Sea state


Sea state

A sea state includes the height, period, and character of waves on the surface of a large body of water. The large number of variables involved in creating the sea state cannot be quickly and easily summarised, so simpler scales are used to give an approximate but concise description of conditions for reporting in a ship's log or similar record.

World Meteorological Organization sea state code

: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 engineering

In 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 S(omega, Theta) which is the product of a wave height spectrum S(omega) and a wave direction spectrum f(Theta). Some wave height spectra are listed below. The dimension of the wave spectrum is {S(omega)} = ext{length^2cdot ext{time}}, and many interesting properties about the sea state can be found from the spectrum.

The relationship between the spectrum S(omega_j) and the wave height A_j for a wave component j is:

: frac{1}{2} A_j^2 = S(omega_j), Delta omega

* ITTC [International Towing Tank Conference] recommended spectrum for fully developed sea (ISSC [International Ship and Offshore Structures Congress] spectrum/modified Pierson-Moskowitz spectrum): [W. J. Pierson & L. Moscowitz, A proposed spectral form for fully developed wind seas based on the similarity theory of S A Kitaigorodskii, J Geophys Res 69 (24) 5181-5190 (1964).]

: frac{S(omega)}{H_{1/3}^2 T_1} = frac{0.11}{2pi} left(frac{omega T_1}{2pi} ight)^{-5} mathrm{exp} left [-0.44 left(frac{omega T_1}{2pi} ight)^{-4} ight]

* ITTC recommended spectrum for limited fetch (JONSWAP spectrum)

: S(omega) = 155 frac{H_{1/3}^2}{T_1^4 omega^5} mathrm{exp} left(frac{-944}{T_1^4 omega^4} ight)(3.3)^Y, where

: Y = exp left(-left(frac{0.191 omega T_1 -1}{2^{1/2}sigma} ight)^2 ight)

and

: sigma =egin{cases}0.07 & ext{if }omega le 5.24 / T_1, \0.09 & ext{if }omega > 5.24 / T_1.end{cases}

An example function f(Theta) might be:: f(Theta) = frac{2}{pi}cos^2Theta, qquad -pi/2 le Theta le pi/2

Thus the sea state is fully determined and can be recreated by the following function where zeta is the wave elevation and epsilon_{jk} is uniformly distributed between 0 and 2pi.: zeta = sum_{j=1}^Nsum_{k=1}^K sqrt{2 S(omega_j, Theta_k) Delta omega_j Delta Theta_k} sin(omega_j t - k_j x cos Theta_k - k_j y sin Theta_k + epsilon_{jk})

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

References

:* Bowditch, Nathaniel original; H.O. pub No. 9: American Practical Navigator, Revised Edition 1938; United States Hydrographic Office; Not Copyrighted 1938.:* cite book
last = Faltinsen
first = O. M.
title = Sea Loads on Ships and Offshore Structures
publisher = [Cambridge University Press]
date = 1990
isbn = 0-521-45870-6

See also

* Beaufort scale
* Cross sea
* Douglas Sea Scale

Footnotes


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