The world's largest gas treatment plant, at Kollsnes, Norway, dries and compresses production from Troll offshore gas field. The plant can handle 89 million cu m/day of production from Troll's gas platform. As much as 3,000 metric tons/day of condensate goes from the plant to Sture oil terminal, 9 km north, for blending into oil cargoes.
Norway's Den norske stats oljeselskap AS has taken over as operator of the giant Troll field platform and onshore gas treatment plant from development operator Norske Shell AS.
The handover to state owned Stat- oil took place during inauguration of the Kollsnes treatment plant north of Bergen. It was a milepost in one of the North Sea's most stunning development projects.
Troll West oil field was discovered by Shell in Norwegian North Sea Block 31/2 in 1979. Later drilling showed the field extended under adjacent Blocks 31/3 and 31/6. In those two blocks lay two thirds of what was revealed as Europe's largest offshore gas field.
Discovery of the larger Groningen gas field in Netherlands in the 1950s sparked development of Europe's natural gas industry. It also encouraged exploration in the nearby Dutch offshore sector and ultimately led to discovery of massive reserves in the North Sea.
Troll's significance is not quite as great as Groningen's, perhaps. But technological advances required to make Troll viable and the fact that the field will meet 10% of Europe's natural gas needs during the next 50 years place it in the same category.
East Troll gas
Shell developed the Troll East gas reservoir by means of the world's tallest, heaviest production platform. Standing in 305 m of water, the platform is 474 m tall. It is designed for a 50-70 year life.
Untreated gas moves ashore to the Kollsnes plant, where it is dried and compressed for export to Zeebrugge, Belgium, and Emden, Germany, through the Zeepipe and Europipe offshore trunk lines.
Troll A platform, installed to produce Troll East gas reserves, is made up of a 369 m high concrete gravity base weighing 670,000 metric tons surmounted by a 22,000 metric ton topsides.
The platform was installed in the field in late May-early June 1995. First gas was produced last May 28. It will prepare offshore and onshore facilities for the start of commercial production Oct. 1.
Gas sales
Long term gas sales contracts were needed before Troll could be judged a commercial project. The Troll gas sales agreement of 1986, between Norway's gas producers and a group of gas suppliers in Central Europe, was an innovative feature of the Troll project.
Nina Graeger, political adviser in Norway's Ministry of Industry & Energy, said, "In the 1970s and for most of the 1980s, Norway's gas was sold under field depletion contracts.
"Under the 1986 Troll agreement, gas was sold in supply contracts that specified gas volumes to be delivered each year. Most Norwegian gas sales contracts have been of this type since then."
Graeger said the Troll sales agreement could lead to 45 billion cu m/ year of gas sales to customers in Germany, France, Netherlands, Belgium, Austria, and Spain.
First deliveries under the agreement were from Sleipner East field, the only other field so far specified under the Troll agreement, which began production in 1993.
Graeger said development of Troll West oil field, which went on stream in 1995, cost 18 billion kroner ($2.5 billion). Troll East development cost 35 billion kroner ($5.5 billion).
So far, Troll has been allocated deliveries of as much as 24 billion cu m/year of gas. Platform and treatment plant capacities can be expanded to 30 billion cu m/year.
Graeger said Troll gives Norway a major role in Europe's future gas industry. Western Europe consumed 285 billion cu m of gas in 1994. Norway's delivery commitments amount to 62 billion cu m in 2005.
Norway's total exports to European buyers could amount to 70-80 billion cu m/year after 2000, depending on new sales contracts to be negotiated.
Production wells were said to be 40% more prolific than expected. Twelve will be available Oct. 1, although only 10 will be necessary to meet maximum demand.
Technical challenges
Troll field holds estimated reserves of more than 1 billion bbl of oil and 1.3 trillion cu m of gas, but development was viable only because of new technologies.
Troll's western oil province contains most of the field's liquids in a thin reservoir. Without horizontal drilling, oil development would have been impossible.
Norsk Hydro AS placed Troll West on stream last year with a production semisubmersible.
Prior to start of oil production, Hydro disclosed a plan to develop oil in the gas province, effectively doubling the field's oil reserves (OGJ, Nov. 27, 1995, p. 25).
While the Troll East gas platform is big, it is little more than a giant wellhead platform. Production is controlled from shore, and the platform's operating crew is made up of only 20 persons. There is room to accommodate up to 108 persons during drilling.
Shell said the platform's drilling module is the most advanced in the world, with mechanized pipe handling and control of drilling operations from a soundproof control room.
During normal drilling and casing operations, no one is allowed on the rig floor.
While it would have been just about possible to build Troll's enormous gas treatment plant on a platform, Shell decided an onshore site afforded more room for expansion.
The Troll East platform will produce from 39 wells, each capable of flowing as much as 3.4 million cu m/ day of gas, although normally restricted to 2.8 million cu m/day.
Production capacity of the platform is 100 million cu m/day of gas. The terminal capacity is 89 million cu m/ day, but this can be expanded as further gas sales agreements with Europe are signed.
Wet gas shipments
Shell said one of the greatest challenges of the Troll gas project was developing technology to allow wet gas to be shipped over long distances. Troll production moves to shore through two 36 in. pipelines.
Key to successful multiphase flow was the design of slug catchers at Kollsnes and addition of glycol to the wellstream to prevent corrosion from carbon dioxide. An intelligent pig will help monitor pipeline corrosion.
For the last 3.6 km to shore, the pipelines run through tunnels more than 200 m below the seabed because of the uneven terrain just off the coast.
As much as 3,000 metric tons/day of condensate can be removed from the wellstream entering Kollsnes. Liquids are stabilized before being sent 9 km by pipeline to Sture oil terminal north of Kollsnes.
Aker AS of Oslo and M.W. Kellogg Co. of Houston built the Kollsnes terminal jointly under a 4 billion kroner ($640 million) engineering, procurement, construction, and management contract.
The project covers 700,000 sq m and processes Troll production in three process trains and five compressor trains. Gas leaves the plant at 170 bar through a pipeline to Sleipner field facilities to join trunk lines to Emden and Zeebrugge.
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