GHS BULLETIN
Tank Modes and Types
Updated: 4/24/95
(Applies to GHS/BHS versions 6.20 and later)
GHS models the various ways in which tanks can behave by assigning to each
tank a Type through the use of the TYPE command. The LOAD and CONTENTS
commands are used to set the volume of fluid within the tank and the density
of the fluid, respectively.
The most common and simplest tank type is INTACT. An Intact tank contains a certain
volume of liquid and the surface of the liquid is maintained such that it lies in a plane parallel
with the external waterplane.
Other tank types may have variable volumes, but there is always a Nominal volume associated
with each tank. For an Intact tank, the actual volume is the same as the Nominal volume.
Nominal volumes are always set by the LOAD command and remain as set until changed by
another LOAD command.
Since some tank Types involve complex behavior, it is useful to refer to the Mode in which a
tank is operating. Following is a list of the tank Modes and Types.
Tank Modes
Constant-Volume Mode
Volume: Equal to the Nominal volume
Surface: Parallel to the external waterplane
Weight: Adds to the ship's total weight
Free surface: Adds to the total FSM
Frozen Mode
Volume: Equal to the Nominal volume
Surface: Fixed at a prescribed orientation
Weight: Adds to the ship's total weight
Free surface: None
Spilling/Filling Mode
Volume: Varies
Surface: Passes through Tank's Ref. Point & Parallel to external WPL.
Weight: Adds to the ship's total weight
Free surface: Adds to total FSM
Flooded Mode
Volume: Varies
Surface: Coincident with external waterplane
Weight: Subtracts from ship's buoyancy
Definition: Balanced
Internal and external pressures are equal at tank's Reference Point.
External pressure is determined by Reference Point depth.
Internal pressure is determined by the gas at top plus column height.
Level within the tank is not allowed to go below Reference Point.
Balanced Mode Sealed
(Pressure of gas at top is inversely proportional to void volume.)
Volume: Varies; Nominal volume is where pressure = 1 atmosphere
Surface: Parallel to external waterplane
Weight: Adds to ship's total weight
Free surface: Adds to total FSM
Balanced Mode Vented
(Pressure of gas at top is constant at one atmosphere)
Volume: Varies
Weight: Subtracts from ship's buoyancy
Tank Types
Intact Type
Unconditional: Constant-Volume Mode
Frozen Type
Unconditional: Frozen Mode
Surface orientation is set parallel to external waterplane at time of TYPE
or LOAD.
Spilling Type
Ref. Pt. above interior level: Const Volume Mode
Ref. Pt. at interior level: Spilling/Filling Mode
Ref. Pt. below interior level: never
Bubble Type
Unconditional: Balanced Mode Sealed
Damaged Type
Ref. Pt. above water: Same as Spilling Type
Ref. Pt. below water: Same density as external: Flooded Mode
Different density: Balanced Mode Vented
Flooded Type
Unconditional: Flooded Mode
Side Effects of Type Changes
When the tank Type is set by the TYPE command, Nominal volume remains unchanged.
Contents also remains unchanged except as follows:
TYPE FLOOD sets Contents density equal to external density and Contents description to
"SEA".
Changing Type from FLOOD to another Type restores the original Contents density and
description which were in effect prior to the TYPE FLOOD command.
(Note: These side effects did not exist in versions prior to 6.20.)
Equations of Pressure Balancing
The tank Types BUBBLE and DAMAGED both involve determining the level inside the tank
such that the pressure at the tank's Reference Point is the same inside the tank as it is outside
the tank. The tank's Reference point is set by the REFPT command and normally would be
located on the tank's actual boundary, though it can, in fact, be located anywhere.
Ps is the pressure at the surface of the liquid in the tank.
Pr is the pressure at the Reference Point.
Pc is the pressure contributed by the column of liquid in the tank from its
surface to the Reference Point.
Pc = ht * C * SGt
Pr = 1 + d * C * SGs
where ht is the height of the column,
d is the depth of the Reference Point below the external waterplane,
C is the pressure per unit depth of fresh water in atmospheres,
SGt is the specific gravity of the tank contents,
SGs is the specific gravity of the sea water.
In order to balance the pressures,
Pr = Ps + Pc
In the Balanced Mode Vented,
Ps = 1 atmosphere.
Therefore,
ht * C * SGt = d * C * SGs
ht = d * SGs / SGt
In the Balanced Mode Sealed, the Nominal Volume is defined such that Ps = 1 atmosphere at
Nominal Volume.
Load is defined as
Load = Actual Volume / Maximum Volume.
Since for a gas, pressure * volume is constant,
Ps = (1 - Ln) / (1 - La)
where Ln is the Nominal Load,
La is the Actual Load.
Therefore,
1 + d * C * SGs = (1 - Ln) / (1 - La) + ht * C * SGt
ht = C0 * (C1 - C2 / (1 - La))
where C0 = 1 / (C * SGt)
C1 = 1 + d * C * SGs
C2 = 1 - Ln
This shows that ht is a function of La which is also a function of ht and the geometry of the
tank. Thus an iterative procedure is applied to find the solution.
Setting the Bubble Pressure
In the case of a BUBBLE-Type tank, the LOAD command can also set the pressure at that load
by means of the /PRESS parameter. For example,
LOAD = 0.75 /PRESS = 1.1
sets the load and pressure such that the pressure would be 1.1 atmospheres at 75% load in the
tank.
When the LOAD command is used without giving a value, it displays the present load as well
as pressure.
Liquid-Loss Calculations
The loss of liquid when one or more tanks are ruptured at known locations can be accurately
calculated by using the DAMAGED type. For example,
STATUS
TANK name
REFPT = damage location
TYPE = DAMAGED
`(repeat above three commands for additional tanks)
SOLVE
STATUS
The difference between the load in the tank(s) as of the first STATUS and the second STATUS
is the amount of liquid lost.
By using a suitable macro and variables, this process can be automated for more convenience.
For example,
MACRO SPILL
`Uses System variable TVOLUME to get VOL before & after damage
`Assumes that Reference Points have been set at points of
damage
VARIABLE VOL
TYPE (*) = INTACT
SOLVE
TANKS %1
SET VOL = {TVOLUME}
TYPE = DAMAGED
SOLVE
SET VOL = {VOL} MINUS {TVOLUME}
\ Oil spilled from {PNAME}: {VOL} CUBIC {LUNIT} /
Then
.SPILL "tank list"
shows the volume lost.
True Downflooding
Using the DAMAGED tank type it is possible to have a tank automatically flood when its point
of downflooding is reached. Such a tank behaves as a spilling tank (or an empty tank if its
Nominal load is zero) while the Reference Point is above water and a flooded tank when the
Reference Point is below water. If the heel angle is such that the Reference Point is initially
above water and then increased until it is below the surface, the tank will go through the
transition of suddenly becoming flooded.
Since the contents (density) does not change at the transition, this downflooding will be more
realistic if the contents is seawater.
Hopper Spilling and Flooding
Spilling from a hopper which contains a combination of seawater and a denser liquid can be
simulated by superimposing two identical tanks where one contains sea water and is of the
DAMAGED Type while the other contains the residual density and is of the SPILLING Type.
The residual density would be the difference between sea water and the denser liquid.
If you would like to see another bulletin created regarding a specific topic,
please email Creative Systems, Inc. at support@ghsport.com.
Copyright (C) 2011
Creative Systems, Inc.