Ocean Research 9 (4): 186–191, doi:10.1016/0141-1187(87)90001-0

References• Bowditch, Nathaniel (1938), American Practical Navigator, H.O. pub No. 9 (revised ed.), United States

Hydrographic Office, OCLC 31033357• Faltinsen, O. M. (1990), Sea Loads on Ships and Offshore Structures, [Cambridge University Press],

ISBN 0-521-45870-6

Article Sources and Contributors 4

Article Sources and ContributorsSea state  Source: http://en.wikipedia.org/w/index.php?oldid=449327698  Contributors: A2Kafir, Amgine, Boatman, Bobblewik, Brad101, Colonel Warden, Crohnie, Crowsnest, Cynox,Dammit, Daven200520, Dhaluza, Epipelagic, FrisB33, Inike, Kounos1!, Kraaiennest, LeheckaG, LorenzoB, Michael Hardy, Molly-in-md, Picaroon, Rjwilmsi, Runningonbrains, Sas1975kr,Slaunger, Thunderbird2, To Serve Man, Unara, User2004, Wang ty87916, Wikipelli, Xzapro4, 1971 إماراتي, 柴崎力栄, 20 anonymous edits

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