Following the accident, the platform and the separated column D remained afloat. Column D was towed to Stavanger and divers removed all the fracture faces from the capsized platform for investigation.
The Commission responsible for the inquiry into the disaster concluded that the structural failure had occurred in the following stages:
- Fatigue crack growth in brace D6 initiating from pre-existing cracks in the fillet welds between a hydrophone support and the brace
- Final, mainly ductile, fracture of brace D6
- Subsequent failure of five remaining braces joining the column to the structure by plastic collapse
Brace D6 and the hydrophone support were both made from a C-Mn structural steel (equivalent to a Lloyds' ship steel Grade EH) with a minimum specified yield strength of 355N/mm 2. The brace was 2.6m in diameter with a wall thickness of 26mm. The hydrophone support was 20mm thick with a diameter of 325mm and was set-through the brace. It was attached to the brace by two fillet welds, one on the outside of the brace and the other on the inside. Examination of these fillet welds revealed poor penetration into the hydrophone tube material and an unsatisfactory weld bead shape. Significant cracking was also found which was dated to the time of fabrication by the presence of paint on the fracture surfaces.
Fatigue crack growth in brace D6 originated at the hydrophone support weld and extended, in the latter stages partly by ductile tearing, around approximately 2/3 of the circumference of the brace until final failure took place by brittle fracture.
The chemical compositions of the brace and hydrophone material were within specification, as were the Charpy and in-plane tensile properties. The through-thickness ductility of the hydrophone material (which was not specified) was, however, poor. This, combined with its through-thickness tensile strength being lower than the in-plane strength of the brace material and with sub-standard welding, led to partial cracking of the fillet weld during fabrication.