Byford Dolphin

Byford Dolphin was a semi-submersible, column-stabilised drilling rig operated by Dolphin Drilling, a subsidiary of Fred Olsen Energy. Byford Dolphin was registered in Hamilton, Bermuda, and drilled seasonally for various companies in the British, Danish, and Norwegian sectors of the North Sea. In 2019, Dolphin scrapped the rig.

The rig was the site of several serious incidents, most notably an explosive decompression in 1983 that killed four divers and one dive tender, as well as critically injuring another dive tender.

Built as Deep Sea Driller, Byford Dolphin was the first-of-class in the highly successful Aker H-3 series, designed by Aker Group and completed at the Aker Verdal shipyard in 1974.

Byford Dolphin had an overall length of 108.2 metres (355 ft), breadth of 67.4 metres (221 ft) and depth of 36.6 metres (120 ft). It had a maximum drilling depth of 6.1 kilometres (3.8 mi), and it could operate at a water depth of 460 metres (1,500 ft). As a drilling rig, Byford Dolphin was equipped with advanced drilling equipment and originally met strict levels of certification under Norwegian law, though in later years it was banned from Norwegian waters. Byford Dolphin was able to manoeuvre with its own engines (to counter drift and ocean currents), but for long-distance relocation it had to be moved by specialized tugboats.

On Saturday, 5 November 1983, at 4:00 a.m., while drilling in the Frigg gas field in the Norwegian sector of the North Sea, four divers were in a diving chamber system on the rig's deck that was connected by a trunk (a short passage) to a diving bell. The divers were Edwin Arthur Coward (British, 35), Roy P. Lucas (British, 38), Bjørn Giæver Bergersen (Norwegian, 29) and Truls Hellevik (Norwegian, 34). They were assisted by two dive tenders, William Crammond (British, 32) and Martin Saunders.

At the time of the accident, decompression chambers 1 and 2 (along with a third chamber which was not in use at the time) were connected via a trunk to a diving bell. The connection made by the trunk was kept sealed by a clamp operated by Crammond and Saunders, who were experienced divers. Coward and Lucas were resting in chamber 2 at a pressure of 9 atmospheres (atm). The diving bell with Bergersen and Hellevik had just been winched up after a dive and joined to the trunk. Leaving their wet equipment in the trunk, the two divers climbed through the trunk into chamber 1.

The normal procedure would have been:

The first two steps had been completed when a mechanical failure caused the clamp that was keeping the trunk sealed to open before Hellevik (diver 4) had closed the door to the chamber. This resulted in both chambers being rapidly depressurized from 9 atmospheres to the ambient external pressure of 1 atm. Air rushed out of the chamber system with tremendous force, jamming the interior trunk door and pushing the bell away, striking the two tenders. All four divers were killed; one of the tenders, Crammond, was killed, while Saunders was severely injured.