ONGC OF NORWAY HAVE INSTALLED 472M HIGH TOWER IN THE SEA
V T LONDHE 08–07–2015
Troll A platform
A view from the South East of 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 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.
Continuous slip formed gravity base structure supports under construction in a Norwegian fiord. The tower cranes delivered concrete to the support cylinders during the continuous pour of concrete to create seamless walls.
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 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$650,000,000 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.
The Troll platform was towed over 200 kilometres (120 mi) from Vats, in the northern part of Rogaland, to the Troll field, 80 kilometres (50 mi) north-west of Bergen. The tow took seven days.
In the fall 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 1996 the platform set the Guinness World Record for ‘largest offshore gas platform’.
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 the Troll A 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.
Condeep (abbr. concrete deep water structure) refers to a make of gravity-based structure for oil platforms developed by engineer Olav Mo in Hoeyer-Ellefsen and fabricated by Norwegian Contractors in Stavanger, Norway. A Condeep usually consists of a base of concrete oil storage tanks from which one, three or four concrete shafts rise. The Condeep base always rests on the sea floor, and the shafts rise to about 30 meters above the sea level. The platform deck itself is not a part of the construction.
The Condeep is used for a series of production platforms introduced for crude oil and natural gas production in the North Sea and Norwegian continental shelf.
Following the success of the concrete oil storage tank on the Ekofisk field, Norwegian Contractors introduced the Condeep production platform concept in 1973.
This gravity-based structure for platform was unique in that it was built from reinforced concrete instead of steel, which was the norm up to that point. This platform type was designed for the heavy weather conditions and the great water depths often found in the North Sea.
The Condeep has the advantage that it allows for storage of oil at sea in its own construction. It further allows for equipment installation in the hollow legs well protected from the sea. In contrast, one of the challenges with the steel platforms is that they only allow for limited weight on the deck — with a Condeep the weight allowance for production equipment and living quarters is seldom a problem.
Troll A platform
The Troll A platform is the largest Condeep to date. It was built over a period of four years, using a workforce of 2,000, and deployed in 1995 to produce gas from the enormous Troll oil field.
With a total height of 472 meters, Troll A was the tallest structure ever to be built and moved. Many sources erroneously state that it is the largest. The largest structure ever moved (also a Condeep) was the Gullfaks C platform in the Norwegian North Sea. The total weight of the Troll A Condeep when launching was 1.2 million tons. 245,000 m³ of concrete and 100,000 tons of steel for reinforcement were used. The amount of steel corresponds to 15 Eiffel towers. The platform is placed at a depth of 300 meters. For stability, it is dug 35 meters into the sea floor.