Although the design of the ship performed as well as its peers against International Association of Classification Societies (IACS) specifications for vertical bending strength of the hull girder, the MOL Comfort performed significantly worse than its peers when lateral load effects were taken into account.
The ClassNK investigation considered possible differences in declared and actual weight of containers, possible deviations from the nominal yield strength of the steel used, and the effect of possible deformations in the girder. Those deformations considered were from manufacture, permissible within Japanese shipbuilding standards, and deformation during service as observed on the MOL Comfort's sister ships.
These uncertainty factors were an improvement on the methodology of a previous investigation by Japanese Authorities, which concluded that the hull girder was strong enough to sustain the conditions the MOL Comfort faced at sea.
The more thorough parameters were fed into a simulation and the MOL Comfort's design was compared to seven other ships of 6,000 teu and 8,000 teu designs in "3-hold model elasto-plastic analyses". The models analysed the designs' performances in two high stress scenarios - one where an entire bay is left empty of containers, the other where the ships has low ballast, both of which increase the stress on the hull girder.
Low ballast conditions are more common on post-panamax container ships as their increased width reduces the need for ballast to improve stability, however, the ballast acts to counter the upward pressure of the sea, so its removal increases stress on the hull girder.
Taking into account those conditions and uncertainty factors, the investigation found that it was possible that the vertical load on the hull girder exceeded its limits by a narrow margin.
Among points in ClassNK's future action plan from the investigation are the evaluation of the strength of ships considering lateral loads in 3-hold model elasto-plastic analyses, and procedures and measures to deal with various loading conditions of post-panamax ships and the possible use of hull monitoring systems to provide data from full scale ships in service.
Results of the investigation will be made available to other classification societies and submitted to IACS with proposals and suggestions to related IACS rules. Last week DNV GL's vice president Knut Dohlie criticised ClassNK and the Japanese authorities for not sharing information during the investigation. Based on its own investigations DNV GL said it would not have given class approval to the vessel design.
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