Sea state

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Sea state refers to 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 summed, so simpler scales are used to give a rough description of current conditions, primarily for reporting in a ship's log or similar record.

1 Sea states in marine engineering
2 World Meteorological Organization sea state code
3 References
4 See also
5 Footnotes

[edit] Sea states in marine engineering

In engineering applications, sea states are often characterized by the following two parameters:

The significant wave height H_1/3 — the mean 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 spectrum $S (ω,Θ)$ which is the product of a wave height spectrum $S (ω)$ and a wave direction spectrum $f (Θ)$ . Some wave height spectres are listed below. The dimension of the wave spectrum is ${S (ω)} = {L e n g t h 2 T i m e}$ , and many interesting properties about the sea state can be found from the spectrum.

The relationship between the spectrum $S (ω j)$ and the wave height $A j$ for a wave component $j$ is:

ITTC^[1] recommended spectrum for fully developed sea (ISSC^[2] spectrum/modified Pierson-Moskowitz spectrum):

ITTC recommended spectrum for limited fetch (JONSWAP spectrum)


where:

and  $σ$  = 0.07 if  and 0.09 if

An example function $f (Θ)$ might be:

Thus the sea state is fully determined and can be recreated by the following function where $ζ$ is the wave elevation and $ε j k$ is uniformly distributed between 0 and $2π$ .

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 H_1/3 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.

[edit] World Meteorological Organization sea state code

0 Calm (glassy) 0 m
1 Calm (rippled) 0 - 0.1 m
2 Smooth (wavelets) 0.1 to 0.5 m
3 Slight 0.5 to 1.25 m
4 Moderate 1.25 to 2.5 m
5 Rough 2.5 to 4 m
6 Very rough 4 to 6 m
7 High 6 to 9 m
8 Very high 9 to 14 m
9 Phenomenal Over 14 m

Character of the sea swell
	0. None
Low	1. Short or average 2. Long
Moderate	3. Short 4. Average 5. Long
Heavy	6. Short 7. Average 8. 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.

[edit] References

Bowditch, Nathaniel original; H.O. pub No. 9: American Practical Navigator, Revised Edition 1938; United States Hydrographic Office; Not Copyrighted 1938.
Faltinsen, O. M. (1990). Sea Loads on Ships and Offshore Structures. [Cambridge University Press]]. ISBN 0-521-45870-6.