Sea state

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"State of the sea" redirects here. For the Venetian Empire, see Stato da Màr.
NOAA ship Delaware II in foul weather on Georges Bank.
Sea State 5 and 8 range

In oceanography, sea state is the general condition of the free surface on a large body of water—with respect to wind waves and swell—at a certain location and moment. A sea state is characterized by statistics, including the wave height, period, and spectrum. The sea state varies with time, as the wind and swell conditions change. The sea state can be assessed either by an experienced observer (like a trained mariner) or by using instruments like weather buoys, wave radar or remote sensing satellites.[1]

The short-term statistics describing the sea state are determined for a time interval in which the sea state is considered to be constant. This duration has to be much longer than the individual wave period, but shorter than the period in which the wind and swell conditions can be expected to vary significantly. Typically, the sea state is assumed to be constant for 15-30 minutes.[1]

The large number of variables involved in creating and describing the sea state cannot be quickly and easily summarized, so simpler scales are used to give an approximate but concise description of conditions for reporting in a ship's log or similar record.

WMO sea state code

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Winter, North Atlantic – Water over deck and hatches, storm with huge waves (1958)

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

graphic of WMO sea state codes, descriptions, and wave heights in meters
WMO Sea State Code Wave height Characteristics
0 0 metres (0 ft) Calm (glassy)
1 0 to 0.1 metres (0.0 to 3.9 in) Calm (rippled)
2 0.1 to 0.5 metres (3.9 in to 1 ft 7.7 in) Smooth (wavelets)
3 0.5 to 1.25 metres (1 ft 8 in to 4 ft 1 in) Slight
4 1.25 to 2.5 metres (4 ft 1 in to 8 ft 2 in) Moderate
5 2.5 to 4 metres (8 ft 2 in to 13 ft 1 in) Rough
6 4 to 6 metres (13 to 20 ft) Very rough
7 6 to 9 metres (20 to 30 ft) High
8 9 to 14 metres (30 to 46 ft) Very high
9 Over 14 metres (46 ft) Phenomenal
Character of the sea swell
  0. None
Low 1. Short or average
2. Long
Moderate 3. Short
4. Average
5. Long
High 6. Short
7. Average
8. Long
  9. Confused
The direction from which the swell is coming should be recorded.

Sea states in marine engineering

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

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 over a given number of years by fitting a extreme value distribution.[3] The extreme value distribution can then inform an engineer about the most extreme significant wave height to be expected during a lifetime of a ship or offshore structure. Using the response amplitude operators of the ship, the engineer can then use the extremes in significant wave height and wave period to calculate the most extreme loads that the ship should be able to withstand. Withstanding significant wave heights that happen on average once in 100 years or once in 1000 years is a common demand for design of ships and offshore structures.[4]

See also

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Citations

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  1. ^ a b Holthuijsen, Leo H. (2007). Waves in oceanic and coastal waters. Cambridge: Cambridge University Press. ISBN 978-0-511-27021-5.
  2. ^ Munk, W.H. (1944). Proposed uniform procedure for observing waves and interpreting instrument records. La Jolla, California: Wave Project at the Scripps Institution of Oceanography.
  3. ^ Aarnes, Ole Johan; Breivik, Øyvind; Reistad, Magnar (2012). "Wave Extremes in the Northeast Atlantic". Journal of Climate. 25 (5): 1529–1543. doi:10.1175/JCLI-D-11-00132.1. hdl:1956/9253. ISSN 0894-8755.
  4. ^ Jonathan, Philip; Ewans, Kevin (2013). "Statistical modelling of extreme ocean environments for marine design: A review". Ocean Engineering. 62: 91–109. doi:10.1016/j.oceaneng.2013.01.004.

General and cited references

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