OFFSHORE EUROPE From Draugen to Aasgard: Mid-Norway projects taking shape

Nick Terdre
Contributing Editor - Norway

Mid-Norway has yet to play a significant role in Norway's oil and gas production, but by the turn of the century its contribution will have become substantial. In fact, it is only two years since the first oil flowed from a permanent production facility. This was Norske Shell's Draugen Field, developed with the world's first concrete monotower. Draugen, located in block 6407/9, is now producing more than 100,000 b/d. Novelty looks set to be a continuing feature of Mid-Norway developments, which to date are all located on the Halten Bank:

Heidrun: This summer sees the start of production operations on Conoco/Statoil's Heidrun Field in 6507/7 and 6507/8. Heidrun sports the world's largest tension leg platform and is the first such to be built with a concrete hull.
Norne: Located in block 6608/10, Norne is the most northerly offshore field yet to be developed, will employ the world's largest production ship, with a daily production capacity of some 173,000 b/d of oil and 7 MMcm/d of gas.
Norne contains an estimated 450 million bbl of oil and 15 bcm of gas, located mainly in the Garn and Ile formations of its reservoir. Some 10 production wells are planned, all of them horizontal. dip gas injectors, pumping produced gas into the gas cap. A solution for gas export will be sought at a later stage.
Njord: South of Norne, Njord lies in blocks 6407/7 and 10. It is estimated to contain some 235 million bbl of oil and 14 bcm of associated gas, but has no gas cap. Hydrocarbons have been found in the Ile, Tilje, and Aare formations of the Jurassic reservoir.
The reservoir is very stratified and highly faulted, especially in the central part. To maximize productivity, some of the production wells - about 10 are planned - will be horizontal, and some even U-shaped. They will be drilled through a number of faults and produce from several different areas.
There will also be four gas injection wells and one water injector. Excess gas will be re-injected into the eastern part of the reservoir, while the water injection will be directed into the central part.
Aasgard: Now in the early stages of planning is Statoil's Aasgard project for the joint development of the Smoerbukk, Smoerbukk South, and Midgard Fields. Aasgard represents a number of challenges, not least in the large volume of reserves to be produced, and the difficult nature of Smoerbukk's high pressure, high-temperature, low permeability reservoir.

Gas export route

For the first time in Mid-Norway, a gas export route is required, either directly to the North Sea pipeline network which gives access to buyers on the Continent, or indirectly via a processing terminal on the Norwegian mainland.

The three fields contain large reserves: Smoerbukk 540 million bbl of oil and condensate and 89 bcm of gas, Smoerbukk South 195 million bbl of oil and condensate and 24 bcm of gas, and Midgard 87 bcm of gas, according to the NPD (or 105 bcm according to Statoil), and 107 million bbl of condensate.

Smoerbukk's oil reserves were almost doubled earlier this year - previously they were estimated at 289 million bbl - following a full-scale reservoir simulation exercise carried out by Statoil which used a recovery factor of 37%.

Statoil's plan is to place the field center on Smoerbukk, with subsea facilities on Smoerbukk South and Midgard tied back to it. A single platform would not be sufficient to contain all the facilities required, so a two-facility scheme has been adopted.

Oil production is planned to start in October 1998 using a production ship, according to project leader Kyrre Nese. Work will now proceed on optimizing the size and capacity of the ship, and the number of wells that will be required at start-up. At present this number is put at 20, but Neso says that further work will be carried out to confirm that this is a practical proposition.

The target date for gas start-up is October 2000. No decision has yet been made on whether the production facility will be a platform with a concrete base or a semisubmersible floater. This in turn will depend partly on whether permanent drilling facilities are to be included.

The gas export route has yet to be fixed, as has the important question of whether the gas will be exported wet or dry - if dry, additional processing on the field will be required. All these decisions are due to be taken by year-end, Nese says.

The peak production levels have been set at 220 million b/d of oil and 33 MMcm/d of sales gas. This suggests that the oil production ship will be considerably larger again than the Norne vessel. The volume of gas export will be of the order of East and West Sleipner fields combined.

At present a total of 61 wells is foreseen. The majority - 41 - will be drilled on Smoerbukk, with 10 each on Smoerbukk South and Midgard. The Smoerbukk South and Midgard wells will all be subsea, and each field is likely to have three templates based on Statoil's standardized four-slot design.

