Editorial Comment - The Naval Architect, May 2014
Focus for the investigation into the causes of the break up and subsequent loss of MOL Comfort has turned to the structural strength of the vessel and the declared weight of containers.
According to investigators from the Committee on Large Container Ship Safety in Japan, which is investigating the causes of last year’s accident in the Indian Ocean, the time has come for the industry to start weighing loaded containers to determine their actual weight, rather than relying on shippers to declare the correct weight of a container.
The committee’s report concludes: “With regards to the proper management of cargo weight on the hull for large container ships in the 8,000TEU class and over in particular, cargo loading planning for actual voyages could be frequently reached to the maximum permissible still waterbending moment (hogging condition). In accordance with the deliberations at the IMO related to the enforcement of container weight verification prior to loading, verification of the actual weight of container cargoes provided by the shipper is recommended as a safety measure for large container ships.”
In fact this is not the first time an investigation into the loss of a vessel has reached such a conclusion. In January 2007, while travelling in the English Channel MSC Napoli encountered heavy weather, with wave heights of up to 9m, while travelling at a speed of around 11knots. At around 11am that morning Napoli suffered catastrophic hull failure causing the crew to abandon ship.
In its report of the Napoli accident the UK’s Marine Accident Investigation Bureau (MAIB) noted: “The load and capacity assessments conducted by DNV and BV show that, in the case of MSC Napoli, the design margin of safety was either insufficient when whipping is taken into account (BV), or non-existent (DNV). The analyses are supported by the fact that the vessel broke her back when within her seagoing limitations and, although the conditions were severe and had a low probability of occurrence, they were nevertheless equivalent to the current UR S11 design value [the longitudinal strength standard set by the International Association of Classification Societies].”
In conclusion the MAIB reported: “The effect of the discrepancies in the declared weights of the containers would not have been sufficient to cause hull failure, but it would have contributed to the reduction of the safety margin between the total bending moment experienced and the strength of the hull.”
As a result MAIB said that the stresses on a vessel’s hull cannot be “accurately controlled” unless containers are weighed before loading. Screening of more than 1,500 container ships by class societies took place following the report on the Napoli loss and a further 12 vessels were found to have localised buckling as a result of insufficient strength for the hull structure to cope with certain loading conditions.
Furthermore, the report concluded: “It is apparent that UR S11 has lagged behind the development of container ship design and operation, and requires immediate revision. Buckling checks must be based on global hull stresses along the entire length of the hull and not left to the discretion of individual societies. The use of common methodologies in this respect would provide greater assurance that the strength of all newbuild container ships is being adequately addressed.”
According to the Napoli report the maximum allowable loading of a vessel’s hull structure relies on the experience of the class society in question. It does not “explicitly take into account factors which increase bending moments such as whipping, or other variables such as inaccuracies in container weights and distribution.”
“The audit of the containers removed from MSC Napoli and the deadload calculated on departure, indicate that the declared weights of many of the containers carried by the vessel were inaccurate. This discrepancy is widespread within the container ship industry and is due to many packers and shippers not having the facilities to weigh containers on their premises.”
However, the MAIB went on to say: “It is also due to shippers deliberately under-declaring containers’ weights in order to: minimise import taxes calculated on cargo weight; allow the over-loading of containers; and to keep the declared weight within limits imposed by road or rail transportation.”
As a result container ships are sailing at either above or close to the permissible the maximum loading limits. “Container shipping is the only sector of the industry in which the weight of a cargo is not known,” says the MAIB.
Mitsubishi Heavy Industries, the builder of MOL Comfort, believe that these issues will eventually be seen as a significant contributory factor to the loss of the vessel. Surly it is time for all containers to be weighed and for accurate measurements to be available to master prior to sailing?
Also see article on IMO to make container weighting mandatory