how to calculate stability of semi-submersible vessel?

Discussion in 'Stability' started by manon, May 30, 2009.

1. Guest625101138Previous Member

OK - in the submerged state it will be more stable and tolerant to higher angles of roll.

If you can calculate BM = I/V you can determine the static stability.

Lets take it step by step.

Draw the waterplane of the submerged vessel and post it here. It will be simply the intersection of the columns.

Rick W

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manonJunior Member

Im Sending You The Section At 12.5 M Draft. Please Let Me Know, For Any Further Details.

Thank You

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3. Guest625101138Previous Member

OK - I agree.

Now can you work out the second moment of inertia of this waterplane?

Rick W

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manonJunior Member

I Have One Problem, That Arises Due To The Two Position Of The Pillar Position. When It Is One Hull, I Will Take Breadth From One Side To Another, Bt Now What? 1st I Find Some Length, Than Some Blank Space And Again Some Length. How To Add Or Combine?

5. Guest625101138Previous Member

You simply subtract what is not there from what is. The calculation can be reduced to:
I = 14/12 * (34^3 - 28^3 + 34^3 - 22^3)
= 53676m^4

Now what is the displacement when submerged to the 12.5m mark?

Rick W

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manonJunior Member

According To Hydromax, Its 8262 Tonne At 12.5 M Draft

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manonJunior Member

So Bm Is 6.7388041636407649479544904381506???

8. Guest625101138Previous Member

Yes- 6.7m

Now you need to do the same calculation for different angles of roll. This will enable you to produce the cross curve.

I would be surprised if your software would not be able to do this for the static case.

Rick W

9. Guest625101138Previous Member

As a matter of interest what is the height of the CoG in the flooded state. Is the vessel statically stable when on the flooded waterline?

Rick W

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GuillermoIngeniero Naval

For intact stability criteria, refer to IMO's Code.

Some pages there are missing. You should buy the booklet (cheap). You may do it directly from IMO (http://www.imo.org/) or then a distributor in your country or close to Bangladesh.

India distributors:

Bogerd Martin (India) Pvt. Ltd.
1A-B-C,Goa Mansion, Ground floor, 58 Sunderlal Bahl Path Fort, Mumbai
Telephone: +91 22 22 62 63 18, Fax: +91 22 22 62 14 88
Email: sales@bogerdmartin.com

C & C Marine Combine
25 Bank Street , 1st Floor, Mumbai 400 001
Telephone: +91 22 2266 0525 /1937, Fax: +91 22 2267 0896
Email: ccmarine@vsnl.com
Sterling Book House

181 Dr. D.N. Road, Fort, Mumbai 400 001
Telephone: +91 22 2261 2521, Fax: +91 22 2262 3551
Email: sbh@vsnl.com
Website: http://www.sterlingbookhouse.com

For probailistic damage stability I'd recommend you to use a good stab software.

Cheers.

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To calculate your I, you calculate the inertia about the axis simply I=ah^2, then add the inertia of its own, bd^3/12. This give you 54135m^4.

If your displacement is 8262, the volume is 8262/1.025 (in SW) = 8060.5

So your BM is 54135/8060.5 = 6.72.

However to calculate your intact stability you need to know the CoG. Assuming you are doing small angles of inclination, note, inclination and NOT roll. Roll is used in a different context not one related to calculating the intact stability.

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Ignore information such as "...However the vessel design you have really requires dynamic analysis because it does not take much roll to cause it to be unstable..." This is totally incorrectly.

You have a low water plane area to decouple the motion of your vessel from the waves. Your ROAs, if you have done them, will verify this for you. This is nothing new and is employed by SWATHs and many other types of marine vessels.

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Rick

"..I expect a sea condition with 3m waves would begin to cause a stability concern.."

And where does this little gem come from???...and what kind of 3m wave?

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manonJunior Member

well, well, well. i am afraid, i cant go through all the specific calculations right now ( RAO.......)
as we have found that the BM = 6.7, for better stability, what should be the Centre of gravity of this vessel for better stability?

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