In June 2013 the container vessel »MOL Comfort« broke into two in the Indian Ocean. Now Japan´s ClassNK issued the Investigation Report, which states different factors as possible reasons for the incident
The topic is intensively discussed throughout the maritime industry. Mitsui O.S.K. Lines (MOL) as the vessels owner and operator is[ds_preview] seeking more than 130 Mill. $ from the shipyard Mitsubishi Heavy Industry (MHI) because of alleged negligence and problems of safety. Built 2008 at MHI in Nagasaki, the broken parts of the 8,110-TEU vessel sank 10 and 24 days after the incident.
The classification society ClassNK led an investigative panel, which tried to explain the casualty. However, a definitive cause could not be uncovered. The investigation was carried out relating to the possibility of the occurrence of the fracture in consideration of deviations of uncertainty factors, including the yield stress, the sea states at the time of the accident and the differences between declared weights and actual weights of the containers. The experts concluded »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«. Several containerships with capacity of 6,000 to 8,000TEU were investigated as well in order to get some general findings.
As evaluation of structural safety, 3-hold model elasto-plastic analyses were carried out on the target ships with consideration of lateral loads such as bottom sea pressure and container loads, and investigation was conducted on the hull girder ultimate strength obtained by the analyses. Significant differences were observed between the Ship and the other target ships on the margin of the hull girder ultimate strength calculated by 3-hold model elasto-plastic analysis against the wave-induced vertical bending moment specified in IACS UR S11.
No substantial difference was observed among the all ships in investigation including the Ship on the margin against the requirements of IACS UR S11 relating to the vertical bending strength and on the margin of the hull girder ultimate strength calculated by IACS CSR method (Smith’s method), which do not consider the effect of the lateral loads.
»It is considered that the difference between the Ship and the other target ships observed in this investigation is mainly derived from the 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«, the report says. To prevent similar fracture accidents, 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, according to the experts.
Post-Panamax container ships have improved their stability in comparison with Panamax container ships and have gained more cargo loading flexibility. As a result, the need to ballast to improve stability is reduced. On the other hand, the reports says, 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 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 8,000TEU class or larger.
»As a result, the minimization of ballast increases the frequency with which the still water vertical bending moment reaches close to the allowable value. Meanwhile, it sometimes happens that the transverse stress of double bottom structure becomes almost equal to the transverse stress in One-bay empty condition without ballast in double bottom even in the normal loading conditions where the ship is in around the full draught«, the experts stated.
MHI has already rejeceted any responsibility. It says weather conditions and problematic safe operation as well as differences between manifested and actual container weights were the main reasons for the incident.
The Japanese Ministry of Land, Infrastructure, Transport and Tourism (MLIT) intends to publish a final report on the incident until March 2015.
Michael Meyer
