how to calculate stability of semi-submersible vessel?

[manon]

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

[Rick Willoughby]

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

Rick W

[manon]

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

[Rick Willoughby]

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

[terhohalme]

Use Steiner's rule.

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

[abohamza]

any drawing may help us to answer your questation

[manon]

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

[Rick Willoughby]

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

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

[manon]

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

[manon]

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

[Rick Willoughby]

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

[manon]

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?

[Rick Willoughby]

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

[Rick Willoughby]

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?

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

[manon]

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.

[Rick Willoughby]

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

[manon]

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

Thank You

[Rick Willoughby]

OK - I agree.

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

Rick W

[manon]

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?

[Rick Willoughby]

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?

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

[manon]

According To Hydromax, Its 8262 Tonne At 12.5 M Draft

[manon]

So Bm Is 6.7388041636407649479544904381506???

[Rick Willoughby]

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

[Rick Willoughby]

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

[Guillermo]

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

http://books.google.es/books?id=SVSvl1znVesC&pg=PA12&lpg=PA12&dq=imo+intact+stability+criteria&source=bl&ots=7nKr054PMu&sig=g0kQl6y6dOZX4yK4Ytu7xpA5bb4&hl=es&ei=hHYqSr67D8qPsAbHpsCeDA&sa=X&oi=book_result&ct=result&resnum=3#PPA40,M1

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.

[Ad Hoc]

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.

[Ad Hoc]

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.

[Ad Hoc]

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?

[manon]

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?

[Ad Hoc]

manon

That depends what roll and pitch period you want the vessel to have in the sea spectrum that the vessel is going to be in...I cannot answer that only your design SOR (statement of requirements) will do that. Once you ahve established the sea spectrum that the vessel will encounter, then you can design the vessels characteristics and fix the VCG, LCG, LCB, LCF. All these affect your motions

[Ad Hoc]

For example, have you calculated either by experiments, model testing or another method, the added mass component of A33, when establishing your motions along with the interia of the waterplan area?

[jimships]

Hi. I work in this field and sell AutoLoad software. You have several options.

One is to model the hulls in Modelmaker and process in Autoships-AutoLoad. Simple and complete with full analysis under ANY loading condition in seconds.

Manually, you would need to calculate the areas by station from centerine out half hull to the load waterline, then you can calculate the righting arm for each station, KGL, KGT, GML, GMT. MTI, etc. The rest is fairly easy.

[manon]

Thank you all my friends, for helping me.
now i have come to some results and some new questions also.

we have calculated here for the submersed condition (12.5 m)

LCB from zero pt 34.942 m
LCF from zero pt 35 m
KB 4.123 m
KG 6.795 m
BMt 6.761 m
BMl 15.821 m
GMt 4.089 m
GMl 13.149 m
KMt 10.884 m
KMl 19.944 m

do you think that this values are ok for good stability?

again, what is the type of this kind of vessel? semi-submersible multipurpose platform supply vessel? i have gone through the IMO code as i have been suggested before, but that was for the MODU (mobile offshore drilling unit), than what would be the criteria?

can anybody help me by supplying me any softwares so that i can calculate stability of such kind of vessels? is it possible for anyone? i have hydromax, but i didnot found any direct criteria that matches to this vessel. then i wanted to edit their criteria for my vessel, but for that purpose, i need to know the exact criteria.

anyway friends, thank you all for your time for me.

[Ad Hoc]

manon

You need to understand what you mean by 'stability' Do you mean in terms of passing a criterion to satisfy a Flag Authority that the vessel will not sink when heeled, or do you mean stability commonly referred as seakeeping, ie how stable (vertical accelerations for example) the vessel is in certain wave conditions? The two are totally different.

As for what criteria? These are commonly used.

1)The Health and Safety Executive, Offshore Installations: Guidance on Design, Construction and Certification, Fourth Edition with amendments, 1993.
2) Norwegian Maritime Directorate, Regulations for Mobile Offshore Units, 1992.
3) International Maritime Organization , Code on Intact Stability for all Types of Ships Covered by IMO Instruments, Resolution A.749(18), 1995.
4) International Maritime Organisation, Code for the Construction and Equipment of Mobile Offshore Drilling Units, 1989 (1989 MODU Code), Resolution A.649(16), 1990.
or
5) Det Norske Veritas, Rules for Classification of Mobile Offshore Units, Part 3, Chapter 2, Special Designs, Equipment and Stability, 1998.

[Ad Hoc]

manon

sorry forgot to add
"...do you think that this values are ok for good stability?.."

That is for you to determine using the above regulations/rules...and then compare against the SOR of the vessel.

[jimships]

I am the Oil and Gas Offshore Representative for AutoLoad...the most advanced software of its kind, approved by all classes and the most rebust and leding in these vessels and semi's. Pease contact me off thread at jimships@yahoo.com for a link. I will quote when I know more about your company, etc. Thanks.

