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


Last week we showed how to transform the contents of a tank. That technique can be used to evaluate intermediate stages of flooding. Partially flooding a tank with seawater is done with the added weight method which calls for tank type INTACT and, you guessed it, a contents transformation. Whether it is realistic to replace the original contents with seawater for intermediate flooding depends are the situation. Perhaps we overpromised with regards to answering life's truly important questions.

LOAD is typically used as a simple input to apply known liquid loads to the model. Just select the input format that matches your information, whether that is the sounding, ullage, volume, weight or load fraction.

LOAD INTERMEDIATE: fraction is easy to use with only one parameter. The fraction is applied to the difference between the current tank load and the external waterplane (which are collected when the command is executed). An intermediate load fraction of 0 would match the current tank load, 1 would match the external waterplane, and 0.5 would be halfway between the two loads. In a simple scenario where an empty tank lies fully below the external waterplane, the intermediate fraction would therefore correspond with the normal load fraction.

Typically, LOAD INTERMEDIATE appears in a macro with the following sequence:
1. Set up the final flooding condition and SOLVE for the waterplane.
2. Change tank contents to seawater. (See COW from last week)
3. Set the desired intermediate load fraction and set the tank TYPE to INTACT.
4. SOLVE and evaluate the intermediate condition.
5. Restore the tank contents, and the original load.

You might be asking why worry about intermediate flooding at all. Perhaps an example will help. Suppose our venerable tanker sustains extensive damage to the shell of a double bottom and wing tank causing immediate flooding of these two compartments. Let's further assume that a small crack has developed in the wall of the centerline tank, creating a slow leak. Since the centerline tank will be partially flooded for some time, it is prudent to evaluate the flooding stages in case one of them has worse stability characteristics. The final flooding condition is shown below followed by an animation of the intermediate flood stages.

The animation shows that the equilibrium heel angle in the beginning stages of flooding exceeds the heel at final flooding. If the animation is a little fast for careful analysis, you can even graph how the equilibrium heel angle changes with intermediate load fraction.

The run file used in this example may be found here.

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