4 Leg vs 8 Leg 1

[GregLamberson]

All:

Is there any technical justification to go with an 8 Leg vs a 4 Leg rig support/production platform if the topsides are the same? This would be in 250' of water.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[JLSeagull]

What size topsides? My shallow water experience is all eight-leg but for 15,000 ton topsides in 150-foot water. They were as solid as a rock. I saw lots of smaller platforms. The structural guys described their movement at over a foot during storms.

 

[GregLamberson]

JLSeagull

Don't have the weight of the topsides yet, but it on the small side - it will have 15 well slots and it's a 1,000 bpd production with 30 GOR and 1,000 bwpd, so it will have some gas processing & water handling.

I was asked what the technical justification would be for one over the other and an approximate cost adder for going to the 8 leg over the 4 leg. On that I am assuming the topside structure would be only slightly larger, but with the addition of 4 legs/pile/skirts - is that generally the right thinking?

FYI - this is GOM.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[BillBirch]

Greg,
I'm guessing the 8 legs is way overkill. However the design consideration that will ultimately decide the outcome is the type of drilling you intend. A tender-assisted rig has the rig (and its weight including that of the drill string supported from the platform.
On the other hand (for example), a jack-up rig can be cantilevered over the slots to be drilled supporting the weight of the derrick, drilling package and drill string.
The jack-up or semi-sub has a more expensive day rate than the tender-assisted package so it is a question of economics. Do you design (and pay for) a more robust, but more expensive platform now to allow cheaper drilling campaigns in future, or do you design a lean platform, but are restricted to fewer, more expensive drilling options in future?
Remember also that the weight of the wells, conductors, strings are not supported by the platform as they are cemented into the sea bed, so essentially you will not have much equipment weight to support - unless the drilling package weight must be considered.
Regards,
Bill

 

[BigInch]

Allowable bearing on seabed and pile length vs resistance might have a small something to do with number of columns... maybe. High wave loads in deep water on a lightly loaded platform could present a worst case design scenario due to the higher tension and compression columns from overturning moments. A heavier platform load would reduce tensile loads on the uplift side piles.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain