A fractured hull girder caused the break-up and sinking of container ship MOL Comfort in June 2013, an investigation has found.
Japanese classification society ClassNK, which classed the 8,110teu ship, released its final investigation report today.
The MOL Comfort split into two and sank in bad weather about 400 nautical miles off Oman on 17 June 2013, while sailing from Singapore to Jeddah, Saudi Arabia.
The hull fracture originated from the buckling collapse of the bottom shell plates in a butt joint in the structure underneath the No. 6 cargo hold in the midship part, the investigation showed.
ClassNK said the Mitsui OSK Lines-owned ship's drawings and hull structure conformed to its requirements and the pre- and post-construction surveys were performed satisfactorily.
It said that cases where collapsed bottom shell plates caused fractured hull girders were unprecedented. The investigation was carried out on a number of large container ships and the margin against the expected loads was checked.
Although the MOL Comfort's hull girder strength was 150% of the estimated vertical bending moment at the time it broke up, uncertainties such as yield stress, sea conditions at the time of the accident, and the differences between declared weights and actual weights of the containers, could still have caused the fracture.
"The investigation concluded that it was actually possible that the load of the vertical bending moment exceeded the hull girder ultimate strength at the time of the accident when the effects of the deviations of the uncertainty factors were taken into account, although the overlap between the strength and the load was very narrow," ClassNK said.
Significant differences in the hull girder strength were also found between the MOL Comfort and other post-Panamax ships that were studied in the investigation.
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ClassNK explained: "The difference between the MOL Comfort and the other ships is mainly derived from the difference in the possibility of a buckling collapse of the stiffened bottom panel adjacent to the keel plate panel … with consideration of the superimposition of local stress in a double-bottom structure due to lateral loads and compressive stress by vertical bending."
To prevent similar fractures, it is necessary to assess the hull girder ultimate strength in proper consideration of the effects of the lateral loads and to assess the buckling collapse strength of stiffened bottom panels in the middle part of the holds.
ClassNK said the other ships studied in the investigation have sufficient structuralsafety against similar fractures.
