# The stability of damaged ship

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

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### xichyuJunior Member

Thanks very much！

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### TANSLSenior Member

In short it is, using the intact GZ curves, determine the equilibrium point of the heel due to the damage, so that the margin line will not fall below the water line at any point.

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### gonzoSenior Member

TANSL: No, the free surface effect can not be ignored like you adviced on post #6. Further on post #9 you say that there is no way water can enter a damaged compartment.
xichyu: The displacement may be constant, but the water moves and affects the stability. You can calculate the worse case. For example when the water moves towards the side the vessel is listing. Also, depending on where the damage is, water may be going in and out which further complicates the problem. In engineering there are no exact answers. However, you can find a value that gives a reasonable margin of safety.

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### TANSLSenior Member

Gozo sorry but you're dead wrong. Not because I say so but because the stability studies after damage are not done as you say. No need to calculate where the fluid moves. You have to calculate how floats a boat with one or more compartments flooded.

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### mchlMCHL Stabilitycalculation

TANSL : I am sorry, but we can't say that the probabilistic stability is a "third" method to calculate damage stability : It is a regulation, for which hundreds or thousands of damage conditions are calculated using the loss of boyancy method (the "second" method).

Concerning the FSM correction :

FSM correction is only calculated in the added weight method, not in the lost buoyancy method because we consider in this case that there is free communication of water between the damage tank(s) and the sea.

GONZO : Have you ever calculated at least one damage stability curve ?

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### TANSLSenior Member

Well, I will not argue. You're probably right, may it be a way to express myself. For me there are two methods (not two regulations): Deterministic and probabilistic. You call them as you like.
How would Gonzo have calculated a damage case, if he does not know what is that ?. He does not know how a damage is defined, its extension, etc.

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### RabahSenior Member

Hi All,
To explanation of RAraujo from Singapore it is necessary to add the following:
On two methods of the unsinkability calculation it is necessary to receive equal outcomes for draft, heel and trim.
On relation of calculation of the damaged stability we have equality of the stability factors /the coefficients of stability/:
1. for longitudinal stability
D*H2 = (D + p)*H1
2. for transverse stability
D*h2 = (D + p)*h1
Here: D - displacement of the ship on the constant displacement method
p - Weight of the joined water in the vessel on the added weight method
h1 and H1 - transverse and longitudinal metacentric height on the added weight method
h2 and H2 - transverse and longitudinal metacentric height on the constant displacement method
As p is a liquid load is necessarily necessary to mean and the corrections to h1 and H1 for effect of the free surface for flooding compartments.
For remaining unimpaired compartments if in them by the project there is a fluid for which at operation has appeared a free surface, irrespective of the method of unsinkability calculation it is necessary to calculate corrections on effect of a free surface.

Classification organizations require calculation of the damaged stability to make on the constant displacement method.
______________________
NA Razmik Baharyan

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### TANSLSenior Member

A nuance that would like to introduce in this discussion is that, in my opinion, the free surfaces and its effect should always be considered, regardless of whether it is a damage case or not. The liquid entering the ship as a result of the damage, as it is in contact with the sea, does not introduce any additional free surface effect.

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### RAraujoSenior Member - Naval Architect

Regarding the free surfaces on the loss of floatability method (constant displacement) - the free surfaces for the damaged compartment(s) do not need to be considered. As those compartments are not considered part of the volume of the ship they do not contribute in any way to its properties.

TANSL, as far as I know there are only these two methods to evaluate the properties of the floating body after damage/flooding. What you are refering as probabilistic/deterministic methods are a different thing -they are ways of "measuring" the characteristics of the floating body and compare them to a standard evaluating their compliance.

Nowadays the added weight method is seldom used (only for rough hand-made estimations).

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### TANSLSenior Member

Resolution MSC.281 (85), which is attached below, I do not know if it is a regulation, a method, a guidance or a procedure of calculation. That's not important. You call it what you deem best. What I do know is that it indicates how it should be studied the stability of cargo ships and passenger ships in the various cases of flood damage that may occur.
This resolution was adopted on December 4, 2008. I guess it has entered into force.
I call this "the probabilistic method". Sorry, if not correct, my apologies.

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### xichyuJunior Member

Yes，physical truth is important！

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### xichyuJunior Member

“Nowadays the added weight method is seldom used (only for rough hand-made estimations).” Thanks！

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### mchlMCHL Stabilitycalculation

For your information, added weights method is used when you calculate intermediate flooding stages, before final equilibrium stage of flooding in the probabilistic damage regulation. It is not possible to use the lost buoyancy method for intermediate stages.

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### xichyuJunior Member

#24
“It is not possible to use the lost buoyancy method for intermediate stages.” Is not it？

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### mchlMCHL Stabilitycalculation

During intermediary flooding stage, water in the damaged compartment is below sea level. This can be done only with added weight method.
With lost of buoyancy method the water level is always the same inside and outside.

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