[manon]

thank you all for your valuable suggestions.

now i am in a state to calculate the wind heel moment, it is needed for the stability criteria of IMO for this vessel. can anybody help with it?

the formula stated in IMO for wind heel force = 0.5*Cs*Ch*P*(V**2)*A, i have calculated, bt, how to calculate for different angles?

[Ad Hoc]

Well, now you have calculated the wind lever, you over lay this wind lever onto the GZ curve. Where the wind heel lever intersects with the GZ curve, you get area's underneath and heel angle.

[manon]

yes, that i know, but how i will be able to calculate F for different angle, as i got the GZ values for different angles?

[Ad Hoc]

You don't. Technically the F does varying because as the vessel heels, the projected area varies. But For the purpose of calculations, it is assumed to be constant. This actually provides a more conservative answer too.

[manon]

in the criterias described in International Maritime Organisation, Code for the Construction and Equipment of Mobile Offshore Drilling Units, 1989 (1989 MODU Code), Resolution A.649(16), 1990, a clause states as follows:

The wind heeling moment curve should be calculated for a sufficient number of heel angles to define the curve. For ship-shaped hulls the curve may be assumed to vary as the cosine function of vessel heel.

i have calculated the wind heel moment according to above convention while the vessel is 0 (zero) degree heeled. now for various angles, should i just multiply the value with different cosine angles only? is that ok?

[Ad Hoc]

yes, that's all.

[Rick Willoughby]

in the criterias described in International Maritime Organisation, Code for the Construction and Equipment of Mobile Offshore Drilling Units, 1989 (1989 MODU Code), Resolution A.649(16), 1990, a clause states as follows:

The wind heeling moment curve should be calculated for a sufficient number of heel angles to define the curve. For ship-shaped hulls the curve may be assumed to vary as the cosine function of vessel heel.

i have calculated the wind heel moment according to above convention while the vessel is 0 (zero) degree heeled. now for various angles, should i just multiply the value with different cosine angles only? is that ok?

With the exposed super structure as you have it, it will have increased windage as it heels not less. It is quite different to a slab sided vessel. The cosine applies to a slab sided vessel.

A more valid area would be the projected area of the superstructure that takes into account the exposed base of the platform. You will get some lift from the structure as well that will alter the stability.

You need to get expert advice on this with those experienced with such platforms.

Rick W

[Ad Hoc]

"...A more valid area would be the projected area of the superstructure that takes into account.."

When you have actually done these calculations many times for real in submission to Flag authorities for a Trim and Stability Booklet, you realise that this is the only way to do it....the formula has the projected area above the waterline as the input. Those who have done this calculation before, understand this immediately.. It is not a case of.."oh don't forget to do this.."...as this is the ONLY way to calculate it.

How many Trim and Stability Booklets ahve you submitted to Flag/Class for approvals Rick?

[Ekaiztea]

Hi all:

This design looks very similar to a floating ship's yard. Try to have a look to the stability criteria for those devices.

[ycl229]

your discuses are useful to me!

[sirhamid]

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

hi dear
you can model your vessel on Rhino and loft surfases on this,so then save as "igs" format for maxsurf usage.

i can do this for u.send me your cad model,for this.;)

[manon]

thanks for your interest sir, but i already have the maxsurf model. but the problem occured while calculating the stability, becasue i cant understand which stability criteria in hydromax will be appropreate for such type of vessels.

[sirhamid]

thanks for your interest sir, but i already have the maxsurf model. but the problem occured while calculating the stability, becasue i cant understand which stability criteria in hydromax will be appropreate for such type of vessels.

oh ,ok
i can send you the modify MODU code on hydromax criteria,but you may be wait about some day.
at the last night,i had a same project on iranian MODU "shahid rajaee" about the modifing the stability booklet on new inclining experiment.i think you have problems same as i had.

[sirhamid]

first read this file:
OFFSHORE STANDARD
DET NORSKE VERITAS
DNV-OS-C301 (http://www.4shared.com/file/116675975/fb9170f/OS-C301.html)

[manon]

sir, the problem is, this vessel is not a drilling unit, this is actually multi purpose semi-submersible unit...........can i use the modu code? if so, i already have the GL class codes.

i a have tried to calculate stability with
1.The Health and Safety Executive, Offshore Installations: Guidance on Design, Construction and Certification, Fourth Edition with amendments, 1993.
2.CODE FOR THE CONSTRUCTION AND EQUIPMENT OF MOBILE OFFSHORE DRILLING UNITS, 1989
.......again, if you could please can send me the MODU criteria for hydromax, it will also be very helpful.

thank you very much

[sirhamid]

yes manon,you could use that code.for the semi-sub vessels ,the rules same used for this types.
but on your calculatio,U must be provide the rigging forces on vessel for meassure the righting arm effect,like you calc the wined force and so on.

[sirhamid]

:idea: this can be help U:
Review of issues associated with the
stability of semi-submersibles (http://www.hse.gov.uk/research/rrpdf/rr473.pdf)

:) :idea: