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

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.

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

Volume: Equal to the Nominal volume

Surface: Fixed at a prescribed orientation

Weight: Adds to the ship's total weight

Free surface: None

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

Volume: Varies

Surface: Coincident with external waterplane

Weight: Subtracts from ship's buoyancy

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.

(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

(Pressure of gas at top is constant at one atmosphere)

Volume: Varies

Weight: Subtracts from ship's buoyancy

Unconditional: Constant-Volume Mode

Unconditional: Frozen Mode

Surface orientation is set parallel to external waterplane at time of TYPE

or LOAD.

Ref. Pt. above interior level: Const Volume Mode

Ref. Pt. at interior level: Spilling/Filling Mode

Ref. Pt. below interior level: never

Unconditional: Balanced Mode Sealed

Ref. Pt. above water: Same as Spilling Type

Ref. Pt. below water: Same density as external: Flooded Mode

Different density: Balanced Mode Vented

Unconditional: Flooded Mode

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.)

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 + PcIn the Balanced Mode Vented,

Ps = 1 atmosphere.Therefore,

ht * C * SGt = d * C * SGs ht = d * SGs / SGtIn 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 - LnThis 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.

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 = 1.1sets 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.

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 TANKnameREFPT =damage locationTYPE = 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 "shows the volume lost.tank list"

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.

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.