Ekofisk is a gravity structure, please explain how that applies to Seasteading, with a goal of floating, mobility, expanding into International Waters, etc.
After years of planning, ConocoPhillips has finally begun abandonment of the Ekofisk tank on its flagship field in the Norwegian sector of the North Sea. Cleaning of the tank’s storage cells is under way, and a contractor has been chosen to remove the topsides, starting next year.
Surrounded by its protective barrier, the Ekofisk tank, 2/4 T , stands like a colossus at the center of the Ekofisk platform complex. Installed in 1973 in a water depth of around 75 m, it was the first offshore installation with a concrete gravity base. The latter is 90 m high and 89 m in diameter, and weighs 290,000 tonnes.
Troll A is the single tallest gravity-based structure ever built.
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 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.
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.
Seamounts, being the result of volcanic activity, are subject to rising, and subsiding, at the whim of geological movement. What happens when the earth shrugs, and your seamount rises, causing your gravity-base structure to topple, or subsides, causing it to sink?