how to calculate stability of semi-submersible vessel?

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?

 

PSV/OSV Cat Stabiliy

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.

 

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.

 

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.

 

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.

 

Stability Software Quote

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.

 

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?

 

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.

 

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?

 

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.

 

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?

 

yes, that's all.

 

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

 

"...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?

 

Stability

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.