Supporting safe steel coil transport
Cargo planners preparing the stowage of steel coils in the cargo hold of a general dry cargo ship or bulk carrier often lack adequate cargo-type-specific information which they need to determine the permissible cargo load and avoid damage to the ship structure.
According to SOLAS Chapter VI, Reg. 5, every vessel must be provided with an approved cargo securing manual. However, most of these manuals do not contain detailed information regarding carriage of steel coils or only particular types of steel coil. This leaves it up to the loading planner to find a way to determine the loading limits. Sometimes 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
Steel coils are loaded with their axis oriented in the ship’s longitudinal direction. They are placed on wooden dunnage arranged in transverse direction. This dunnage ‒ pine planks typically 100–150 mm wide and 30 mm thick, and pine wedges ‒ forms a protective layer between the coils and the inner bottom plating. The weight of the steel coils is transferred by the dunnage onto the bottom structure of the vessel, which consists of the inner bottom plating, the longitudinal elements, the double bottom girders and the hull girder structure.
The force of this weight is therefore not distributed uniformly across the tank top but acts upon the inner bottom structure of the vessel as concentrated, short line loads. This means that the permissible uniform distributed load information in t/m² as provided in the cargo securing manual cannot be used as a basis for determining the maximum load when carrying steel coils.
Reliable strength information for cargo planners
Steel coils come in many sizes that differ in weight, diameter and length. They can be arranged in a variety of ways with regards to the placement of the locking coil, the number of tiers, and the dunnage. The cargo planner needs reliable information to determine quickly whether a given type of steel coil can be transported by the vessel at all, and if so, how the cargo should be stowed to ensure safe carriage. The planner must establish the maximum cargo capacity and the minimum requirements for safe stowage without overestimating the required number of dunnage elements.
DNV GL’s ship structure rules, specifically those in DNVGL-RU-SHIP Pt. 5 Ch. 1. Sec. 3, contain strength calculation formulas for steel coil transport. Cargo planners can use these formulas to calculate many different combinations of the input parameters for steel coil transport as specified above, and to determine in advance whether the inner bottom structure of the ship can carry a specific coil arrangement. What is more, DNV GL supports its customers by performing strength investigation calculations and preparing ship-specific reports which complement the cargo securing manual. Based on customer input, such a report states the structural strength and load-carrying capacity of a given vessel’s inner bottom for a large variety of steel coil types. The result is an allowable steel coil loading table which specifies under which circumstances certain coils can be carried safely.
Optimizing the capacity
When a ship is in motion, its vertical acceleration varies along the length of the cargo hold. Therefore the location of coils in the individual holds of a vessel must be accounted for when calculating loading tables. To optimize the coil-carrying capacity DNV GL prepares a table for each hold to factor in cargo locations subject to lower acceleration. The calculations can be carried out for general dry cargo ships according to DNV GL’s rules, or for bulk carriers based on Common Structural Rules. Allowable steel coil loading tables can be prepared for newbuilds or vessels in service.
This service from DNV GL provides loading planners with access to a class-approved document that gives clear guidance on permissible carriage of steel coils. Covering the full range of possible steel coil weights and dimensions, the tables are easy to understand and use.