Causes of Failure
The 0.6m long crack which occurred during initial hydrotesting originated from a repaired boat-sample site. The brittle crack which caused the collapse of the tank also initiated at a repaired boat sample position in the circumferential weld between the lower two strakes. A very small cavity had been left at the bottom of the boat sample groove when it was repaired. This defect was found to be much smaller than others detected in the shell welds after the failure.
The weld quality was in fact quite variable although this had not been revealed by the inspection during fabrication. Tests on the plate material showed it to meet the specification. Its Charpy impact transition temperature, however, was in the approximate range 0°C to 15°C, hence the tank material did not have good toughness at the hydrotest temperature.
The existence of defects which were significantly longer than the one from which the fracture initiated perturbed the investigators. As no evidence of shock or impact loading which could have triggered the collapse was found, the investigation into the failure did not reach a conclusion regarding the cause of fracture initiation.
Approximately one month after the failure of the crude oil tank, a neighbouring gas oil tank failed during hydrotest. This tank split vertically but remained in one piece. The tank was 45.7m in diameter and 14.6m high built, like the crude oil tank, of BS 13 steel to API 12C. The water temperature was +4°C and the air temperature +9°C at the time of failure.
Examination of the fracture faces revealed that the failure initiated at a partially repaired crack in a vertical weld in the bottom shell course. The surfaces of the crack were blackened indicating that the crack had gone through a heating cycle due to a nearby welding operation.
Subsequent studies indicated that the probable cause of failure was the presence of very low toughness material in the region of the initiating defects. These regions of low toughness would have resulted from dynamic strain-ageing embrittlement at the tip of the flaws during repair welding (or subsequent heat cycling). This type of strain ageing embrittlement, which is intensified at crack tips, is a potential problem associated with repair welds, particularly in coarse grained non-aluminium treated steels.