Often cargo planners who are preparing the stowage of steel coils in the cargo hold of a general dry cargo ship or bulk carrier, do not have the necessary cargo type specific information required to help them decide the permissible cargo load, thus preventing damage to the ship’s structure.
As Jan Rüde, Ship Type Expert MPV, DNV GL explains, according to SOLAS Chapter VI, Reg. 5, every ship must have an approved cargo securing manual.
Nonetheless, the majority of these manuals do not include detailed information about the carriage of steel coils or the only contain only particular types of steel coil.
This fact leaves it upon the loading planner to determine the loading limits, and some planners simply divide the coil weight by the coil surface area and compare the resulting pressure acting upon the tank top with the allowable uniform tank top load (t/m²).
This approach fails to account for the true forces acting upon the ship’s structure when carrying steel coils. The result could be a deformed inner bottom and floor buckling.
Steel coil loads are not uniform
Regarding steel coils, they are loaded with their axis pointed in the ship’s longitudinal direction.
They are located on wooden dunnage arranged in transverse direction. This dunnage creates a protective layer between the coils and the inner bottom plating.
The weight of the steel coils is then transferred by the dunnage onto the bottom structure of the vessel, which incorporates the inner bottom plating, the longitudinal elements, the double bottom girders and the hull girder structure.
Moreover, the force of this weight is not distributed correctly across the tank top.
Instead it acts upon the inner bottom structure of the vessel as concentrated, short line loads.
This leads to the fact that the permissible uniform distributed load information in t/m² as provided in the cargo securing manual cannot be used as a basis for concluding in the maximum load when carrying steel coils, Mr. Rüde adds.
Strength information for cargo planners
Steel coils come in many sizes and weights, and can be arranged in various ways regarding the placement of the locking coil, the number of tiers, and the dunnage.
The cargo planner has to have reliable information to decide fast if a certain type of steel coil can be transported by the vessel.
If this is possible, then the planner must figure out how the cargo should be stowed to ensure safe carriage.
For this reason, the planner should set out the maximum cargo capacity and the minimum requirements for safe stowage without overestimating the necessary number of dunnage elements.
When a ship is moving, its vertical acceleration varies along the length of the cargo hold. Thus, the position of coils in the individual holds of a vessel must be accounted for when calculating loading tables, Jan Rüde concluded.