On 12 February 1952 a large all-welded oil-storage tank collapsed during hydrotest at the Esso Petroleum plant at Fawley in Hampshire. Hydrotesting had commenced on 30 January following completion of the tank, but was halted when a 0.6m long vertical crack appeared in the bottom two strakes. The tank was emptied and the crack repaired. When the hydrotest was recommenced on the 11 February, the air temperature was near freezing and the water temperature +4°C. The tank split when the water reached 90% of the tank height, a continuous vertical fracture running through the parent plate of every strake. The shell was torn from the tank bottom and collapsed on the surrounding band, leaving the roof lying on the base.
The cylindrical tank was 42m in diameter and 16m high. The bottom was conical with a 0.6m fall at the centre and roof was a detached fully floating pontoon. The tank shell consisted of nine strakes made from butt welded plates measuring 1.8m x 7.2m. The strakes were progressively thinner from bottom to top, being aligned to produce a flush internal surface. The bottom strake was 28mm thick and the top 6mm.
The construction of the tank was according to API Code 12C. The material used was a BS 13 steel with specified tensile strength in the range 430 to 510MPa, equivalent to ASTM A7 or A283 steel. Plate edge preparation for welding was carried out prior to rolling the plates to the required radius.
The shell welds were full penetration double or single V welds, depending on the plate thickness. No-preheating was used except to dry the plates or remove frost. Boat shaped samples were cut from the welds of the lower courses for inspection, leaving grooves that were repair welded. All boat samples, bar one, were satisfactory.
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.
These failures raised concern over the weld inspection method specified in the API Code which relied on taking boat samples from the welds. In the case of the crude oil tank, the failure initiated from a poorly repaired boat sample site and in the case of the gas oil tank a significant defect was missed by the inspection method. These concerns led towards the use of radiography for weld inspection in storage tanks. The failures also highlighted the importance of material toughness for storage tanks, and the introduction of the use of materials with minimum Charpy V properties greatly improved the safety of these structures.
This is a case history taken from Report 632/1998 . For further case histories, Industrial Members may consult the full report.
Professional & WJS members and non-members of TWI can obtain further case histories by reading the following article:-
Six case histories of pressure vessel failures
Engineering Failure Analysis, vol 3, no 3. 1996. pp.157-170.