The Ekofisk Tank was the first structure in the Norwegian sector of the North Sea. It was constructed in a fjord in Norway, and towed to the Ekofisk field in 1973.
This photo shows the reinforced concrete structure under construction in a quiet Norwegian fjord. The central part of the structure, consisting of nine sealed chambers, provide the buoyancy that keeps the tank afloat. The perforated wall around the tank is to reduce the forces exerted by waves, which reach a height of 75 feet during the design storm.
Here seven tug boats begin to move the tank from the fjord to the North Sea.
The draft of the tank while floating was 215 feet. When it was in position, water was pumped into the sealed chambers to reduce buoyancy and sink the tank to its position on the dense sand at the bottom.
This photo shows the tank in position, resting on the bottom of the sea. Facilities needed for operation of the oil field are being constructed on top of the tank.
The conditions in the North Sea are very harsh. This photo was taken during a period of windy weather, much milder than the design storm.