# The stability of damaged ship

Discussion in 'Stability' started by xichyu, Apr 12, 2016.

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### TANSLSenior Member

I try to express my best. I agree with what you say, MCHL. I think the effect of free surfaces is simply a weight that moves on its own and, by changing the position of its center of gravity, introduces a heeling moment. This effect, in the case of liquid in contact with the sea does not exist. Not being (the liquid introduced by flooding) a weight whose center of gravity shifts with heel, there is not the so-called effect of free surfaces. Nor is it a weight that must be taken into account to calculate the displacement of the boat.

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### RabahSenior Member

Hello everyone,
I think that it is necessary to straighten out what for at an unsinkability calculation on method Constant Displacement, for tanks/compartments which at accident were charged overboard water up to levels damaged draft, it is impossible to take in account the correction for effects of free surfaces.
We convert to the formula from the Ship theory for extra heeling moment from the free surface in the tank at a definite heel angle:
∆M = γ * i
γ - volume weight of the fluid in the tank
i - moment of inertia of the free surface of the fluid in the tank be relative central axis, a perpendicular inclination plane
The method Constant Displacement is that the flooded compartment together with the internal water is treated as not belonging to the ship.
The injured tank is not included neither in volumetric displacement, nor in the loads of the ship. He becomes as absolutely pervious for overboard water, and the wetted surface of the hull in this part ceases to take up pressure on the part of ambient water.
The square of the water-line in space of the flooding tank calls as lost square. Time he any more does not belong to the water-line and is physical eliminated, then also its inertial moment "i" is equal to null. Or ∆M = 0.
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### xichyuJunior Member

So,if apply lost bouyancy, the center of weight is constant. Then, the heel, trim is also not varid.

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### RabahSenior Member

Hi xichyu ,
If to consider an emergency compartment as a lost part of the volume of displacement then Center of Buoyancy is changed and separately that the coordinates of the centroid and the displacement are stationary values by constant displacement method, the vessel receives new draft, heel and trim.
I recommend you to study well the Ship theory before to set naive questions.
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### Don92Principal Naval Architect

When using the lost buoyancy method the displacement and centre of gravity can actually change. Picture a large tanker with a damaged tank that was once full of crude oil. If we assume that this cargo is lost there will be shift in the centre of gravity and a change in displacement.

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### RabahSenior Member

Hi Don92,
This is for you! See the files...
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### RAraujoSenior Member - Naval Architect

Yes, you are right about intermediate stages of flooding. I was only thinking of final equilibrium condition.

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### Don92Principal Naval Architect

Hi Rabah, the description of the lost buoyancy method in the files you provided is general. If you read IACS Guideline for Scope of Damage Stability Verification for new oil tankers, chemical tankers and has carriers on page 9 it clearly states:

"The new floating position can be determined by assuming that the damaged displacement is equal to the intact displacement (constant displacement) minus the weight of liquids which were contained in the damaged compartments."

According to the aforementioned quote there would be a change in both displacement and centre of gravity.

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### RabahSenior Member

Hello Don92,
Please see the files from Manual of Maxsurf Stability!
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### Don92Principal Naval Architect

The manual from Maxsurf stability gives their definition as used within that software but it is not an absolute definition. Its common/good practice to assume that cargo has been lost when a tank is damaged and an assessment is being conducted using the lost buoyancy method. See attached capture.

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### RabahSenior Member

Hi Don92,
In May 2014 I have made a calculation of the unsinkability of the tug 34m with the programs Maxsurf Stability, Sea Hydro and Dialog-Static.
If to you it is curious see the link http://www.boatdesign.net/forums/bo...ility-unsinkability-asd-tug-fv_34m-50860.html
See the Maxsurf Stability booklet.
That you point that as a normative request of IACS, in effect Maxsurf Stability fulfills major virtuosity, fast and snug.
Extraction for Damaged case 15 - damage in Engine Room, see on the following pictures.
In the table of Load case 15 the program will equate to null of weights of fuel and other fluids in the fuel and other tanks located in the double bottom in the zone of damage Engine Room.
For this purpose if to compare with the Full Displacement Intact Load case we shall see the following:
Name of the case Displac,t LCG,m TCG,m VCG,m
Full Displ-Intact 883,252 17,282 0,000 3,952
Damage case15 841,391 17,347 0,000 4,111

These 841,391t are constant displacement which remains without change before ultimate equilibrium position of Damaged Water-line.
As itself you see in the concrete event of the variance in LCG and VCG is very minor.
For me that does the program Maxsurf Stability is as precomputation of the loads as introduction in the main method “ Lost of Buoyancy ”.
______________________
NA Razmik Baharyan

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