Slow Steaming “can damage containers” - BMT
Fuel saving measures can have a negative effect on the lashing forces on the containers, according to Maritime London member BMT Surveys. The company's risk & quality manager and marine surveyor, Olivier van der Kruijs, bases this warning on audits on more than 100 container vessels last year.
He says that the latest generation of container ships have been designed not only to increase capacity but also to improve energy efficiency and environmental performance. The rise in fuel prices in combination with a continuing pressure on freight rates has forced ship owners and operators to look closely at the amount of fuel being used. This has resulted in economical steaming and other fuel efficiency measures.
According to BMT, fuel efficiency monitoring can be achieved by a number of ways; for example, by using computer and communication software which monitors and analyses the ship’s performance and operational parameters in real time. The results of these analyses may then suggest, for example to change speed, trim and draft. The optimal trim, varies with speed, displacement, weather and underwater hull shape and can be a significant factor in saving fuel. One study suggested that fuel consumption could be reduced by as much as 5% using this technology. 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.
BMT carries out regular inspections of container ships, with a point of attention during these surveys being the requirement to review the lashing computer data and establish if there is a situation on board whereby container lashing forces are exceeded.
Usually, for the preparation of a stowage plan, stability and lashing forces are calculated. These calculations take into account the usual changes to stability as a consequence of expected fuel consumption or changes to the ballast water quantity, whilst sailing. It has become apparent that during the voyage, the ship is sometimes instructed by the owners (or the charterers) to make adjustments to improve fuel efficiency.
BMT cautions: “These (unplanned) adjustments of draught and trim increased the GM (metacentric height) on 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.”