Report says design specifications must deal with loading condtions.
The Japanese classification society ClassNK has released a report on “The Structural Safety of Large Container Ships,” undertaken following the sinking of the MOL Comfort after the ship broke in half in June 2013.
The report concluded it was “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.”
Uncertainty factors included yield stress, the state of the sea at the time of the accident, and the differences between declared weights and actual weights of the containers.
The report said at the time of the accident, there were significant waves of 5.5 meters in height, and that the encountered wave direction was 114 degrees, an oblique sea from the bow and port side.
It said when a sophisticated analysis was carried out and lateral loads were considered, “significant differences were observed” between the MOL Comfort and other large contanerships considered in the study. On the other hand there was “no substantial difference” between the MOL Comfort and other large containerships in the study against the requirements in the International Association of Classification Societies rule using a method that did not consider the effect of lateral loads.
ClassNK said it was important that “design conditions can deal with various loading conditions in service adequately, and such design conditions are to be understandable to both operators and ships.”
The report notes post-panamax container ships have improved stability in comparison with panamax container ships and have more cargo loading flexibility in complying with stability requirements.
While the need to ballast in the double bottom tanks in ships to improve stability is reduced, the report noted, “On the other hand, from the strength viewpoint, the occasion increases where the load acting on double bottom structure become almost equal to the load in one-bay empty condition without ballast in double bottom even in the case of normal loading conditions, which means the load acting on double bottom structure has become severe conditions in the transverse strength.”
The report found this trend “is especially obvious in post-panamax containerships of 8,000 TEU or larger.”
It said “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 streff 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.
It said that with post-panamax containerships “it is important that design conditions can deal with various loading conditions in service adequately, and such design conditions are to be understandable to both operators and ships.”
ClassNK releases report on MOL Comfort accident
