Sea State - Sea States in Marine Engineering

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 which is the product of a wave height spectrum and a wave direction spectrum . Some wave height spectra are listed below. The dimension of the wave spectrum is, and many 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 recommended spectrum for fully developed sea (ISSC spectrum/modified Pierson-Moskowitz spectrum):
\frac{S(\omega)}{H_{1/3}^2 T_1} = \frac{0.11}{2\pi} \left(\frac{\omega T_1}{2\pi}\right)^{-5} \mathrm{exp} \left
  • 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}\right)(3.3)^Y,

where

and

\sigma = \begin{cases} 0.07 & \text{if }\omega \le 5.24 / T_1, \\ 0.09 & \text{if }\omega > 5.24 / T_1. \end{cases}

An example function might be:

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

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.

Read more about this topic:  Sea State

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