34th Anniversary of Norway’s biggest offshore disaster
Today marks the 34th anniversary of the accommodation platform Alexander L. Kielland capsizing in the North Sea, offshore Norway.
On Thursday evening of 27 March 1980, one of the five columns of the ‘Alexander L Kielland’ broke off. The five columns, overall height of 35.6m, mounted on 22m diameter pontoons, were the principal buoyancy elements of the platform.
After the column broke off, 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 column D, 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 (one landed upside down in the water). Some inflatable rafts launched themselves due to the listing of the platform. A massive international air and sea rescue operation was undertaken.
A year later in March 1981, the investigative report concluded that the rig collapsed owing to a fatigue crack in one of its six bracings (bracing D-6), which connected the collapsed D-leg to the rest of the rig. This was traced to a small 6mm fillet weld which joined a non-load-bearing flange plate to this D-6 bracing. This flange plate held a sonar device used during drilling operations.
The poor profile of the fillet weld contributed to a reduction in its fatigue strength. Further, 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.
The rig was recovered in 1983 at the third attempt. The rig was scuttled later that year after a search for missing bodies had been completed, as well as several tests to determine the cause of the disaster. The fatigue crack had grown over time from a hydrophone port in the bracing tube. Judging by paint on part of the fractured surface the crack was probably due to improper labour at the plant in Dunkerque, France where the rig was built in 1976.
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.