Artificial seabed deepwater extension of TLP technology

Statoil may have exploration test prototype by 1998

Schematic showing the field development version of the Atlantis system. [21,206 bytes]

A new solution to reduce the cost of development in deep waters is being jointly marketed by Proffshore and Aker Maritime. The partners are proposing to take the "deep" out of deepwater by creating an artificial buoyant seabed (ABS) at a shallow water depth, from which subsea wells can be drilled. The concept, which is dubbed Atlantis, was developed by Terje Magnussen of Proffshore, a Stavanger-based consultancy.

The ABS makes use of the same principle as comes into play if you push an upturned bucket down into water - the air trapped inside the bucket gives it great buoyancy. The ABS is essentially a large buoyancy tank with a ballast system controlled from the surface. It is held in place by the surface casing of the well or wells it serves - the casing is extended from the seabed, where the wellhead would otherwise be placed, to the ABS.

The ABS itself is located about 200 meters below the sea surface, beneath the wave zone in which a lot of environmental forces are at play, but at a depth at which conventional subsea equipment and techniques can be used, says Jon Erik Reinhardsen, Aker Maritime's executive vice president with responsibility for Atlantis.

If the concept is to fly, as it were, the casing must be strong enough to bear the upward force of the ABS, while at the same time minimizing its lateral movement. Extensive simulations, mainly carried out at the Marintek marine research laboratory in Trondheim, has shown this to be the case, according to Reinhardsen. "All the calculations show that you use less than half the tensile strength of ordinary casing," he says.

Marintek's hydrodynamic simulations of a 100-year Voering current profile at water depths of 800-1,600 meters also showed that there is no absolute limit to the depths in which Atlantis can be used.

Drilling applications

Proffshore is also investigating the potential of the concept for drilling deepwater exploration wells, and has been assisted in its initial studies by Smedvig. The main sponsor of the work for both exploration and development versions is Statoil, which is engaged in deep-water activities in a number of countries.

The ABS is a circular structure. In the developmental version it supports a number of wells spaced at equal intervals around the circumference. These are then connected via a production manifold module installed in the centre to flexible risers which carry production to a floating production unit at the surface.

As an alternative to offshore loading, one or two of the well slots on the ABS could be used to accommodate an export riser connected to a pipeline.

By avoiding the need for an extremely long riser, Atlantis opens the way to the use of floaters such as production ships and semisubmersibles in the very deep waters in which tension-leg platforms and spar platforms have so far predominated. In Magnussen's view, Atlantis can be regarded as the final development from TLPs, mini TLPs, and submerged TLPs.

In fact, Aker Maritime's close involvement with very deep-water developments in the Gulf of Mexico probably weighed heavily in its favor when Proffshore was seeking an industry partner. In turn, the concept extends the contractor's range of floater solutions.

Existing technology

A series of advantages are claimed for Atlantis. It uses only existing technology and involves a minimum of new operational practices. It is low-cost, requiring for development purposes about the same capex as a subsea development in shallow waters.

But one of the most important benefits, according to Magnussen, is that it enhances rig availability at a time when the market is very squeezed. By in effect bringing the seabed to within 200 meters, it is possible to use a drilling rig without the costly upgrading now required for deepwater operations, with the exception of modifications to the mooring system.

Rig loads are also reduced in terms of the volume of consumables needing to be stored. Moreover, dispensing with the deepwater riser also greatly decreases the amount of mud which must be used, and makes emergency disconnect operations much safer by eliminating the riser margin problem.

"The resulting rig, well design and drilling advantages will contribute to lower costs and a higher success ratio for future deep-water exploration wells," Magnussen concludes. In the case of a hydrocarbon leak or blow-out in the well, control possibilities with Atlantis's conventional riser and BOP are far better than in the deep-water scenario.

Statoil is hopeful of having a prototype of the exploration version ready for testing in 1998, Magnussen says. The development version will probably take somewhat longer, but according to Reinhardsen the first system could be delivered before the turn of the century. Two studies looking at specific development scenarios are currently under way, one for a Gulf of Mexico field and one for a mid Norway field.