As a drilling rig, Byford Dolphin is equipped with advanced drilling equipment and has to meet strict levels of certification under Norwegian law.
Byford Dolphin is able to maneuver with its own engines (to counter drift and ocean currents), but for long-distance relocation, it must be moved by specialist tugboats.
Byford Dolphin has a length of 108.2 metres (355 ft), breadth of 67.4 metres (221 ft) and depth of 36.6 metres (120 ft).
It has a maximum drilling depth of 6,100 metres (20,000 ft), and it could operate at a water depth of 460 metres (1,500 ft).
This connection was sealed by a clamp operated by two tenders, who were experienced divers.
A third chamber was connected to this system but was not involved.
On Saturday, 5 November 1983 at 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 attached by a trunk (a short passage) to a diving bell.
The divers were Edwin Coward (British, 35 years old), Roy Lucas (British, 38), Bjørn Giæver Bergersen (Norwegian, 29) and Truls Hellevik (Norwegian, 34).
The obsolete Byford Dolphin diving system, dating from 1975, was not equipped with fail-safe hatches, outboard pressure gauges, and an interlocking mechanism, which would have prevented the trunk from being opened while the system was under pressure.On this day, Coward and Lucas were resting in chamber 2 at a pressure of 9 atm.