At about 1900 hours local time, the nearby Sedco 706 experienced a large, powerful wave which damaged some items on deck and caused the loss of a life raft. Soon after, radio transmissions were heard from the Ocean Ranger, describing a broken portlight (a porthole window) and water in the ballast control room, with discussions on how best to repair the damage. The Ocean Ranger reported experiencing storm seas of 55 feet, with the odd wave up to 65 feet, thus leaving the unprotected portlight at 28 feet above mean sea level vulnerable to wave damage. Some time after 2100 hours, radio conversations originating on the Ocean Ranger were heard on the Sedco 706 and Zapata Ugland, noting that valves on the Ocean Ranger's ballast control panel appeared to be opening and closing of their own accord. The radio conversations also discussed the winds and waves up to high. Through the remainder of the evening, routine radio traffic passed between the Ocean Ranger, its neighbouring rigs and their individual support boats. Nothing out of the ordinary was noted.
At 0052 hours local time, on 15 February, a MAYDAY call was sent out from the Ocean Ranger, noting a severe list to the port side of the rig and requesting immediate assistance. This was the first communication from the Ocean Ranger identifying a major problem. The standby vessel, the M/V Seaforth Highlander, was requested to come in close as countermeasures against the 10-15 degree list were proving ineffective. The onshore MOCAN supervisor was notified of the situation, and the Canadian Coast Guard and Mobil-operated helicopters were alerted just after 0100 hours local time. The M/V Boltentor and the M/V Nordertor, the standby boats of the Sedco 706 and the Zapata Ugland respectively, were also dispatched to the Ocean Ranger to provide assistance. At 0130 hours local time, the Ocean Ranger transmitted its last message: 'There will be no further radio communications from the Ocean Ranger. We are going to lifeboat stations'. Shortly thereafter, in the middle of the night and in the midst of atrocious winter weather, the crew abandoned the rig. The rig remained afloat for another 90 minutes, sinking between 0307 and 0313 hours local time.
While the rig was provided with an Emergency Procedures Manual which detailed evacuation procedures, it is unclear how effectively the rig evacuation was carried out. There is evidence that at least one lifeboat was successfully launched with up to 36 crew inside, and witnesses on the M/V Seaforth Highlander reported seeing at least 20 crew members in the water at the same time, indicating that at least 56 crew successfully evacuated the rig. The United States Coast Guard report speculated that 'these men either chose to enter the water directly or were thrown into the water as a result of unsuccessful lifesaving equipment launching'. Rescue attempts by the standby vessels were hampered by the adverse weather conditions and the conclusion that the standby boats were neither equipped nor configured to rescue casualties from a cold sea. As a result of the severe weather, the first helicopter did not arrive on scene until 0430 hours local time, by which time most if not all of the Ocean Ranger's crew had succumbed to hypothermia and drowned. Over the next week, 22 bodies were recovered from the North Atlantic. Autopsies indicated that those men had died as a result of drowning while in a hypothermic state.
The remains of the rig were found by sonar search over the following weeks, resting in an inverted position approximately south-east of the wellhead, surrounded by major items of debris such as the derrick. The rig had capsized bow-first, turning over and striking the sea floor with the forward ends of the rig's pontoons. The United States Coast Guard Marine Board of Investigation report into the Ocean Ranger sinking summarised the chain of events leading to the loss of the Ocean Ranger as follows:
A Canadian Royal Commission spent two years looking into the disaster. The commission concluded that the Ocean Ranger had design and construction flaws, particularly in the ballast control room, and that the crew lacked proper safety training, survival suits and equipment. There was no suggestion that it was caused by a rogue wave, as some recent articles have suggested. The Canadian Royal Commission also concluded that inspection and regulation by Canadian government agencies was ineffective. In addition to key recommendations for Canada's offshore oil and gas industry, the commission recommended that the federal government invest annually in research and development for search and rescue technologies, such as improving the design of lifesaving equipment—a commitment that has been met in every fiscal year since 1982.
Note: the bulk of the Ocean Ranger article is based on this report.