Location of VCG/KG

Dear Gents,
Can some one point me to how to calculate the maximum VCG (allowable KG) for a drilling rig. I have tried autohydro tutorial but it gives me off results than the the original designer sheet, Highly appriciate ur answers.

 

"..I was advised by a Naval Architect that a good place to start with VCG/KG is on the proposed waterline..."

What do you mean by a good place to start....are you referring to preliminary design to establish a boats stability, in the absence of any 'weights and centres' calculations,??, or some other definition?

 

"...So where would you place it so that she rolled easy, came back easy?.."

Has the designer/naval architect not told you where it should be or will be?..since the design has already been done.

 

"...we placed it at .80m,.."

This is all reactive not proactive and it is not design per se.

Well, there is your problem, you have assumed a location for the VCG (probably assumed a weight/mass too?) but not actually calculated it. As such you don't know if it is sufficient to pass ISO/RCD for stability/freeboard criteria until the boat is finished and you will only know once you perform an inclining expt. Without any weight and centres data all you can do is guess. But whether the guess is right or wrong, for the VCG, it still doesn't answer your Q about what the roll period/seakeeping will be like.

Without knowing the lines, the LCG/LCB, the duty the draft etc etc and its SOR hard to say with any degree of certainty to use the limited data given as a basis for comparison of 'similar' type of boats to provide a 'realistic' answer. Since that is all anyone can do, use previous data to provide a "rough idea".

I don't image it is a fast boat, as such, round bilge is far more forgiving and provides a far better roll characteristic than a hard chine. I'm assuming the hard chine is purely for ease of construction rather than any beneficial aspects for design. If the draft is not in a favourable location, then the motions could be a real pig, whilst actually passing all the stability criteria.

That is the compromise/balance of design, since the two requirements are, in general, diametrically opposite each other.

Build a small model, using the VCG and LCG where you have "placed it" on your computer model, and test it. That is all you can do for now if you want some 'hard data'.

No idea what "..80--100 avs.." means????!

 

i thought you were a NA? it is the angle of vanishing stabilty
and we have calculated all that,

 

I don't use avs as a measure of a boats seakeeping nor stability. Haven't done so since i was a student. If a boat passes the required criteria and has acceptable seakeeping, following a design review, then the avs is irrelevant, as it this is dictated by the aforementioned. It does not alter nor change the design. Hence never even consider it...

 

Ad Hoc,
I wouldn't say AVS is irrelevant. It is taken into consideration in quite a few of the stability criteria for several types of vessels, among them ISO 12217 for sailing boats.

dmetawie,
Firstly, I have not experience with drilling rigs, but I think it is probably not easy to answer as it depends on many factors, load condition among them. You'd need to check the VCG position against any and all stability criteria for rigs and build the correspondent curves for all load conditions. I'm not at the office, so cannot check it: Have you seen if there is something at IMO's Code on Intact Stability?

Cheers

 

guillermo
I had orignally posted in here, but Ric advised start new thread, I would be greatful any input you can give
cheers Stu

 

Guillermo
Yes, that is correct. But this boat is not a sailing boat/yacht. Hence when designing it, the avs is not a driver for the design.

 

strange answer AH from somebody who did not not know what AVS was til 5 hrs ago

 

It took me awhile to figure out what "AVS" meant, too. And it's really not applicable to evaluating the stability of a motor vessel. Naval architects are more concerned with the downflood angle when doing stability calculations for motor vessels.

We're used to terms like "the largest angle corresponding to a positive righting arm" being written in stability criteria. The initials "AVS" is new to me.

 

glad to be of help:)
google it up, much has been written on the subject and now a stabilty curve showing that angle, must be produced to race, and also for sale of sailing yachts in Europe
I think that USA does not adopt these standards

 

Paul
"..It took me awhile to figure out what "AVS" meant, too. And it's really not applicable to evaluating the stability of a motor vessel.."

Exactly, hence my post #21.

"..much has been written on the subject and now a stabilty curve showing that angle, must be produced to race,.."
This is nothing new, it is applicable to sailing vessels, not motor vessels.

"..We're used to terms like "the largest angle corresponding to a positive righting arm" being written in stability criteria. The initials "AVS" is new to me..."

Likewise, never seen the acronym 'avs' before...and certainly not in relation with a vessels "stiffness" as was originally posted but now removed.

Min GZ, GM, max angle of GZ, areas under the the GZ curve up to 30 and 40 degrees, downflood angles, when heeled and flooded etc etc, are all the drivers of a design, not "avs", for 'motor' vessels.

 

AVS is not only applied to sailing boats, but also to some motor vessels, like commercial fishing ones. There have been and still are some national reccommendations about it not being less than a certain amount (around 80º) depending on type of vessel and nation.

Cheers.

 

Guillermo

As i noted in #21, i haven't used 'avs' since i was a student some 25years ago. All the boats i design (commercial) avs is never a consideration.

I know that sailing boats use them as a basis for a measure of their "stability"....and I'm aware that there is "some use" as such in fishing boats. But exactly their usage in fishing boats to a criterion, I'm not fully aware of, since not my field. The boat shown was a small pleasure craft, which doesn't fall into to either category.

I just thought that since the boat in question doesn't fall into either, it would become confusing debating avs when it doesn't apply to the design, and certainly not to the original question of "stiffness" or "seakeeping".

But for 100% completeness, yes avs does apply to some design categories....but not many! I've never used it as a design 'driver'...may be you have??

I've reviewed several papers on fishing boats, but their concern was more about green water and freeboard on the fore deck and the reduction in GM/GZ owing to said, on the capsizing probabilities and referring to the Torremolinos Convention. Again, no avs metioned.