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Supporting safe steel coil transport - DNV

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Supporting safe steel coil
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.
5 Minutes
20 February, 2019
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
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.
Deformation of inner bottom structure due to improper steel coil loading.
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.
Schematic (side view) representing a typical steel coil cargo arrangement using
wooden dunnage.
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 shipspecific 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.
Example of an allowable steel coil loading table prepared by DNV GL.
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.
DNV Expert
Jan Rüde
Ship Type Expert MPV
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