ClassNK established the Investigative Panel on Large Container Ship Safety, comprised of experts from shipbuilders, shipowners and academic institutions, to investigate the possibility of casualty occurrence and consider and examine safety of the large containership’s structure.
The casualty of MOL Comfort which occurred on 17 June 2013, has been under investigation for over a year now by various bodies and based on preliminary findings, the fracture in the vessel’s hull is considered to have originated from the bottom part of the vessel.
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 like sea state of the time, yield stress etc were taken into account. Although, the report indicates that the overlap between the strength and the load was very narrow.
Investigation that compared MOL Comfort and its sister ships showed a difference in possibility of buckling collapse of stiffened bottom panel adjacent to the keel plate panel under bi-axial compression with consideration of the superimposition of local stress in double bottom structure due to lateral loads and compressive stress by vertical bending.
To prevent similar fracture accidents, ClassNK said it was 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 confirmed that the target ships other than MOL Confort have the sufficient structural safety against the occurrence of similar fracture accidents.
“Post-Panamax container ships have improved their stability in comparison with Panamax container ships and have gained more cargo loading flexibility in complying with the stability requirements. As a result, the need to ballast in the double bottom tanks to improve stability is reduced,” the report read.
As explained in the report, from the strength view point, the occasion increases where the load acting on double bottom structure becomes almost equal to the load in One-bay empty condition without ballast in double bottom even in the case of normal loading conditions. This means the load acting on double bottom structure has become severe conditions in the transverse strength. This trend is especially obvious in Post-Panamax container ships of 8,000 TEU class or larger.