Design of Fixed Steel Jacket -Legs

[Moush]

Hi all,

This is one for the structural engineers out there.
I have just begun my first job at an oil and gas company as a mechanical engineer and am learning about all the different types off offshore structures.
In regards to fixed platforms, I was curious to know what dictates how many legs the jacket is going to have. For example why would a jacket have 4 legs instead of 3?
Ive tried researching but havent had much luck. Can anyone enlighten me with regards to this or maybe point me in the direction of where I can read more about this.

Regards.
Ps this forum is magnificent, I cant believe I didnt find it sooner.

 

[BigInch]

Weight of topsides.
Lateral loads from waves and currents.

you must get smarter than the software you're using.

 

[London]

The behaviour of a Jacket will change depending on the number of legs it has. The dynamics, sway, and natural frequency will change. This could be driven by the seastate, water depth and to a degree, the stiffness of the foundations (The Jacket does not stop at the mudline, but is considered with the foundations and soils).

The other aspect, as BI noted, is the topsides layout and design. It the topsides is a square, standard trussed layout with 4 columns as lifting points, these columns would stab into the legs and welded out offshore.

 

[Angre]

There is no simple answer to this.
As indicated by Biginch weight of topsides and lateral loads are the major factors.
Apart from that facilities layout, no of well slots(if wellhead platform).Please refer to
Handbook of Offshore Engineering by Subrata Chakravarty to get a fuller idea on this aspect.

One more thing, since you are a beginner, this is not going to be your call for a long time to come, instead devote your
resources to strengthening the basics of mechanics and design of elements.

Rgds
Angre

 

[Moush]

Thank you all very much for your replies, very helpful.
Angre, I totally agree with you but Im the kind of person that needs to figure out the answer to a question or atleast gain a better understandiung of the topic or it will drive me mad. Its a curse.

Again thank you all.
Cheers
Moush