Troll A platform

FACTSFact sheet

01A gas field in 303 meters of water

The Troll field was discovered in 1979, in the Norwegian sector of the North Sea, northwest of Bergen. It held enough gas to supply part of Europe for decades: according to Equinor, in 2026 Troll still met about 10% of European demand. The problem was the water. At the spot chosen for the platform there were 303 meters of sea, deeper than any fixed concrete platform had faced until then.

Norway had a proven answer for the North Sea: the Condeep platforms of Norwegian Contractors. Since 1975 the company had been building, in fjords, concrete bases made of large cylindrical cells, from which hollow columns rose to the deck. The structure was floated out to the field and deliberately sunk there, by filling the cells with water, until it came to rest on the seabed. It needed no piles: its weight held it in place. Troll A would be the last and the tallest of them.

02Eighteen minutes in the Gandsfjorden

In March 1991, Norske Shell signed the contract for the Troll A base with Norwegian Contractors: 4.15 billion kroner. On August 23 of the same year, in the early morning, another base by the same builder, the one for the Sleipner A platform, was undergoing a ballast test in the Gandsfjorden, near Stavanger. The structure was being lowered into the water in a controlled way when a concrete wall failed. Almost a thousand tonnes of water a minute poured in. In about eighteen minutes the base was on the bottom of the fjord, at almost 200 meters. The 22 people on board got off unhurt.

The investigations, with technical advice from the SINTEF institute, found the cause: the joint between the cells, a triangular element called a tricell, had been undersized because of errors in the finite element analysis, the computer calculation that divides a structure into small pieces. The water pressure broke through a wall that was thinner than it should have been. The loss is given as 1.8 billion kroner, according to the Norwegian Petroleum Museum, or US$ 700 million, in other sources. Shell, which had another base under construction with the same company, the one for the Draugen platform, ordered work halted and the calculations reviewed; construction resumed only three months later.

03A tower that grows while floating

Work on the Troll A base began the same year, in the summer or in September 1991, depending on the source, in a dry dock at Hinnavågen, in Stavanger. When the lower part was finished, the structure floated out. The rest went up in the fjord itself, near Vats, in northern Rogaland, with the concrete poured into slipforms that climb continuously, until the columns reached their final height.

The whole structure used 245,000 or 250,000 cubic meters of concrete, depending on the source, and 100,000 tonnes of reinforcing steel; the Norwegian Petroleum Museum compares the reinforcement to the weight of thirteen Eiffel Towers. According to the cement supplier, more than 2,000 people worked on the construction. How many at the peak, on what shifts and for what wages, the sources consulted do not say. Nor do they record deaths or serious accidents on the job.

There was also a risk that emerged along the way. In the early 1990s, model tests of other Norwegian platforms showed that steep waves could make vertical columns suddenly vibrate, an effect dubbed ringing, like a church bell after it is struck. A paper presented in 1994 at the Offshore Technology Conference, in Houston, in the United States, reports that the Troll model tests showed larger responses than design practice predicted, and describes the work done to assess the integrity of the base.

04The deck and the wait

While the base grew, Aker Stord built the deck, with 22,500 tonnes of processing and accommodation equipment, according to Offshore magazine. On January 25, 1995, it was brought to Vats and mated to the top of the columns. From then on the platform was complete, standing in the fjord, and all that remained was to take it out to sea.

The tow was scheduled for May 2, 1995. The weather would not allow it. According to the Norwegian Petroleum Museum, each day of delay cost about 2 million kroner.

05Seven days under tow

On the afternoon of May 10, 1995, Troll A left Vats. Ten tugs did the work: eight pulled and two, at the back, steered. The speed was one to two knots, a few kilometers per hour. About 245 meters of the platform stood above the water and 225 below, and the route had to avoid stretches where the seabed rose: the shallowest point was 237 meters, according to the engineers in charge of the installation. The next day the wind reached 40 knots, with five-meter waves. Eighty journalists from nine countries followed the voyage.

On May 15, the convoy was twenty hours ahead of schedule. On May 17, Norway's Constitution Day, the platform was ballasted over the field and lowered to the seabed, at 303 meters. The concrete skirts under the base, designed to hold it in soft soil, penetrated 36 meters into the seabed, a depth the engineers described as a record. Gas production began on February 9, 1996, and King Harald V opened the platform in June of that year.

06Thirty years later

Over time, the reservoir lost pressure, and the gas began to need help to rise. In 2005 Troll A became the world's first platform powered from shore by high-voltage direct current, through cables of about 70 kilometers from Kollsnes, to drive compressors. In 2015 two more, of 50 MW each, went into operation. In 2006 the singer Katie Melua gave a concert inside one of the columns, 303 meters below sea level.

