Investigation
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. It 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.
Other major structural elements then failed in sequence, destabilising the entire structure. The design of the rig was flawed owing to the absence of structural redundancy.
(Dave Myles B.E.M. of J.R.C. in Aberdeen, Scotland, designed the charges to fit around the legs and carried out the demolition/scuttling of the rig in the Nedstrand Fjord)
Read more about this topic: Alexander L. Kielland (platform)