Learning from our mistakes...
TWI Bulletin, September/October 2004
Older and wiser - three brittle weld fracture cases in the dock
Bridget Hayes
Bridget Hayes joined TWI in 1984 and works in the area of structural integrity as a principal project leader. She has been involved in numerous projects concerned with small and large scale fracture testing, failure investigation and defect assessment.
Fortunately catastrophic structural failures are rare, however when significant structures fail, the social and economic consequences can be considerable. Welding problems have contributed to many catastrophic failures and, in order to reduce the future incidence of costly failures, it is important to learn from these events. Bridget Hayes raises awareness of past structural failures by looking at three cases of brittle fracture where welds played a major role.
M V Kurdistan
Summary facts
| Failed structure: | all welded tanker built to construction category Ice Class I |
| Date: | 15 March 1979 |
| Place: | south of the Cabot Strait, off Nova Scotia |
| Conditions: | moderately high seas, air temperature near 0°C, cargo temperature ~60°C |
| Failure mode: | brittle fracture |
| Cause: | presence of defect in bilge keel welds combined with high thermal stresses |
| Consequences: | loss of ship by intentional sinking |
Background
On the morning of 15 March 1979 the motor tanker M V Kurdistan left Point Tupper in Nova Scotia bound for Sept-Isles, Quebec. The tanker was carrying a heated cargo of oil for the first time. The weather conditions were not good and the ship was rolling heavily. At about 12.30 the Kurdistan came to the edge of an ice field but, after travelling 2.5km into the ice, the ship was brought to a halt. The ship was turned around and then headed back towards the open sea. At 13.50 the Kurdistan cleared the edge of the ice belt and put full ahead. Almost immediately there was a thud and a shudder during a downward pitch of the vessel. (The sea conditions were described as 'very heavy swell'). Oil started to escape from a vertical crack in the sides of No.3 wing tanks that came up to about 3.6m below the main deck level.
To reduce the loss, oil was transferred from No.3 wing tanks to the No.4 tanks while the ship continued on its course. At 18.40 a second shudder was felt and the transfer of oil was stopped. The weather conditions had improved and the wave height was two metres. At 21.30 hrs the ship broke in two: a shudder was felt and the bow rose, hingeing about the deck at the No.3 cargo tanks before finally separating from the stern. ( Fig.1) Almost eight hours had elapsed between the initial fracture of the vessel's shell and its breaking in two.
Fig.1. The retrieved stern portion of the MV Kurdistan
The Kurdistan was built to construction class 'Ice Class I' and completed in 1973. The vessel was longitudinally framed except for the sides where the framing was transverse. With six cargo tanks, each divided into two wing tanks and a centre tank, the overall length of the ship was approximately 182m. The Kurdistan was built almost entirely in Grade A steel (no Charpy requirements). The bottom shell was 19.5mm thick and the bilge strake 14.7mm.
The bilge keel over a length of ship including the failure region consisted of 125 x 11mm ground flat bars butt welded end to end and overlapped on the underside by 300 x 13mm bulb plates, attached by intermittent welding. The bilge keel was connected edge-on to the bilge strake by continuous fillet welds above and below. The design of the keel called for a 25mm crack arrestor hole to be drilled in each butt weld joining the ground bars.
Cause of failure
Examination of the fracture faces revealed that the initial fracture through the bottom and side shell plates was brittle ( Fig.2). The origin of the crack was a defective butt weld in the port bilge keel. There was lack of penetration in the butt weld and, where the bulb plate overlapped the underside of the ground bar, there was no weld at all. The bulb plate was misaligned and the crack arrestor hole was missing. This region of the bilge keel had been damaged in 1975 and repaired in 1977. Areas of fatigue crack growth along the lack of penetration at the weld root were found.
Fig.2. Section through the origin of the failure region
The inquiry into the failure of the Kurdistan did not establish precisely the sequence of failure of the ship's longitudinal structure, which showed both brittle and ductile fracture. Given that the ship's shell plates were found to have 27J Charpy transition temperatures of between 5°C and 20°C, the steel in contact with the sea water was close to or below its transition and that in contact with the heated cargo was above. The displacement of oil by water entering the cargo tanks lowered the steel temperature to below its ductile/brittle transition.
Calculations of the thermal stresses in the ship resulting from the carriage of a warm cargo in a cold sea indicated that a high tensile stress level would have been present in the shell and bilge keel. It is thought that the stresses due to the impact of a wave on the bow, superimposed on the high thermal stress and the stresses due to the moderate wave bending moments, triggered the fracture of the Kurdistan's bilge keel.
The initiation of the fracture was due to the classic combination of poor weld metal toughness and high stresses in the presence of a defect. This failure showed how critical the quality of workmanship can be even for a detail of apparently little significance such as the bilge keel.
