NORWAY: Artificial buoyant seabed to undergo trials for deepwater drilling project

The Atlantis deepwater technology is close to becoming a reality, with the testing of a prototype scheduled to take place in Norway late this year. Meanwhile, in a display of confidence in the concept's commerciality, Statoil has taken a stake in the ownership group.

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The Atlantis deepwater technology is close to becoming a reality, with the testing of a prototype scheduled to take place in Norway late this year. Meanwhile, in a display of confidence in the concept's commerciality, Statoil has taken a stake in the ownership group.

The Atlantis concept is based on an artificial buoyant seabed (ABS) installed at a depth of 200-300 m below the sea surface on top of the 20-in. well casing. On this base are installed the wellhead, BOP, and in the case of a production well, subsea tree. Drilling operations can then be carried out in a conventional water depth regardless of the depth to the true seabed.

The concept has been extensively analyzed in the six years since it was originated by Terje Magnussen, who is now technical director of Atlantis Deepwater Technology Holding AS. All the study and qualification work to date indicates that it will work, says Managing Director Pal Norheim. Its application has also been analyzed for nearly all deepwater provinces, the latest being offshore eastern Canada in a water depth of 2,750 m.

Atlantis was originally conceived as a tool for exploration drilling, but its application has been broadened to include field development and riser tower applications. The most recent stage of development has taken place under the auspices of the Demo 2000 program in Norway. This is a NKr 50-million project, funded by the licensees of block 18 in Angola (BP and Shell), the government, and the Atlantis owners, under which a prototype ABS exploration buoy has been designed and is now being built at the Nymo yard in southern Norway. The 250-ton unit, which is 15 m in diameter and 17 m high, will be equipped with a control system supplied by Atlantis.

The prototype, designed to functional requirements from block 18, including a water depth of 2,000 m, is due to be tested late this year. This exercise, which will probably take place in the sheltered waters of the Gands Fjord near Stavanger in western Norway, will focus on marine installation aspects such as towing, deployment, stability, and ROV operations. The trial will take about 25 days and will most likely be carried out by anchor-handling vessels. A contract for the marine operations has been under negotiation.

"We believe the technology is now mature enough to be offered to the market," says Norheim. For exploration drilling and short-term field development, the units will likely be rented on a dayrate basis. For larger field developments, the field owners would be expected to acquire the equipment. For development purposes, studies indicate that the optimal upper size would be a 650-ton ABS able to accommodate three wells.

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The lower section of the prototype Atlantis ABS under construction at the Nymo yard in Norway.
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Statoil acquired a 12% stake in ADTH in April. "It's a good investment. We believe in the technology and the market for it," says Jan Inge R yland, project manager in Statoil's industrial development division and an ADTH board member.

Statoil originally became involved with Atlantis in the late 1990s, when it planned to use it to drill an exploration well West of Shetlands in UK waters. However, the collapse in oil prices led to the well being cancelled.

"We don't have an application at present," says R yland, "But we see we could also benefit as a user." The company holds various deepwater licenses, including offshore Angola, Nigeria, and Brazil.

Use of the Atlantis technology opens the way to numerous savings opportunities, which tend to increase in deeper water. Considerable time is saved in preparing for drilling operations, as the BOP and riser have to be run only to the ABS. In extreme cases, this could save more than a week's rig time, ADTH says.

Because drilling is effectively taking place in a conventional water depth, a second- or third-generation rig can be used, in combination with a taut-leg polyester mooring system. This implies day-rate savings of up to 50% compared with the use of a fourth- or fifth-generation unit. The time taken to drill wells would also be reduced, thus accelerating the time to start-up.

A generic case study by ADTH calculated a saving of eight days, worth $2.6 million, on drilling and completion work when comparing the cost of a deepwater seabed well with an ABS well, both drilled by a deepwater rig, and a further savings of $3.4 million through using a second- or third-generation rig instead of a deepwater rig. This gives a total potential saving per well of about $6 million. And well costs tend to account for a growing proportion of overall development costs as the water depth increases, the company points out.

Some support is required during the installation sequence. Once the tophole of the well has been drilled and the casing installed, an operation that does not require a deepwater rig, the ABS is towed to the location and held by two vessels while it is submerged to the desired depth beneath the rig. For a mild climate like West Africa, a system has been developed for holding the buoy in place using only one vessel, Norheim says.

Next the tie-back casing is run from the rig through the ABS and connected to the casing already installed in the seabed well. Via an air hose from the rig, air is injected into the ABS, which is open at its base. At this point, it has been described as an upside-down bucket. The riser and BOP are run and connected to the wellhead mounted on the ABS. As the weight on the ABS is increased or decreased, the buoyancy is adjusted appropriately.

For field development operations, the use of a lower-rated rig or light well intervention vessel also implies lower well intervention costs. In areas such as Angola, where a high rate of sand production is anticipated, relatively frequent well intervention is expected.

Further savings would accrue in the case of a more complex field development involving a number of ABS units linked by floating pipelines such as SBM's gravity actuated pipe system. By transporting the well stream at relatively shallow water depths, the flow assurance problems met in pipelines installed on the seabed in deep waters would be avoided.

The ABS concept could also be employed as a riser tower, from which only short flexible risers would be required to carry the well stream to the production unit at the surface.

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