General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

Long-Term Wave Statistics, Part II
(Requires GHS version 18 or later)

In COW192, we introduced the Long Term Statistics Wizard (or LT Wizard). We didn't say much about it, other than briefly mention that it will handle wave scatter data. Apparently, we left John Bonn hanging in suspense by promising a future COW on the LT Wizard. We felt bad that we made him wait so long, but in that time we've made a number of updates to the LT Wizard, and it is finally time to share some of it.

A common application of the LT Wizard is the use of environmental data that is presented as a wind or wave rose. This is a graphical representation of long-term environmental conditions that might be found in the output from environmental databases. An example wave rose is given below:


An example wave rose. Each color represents a different wave height and the amount of color indicates the probability of occurence. Source: Marine Response Consultants/Donjon Marine. Used with permission.

Roses like this show not only the direction the waves (or wind) are coming from but also the probability and severity in each of those directions.

If all we have is a wave rose, it takes a bit of work to get the data out of the image. But often roses will accompany a data table with the numerical values for each heading, like that given above. Using this data as an example, we can prepare the following scatter table in the LT Wizard. Note that we have converted the wave height from meters to feet in this instance.


Note the headings in the scatter table are heading relative to the ship. In this case, we assume a wave heading of 180 degrees corresponds to due North. For more information on how headings should be assigned, see Page 18 of the SK User's Manual. In addition, the data are divided over 1000 observations, where the rose data only gives the percentages. These are small details, but it is important to confirm heading convention and to check the scatter totals as you work to avoid confusion.

There are many ways to utilize the scatter data given here, but since we have wave heights in terms of headings, let's say we want to know the most-probable extreme significant wave height for each heading. To do this we can use the "Row-wise CDFs" option. When this option is selected, the Cumulative Distribution Function (CDF) buttons for each row in the table become available, as shown above. Clicking on the button for the -90 degree heading brings up the CDF dialog for that row. By clicking "Check Regression" and "Compute" we can see a Weibull CDF for the wave data in that row.


The "Confidence Threshold" determines how confident we are that the computed wave height won't be exceeded for the given heading. In this case, we use 0.99. This means 99% of the time, all expected waves at a -90 degree heading will be smaller than or equal to the computed wave height of 10.82 feet. Pretty handy! You can compute these types of statistics for each heading individually, or use the "CDF All" dialog to compute them for all headings simultaneously.

With these statistics computed, we can quite easily use the other options in the LT Wizard to set up a seakeeping run to obtain responses in this seaway. One major benefit to using the CDF method is we only have to run a single seakeeping run at each heading, instead of multiple runs in each possible seaway. This can be a great time savings and yield useful (and conservative) design values.

Questions, comments, or requests?
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