Statoil searching for well to test artificial seabed concept

Sept. 1, 1999
Statoil plans to use the Proffshore's Atlantis Artificial Buoyant Seabed (ABS) to drill its next deepwater well.

Statoil plans to use the Proffshore's Atlantis Artificial Buoyant Seabed (ABS) to drill its next deepwater well. Had it not been for the postponement of drilling projects impelled by low oil prices, the company would probably have been putting the concept to its first test already.

"If successfully deployed, the Atlantis-ABS concept represents the greatest step-change to date in deepwater drilling technology," says Johan Eck-Olsen, Project Manager for business development in Statoil's drilling and well technology department.

The concept is centered on a submerged buoy deployed at a conventional depth under the drilling rig, which acts an artificial seabed on which the wellhead and BOP are installed. It was invented by Terje Magnussen.

"We're now hard at work looking for a candidate for trying out Atlantis," says Eck-Olsen. "We would have used it on a deepwater well we had planned in Norway, but this has now been postponed for two years. We have other wells coming up where Atlantis is now being considered. We have informed Mobil about the concept and we are now discussing the possibility of using Atlantis on a well in the UK or Ireland, but this well has also been postponed.

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We have also presented the concept to Texaco and Shell for a well in Nigeria, and there are upcoming wells in the Gulf of Mexico which could also be candidates. We are now hoping to firm up a well by the end of the year."

Statoil has provided NKr 12 million in funds for developing Atlantis, and has an option of 15% ownership of the technology. The wide range of studies has included analysis of rig-ABS interaction, anchor line failure and vortex induced vibrations in the casing.

The ABS acts on the same principle as an upturned bucket which is made buoyant by the air trapped inside it. The upward force from the ABS buoy keeps the well casing in tension and the ABS in position. The ABS has a cylindrical shape with a circular opening through the center, around which are a number of separate buoyancy chambers connected by individual air supply lines to the rig, from which it is controlled. A skid-mounted compressor on the rig provides the air supply.

When the ABS has been positioned some 200-300 meters below the rig, the anchor casing is stabbed through its center opening and extended into the top-hole already drilled at the seabed. When the casing has been cemented in the top-hole, the ABS is pulled up to mate with the wellhead, which at this point is a few meters above it on the string. The buoyancy of the ABS is then increased to tension the casing below it. The casing running string can then be pulled, and a conventional shallow-water BOP and drilling riser run and connected to the wellhead. The system is then ready for drilling.

The Atlantis concept overcomes a series of deepwater challenges. Among these are the need for a long, heavy-duty drilling riser and for long choke-and-kill lines. It also requires a lesser volume of mud than a typical deepwater well, and bypasses the problems of poor seabed soil conditions and tricky conductor installation. Because the well is in effect in a conventional water depth, it makes it possible to use a second or third-generation rig equipped with a deepwater mooring system.

Studies by Statoil have shown that an Atlantis well drilled with a shallow-water rig has the potential to save 30% of the cost of a conventionally drilled well in 1,000 meters water depth, and 60% in 2,000 meters.

Proffshore prepares to build first ABS

The first Atlantis ABS will be built by a partnership between Proffshore and Statoil. Detailed engineering has been completed and the construction specification is finalized. The only remaining engineering is some site-specific analysis.

Proffshore has also received bids from five companies for construction of the buoy, including the compressor package and umbilical. "We are pleased to say that the bids confirmed our estimates of the cost," says Kristian Kudsk Andreasen, Proffshore's Project Manager for the drilling version of Atlantis. The budget for the prototype is NKr 80 million.

Typical dimensions for the first ABS buoy are: diameter - 15.6 meters, height - 11 meters, and steel weight - 350 tons. The buoy will give an effective upward force on the casing of 1,250 tons.

Proffshore has carried out studies for a number of operators, says S ren Gudmann, a key member of the Atlantis team. These included a feasibility study for a Saga well in 2,000 meters water depth in the Irish Sea, and for Saga's recently drilled Gjallar Ridge well in 1,352 meters off mid-Norway.

A conceptual study was performed for a possible Gjallar Ridge development involving eight wells. For this application, the buoy would be 40 meters wide and 8 meters high.

Rights to the field development version of Atlantis are held by Maritime Tentech, to which Proffshore supplies engineering. The two companies are investigating the possibility of deviating the wells in the water column between the buoy and the seabed, which could significantly increase the reach of the wells. This work should be completed by autumn, after which a field-specific study will hopefully follow, says Gudmann.