FREE SURFACE EFFECT: WHY? WHAT?

FREE SURFACE EFFECT: WHY? WHAT?


Picture this...

A half filled tank on ship ->
Vessel Heels ->
Fluid in tank moves in direction of heel ->
forms a wedge ->
CG (centre of gravity) shifts ->
Reduces the rightening lever (GZ) ->
Reduces Metacentric Height ->
increases angle of heel ->
thus making it hard for ship to retain original position ->
RESULT: UNSTABLE SHIP!

• Initial Stability of a ship is measured by its Metacentric Height
• Due to FSE (Free Surface Effect).

GG2 is the virtual reduction in Metacentric Height due to free surface effect.

 

Here, P(read rho) = density of the liquide n tank

P1 = (read rho 1) = density of water

i = second moment of area of free surface about the centre line of the tank

V (read vol of displacement) = vol of displacement

Factors Effecting Free Surface Effect

Pocketing

Free Surface Effect can be reduced, to some extent, by creating pocketing. Pocketing occurs when the surface of the liquid contacts the top or bottom of the tank, reducing the breadth (B) of the free surface area.

Since the effects of pocketing can not be calculated, it is an indeterminate safety factor. The Free Surface correction will therefore indicate less overall stability than actually exists.

Surface Permeability

Impermeable objects (engines, pumps, piping systems, etc) inside a flooded space project through and above the liquid surface. These objects inhibit the moving water and the “shifting of the wedge” may or may not be complete, thus reducing Free Surface Effect. The impermeable objects also occupy volume, reducing the amount of flooding water (movable weight) that can fill the space.

Swash Bulkheads (Baffle Plates)

In addition to some structural support, these bulkheads are designed to reduce Free Surface Effect. They are longitudinal bulkheads that hinder, but do not prevent, the flow of liquid from side to side as the ship rolls or heels. They are found in tanks, voids, double bottoms, bilges, etc.

Sluice Valves

Free flow (Sluice) valves on tankers allow opposing tanks to be cross-connected. When large, partially filled tanks are connected, Free Surface Effect increases, and the vessel becomes less stable.

Tank Divisions reduce FSE, when divided Longitudnally

Consider a rectanguar tank with lenth l, breath b,and semi filled.

Condition 1:

Without any divisions, 

Condition 2:

With Transverse subdivisons,

Condition 3:

With Longitudinal subdivision, 

Hence, we determine that when we divide a tank longitudinally, then its virtual reduction in metacentric height reduces and thus there is less movement of the liquid inside the tank, which results in Better Stability!

Condition 4: 

n longitudnal divisions , causes, better stability as from above GG2 formula, 

we conclude, 

IT MAY BE SEN THAT FSE REDUCES UPON MORE AND MORE LONGITUDNAL TANK DIVISIONS!

Conditions of Free Surface Effect

• FSE increases with increased length and width of compartment
• FSE increases when displacement decreases (de-ballasting)
• FSE is independent of the depth of the liquid