When bulk cargo becomes liquid
The IMO’s IMSBC Code addresses the safety threats resulting from cargo liquefaction on board bulk and ore carriers. A new DNV GL class notation defines appropriate stability and hull strength requirements for specialized ships.
Transporting large quantities of bulk cargo can be a tricky affair: depending on its physical properties, granularity and water content, the material may change its behaviour under the influence of ship movements and vibration and adopt the properties of a liquid. This means that it will develop the so-called free-surface effect and begins to move similar to a liquid in response to the vessel’s attitude, which can cause the ship to lose stability and capsize. Between 2005 and 2015, cargo shifting and liquefaction caused eleven fatal ship accidents with 102 human lives lost at sea. Cargoes that are especially affected by this phenomenon include iron ore fines, nickel ore and bauxite, when the water content exceeds the transportable moisture limit (TML).
Another challenge associated with liquefied cargo is the hull strength. The weight of the compacted cargo can put extreme pressure loads on the sides of the cargo hold that can exceed the yield strength of the structure.
Containing the free-surface effect
The IMO addresses these issues in its International Maritime Solid Bulk Cargoes (IMSBC) Code which became mandatory in 2011. It requires cargoes with a moisture content above the TML to be carried on board specially constructed cargo ships (SCCS) that are to be approved by flag administrations. DNV GL’s new class notation BCLIQ, published in January 2018 and due to enter into force on 1 July 2018, interprets the IMSBC Code and specifies stability and strength requirements for these ships. It is mandatory for SCCS Ships but may also be applied on a voluntary basis to any other bulk or ore carrier. It has already been confirmed by major flag states that the set of requirements in the class notation complies with the IMSBC Code.
The BCLIQ notation confirms that the vessel is built to minimize the free-surface effect, for example by featuring wide wing tanks and reinforced structural focus areas on the cargo hold side walls, and is consequently able to carry cargoes with a moisture content exceeding the TML.
The notation confirms compliance with the SCCS requirements of the IMSBC Code and enhances the ship’s loading flexibility, efficiency and safety. There are two versions of the class notation: BCLIQ(Cat1) and BCLIQ(Cat2), distinguishing between cargoes that resettle in a stable condition after becoming liquefied, such as iron ore fines, and those that stay liquefied, including nickel ore, bauxite fines and similar materials. The latter are subject to stricter requirements.