Amine Absorber Tower
Summary facts
| Failed structure: | 16.8m high monoethanolamine (MEA) absorber vessel |
| Date: | 23 July 1984 |
| Place: | Lemont, Illinois, USA |
| Conditions: | normal operating conditions: 1.4N/mm 2 internal pressure, 38°C |
| Failure mode: | very low energy ductile failure (very little cleavage present) |
| Cause: | hydrogen induced heat affected zone (HAZ) cracks combined with hydrogen embrittled material |
| Consequences: | loss of 17 lives; over $100 million of property damage |
Failure event
On the evening of Monday 23 July 1984, the Union Oil Co refinery near Lemont, Illinois, USA was seriously damaged by an explosion and fire ( Fig.3) Seventeen people working at the refinery were killed and the property damage was estimated to be over $100 million. The explosion was caused by the ignition of a large cloud of flammable gas (a mixture of propane and butane) which had leaked from a ruptured amine-absorber pressure vessel.
Fig.3. The Amine Absorber Tower scene some hours after an explosion which claimed 17 lives
An operator working near the absorber tower noticed gas escaping from a horizontal crack about 150mm long near the bottom of the vessel and tried to close off the main inlet valve. The crack grew to 600mm and he initiated evacuation of the area. As the company fire fighters arrived, the absorber tower cracked further and a large amount of gas was released. The gas ignited in a massive explosion which sent the upper part of the tower into the air, landing over a kilometre away. The explosion was felt over 20 kilometres away and the blaze which followed sent flames 150m into the sky.
Background
The absorber tower first went into service in 1970. It was a cylindrical vessel 2.6m in diameter and of overall height 16.8m. The shell section consisted of six courses of 25mm thick ASTM A516 Grade 70 steel. These were joined by full penetration submerged arc welds in the as-welded condition. The vessel, built to ASME Section VIII, was designed to strip H 2 S from the propane/butane gas mixture passing through it. Monoethanolamine (MEA) was fed through the tower as part of this process. The operating conditions were 1.4N/mm 2 internal pressure at 38°C. The environment in the tower was corrosive.
Cause of failure
The investigation into the failure found that the tower fractured at the circumferential weld between the replacement ring and the lower course (Fig.4). Four large cracks in the heat affected zone (HAZ) had been present prior to the failure, originating at the inner surface of the tower and extending almost through the wall thickness. About 35% of the vessel circumference was affected. The location of the first leak observed corresponded to one of these HAZ cracks which was approximately 800mm long.
Fig.4. Section through the tower
Microhardnesses measured in the HAZ near the surface exceeded 29 HRC and peak hardnesses of 40 to 48 HRC were found near the fusion line. These facts, taken with the in-section appearance of the pre-existing cracks (straight in the HAZ near the surface and then zig-zagging through the base material at the limit of the HAZ), pointed to the cracks initiating by hydrogen cracking and then progressing by hydrogen-induced stepwise cracking (HISC). Tests according to a NACE standard procedure confirmed that the material was susceptible to HISC.
The fracture ran around the HAZ of the circumferential weld at right angles to the axial stress of 35N/mm 2 . The fact that this stress level was so low and the crack did not change directions to run in a direction perpendicular to the higher hoop stress, indicated very low toughness material in the HAZ.
Charpy V notch tests of the replacement course material and the weld between the replacement course and the upper part of the tower showed the weld metal and HAZ to have superior notch toughness to the base material. (20J transition temperatures: 0°C for parent plate, -51°C for weld metal, -40°C for HAZ). Fracture toughness tests measuring crack tip opening displacement (CTOD) in the HAZ material gave much greater critical CTOD values than the applied CTOD in the tower at the time of failure, estimated ignoring any residual stresses as 0.064mm. Tests on hydrogen charged specimens did, however, reveal much reduced CTOD fracture toughness values in the range of approximately 0.070-0.080mm at 38°C.
Taking all of these findings into account, it can be concluded that this failure occurred because the welding procedure used when replacing a section of the vessel caused the formation of a hard microstructure in the HAZ of the weld. This hard region was susceptible to hydrogen assisted cracking resulting in growth of large cracks in the vessel. The uncracked material in the vicinity of the existing cracks had low toughness due to hydrogen embrittlement and failed at the applied CTOD in the vessel arising from the operating pressure and residual stresses associated with the weld.
For operation in corrosive conditions, the control of weld properties is critical. Welding procedures, particularly for field repair welds, need to be formulated to avoid the formation of high hardness microstructures for service in hydrogen environments. Furthermore the presence of welding residual stresses can make a significant contribution to the applied CTOD at a flaw present in a structure.
Ashland Tank
Summary facts
| Failed structure: | 16000m 3 capacity fuel oil storage tank |
| Date: | 2nd January 1988 |
| Place: | Floreffe, Pennsylvania, USA |
| Conditions: | filling of tank to near capacity with oil at 8°C; ambient temperature of -3°C |
| Failure mode: | brittle fracture |
| Cause: | very low toughness region (strain aged embrittled) associated with flaw |
| Consequences: | major environmental pollution for which Ashland Petroleum Co. assumed full financial responsibility |
Failure event
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.
Background
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 two to five 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.
Cause 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. 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 ( Fig.5). The flaw was described as being 'dime-size' or about the size of a five 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.
Fig.5. The flawed region believed to be the origin of failure
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 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.
