In-depth | SHIP STABILITY Dealing with dry cargo liquefaction
Liquefaction of cargo is a widely recognised phenomenon that has been implicated in a number of recent casualties which have resulted in significant loss of life. Daria Cabai and Simon Burnay of BMT Marine & Offshore Surveys highlight the issues that surround cargo liquefaction and suggest how naval architects could assist in addressing the problem.
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n recent months there has been a spate of bulk carrier related casualties that have involved vessels either sinking or
experiencing significant stability problems. Within the period of 39 days, three bulk carriers sunk and 44 seafarers lost their lives. All the vessels involved were carrying bulk mineral ore and although the investigations of these casualties are on-going, the liquefaction of the cargoes with the resulting free surface effects and loss of stability has been identified as a likely cause. Table 1 summarises the recent incidents: All three vessels were reportedly carrying
the same cargo (nickel ore), were loaded in the same country and were bound for China to deliver the nickel ore for use in the steel industry. Similar incidents have occurred in previous years with iron and nickel ore cargoes from India, The Philippines and Indonesia. Ships have long carried liquid cargoes and
consequently vessels dedicated to the transport of liquids are designed accordingly. The problem arises when a ship designed to carry a dry bulk cargo suddenly finds itself carrying a bulk liquid as well. So how does an apparently dry cargo turn
into a liquid? Liquefaction of cargo can affect many
types of material being transported in bulk. Iron ore, nickel ore, coal slurry, sand slurry, and other wet minerals or fines are all susceptible. Liquefaction is a particularly dangerous issue as it turns what appears to
Vessel Jian Fu Star Nasco Diamond Hong Wei 16
has a detrimental effect on ship stability which can lead to capsize. Another issue that makes liquefaction of cargo so dangerous is the rapid change from the stable to the unstable condition with little warning, potentially giving crews very little time to abandon ship. Te cause of cargo liquefaction is not a
Daria Cabai, BMT Marine & Offshore Surveys.
new problem. Current regulations therefore reflect this in terms of transport, loading and shipping. Te International Convention for the Safety of Life at Sea (SOLAS) provides general guidance on the carriage of all cargo types including dangerous cargoes. Recently enforced regulations - International Maritime Solid Bulk Cargoes (IMSBC) Code (which superseded the Code of Safe Practice for Solid Bulk Cargoes or “BC Code”) - provides specific requirements for the carriage of a range of bulk cargoes. With the problem of this liquefaction
be an apparently safe cargo into something that can have a significantly detrimental effect on a vessel’s stability. Te mechanism that causes liquefaction is oſten triggered by a ship’s motions. It can also be affected by other cyclical loads such as vibration. Bad weather will therefore exacerbate the effect. Liquefaction occurs when the combination
of cyclical movement and vibration compacts the spaces between cargo particles. Tis causes the loss of inter-particle frictional force to the point where the cargo can behave like a liquid. Te resultant cargo shiſt and free surface effect
Date of Incident 27/10/2010 10/11/2010 03/12/2010 Loss 13 fatalities 21 fatalities 10 fatalities
of certain cargo types in sharp focus, the International Group of P&I clubs and the Association of Dry Cargo Ship owners (INTERCARGO) both expressed their concerns about the risks that those cargos present at the last International Maritime Organisation (IMO) Maritime
Safety
Committee (MSC). Ironically, with the IMSBC coming into force on 1st January 2011, the shipper is now obliged to provide information on the cargo in advance and the Master of the vessel is entitled to refuse the cargo if the certification is not adequate or if, he/she is not happy with the information provided. The IMSBC code specifies that Group A bulk cargos that are liable to succumb to liquefaction must be accompanied by a certificate specifying the transportable moisture limit (TML), which is calculated as 90% of the flow moisture point (FMP). Te reality of complying with this is that each cargo to be loaded should be subject to sampling and testing to identify key physical properties in a controlled environment such as a testing laboratory.
The Naval Architect April 2011
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