Top-Heavy stacking on deck is causing increasing numbers of containers to be swept overboard as shipping lines take more onboard than they should in last minute loading in the face of rocketing fuel prices, says a new study.
"More heavy containers being loaded on top of light containers because of the last-moment order when vessels with fixed schedules have a closing window of a few hours before departure," commented Blue Star CEO Harold Krill.
The American Institute of Marine Underwriters' (AIMU) study, "Ondeck Storage of Containers" ascribed losses to overweight containers, bad weather, twistlocks, parametric rolling caused by the motion of waves and the design of new mega-ships.
Containerised grain exports are also intensifying container weight because it is railed unweighed to the port of departure, Only trucked grain is weighed before loading.
Commented National Cargo Bureau president James McNamara: "The vibrations of going over the road or the railroad may cause metal fatigue and stress the welds. If the containers are weakened, they could give way when stacked on deck."
The loss of 100 containers in 2006 from the decks of P&O Nedlloyd's 7,500-TEU Mondriaan appears to have resulted from the lower tier collapsing in bad weather. Twistlocks would not have helped top-heavy boxes in rough seas. Once the twistlocks fail, everything goes too, said Blue Star's Mr Krill.
Parametric rolling from waves has been studied in 2004 by The American Bureau of Shipping with results showing container vessels are susceptible to parametric roll due to the fine form of the underwater hull geometry, combined with the full form above the waterline.
Reduction of parametric roll can be achieved by small anti-roll tanks that absorb surplus energy and fin stabilisers that counteract the roll motions. But the study concluded the solution to roll is not a simple one.
With no data repository on overboard containers, losses are unknown, but a reported 1,600 boxes were lost that sea last year, according to New York's Journal of Commerce.
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