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M V Kurdistan tanker

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 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. The crack came up to about 3.6m below the main deck level. 

To reduce the loss, the transfer of oil from No.3 wing tanks to the No.4 tanks was undertaken `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 2m. At 21.30 the ship broke in two: a shudder was felt and the bow rose, hinging about the deck at the No.3 cargo tanks before finally separating from the stern. Almost eight hours had elapsed between the initial fracture of the vessel's shell and its breaking in two.

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 region failure 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 arrest hole to be drilled in each butt weld joining the ground bars.

Examination of the fracture faces revealed that the initial fracture through the bottom and side shell plates was brittle. 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 arrest 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. 

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° 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 toughness of the shell plate was insufficient to arrest the propagating crack and complete failure ensued. 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 two important failings of the requirements for ships of the size of the Kurdistan built as First Year Ice Class vessels. Firstly that the ship could be built entirely of Class A steel with no notch impact requirements and, secondly, that no calculation of thermal stresses was required for cargoes at temperatures below 65°C. Furthermore, this failure showed how critical the quality of workmanship can be even for a detail of apparently little significance such as the bilge keel.

This is a case history taken from Report 632/1998 . For further case histories, Industrial Members may consult the full report. For more information please email contactus@twi.co.uk. 

M V Kurdistan tanker failure
M V Kurdistan tanker failure
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