Troll A platform
The Troll A platform is a condeep offshore natural gas platform in the Troll gas field off the west coast of Norway. It is the tallest and heaviest structure that has ever been moved to another position, relative to the surface of the Earth, and is among the largest and most complex engineering projects in history. The platform was a televised sensation when it was towed into the North Sea in 1996, where it is now operated by Statoil.
The Troll A platform has an overall height of 472 metres (1,549 ft), weighs 683,600 tons (1.2 million tons with ballast) and has the distinction of being the tallest and heaviest structure ever moved by mankind. The platform stands on the sea floor 303 metres (994 feet) below the surface of the sea and one of the continuous-slip-formed concrete cylindrical legs (the leg containing the import and export risers) has an elevator that takes over nine minutes to travel from the platform above the waves to the sea floor. The walls of Troll A's legs are over 1 metre thick made of steel reinforced concrete formed in one continuous pour (slip forming) and each is a mathematically joined composite of several conical cylinders that flares out smoothly to greater diameters at both the top and bottom, so each support is somewhat wasp-waisted viewed in profile and circular in any cross-section (see picture at right). The concrete legs must be able to withstand intense pressure so are built using a continuous flow of concrete, a lengthy process that takes 20 minutes per 5 cm laid.
The four legs are joined by a "chord shortener", a reinforced concrete box interconnecting the legs, but which has the designed function of damping out unwanted potentially destructive wave-leg resonances by retuning the leg natural frequencies. (Not present in the picture at right.) Each leg is also sub-divided along its length into compartments a third of the way from each end which act as independent water-tight compartments. The legs use groups of six 40 metres (130 ft) tall vacuum-anchors holding it fixed in the mud of the sea floor.
Troll A was built by Norwegian Contractors for Norske Shell, with base construction beginning in July 1991 at a cost of 4150 million NOK, or approximately US$650 million at the time. The base and the deck were built separately, and were joined in 1995 while the base was partially submerged. The base is a Condeep gravity base structure built from reinforced concrete.
In the Autumn of 2010 the Troll A platform was expanded with a new module. This module contains living quarters and a new control room. The new control room and expanded facilities are needed to support the new compressors (a 3rd and 4th are planned and being installed in 2014). These compressors are needed due to the dropping reservoir pressure in the wells.
On 18 June 2013 the new support module M12 for compressor 3 and 4 was lifted on board Troll A. The increased electricity consumption of these compressors required new power cables from shore, and due to grid constraints the platform is cut off from shore power if a grid emergency arises.
Gas rises from 40 wells, and is exported through a number of pipes, to a processing plant at Kollsnes. Troll A is scheduled to receive two additional gas compressors to increase production volume and compensate for dropping reservoir pressure in the wells.
In 2006, the 10th anniversary of Statoil's operatorship of Troll gas production was celebrated with a concert by Katie Melua held at the base of one of the hollow legs of the platform. As well as entertaining the workers on the rig, the concert set a new world record for the deepest underwater concert at 303 metres below sea level.
- HVDC Troll
- Ursa tension leg platform, another, taller record-breaking platform
- Magnolia extended tension leg platform, the world's deepest ETLP
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- Øyvind Lie. "Troll A blir B-kunde" Teknisk Ukeblad, 27. september 2011. Accessed: 14 November 2013.
- "Guinness World Records - largest offshore gas platform". Guinness World Records. 1996.
- "Melua's deep sea gig sets record". BBC News. 2 October 2006.