In 2026 the platform completed 30 years of production. On August 22, Troll Phase 3, stage 2, came on stream, a subsea project that brings gas from another part of the field to Troll A and brings forward the production of 55 billion cubic meters, almost two years of France's consumption, according to Equinor, which completed it ahead of schedule and under its budget of 12.3 billion kroner. In June the partners had already approved another, TWIN, worth 4 billion kroner, with production scheduled for 2028. The new gas still rises through the same concrete columns that crossed the sea in May 1995.

MYTHSWhat people say and what the record shows

• What people say

  Troll A is the heaviest object ever moved by humans.

  What the record shows

  The Guinness record for the heaviest object ever moved belongs to another Norwegian concrete platform, Gullfaks C, towed in 1989 with a displacement of 1.5 million tonnes. Troll A's record is for height: it is the tallest structure ever moved.

• What people say

  It weighs 1.2 million tonnes.

  What the record shows

  That is the weight with ballast water, in operation. The structure itself weighs between 565,000 and 684,000 tonnes, depending on the source and on what is measured: dry weight, weight under tow or weight without ballast.

• What people say

  The concrete was tuned to avoid acoustic resonance on the seabed.

  What the record shows

  No technical source mentions acoustic resonance. The real problem was ringing, the sudden vibration of columns struck by steep waves, which led the engineers to examine the structure in model tests.

• What people say

  Troll A is a floating platform.

  What the record shows

  It floated only during construction and towing. Since May 17, 1995, it has rested on the seabed, at 303 meters, held to the soil by concrete skirts of 36 meters.

QUESTIONSFrequently asked questions

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SOURCESSources

1. Ole Jone Eide, Norsk Oljemuseum, "Troll A – giant move from fjord to field", 2022 — equinor.industriminne.no
2. Norsk Oljemuseum, "How large is a troll?" — equinor.industriminne.no
3. Björn Lindberg, Norsk Oljemuseum, "Sleipner A GBS lost and replaced" — equinor.industriminne.no
4. Finn Harald Sandberg, Norsk Oljemuseum, "The Sleipner sinking and Draugen", 2018 — draugen.industriminne.no
5. Norsk Oljemuseum, "The ringing phenomenon", 2018 — draugen.industriminne.no
6. R. V. Ahilan, Ø. M. Klaas and I. Langen, "Analysis of Dynamic Transient Response (Ringing) in a Gravity-Base Structure", Offshore Technology Conference, 1994 — onepetro.org
7. E. Andenæs, E. Skomedal and S. Lindseth, "Installation of the Troll Phase I gravity base platform", 1996 — osti.gov
8. Aker Solutions, "Troll A (Condeep) concrete gravity-based structure" — akersolutions.com
9. Heidelberg Materials (Norcem), "Troll A" — sement.heidelbergmaterials.no
10. Guinness World Records, "Heaviest man-made object moved" — guinnessworldrecords.com
11. Store norske leksikon, "Troll – petroleumsfelt" — snl.no
12. Store norske leksikon, "Norwegian Contractors" — snl.no
13. Offshore Magazine, "Troll and Heidrun: end of an era or forerunners of a modern world?", 1995 — offshore-mag.com
14. ABB, "ABB completes Troll A platform's second power-from-shore system", 2015 — new.abb.com
15. Equinor, "More gas for 30-year-old Troll A", 25 August 2026 — equinor.com
16. Equinor, "Expanding Troll: more gas to Europe", 19 June 2026 — equinor.com
17. Offshore Technology, "Equinor starts production at Troll phase three, stage 2 in North Sea", 2026 — offshore-technology.com
18. Martin Jansen, Vi Menn, "Slepet av Troll A – en utrolig norsk ingeniørbragd", 2025 — klikk.no

Images

• Fig. 01 — Troll A in the North Sea in July 2016. What you see is the smallest part: below the surface, the concrete goes down 303 meters to the seabed: Norsk olje og gass · CC BY-SA 2.0
• Fig. 02 — The four columns and the deck in June 2008: Swinsto101 · CC BY-SA 3.0
• Fig. 03 — Crew members of a ship looking out at the platform, in July 2016: Norsk olje og gass · CC BY-SA 2.0
• Fig. 04 — Closer up: the concrete columns meet the deck, in July 2016: Norsk olje og gass · CC BY-SA 2.0
• Fig. 05 — The cell base of a Condeep platform under construction in a Stavanger dry dock, between 1990 and 1992. The archive identifies the project as Sleipner: Knudsens Fotosenter / DEXTRA Photo, Norsk Teknisk Museum · CC BY 4.0