Precision Work – Stable Stowage

Secure cargo for Safety

Based on the many millions of containers that are shipped world-wide every year, containers that go overboard are truly isolated cases. Nevertheless, it is important for the safety of crew, cargo and the environment to place special emphasis on securing cargo properly on board.

Containers carrying cargo are, of course, also exposed to the forces acting on a fully laden container ship. Every single one of them has to withstand vertical and horizontal stresses without deforming. On the one hand, there are static forces resulting from the gross weights of the containers on board during vessel motion. In addition, there are dynamic forces, such as the rolling, pitching and heaving motions of a ship and the influence of the wind.

In contrast to the containers stacked below deck that are held by cell frames – they form a counterbalance to the vessel’s movements – containers stacked on deck are completely at the mercy of these forces. They cannot be supported horizontally.

To ensure that the high stack of containers on deck does not lose its balance and individual containers are not deformed by the load applied from above, lashing and support forces as well as the permissible stacking weights have to be specified precisely. These calculations are complicated and must conform to the characteristics of the respective ship. After all, the permitted stacking weights depend on the transversal acceleration forces that differ according to the size and rolling behaviour of the vessel.

Not everything is calculable

As the cargo officer of a ship, the Chief Mate is responsible for correct stowage. This is complex work since a vessel is never completely discharged – heavy containers that are to be unloaded in the next port cannot be stowed below deck. In addition, certain cargo may not be stacked on top of each other. In the case of containers with dangerous goods, minimum spacing from other containers has to be maintained. Furthermore, containers may arrive late at the port, making restacking necessary at short notice.

The cargo officer must take this into consideration when he calculates the departure and arrival stability of the ship with the cargo computer according to international stability criteria and on the basis of the containers to be loaded and discharged, which he is informed about by the agent in advance via the stowage plan, including weight and port of discharge of each individual container. The vessel may not depart until this has been done. The Hansa Mare Reederei uses the EASECON programme for this purpose.
However, the cargo computer has to be supplied with reliable data. To ensure containers from exceeding the weight specified in the stowage plan, many gantry cranes at terminals are now equipped with weight indicators so that an impermissibly heavy container is noticed immediately. During the loading operations the cargo officer personally makes sure that the containers are at the right place. If a 30 tons-container is in the upper section of a stack instead of in the lower section, this imposes an uncalculable risk of damage. However, even a container that is correctly positioned based on its weight can become a risk in a storm if it has not been stuffed properly by the shipper and nothing inside can shift.

Given stack heights of up to eight layers on deck, the technical scope of shipping companies and lashing gear manufacturers has nearly been exhausted. Due to high stowage, increased lifting forces occur when the ship rolls and high compressive loads act on the bottom deck container. For this reason only empty containers can be carried in the upper layers in most cases so that the centre of gravity of the vessel is not shifted upwards too far.

To be able to transport even higher stack weights, containers would have to be reinforced accordingly. The sector is already working on solutions. Additional lashing devices, such as lashing bridges that are one to two layers enabling to lash the containers in the third and fourth layer with a cross lashing, would be a tentative solution. In any case a container stack depends on meticulous and regularly monitored lashing.

top

Archive ON BOARD

  • Change through Trade
    (ON BOARD 66)
  • Safety First!
    (ON BOARD 65)
  • A Power Plant on the Ship
    (ON BOARD 64)
  • Preparation for Growth
    (ON BOARD 63)
  • A place to anchor
    for Seafarers
    (ON BOARD 62)
  • Low Sulphur Regulation
    (ON BOARD 61)
  • The Course is set for 2010
    (ON BOARD 60)
  • Challenge of Ballast Water
    (ON BOARD 59)
  • With the current
    (ON BOARD 58)
  • Tackling the Challenges
    (ON BOARD 57)
  • EU approaches Arctic
    (ON BOARD 56)
  • Integrated Terminal-Ship System
    (ON BOARD 55)
  • Precision Work – Stable Stowage
    (ON BOARD 54)
  • New Dual Leadership
    at Hansa Mare
    (ON BOARD 53)
  • Drive for new Drive
    (ON BOARD 52)
  • Lubricating systematically “Pulse Lubricating System”
    (ON BOARD 51)
  • ON BOARD – the 50th!
    (ON BOARD 50)

 

Back to Corporate News