Container ships travel on round-robin routes where at each port of destination containers may be loaded and additional containers destined for subsequent ports may be loaded. Determining a viable arrangement of containers that facilitates this process in a cost-effective way makes up the container stowage problem.
Stowage planners determine stowage arrangement for a container ship. The planners ascertain the placement of containers so that following constraints are satisfied.
- Restrictions placed upon where and how containers can be stowed and
- Material handling costs i.e. costs associated with loading, unloading and transporting cargo are minimized. The most important aspect is the re-handle. A re-handle is a container movement made in order to permit access to another or to improve the overall stowage arrangement.
In a container ship, a stowage plan is prepared as per the container to be discharge and loaded on a particular port along with the tanks condition i.e. mass carried by the ship other than cargo. This is done to maintain the stability of the ship at all times.
Objectives when cargo is stowed in the ship,
- To protect the ship.
- To protect the cargo.
- To obtain the maximum use of the available capacity of the ship.
- To provide for rapid and systematic discharging and loading.
- To provide for the safety of crew and shore men at all times.
Key issues to be considered when loading cargo container on ships,
- Over stowage should be avoided and cargo planning to be done as per the latest cargo, i.e. cargo for a later port should not be placed over that of an earlier port.
- Loading conditions must be calculated for intact stability, shearing force, bending moment, torsion moment, trim and draft etc. Torsion moment, bending moment and shear force values must not exceed 100% at any time.
- The IMO visibility line should be taken care of when planning the stowage of containers on deck.
- The stowage of IMDG containers to be done as per ships Document of compliance with the special requirements for ships carrying dangerous goods
- The GM value is affected (increases/decreases) by means of stowing light containers on top of heavy containers respectively and vice-versa. GM is the Metacentric height which is the distance between the center of gravity of the ship and its metacentre.
- The GM is responsible for deciding the stability factor of the ship. In a low GM situation, it is preferable for light containers to be stowed on top.
- However, usually the GM values for ship are high and stowing light containers on top of heavy ones will only increase GM leading to a “stiff” ship with short rolling periods, which increases the stresses on the lashing.
- In this situation, it is preferable for the heavy containers to be loaded on top but with due regard to lashing stresses and staking weight.