In March 1981, an investigation report concluded that the Kielland collapsed due to a fatigue crack in one of its six bracings (bracing D-6), which connected the collapsed D leg to the rest of the deck.[4] This was attributed to a small 6 mm fillet weld joining 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 resistance.

Additionally, the investigation found a considerable number of lamellar tears in the flange plate and cold cracks in the underlying groove weld. Cold cracks in the welds, increased stress concentrations due to weakening of the flange plate, poor weld profile and cyclic stresses (which would be common in the North Sea) appeared to play a collective role in the shelf collapse. Judging from the paint on part of the fractured surface, the crack was probably due to improper work during the construction of the platform in 1976. Then, other major structural elements failed in sequence, destabilizing the entire structure. The platform design was flawed due to the lack of structural redundancy.

The Kielland was recovered in 1983, on the third attempt. The platform was sunk later that year in Nedstrand Fjord, after the search for the missing bodies and various tests to determine the cause of the disaster were completed.

In March 1981, an investigation report concluded that the Kielland collapsed due to a fatigue crack in one of its six bracings (bracing D-6), which connected the collapsed D leg to the rest of the deck.[4] This was attributed to a small 6 mm fillet weld joining 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 resistance.

Additionally, the investigation found a considerable number of lamellar tears in the flange plate and cold cracks in the underlying groove weld. Cold cracks in the welds, increased stress concentrations due to weakening of the flange plate, poor weld profile and cyclic stresses (which would be common in the North Sea) appeared to play a collective role in the shelf collapse. Judging from the paint on part of the fractured surface, the crack was probably due to improper work during the construction of the platform in 1976. Then, other major structural elements failed in sequence, destabilizing the entire structure. The platform design was flawed due to the lack of structural redundancy.

The Kielland was recovered in 1983, on the third attempt. The platform was sunk later that year in Nedstrand Fjord, after the search for the missing bodies and various tests to determine the cause of the disaster were completed.