Ashland storage tank (fracture face) (Reproduced courtesy of Ashland Petroleum Co and Battelle Columbus Division)
On the 2 January 1988, tank No. 1338 at the Ashland Petroleum Company's Floreffe terminal in Pennsylvania was being filled to capacity with diesel fuel oil for the first time since its re-erection at this site the previous August. The temperature of the oil was 8°C and the air temperature was -3°C. At 5.00pm, when the oil level was almost at the operating maximum, the tank shell fractured vertically without warning. The tank shell parted from the bottom plate at the connecting welds and, under the force of the escaping oil, moved sideways about 35m. The tank roof to shell joint remained sufficiently intact for the roof to move with the shell.
The escaping oil flowed over the surrounding dykes damaging an adjacent tank and passed through storm sewers into the Monongahela River and then the Ohio River. The total spillage was estimated at 15.2 million litres, causing severe harm to the environment and affecting the drinking water supply.
The tank had been built originally at Whiskey Island in Ohio some time in the 1930s-1940s. It was a 36m diameter cylindrical tank with a flat bottom and supported conical roof. The shell was approximately 14.4m high and consisted of six courses of welded plate, each plate being about 2.4m x 9.6m. The plate thickness in the bottom course was 21mm and 6mm in the top course. The thicknesses of courses 2 to 5 lay in between these.
The tank capacity was 16000m 3 or 16 million litres and until 1986 it had been used to hold distillate oils and heavier distillates. In 1986 the tank was taken down by oxyacetylene cutting adjacent to the original welds and then reassembled by welding in Floreffe, keeping the plates in the same order.
Causes of Failure
Examination of the fracture faces in the tank shell showed them to be flat and perpendicular to the plate surfaces, with the characteristic chevron markings of brittle fracture (see Fig.). The chevron markings pointed back to a flaw below the weld between the 1st and 2nd courses, at the point where a vertical weld on the 2nd course met the circumferential weld. The flaw was described as being 'dime-size' or about the size of a 5 pence piece and its orientation was in the vertical direction of the tank. Metallographic studies of the flaw revealed it to be due to flame cutting, rather than welding but, surprisingly, not the flame cutting of the dismantling procedure. The flaw had been present in the steel plate prior to being welded when the tank was originally built.
Charpy V notch tests and drop-weight tests (Pellini) to measure the nil-ductility transition (NDT) temperature were performed on the shell plate. The parent material was an ASTM A10 steel, either rimmed or semi-killed. The NDT temperature was found to be +10°C and at +3°C, the estimated temperature of the tank wall at failure, the Charpy tests showed low energy absorption. However, engineering defect assessments using fracture toughness values measured at +3°C indicated that the stress due to the hydrostatic pressure alone (approximately 80N/mm 2) would not have been sufficient to trigger failure. Soil foundation analyses were carried out which ruled out subsidence as a contributory factor.
Attention was then turned to the influence that the weld adjacent to the flaw may have had. Welding residual stresses may be as high as yield strength level. In the case of the Ashland tank this could have meant that the flaw was subject to a stress level of approximately 240N/mm 2. Furthermore, the effect of the welding heat cycle on the material at the crack tip was thought to have caused locally intensified strain-ageing embrittlement to which steels of this type and vintage are susceptible. This was confirmed when low fracture toughness values were measured on shell plate samples simulating this form of embrittlement.
It was finally concluded that the failure was due to the material immediately surrounding the flaw being of particularly low toughness, with crack initiation occurring under the combined effect of hydrostatic and residual stresses. As the tank was operating below the NDT temperature of the shell plate, the crack emerging from the locally embrittled area could not be arrested.
This failure highlighted the potentially serious problem of locally intensified strain-ageing embrittlement associated with re-welding and weld repairs of older steels. In these situations, the material toughness can be less than expected
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:-
Classic brittle failures in large welded structures
Engineering Failure Analysis, vol 3, no 2. 1996. pp.115-127.