7. Freesurface Effect

Question 1

Factors affecting FSE

Answer 1

• Relative Density
• Length of tank
• Breadth of tank
• Displacement of the vessel

If water is allowed to accumulate in a ship at sea, it will move towards the low side, increasing the roll and reducing stability

• Tanks which are full are said to be pressed up, are not subject to Free Surface Effect and are not a problem
• Empty tanks are not subject to Free Surface Effect and are not a problem

• A tank is generally considered to be slack if it is between 5% and 95% full.
Slack tanks are subject to Free Surface Effect

• 0-5% and 95-100% full tanks may be subject to FSE dependent on shape of tank and other factors
• Slack tanks (partially filled tanks) can cause problems whether the contents be water or oil
• For a given angle of heel the more dense the liquid the greater the mass that moves and hence the greater the heeling moment caused. Thus FSE varies directly as the density of the liquid in the tank.
• A given moment applied to a vessel will cause a greater effect as the vessel’s displacement decreases. Thus FSE varies inversely as displacement.
• Position of tank in the vessel makes no difference to the free surface effect. Whether the tank is on the vessel’s centerline or to one side, or whether the tank is above or below the vessel’s centre of gravity.

• In a rectangular tank the same volume of liquid will be moved whatever the depth of the liquid in the tank, providing the liquid surface does not intersect the tank top or bottom, and the tank is heeled to the same angle.
Thus FSE is not affected by depth of tank nor depth of liquid.

• The construction of the tank usually includes measures to reduce free surface effect.
• In the design stage, free surface can be minimized by fitting more and smaller tanks instead of having few large ones. The length of the tank is not the major issue
• It is the beam of the tank (actually the cube of the beam). If the breadth of the tank is doubled not only does the mass of liquid moved change, but also the distance through which it moves. It may be shown that FSE varies proportionately with the breadth of tank cubed.
• Therefore, reducing the beam of the tank is the most effective measure in reducing Free Surface Effect.
• Effective management of tanks, keeping partially filled tanks to a minimum will reduce free surface effect.

Question 2

Flume Tank Stabilisation System

Answer 2

• Water is allowed to flow across the ship but is delayed by a series of baffles.
• As the ship roll to port, the water also flows to port, but is delayed.
• By the time the water reaches the port side, the ship has rolled to starboard.
• The water is delayed and is acting opposite to the roll, thus reducing the amount of roll.

Question 3

What is Free Surface Effect?

Question 4

Free Surface Effect Calculation formulae

Answer 4

Question 5

FSE – Why Breadth Is Critical?

Answer 5

• The larger the beam of the tank, the farther the fluid is allowed to move, thus causing a greater movement of G, which can affect the stability of the vessel.
• The beam of the tank affects the FSE to the power of 3.

Question 6

Free Surface Moment terminology explained.

Answer 6

If given Moment of Inertia instead of FSM (Free Surface Moment) of a tank just substitute I in place of FSM in your equation.

FSM and I are the same value, just different symbol.

FSC = FSM x RD / Displacement is the same as FSC = I x RD / Displacement.