A considerable degree of uncertainty surrounds the number of wells required on Smoerbukk, Nese says. No horizontal wells are yet planned for, but it is on the cards that this development will provide the opportunity to push back the limits of extended reach drilling.

Even so, Smoerbukk's areal extent is so great that some subsea wells will be required. A pilot producer is to be drilled next year to provide further orientation. Processing is also a challenge, in that the Smoerbukk liquids consist of a mixture of light and heavy condensate and oil, which will be difficult to stabilize.

The main reservoir on Smoerbukk South is in the Garn formation, with additional reserves of gas/condensate in the Ile formation and oil in the Tilje. Gas injection, mainly into the Garn formation, is also planned to increase recovery.

The Midgard reservoir conforms with a tank model and is relatively straightforward from a development point of view. The gas is also relatively dry. Gas injection is planned to improve the drainage of liquids.

Testing vessel designs

An important role in verifying and model-testing production facility concepts is played by Marintek, the Norwegian Marine Technology Research Institute based in Trondheim. As events move into deeper waters, its involvement is likely to grow.

The institute performed a long series of test programs and numerical studies for the Heidrun TLP, which was due to be installed in July. These included the installed platform system at both early and final design stages, and marine operations related ot the towout and installation of the platform itself, the tethers and the tether bottom foundations.

The final verification testing of the platform installation involved what was probably the largest model ever tested in an indoor basin, according to Terje Nedrelid, manager of the offshore floating structures division - the model was built to a scale of 1:25 and had a displacement of 18 tonnes.

It was Marintek which discovered the ringing problem with the Heidrun TLP, which led to the modification of both that structure and the Draugen platform. "These effects can't be found through theoretical studies," Nedrelid commented. "It shows the importance of model testing with a total system set-up."

Marintek also performed model tets for the Norne production ship, and for Golar-Nor's Petrojarl IV, which BP is to use on its Foinaven Field. BP will be returning to have its second West of Shetlands production ship model- tested.

"We do most of our model testing for water depths over 350 meters because other facilities cannot cope with these depths," said Nedrelid. The Norne ship will be installed in 400 meters of water and Petrojarl IV in 500 meters.

The institute has the deepest ocean tank in the world, measuring 80 meters long by 50 meters wide by 10 meters deep. Waves in two directions - current and wind - can be created. Model testing clients include leading deep water operators like Petrobras and specialized contractors such as Sofec, SBM and Bluewater.

Marintek has two advanced tools for analyzing marine systems. SIMO is a computer program for time-domain analysis of multibody systems, and can simulate scenarios such as TLP installation, offshore crane operations, dp systems for ships and floaters connected by a gangway, and the offloading by a shuttle tanker of a storage vessel connected to a loading buoy.

RIFLEX is a finite element programme tailor-made for static and dynamic time-domain analysis of flexible risers and slender marine structures in general.

New features include an advanced load model which can be used for the analysis of floating hoses in offshore loading systems, and a general contact formulation for analysis of stinger/pipe contact during pipelay operations, and of riser/vessel analysis in the moonpool area. Marintek now plans to merge the two programs into a single system for comprehensive analyses.

Not surprisingly, the institute is already involved in forthcoming activities in the Voering basin. Discussions have been held with rig owners contemplating drilling in the Voering about the problems posed by the stormy seas and strong currents. One area of concern is mooring systems, for the analysis of which Marintek uses the Mimosa program.

Another area, which is currently the subject of much analytical work, is the behavior of risers in deep waters. In addition to studying single risers as used by drilling rigs, the institute has also investigated arrays of risers as would be required for a production operation.

Apart from its own facilities, Marintek can also use the deep waters of the fjords for test purposes. One occasion came when it was involved in a joint industry project carried out in 1992/93 to verify a numerical method for predicting current interaction in deep sea riser arrays. The project was intended to throw light on the probability of risers colliding and suffering damage.

Tests were performed at Skarnsund, the mouth of a fjord 100 km north of Trondheim. Built to a scale of 1:30, the model riser array was nearly 50 metres high. Current velocity was on a scale of 1:1 - the tidal current at Skarnsund can reach three knots.

This was the first time tidal currents in a deep fjord had been used for such experiments, and it was concluded that it provided good conditions for testing current induced loads and the response of deep-sea riser systems.

The Sintef group, of which Marintex is part, is currently carrying out a study for a joint industry project on the behavior of titanium risers in deep waters. Titanium has some interesting properties in this application, Nedrelid says: "It is light and has the same elasticity as aluminium, but twice the tension strength of steel."