Building at sea

Ekofisk is the name given to a geological structure under the North Sea—to be more precise a vast strata of limestone lying 10, 000 feet underground and stretching eight miles from north to south and four miles from east to west. This limestone is saturated with oil and gas. To produce this oil and gas a hole is drilled down into the limestone reservoir. The whole of the oil reservoir is under pressure. This pressure will force the oil and gas out of the rock and up through the cased hole to the surface.

This operation is very simple. The problem at Ekofisk is that the area of operations lies 180 miles out in the North Sea and water depths often exceed 200 feet.

This makes everything connected with extracting oil very difficult and very expensive. To justify such expense the volume of oil produced has to be very large.

The central part of the Ekofisk installation consists of a series of platforms connected by bridges and raised 60 feet above sea level. It stretches more than half a mile from north to south in the central part of the field.

At one end of the Ekofisk complex a gigantic concrete tank is installed. It can store 1, 000, 000 barrels¹ of oil. Without it many days' production would be lost each year during bad weather. This is because the North Sea is sometimes so stormy that tankers cannot load safely. As soon as the storm subsides and during fine weather tankers will empty the tank.

The tank is a huge reinforced concrete structure that sits squarely on the sea bed. It is so tall that it projects above the sea to the same height as the other platforms at Ekofisk. It consists of a group of nine cylindrical concrete tanks sitting on a concrete and steel base surrounded by a

perforated concrete breakwater wall. The whole tank is topped by a structural steel platform. The breakwater is 270 feet high. The complete tank is also 270 feet high and weighs 235, 000 tons.

The foundations were constructed in a drydock in Norway. When they were complete they were towed out to a harbour and grounded on the sea bottom.

The rest of the Ekofisk facilities are of steel, welded together as in ships. It was built in separate parts in different shipyards in Europe and in the USA. This cut construction time. The undersea part of the Ekofisk platforms are called jackets.

The superstructure was split into units—vast blocks of finished equipment that had simply to be connected together. Each one was constructed and fitted out on shore, and then taken out to the worksite and slotted into place. 'Workmen were needed to put the sections into place and connect them all together, but far fewer than would have been needed to " fit out" this equipment on site. Naturally, the bigger the unit that could be installed the better. The limit was the capacity of the crane barge 1, 200 ton.

It took two years to bring all the jackets to the site and launch into place with the help of the crane barges. The largest jacket was 250 feet long and weighed 2, 500 tons.