|Operating deck load||3025 MT|
|Crew quarters||102 persons|
|Operating water depth||1500 feet maximum|
|Derrick||160 foot Shaffer Top Compensator|
|Mooring system||12 point|
|Blow-out preventer||Hydril 18¾ inch, 15,000 PSI|
|Sub Sea Handling System||Christmas tree|
|Deck cranes||2 x 40 tons|
The Byford Dolphin is a semi-submersible Norwegian oil exploration rig converted from a diving rig. It floats in the North Sea to find and drill crude oil deposits. Built on the "Aker H-3" design, the rig is operated by Dolphin Drilling, a Fred Olsen subsidiary. It is able to manoeuvre with its own engines (to counter drift and ocean currents), but for long-distance relocation it must be moved by specialist tugboats.
As a drilling rig, the Byford Dolphin is near the top of its class. It is equipped with advanced drilling equipment and has to meet very high levels of certification under Norwegian law. However, the rig has suffered some serious accidents, most notably an explosive decompression accident in 1983 known as the Byford Dolphin diving bell accident that killed five workers and badly wounded one more.
Table of contents
Diving Bell Accident
At 4:00 AM on November 5, 1983, four divers were in a compression chamber system attached to a diving bell on the rig, being assisted by two dive tenders. One diver was about to close the door between the chamber system and the trunk when the chamber was explosively decompressed from a pressure of 9 atm to 1 atm in a fraction of a second. Five of the men were killed; the other was severely wounded.
The situation just before this accident occurred was as follows. Compression chambers 1 and 2 were connected via a trunk to a diving bell. This connection was sealed by a clamp operated by two tenders (T1 and T2), who themselves were experienced divers. A third chamber was connected to this system, but was not involved. On this day, divers D1 (35 years old) and D2 (38 years old) were resting in chamber 2 at a pressure of 9 atm. The diving bell with divers D3 (29 years old) and D4 (34 years old) had just been winched up after a dive and joined to the trunk. Leaving their wet gear in the trunk, the divers then climbed through the trunk into chamber 1.
The normal procedure would have been to (a) close the bell door, (b) The diving supervisor would then slightly increase the bell pressure to seal this door tightly, (c) close the door between the trunk and chamber 1, (d) depressurize the trunk to 1 atm, and (e) open the clamp to separate the bell from the chamber system.
Operations a and b had been completed and D4 was about to carry out operation c when, for some inexplicable reason, one of the tenders opened the clamp. This resulted in the high pressure within the system being released into the external atmosphere, causing explosive decompression. A tremendous blast shot from the chambers through the trunk, pushing the bell away and hitting the two tenders. The tender who opened the clamp died, and the other was severely injured.
Diver D4 was shot out through the small jammed hatch door opening, being ripped apart. Subsequent investigation by forensic pathologists determined that diver D4, being exposed to the highest pressure gradient, exploded with violence due to the rapid and massive expansion of internal gases. All of his thoracic and abdominal organs, and even his thoracic spine, were ejected, as were all his limbs. Simultaneously his remains were expelled with force through the narrow trunk opening left by the jammed chamber door, less than 60 centimetres (24 inches) in diameter. Chunks of his body were found scattered about the rig. One part was even found lying on the rig's derrick, 10 metres directly above the chambers. His death would almost certainly have been painless and instantaneous.
Medical investigations were carried out on the four divers' remains. The most conspicuous finding of the autopsy was large amounts of fat in large arteries and veins and in the cardiac chambers, as well as intravascular fat in organs, especially the liver. This fat was unlikely to be embolic, but must have "dropped out" of the blood in situ. It is suggested that the boiling of the blood denatured the lipoprotein complexes, rendering the lipids insoluble.
The rigor mortis was unusually strong. The hypostases were light red, and in two cases there were numerous hemorrhages in the livers. All the organs showed large amounts of gas in the blood vessels, and scattered hemorrhages were found in soft tissues. One of the divers had a large sub-conjunctival bulla.
The committee investigating the accident concluded that it was due to human error on the part of the dive tender who opened the clamp. It is not clear whether the tender who opened the clamp before the trunk was depressurized did so by order of his supervisor or on his own initiative, due to miscommunication. At the time the only communication the tenders on the outside of the chamber system had was through a bullhorn attached to the wall surface; with heavy noise from the rig and sea, it was hard to listen in on what was going on. Fatigue from many hard hours of work also took its toll on awareness amongst the divers, who would often work 16 hour shifts.
This incident was also an engineering failure, in that the system was not equipped with fail-safe hatches. Subsequently a law was passed in Norway requiring such systems to have fail-safe seals that would close automatically.
Some individuals have alleged that the investigation was a cover-up, and that the accident was due to of a lack of proper equipment including clamping mechanisms equipped with interlocking mechanism (which would be impossible to open while the chamber system was still under pressure), outboard pressure gauges and safe communication system, which had been held back due to dispensations done by the Norwegian Petroleum Directorate.
- In late November 2001, the rig was left adrift without control in a storm. Although 17 of the 71 workers were evacuated by helicopter, the company claimed there was no serious danger.
- On April 17 2002, a 44 year old Norwegian worker on the rig was struck on the head and killed in an industrial accident. The accident resulted in the Byford Dolphin losing a exploration contract with Statoil, who expressed concerns with the rig's operating procedures. The incident cost the company millions of dollars in lost income.
Giertsen, J.C. et al., "An Explosive Decompression Accident", The American Journal of Forensic Medicine and Pathology, 9(2):91–101, 1988.