# how to calculate stability of semi-submersible vessel?

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

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### manonJunior Member

hello everybody,

i now have a problem about calculating the stability of a semi-submersible plat form supply vessel ( twin hull ). it is priniciple dimension Lbp = 70 m, beam per hull = 10 m, gap between two hulll = 18 m, platform height = 21 m, hull depth = 5m. light condition depth is 2.7m and submersed condition depth is 12.5 m.

please help me to calculate the stability of this vessel manually. i cant calculate with my software (hydromax), because, there has no criteria that could fully support this vessel while calculating.

thank you very much

2. ### Guest625101138Previous Member

Have you got a drawing of the waterplane in the loaded condition?

Rick W

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### manonJunior Member

yes, i have the water plane in loaded condition or maximum operated draft condition (12.5m).

but, let me tell you one thing, this two hull is connected by sum heavy girder arrangment only. its possible for me to calculate stability for single hull, but here i dont know, how to combine.

one more thing please, as i dont know any specific criteria ( this vessel is not supporting the stability criteria of MODU, offshore platform vessel), will you please let me know the stability criteriafor such kind of vessels?

thank you very much

4. ### Guest625101138Previous Member

How about drawings of what you are trying to work out. If you have the waterplane and the hulls are rigidly connected it is a matter of doing the integration.

Please provide a drawing of the water plane.

Confirm the hulls are rigidly connected - I imagine it is a catamaran!

Rick W

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### terhohalmeBEng Boat Technology

Use Steiner's rule.

Or draw the wateline sections in any CAD and ask area moments.

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### abohamzaJunior Member

any drawing may help us to answer your questation

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### manonJunior Member

attached drawing

thank you for your response. as you have wanted, i have attached the pdf file of the GA of the vessel. please have a look and suggest me.

thank you very much

File size:
1.3 MB
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8. ### Guest625101138Previous Member

The GA has inconsistent data. The drawing shows the vessel as being a little over 110m long. The MAIN PARTICULARS list has the length as 73m.

The displacement of 2000T is for a single 73m hull. I get a displacement of 5500T for the vessel drawn.

For the vessel drawn, rather than the MAIN PARTICULARS, the KMT is 107m.

Rick W

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### manonJunior Member

no my dear, thats the frame no, and the frame spacing is 600 mm.

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### manonJunior Member

upss, sorry about the displacement term, that will actually the light weight it prelimenery the light weight is 2000 tonne, bt, it could be vary little more. bt the length is 73 m over all and the hull length is 70 m

11. ### Guest625101138Previous Member

Reducing the length will not change the KMT. I determine the loaded displacement is 3300T. Based on this value the KMT is 107m.

As you can see I have a much larger displacement than you have calculated. What is your revised displacement?

Rick W

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### manonJunior Member

well, please take cb = 0.8
and, it would be helpful more, if you could give me the exact stability criterias for this kind vessels

p.s: i have maxsurf, hydromax....................can you suggest any more please?

13. ### Guest625101138Previous Member

The value I have provided is only for initial stability of course.

I would be concerned about the reserve buoyancy in the hulls. It is not much more than 100%. Hence it will be possible to submerge an entire hull under dynamic conditions. Once this happens the stability deteriorates rapidly. It only takes 6 degrees before the stability begins to reduce. At 12 degrees of roll one hull will be submerged with the other clear of the surface. You are then relying on the platform struts to provide waterplane area. Are these designed to be water tight?

This vessel will have quite high roll inertia so the possibility of submerging a hull needs to be examined. The dynamic analysis is more complex than static analysis. I expect a sea condition with 3m waves would begin to cause a stability concern.

The internet is not the place to be getting advice on the stability of this vessel. It needs proper dynamic analysis related to the sea conditions it will operate in.

Rick W

14. ### Guest625101138Previous Member

The exact static stability criteria for any vessel is:
BMT = IT/V
Where BMT is the transverse metacentric height above the centre of buoyancy
IT is the second moment of area of the waterplane about the longitudinal axis, vertically aligned with the centre of buoyancy
and V is the volume of the submerged portion of the hull/s

However the vessel design you have really requires dynamic analysis because it does not take much roll to cause it to be unstable. You really need to have expertise in its use unless you are just playing about for learning purposes.

By the way the longitudinal stability is not particularly good either. This may actually be worse than roll stability when steaming.

Rick W

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### manonJunior Member

thank you for your quick response.
submersed condition of the vessel will be achieved by ballasting the vessel. its parallal middle body of the hull is used for water ballasting and the pillars provided are also hollow and 60% of the height of the pillar can be used for ballasting to achieve the submersed draft (12.5 m).

this vessel will be submerged only in the rough weather, because this design we are trying, so that it can operate in the rough weather also.

now???

again i m requesting to give me some idea about the criteria of stability for this vessel, i.e, max/min values of GZ/GMT/BMT, rolling angle, wind heel..........and so on. we have examined the modu code of germanischer lloyd's but, the modu code is not for this kind of vessel. i am able to calculate manually BM = I/v...........but, i dont know the standard limits of stability criteria for this kind of vessel.

is it possible for you to give a format in for calculating initial stability of multihull vessels which are connected with some heavy girder arrangements?

thank you again for your response.

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