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

(Requires GHS version 17.10 or later with SK)

The NATO STANAG 4154 seakeeping criteria are often cited in more than just naval ship design. Evaluating the requirements means satisfying certain dynamic limits and finding a maximum significant wave height. All of this can be done in GHS rather efficiently with the new SEA DATA feature.

One especially useful aspect of SEA DATA is that you can input an RAO data file and then specify a seaway to compute motions using the given RAO. This means once you have RAO data, it is both efficient and easy to run different seaways for comparison.

In the case of NATO STANAG requirements, determining the maximum operable significant wave height requires iterating over increasing wave heights until a limiting condition is found. To make this process faster, we can use our first SEAKEEPING run to create RAO data files for all headings and speeds of interest, and then use SEA DATA as we continue to increase the significant wave height in a macro:

` geometry and establish loading condition

crt (1) "bridge" 35.913a 0 8.824

macro wave_height
set hs={hs} PLUS 1.0
wave (spe) p2 {hs}
sea data data\sk*-response.dat /crt:1 /polar:RMS /gu

vari (str) sks="/samp:50,0.2,3.0 /crt:1 /polar:RMS /gu"
vari hs=1.0

sea limit rms roll p < 4
sea limit rms pitch p < 1.5
sea limit rms heave a < 0.2g
sea limit rms sway a < 0.1g
sea limit combine 1,2,3,4

wave (spe) p2 {hs}
sea /speed:0,8,16,24 /head:0,30,...,180 {sks} /data:data\re
report close /preview

You'll notice we used SEA LIMIT statements to define the NATO STANAG requirements in RMS roll, pitch, and vertical and lateral acceleration. We then combine all four limits into a single limit using the SEA LIMIT COMBINE statement. After that, it is all number crunching...

As discussed in COW112, the SEA LIMIT output is simply a polar plot with a marker indicating pass or fail for the specific speed/heading combination. We obtain these plots for each wave height considered:

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