# the free surfaces effect in tanks

Discussion in 'Stability' started by santoo, Mar 30, 2011.

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### santooJunior Member

Hi... All members of boatdesign.net forum,
Can anyone help me to explain to me about the information below :

" In calculating the free surfaces effect in tanks containing consumable liquids, it is to be assumed that for each type of liquid at least one transverse pair or a single centreline tank has a free surface and the tank or combination of tanks taken into account are to be those where the effect of free surface is the greatest ".

I get this information in RINA Rules Pt B, Ch 3, Sec 2.
I hope all of you can help me...

Best Regards,
Santoo

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### RAraujoSenior Member - Naval Architect

Santoo,

The text comes from the IMO Code on Intact Stability 2008 and its predecessors.

In practical terms what it means is that you must consider corrections, in GM and GZ, due to the effect of free surfaces you might have on tanks.

The above assumes that, for each liquid, one tank (if centre line tank) or a pair of tanks is to be used at a time.

In order to maximise the effect, on a conservative approach, only the tank, or combination of tanks, for each type of liquid, that produces the greatest detrimetal effect are to be considered.

For instance considering that you have the following fresh water tanks:
1 (C) with a fsm of 345 t.m
2 (P) + 2 (S) with a combined fsm of 450 t.m
and
3 (P) + 3 (S) with a combined fsm of 220 t.m
you only need to consider the effect of 2 (P) + 2 (S) totalling 450 t.m.

The same for all other liquids...

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### santooJunior Member

Thanks RAraujo,

Hm.... Noted,
So, if in my Load case there are 3 tanks which are not full (less than 98% full), only one to be consider for calculation?? Because in my booklet, there are 3 tanks less than 98% full, and i was calculate FSM for every tank which are not full.

That's mean, I false if I calculate FSM for all tank which are not full?? Right??

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### RAraujoSenior Member - Naval Architect

It's not common to be consuming the same from 3 different tanks.

Are the three tanks of the same type of liquid? If yes then you should consider the three tanks (the assumption in the Code is that only one tank or pair of tanks for each type of liquid is being used simultaneously). If not you should consider one tank, or pair of tanks, for each liquid type.

Of course you can introduce the free surface correction considering all tanks on board but that might be over-conservative.

Maybe if you post your tank arrangement, with fsm, and sequence of consumption it would be easier to explain.

Rodrigo

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### santooJunior Member

Ok Rodrigo...

I have consider three tanks for free surface effect, but my class comments my booklet which says like I write in the first time of this forum.

Actually, I don't know what it's that mean...

You have any suggestion?

Ow sorry, I'm not in office today, maybe tomorrow I will post it.

Thanks a lot...

Susanto Agusta Herlambang

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### ontwerpNaval Architect

tank free surfaces

I just want to add that what is said above is true for tanks with consumables, such as fresh water, gas oil, lube oil, but not for water ballast tanks.

For ballast water tanks you need to use the maximum FSM of each tank that is filled or emptied during your voyage.

1. If a BW tank is kept empty or full during the voyage you can consider it 0 FSM.
2. If a BW tank is partially filled or emptied (go from 60 to 40 % filling) you only need to use the maximum FSM for that range of filling (between 60 and 40 for this example).
3. If a tank is kept partially full you can consider the real FSM for that filling condition. ​

Example:

Departure Condition:
BW1=Full
BW2=Full
BW3=Empty
BW4=60%
BW5=77%

Arrival Condition:
BW1=Full
BW2=Empty
BW3=20%
BW4=40%
BW5=77%

FSM Values for tanks for voyage:
BW1=0 (no change)
BW2=Max (total range of filling)
BW3=Max between 0-20% (range of filling)
BW4=Max between 40-60% (range of filling)
BW5=Real FSM at 77% (no change of filling)​

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### RAraujoSenior Member - Naval Architect

Ontwerp,

You are absolutely right - that's the way it should be. I was just concentrating on the extract above on consumable liquids.

Rodrigo

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### santooJunior Member

wah wah wah...
Ok ok Rodrigo...
Noted noted...
thank you thank you...

I got it...!!!

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