Situated about 80 km off the west coast of Norway is Troll A, a colossal natural gas platform and the tallest structure ever moved by mankind. It’s construction is regarded as one of the largest and most complex engineering projects in history.
Operated by Statoil, the platform was towed over 200 km, from the village of Vats to the North Sea in 1996, where the event was a televised sensation in Norway.
The Troll A platform has an overall height of 472 metres (1,549 ft) and weighs 683,600 tons (1.2 million tons with ballast). 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. [source]
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. 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. [source]
Troll A was built by Norwegian Contractors for Norske Shell, with base construction beginning in July 1991 at a cost of $650 million USD. 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 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. [source]