Offshore Shallow Water Structures for oil & gas platform

[Ariel26th]

Hi Everyone….

I have several issues regarding a kind of structures that I should design in an environment described below

For introductions, my company currently still in the Conceptual Offshore Oil and Gas Development in Asia, The reservoir that we have lies below mud flats and Peninsula that I should say very Challenging in term of Execution to build Structure/Platform on the land since there are no access to the area from the land (Mangrove area), if we would have to do it from offshore it cannot be far from the reservoir target (Drill Cost!) so we have to build them around the area near the target or I should say mud flats ranging from 2 to 4 meter water depth.

1. Soil analysis indicates that dredging will be ineffective due to the lack of cohesiveness of the soil.
a. Extremely Soft soil until 100’ of soil depth(Soft Clay > 80% water) based on soil survey
b. Stiff Clay only exist beyond 100’ (Stiff Clay < 50% water)

2. Dredging would be required for installation Vessel access of the structures and Drilling Rig access (Swamp Drill Barge operates minimum 5 meters)
a. I guess based on soil survey above dredging nearly impossible
b. Possibility of using sheet pile around to coffer dam the area and use Suction dredging
3. Soil improvement to support the drill rig position when it grounded down
a. We try to use pile and compacted soils and sand bags

I have another thread submitted in soil subject to resolve the issue above, but in terms of structures type below are the problems.

If we would have to build conventional Monopod Structures platform the soils may not have a sufficient strength to support the structures, it might have to be developed in tripod or 4 legged jacket.

a. But how do we install in the structures in this very shallow water if we have to use heavy Crane barge that usually have draft more than 5 meters and for operational we have to have access all the time for drilling rig and operational boat
b. If we have to do dredging first to get the access the issues still lies on the possibility of dredging the soft clay on 80% water content.
c. We try to design in situ shallow structure to avoid using heavy lift crane barge, but this will have huge Offshore Spread Cost instead of pre-fabricated in a fabrication yard

Sorry for the long intro and thanks for your insight….

 

[BigInch]

Why a fixed structure? Answer on your other post.

Independent events are seldomly independent.

 

[JedClampett]

I won't pretend to understand everything that you presented, but could you build a floating structure remotely, sink it in place and use it to mount your excavation equipment. As you muck out the insides, it will sink into final position on sound material. This was called caisson construction in the past.
The Wikipedia entry linked below, shows the pressurized hand excavated type. This is scary dangerous. I would propose using floating excavation equipment or mounting them to the caisson.

 

[dhengr]

Ariel26th:
This area of work is certainly not my forte, so I’m just offering some food for thought. I would move lightly loaded shallow draft barges into the needed drilling positions and drive corner spuds/piles to hold them in position laterally. You can couple several of these barges together to make a larger working area. And, one can be made with a large void in the middle of the deck to work and drill through. You could drive a sheet pile skirt around a barge, depth to be determined, suction dredge the mud out from under it, so the barge floats and partly hangs off the sheet piling. This takes mud settlement (squeeze-out) out of the equation for the barge working elevation. This deeper water well offers several other improvements. Then you start to populate the working barge with equipment brought in on other shallow draft barges. This is no different than they do in other remote areas where they truck the entire drilling rig in, in fairly large pieces which are then set atop each other. If you don’t somehow contain your dredging area, I suspect you’ll pull out a cubic meter of water and mud, and an adjacent cubic meter of mush will just slough in to take its place. The skirt can probably be partly dewatered for work when needed. And, if you could place your offloading piping 10', depth to be determined, below the mud line, (BigInch calls this a pipeline end manifold, a PLEM) and run it out to another fixed working barge, in deeper water; you would then have short term storage and a location to load larger barges.

You said 6 to 13' of water (at low tide?) over mud flats. Then about 100' of mud (clay, etc.) which is about 80% water; then a firmer mud but still 50% water. This will cause long term settlement problems, almost whatever you do to try to build a solid, confined, island for drilling equipment. How far from deeper water to the drilling locations? That is, navigable water with fairly shallow draft equipment, tugs, barges, etc. Depending upon the environmental regulations at the site, large scale water jetting may be effective at keeping the mug (a narrow channel) fluid enough so that equipment can be moved into place. But, as soon as the jetting stops the channel will fill back in and have to be opened up again.

It seems to me that your company probably shouldn’t be in the oil drilling business if you don’t have a fairly good handle on these type of matters. That’s a fairly risky business even when you do know what you are doing, but of course, everyone has to start someplace. Obviously, part of your work at the moment is to determine how the economics work out. If the economics are right, I would guess there are plenty of people in the oil industry who could provide knowledge, experience and equipment to make this work. I think BigInch has given you some pretty good advice in the other thread.