On May 1975, the semi-submersible Pentagone 84, owned by Schlumberger’s subsidiary Forex Neptune entered the foggy Iroise Sea after a short campaign in the Alwyn field in the UK North Sea. A month earler, on April 3, 1975, the French company Elf Aquitaine obtained the authorization to conduct oil exploration in the Channel and the Atlantic, an area including the Celtic Sea, the Armor and the Sea of Iroise.
The Pentagon 84 legs were skinny, the derrick too, but it was a state-of-the-art drilling rig at that time, one of the first to be fitted with a crown heave compensator. The rig was built at the Rauma Repola shipyard in Finland and delivered in the first months of 1975. The rig remained assigned to operations in the North Sea, visiting the shipyards of Cromarty, Methil, Haugesund and Stavanger for regular wet dock inspections. The reason for its very short life is unclear; the Alexander Kielland in 1980 offshore Norway ternished the image of the Pentagone rigs. In 1987 the Pentagone 84 was broken up near Saltburn Jetty on the Cromarty Firth, Scotland. Her pontoons were towed to the Highland Deephaven at Evanton to be beached and scrapped.
Thirteen wells were drilled in the Sea of Iroise, but no oil was found. The final exploration well was drilled in 1985 by the semi-submersible Ocean Bounty at about 260 km from Brest, not far from the watershed between France and Great Britain.
(Image: June 1975, the Pentagone 84 on tow entering the Sea of Iroise)