Alexander Kielland oil platform disaster

On the evening of 27 March 1980, one of the main horizontal braces supporting the Alexander L Kielland accommodation platform anchored in the North Sea broke off. The failure ocurred during a storm with wind gusts up to 40 knots with waves up to 12 meters high. The platform immediately heeled over to an angle of 30-35° and then continued to heel and sink slowly. Twenty minutes after the loss of the supporting column, the platform capsized. Of the seven lifeboats on board, only two were launched successfully albeit with great difficulty in part due to bad weather conditions . A massive international air and sea rescue operation was undertaken. Of the 212 men on board the platform when it failed, 123 died.

The Alexander L Kielland was a semi-submersible mobile rig of the Pentagone type, a design which had been developed in France. The rig was built between 1973 and 1976 in France for an American operator. Although it was designed as a drilling rig, it was only ever operated as an accommodation platform during its four years in service. At the time of the accident it was being used as living quarters for the nearby Edda platform.

The investigative report concluded that the rig collapsed owing to a fatigue crack in one of its six bracings, which connected the collapsed D-leg to the rest of the rig. This was traced to a small 6 mm fillet weld which joined a non load-bearing flange plate to this D-6 bracing. This flange plate was used to hold a sonar device used during drilling operations. The poor profile of the fillet weld contributed to a reduction in the fatigue strength of the weld. Furthermore, the investigation found considerable amounts of lamellar tearing in the flange plate and cold cracks in the butt weld. Cold cracks in the welds, increased stress concentrations due to the weakened flange plate, the poor weld profile, and cyclical stresses (which would be common in the North Sea), seemed to collectively play a role in the rig's collapse.

Attempts were made to right the rig in 1980 but were abandoned due to the rig's dangerous state. In 1983, the rig was successfully righted and investigated before being towed to Nedstrand Fjord, where the remains of the rig were deliberately sunk.

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