World Concord tanker failure
The World Concord left the Mersey on the afternoon of the 26 November 1954 and set off southward down the Irish Channel. The vessel was in 'Winter Ballast Departure Condition', as recommended by the builders, carrying about 18000tonnes water ballast. In the evening, warnings of severe south-westerly gales were received. The master took on more ballast and reduced the engine speed. By midnight the wind force was 8-9 on the Beaufort scale and waves were about10m high. The engine speed was reduced further.
In the early hours of 27 November, two very large waves hit the ship. The master estimated that the crest of the first wave was under the centre of the ship when the second wave broke over the fore of the ship. There was a loudrumbling noise and the vessel broke in half (see Fig.). Both parts remained afloat, although they did collide. No casualties were reported.
The World Concord was built by Vickers Armstrong Ltd in Barrow-in-Furness. It was a single deck, all-welded, single screw steam turbine oil tanker, 199m long with a registered tonnage of 11700 tonnes. At the time of its failure, it was the largest tanker in the world. The vessel framing consisted of longitudinals approximately 0.8m apart and transverse webs positioned about 3m apart. Transverse bulkheads were located every 12m along the vessel's length. There were ten cargo tanks each with starboard, centre and port compartments.
The steel plating for the hull and deck was 20 to 31mm thick. The bilge strakes, deck stringers and shear strakes were of 'special quality' steel meeting the requirements of Section 7 of the Rules for Quality and Testing Materialsused by Lloyd's Register in 1950 with the extra condition of 0.23% maximum C content. The remaining plates were of 'ordinary shipbuilding quality' by early 1950s standards.
The World Concord was repaired and operated under another name until 1974.
Causes of Failure
The break in the vessel occurred approximately amidships at the position of a transverse bulkhead. The T-shaped bottom longitudinal stiffeners were joined to the transverse bulkheads by welded vertical brackets. Also, as the longitudinals were scalloped, filling-in flat bars were welded between the bottom plates and the longitudinals at the bulkhead connections. This resulted in two regions of stress concentration at the bulkhead position, one at the vertical bracket and one at the filling-in plate.
It was thought that the fracture initiated at one of these two stress concentration regions associated with a bottom longitudinal near the starboard side. The fracture propagated in a brittle manner in two directions along the line of the transverse bulkhead. In one direction it ran across the bottom of the ship, up the port side and back across the deck to the starboard side and in the other direction along the bottom of the vessel and up the starboard side. The final separation of the two halves of the tanker took place at the starboard deck to hull angle where the sheer strake and deck stringer plate were severely distorted.
The investigation into the failure revealed that the Charpy V notch properties of much of the hull plate were poor at the casualty temperature (~12°C) with absorbed energies less than 27J. (Most of the 'special quality' plates had Charpy energies above 40J at 12°C). Furthermore the ballast condition of the ship, which was considered to be correct at the time, did in fact result in high stress levels in the bottom plating. Although the quality of welding was good with no significant defects, the combination of low toughness material, high stress levels and shock loading due to wave action resulted in conditions that the structure could not withstand.
This failure, and others, led to the changes in classification society rules regarding notch toughness of steels used in welded ships, the design of structural details and control of welding quality. In 1957 Lloyds Register incorporated mandatory minimum Charpy requirements into its Rules. These requirements were for at least 47J absorbed energy and 30% fibrous fracture appearance at 0°C. Maximum permitted levels of carbon, silicon, manganese,phosphorous and sulphur were also introduced to ensure weldability.
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.