Damage Stability

Damage Stability

Absence of hull damage

• Stability is calculated still in the usual way by using added mass and making allowance for free surface liquid.

• Without hull damage, the ship’s arrangement is still not revised and the principles of every spaces is still in that way.

• The calculation will be normal without hull damage on the ship

Bilging and Flooding

• Freesurfacemoments for rectangular compartment that is flooded by saltwater is approximated by multiplying the length by the cube of breadth and the density of salt water divided by 12.

Free Surface Moment = Length x Breadth^3 x 1.025

12

Loss of bouyancy

• When compartment is holed the ship will sink deeper in water until intact colume displaces waater equivalent to the mass od the ship and its contents.

• Loss of buoyance of a holed compartment is the mass of water entered the compartment up to the original waterline after the appearance of hole.

• It is computed by the volume of compartment multiplied by the permeability of the compartment.

Calculation:

• 1. SF = 45 lb/ft3 Density: 3

• 2. SF = 47 lb/ft3 Density: 1.5

• 2. SF = 46.5 lb/ft3 Density: 2.7

Loss of bouyancy

• If the lost buoyancy is greater than the reserve buoyancy, that means that the ship is already sinking.

• The draft will be until the top of the upper deck.

• The reserve buoyancy can’t support the lost buoyancy due to hole/s in compartment/s.

Movement of points and displacement

• The center of buoyancy moves to the centre of immersed portion of the ship because of the flooding and bilging.

• The sinkage is of ship considered flooding and bilging that is caused by the holes and damages in the hull of the ship.

• The center of buoyancy is depending on the immersed part of the vessel.

• Height of centre of buoyancy above the keel increases by about half the increase in draught due to flooding.

• According to the stability, The centre of Gravity and displacement is remained unchanged.

• Due to the separation of centre of gravity and centre of buoyancy of the upright ship, the heeling arm is produced that cause the ship heeled in a side on which the centre of buoyancy is concentrated.

• The are of the intact waterplane is reduced by the area of the flooded spaces at the level of the flooded waterline multiplied by the permeability of the spaces.

• Reduction of WPA leads to a reduction in the 2nd moment of area BM=I/V , wherein BM of a ship is generally less when bilged that when intact.

• According to the stability notes of damage stability, the compartment flooded is not anymore considered a part of the ship if the hole is in the waterline or the water is going in and out.

• Flooded space is entirely below waterline there is no reduction in intact waterplane. The space is only considered to be loading additional weights on the ship and still be considered part of the ship.

Calculate the increase in mean draught

• Draft = Volume of lost buoyancy/ area of intact waterplane

1. Compartment 15x20x10 TPC = 50

2. Compartment 20x25x15 TPC = 70

GM changes due to chang in KB and BM

• Gm decreases because:• Large loss of intact WPA

• intact buoyancy below flooded space

• Flooded surface has a high permeability

• GM increases because:• Bilging of empty double bottom tanks

• Deep tanks that are wholly below the waterline