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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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PERMEABILITY
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The permeability factor reduces the capacity of a tank in order to account for internal structure and
impermeable elements. Naturally permeability values are carried in the Geometry File since they
represent an aspect of the physical geometry of the ship.
It is implied in report documents that geometry is fixed, just as the physical vessel is one thing and not many. Therefore GHS makes it difficult to change the geometry model in a way that the physical vessel cannot change. The PERMEABILITY command violates this rule. Each Geometry file represents a particular ship or version of the ship. If some element of geometry is movable, such as a crane boom or jackup legs, these changes can be made readily, and the weight locations in the report will reflect the change. Likewise, compartments can be switched from being intact to being flooded, and this is clearly indicated in reports. Usually permeability is fixed, but there are reasons to change it. If you want to change permeability within a tank or compartment without redoing the Geometry File, the PERMEABILITY command is available for that purpose. This command is unique among commands that set the value of some property: it automatically reports the change so the reader is alerted to the fact that the geometry has effectively been changed. Beyond that, the PERMEABILITY command also facilitates having a different permeability automatically switched in when the compartment is flooded. Here's an example for a selection of tanks: tanks deep1*, deep2*,deep3*, cmtcmpt.c PERMEABILITY 0.8 FLOOD |
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(The volumes shown are capacities out of the permeability shown in the Normal column,
not the volumes in the current condition.)
Setting a different permeability for the cement-tank hold requires an additional command, and we get an additional report because you cannot change a permeability without it being reported. PERMEABILITY (cmtcmpt.c) 0.6 FLOOD |
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We now have two permeabilities defined for these tanks: the permeability in use depends on
whether the tank is intact or flooded.
But the intact permeability can be changed too: PERMEABILITY (cmtcmpt.c) 0.7 |
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Now if we flood a tank, there is no need for a new permeability report since
the previous one showed both intact and flooded values; but there is
a nice feature where the flooding is marked and the flooded volume is shown.
To take advantage of that you have to explicitly call for the report:
type (deep3.s) flood PERMEABILITY REPORT |
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In other words, the report above shows the current capacities and indicates which
tanks have had their permeabilities changed (for flooding or some other reason.)
There is a convenient way to restore permeabilities to the original value from the Geometry file: PERMEABILITY RESTORE |
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Changing permeability is a bit hard to justify except in cases where various volumes
of solid cargo occupy part of a hold, leaving variable volumes for flooding water.
Beware that tanks which have been modeled as a combination of two or more components might have a different permeability factor assigned to each component. When the PERMEABILITY command modifies permeability, it is addressing the tank as a whole, so all it can do is make the permeability in every component equal to the given value. Damage stability regulations may allow a lesser permeability factor. GHS doesn't ask and you don't have to tell whether it is reasonable; it just publishes the fact. A bulletin on GHS Permeability is available from the GHSport web site: ghsport.com/support/PERM.HTM |
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Questions, comments, or requests?
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