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General HydroStatics
Ship Stability Software |
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Command of the Week
(New or interesting aspects of GHS that you may not know about) |
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CRITPT /INSIDE
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A ship that meets stability standards will keep above water any openings that would cause
downflooding. In other words, all of the requirements having to do with capsize resistance
will have been met before a downflooding point touches the water. After the "angle of downflooding"
is reached, the game is over as far as most stability modeling and methodology is concerned.
In GHS we use Critical Points to represent places on the ship where downflooding would occur. The user is responsible for assigning critical points to mark such openings. This is simple and straightforward until you check out what happens if the integrity of the hull is breached, or what we call "damage stability." What was once a downflooding point is no longer a factor if the space it would flood is already flooded by the damage. So GHS allows you to link a critical point to a compartment. When the hull is damaged, generally a secondary barrier is exposed; but such bulkheads may have openings in them through which "progressive flooding" may occur. To conveniently model this without having to constantly activate and deactivate critical points, GHS provides a way to indicate that a critical point is of no account unless a particular compartment is flooded. Here is a simplified example of heeling up to the point of downflooding: |
CRITPT "Engine room air intake" 35.0, 10.0, 14.0 /TANK: VENTDUCT.S CRITPT "Vent to engine room" 35.0, 6.5, 10.0 /INSIDE: VENTDUCT.S MACRO InclineToFlood HEEL 0 RA /STOP:FLD /HOLD /GRAPH:NONE DISPLAY (*) STATUS body@35.0 /YesEmpty / .InclineToFlood |
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| Now look what happens if we run the same thing with VENTDUCT.S flooded: |
TYPE (VENTDUCT.S) FLOOD .InclineToFlood |
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If you want to
model actual flows through openings the /OPENING:size parameter will do that
in conjunction with LOAD FLOW—for the benefit of salvage engineers
at least.
Critical points can do other things too: they are not restricted to representing downflooding points. Another common application is to mark weathertight openings, which have closures preventing flooding when intermittently immersed. We call these "Tight" points for brevity. You can easily move a critical point up or down by a given amount relative to the current waterplane without having to calculate the new coordinates. Then if you would like to get access to those new coordinates, the /ACCESS parameter gives you a way to load those values into your own variables. And there is a /SYMM parameter that automatically generates the mirror image of your critical point on the opposite side of the ship. Oh, yes, if you arrange critical points along a coaming you can get your reference point for flooding or spilling to relocate automatically to the lowest one. We could go on raving about the CRITPT command till the cows come home. It's enough to make you wish for the days when they did damage stability with reduced permeability instead of runoff and called it good—and when salvors did their calcs on the back of an envelope. |
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Questions, comments, or requests?
Contact Creative Systems, Inc. USA phone: 360-385-6212 Fax: 360-385-6213 Office hours: 7:00 am - 4:00 pm Pacific Time, Monday - Friday Mailing address: PO Box 1910 Port Townsend, WA 98368 USA Click here for an index to this and previous COWs |