French win Ekofisk tank job

Offshore loading was vulnerable to strong winds and high waves, when the tankers had to disconnect from the buoys and production was forced to stop.

To keep development plans for the field on track, the decision was taken to build a big concrete oil storage tank for the Ekofisk Complex. That would allow the field to stay on stream in every kind of weather.

French company C G Doris was awarded the main contract to build this installation, and everything then happened at record speed.

Norway’s Ing F Selmer company won the actual construction job and could draw on extensive experience with concrete structures for reservoirs, grain silos and bridges. It also joined forces with fellow Norwegian construction specialist Høyer-Ellefsen.

But where was the world’s first concrete oil storage tank to be built? Dirdal in Gjesdal local authority north-east of Stavanger was initially assessed, but local opposition and geotechnical problems got in the way.

In Stavanger, however, all objections were swept aside by Arne Rettedal, the dynamic mayor, and construction of the base section began in a dry dock at Jåttåvågen south of the city in mid-June.

The work continued at full pitch throughout the summer and into the autumn, before the structure could be towed to deep water at Hillevåg to complete slipforming of the walls.

Plans originally called for the tank to be ready by 1 August 1972, but this timetable fell behind by almost a year as a result of several factors.

These included uncertainty at classification society Det Norske Veritas (DNV), which was due to approve the tank’s structural integrity, over whether it was strong enough.

That prompted demands for more reinforcement bars and a thicker base. Tests in a ship model tank at the Norwegian Institute of Technology (NTH) in Trondheim simulated the forces which would be imposed on the structure by a 100-year wave.

Nine storage cells were incorporated in the centre of the tank, surrounded by a protective breakwater with large circular openings which allowed water to flow in and out.

Although the structure topped out on 21 June 1972, a lot of equipment had to be positioned on board before the construction job was finished.

The tank had the capacity to accept 350 000 barrels of crude per day. It was also decided to install oil and gas processing equipment. The extra outfitting meant that towout to the field was 11 months behind schedule, with installation on 1 July 1973.