Fuel saving measures aimed at boosting efficiency can have a negative effect on the lashing forces on the containers, according to recent findings from the audits conducted by BMT on more than one hundred container vessels last year.
Fuel efficiency monitoring and analyses of the ship’s performance and operational parameters in real time may suggest change of speed, trim and draft during a voyage to save fuel.
One study suggested that fuel consumption could be reduced by as much as 5% by using computer and communication software which monitors and analyses ship performance.
“However, as an unwanted side effect, this fuel saving method may increase the calculated dynamic forces to the containers and lashings, possibly exceeding maximum permissible levels,” explains Olivier van der Kruijs, Risk & Quality Manager and Marine Surveyor at BMT Surveys.
As regards maximum permissible forces, stipulated in ISO standards, there are limitations resulting from the strength of the container itself.
” It is important to appreciate that there is no safety margin on these limits. Theoretically, a container may thus distort as soon as these force limits are exceeded. This is different for the safe working loads on the lashings, which do have a safety margin,” he continued.
During the voyage, the ship is sometimes instructed by the owners (or the charterers) to make adjustments to improve fuel efficiency.
According to van der Kruijs, these (unplanned) adjustments of draught and trim increased the GM (metacentric height) at various occasions and, as a result, also the dynamic forces acting on the containers and lashings. This could lead to a situation whereby the ship left port with the calculated lashing forces being within design limits, but exceeding the limits at a later stage when the trim adjustments were made.
For vessels enjoying a voyage with good weather, exceeding the designated maximum lashing forces is unlikely to result in any damaged cargo. However, if the ship was to encounter its “design motions criteria”, damage to the container stacks and cargo could occur, thus as an indirect result of saving fuel, van der Kruijs concludes.