In-depth | SHIP STABILITY
However, there is evidence that in some
cases, the data shown on the cargo certificate may not be accurate for the actual physical properties of the material loaded into the hold. Errors might come from genuine mistakes in sampling, the fact that the cargo might originate from several places and it is not homogenous, or even that heavy rain between sampling and loading has changed its physical characteristics. Tere is also the possibility that the certificate provided has purposefully been produced to demonstrate compliance with the IMSBC Code but in reality is not from the cargo being loaded. Te significant commercial pressure that the ship and her Master are under to ship the dangerous cargo without a proper certification has been widely reported, with stories of crew’s being threatened if they did not accept the cargo. So, given that we have seen the same types
of incidents regularly each year, what should be done to stop what is an entirely preventable loss of life? Clearly for a long term solution, it is better to
treat the cause (cargo with too high a moisture level) rather than the symptoms (a dramatic loss of stability), but we as naval architects have a role to play in finding a solution to ensure safe ship operations. Aſter all, many of these ships are well into their service lives and have long since forgotten the ‘drawing board’ but we have a responsibility for operational improvements as well as the initial vessel design. Te following does not intend to solve the
issue by itself, nor is it an exhaustive list but hopes to provide some points and ideas that might be explored to help solve this issue: A better understanding of stability: Ship’s
crew’s are taught ship stability as it is critical to their safety, but how many really understand the implications of free surface effects. It is the author’s view that a more rigorous programme of improving the understanding of ship stability and free surface would help. With the focus on the risk of free surface effects, the crew should be able to make the decision not to accept a cargo if they are concerned about its tendency to liquefy. Tis programme needs to be industry wide, worldwide and led by ‘Class’, P&I Clubs and nautical colleges. Longitudinal hold division: As with grain
cargoes, there may be possibilities to put longitudinal hold divisions in place to reduce the free surface effects. However, this needs careful consideration due to the trading nature of many bulk carriers where different cargoes
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GZ curves demonstrating free surface effects of cargo shifting.
may be carried and hence whether such division is permanent or temporary will need to account for the practicalities of loading and discharge. Hold Pumping Systems: There are bulk
carriers that trade loading slurries or water / mineral mixtures. Te cargo is loaded and then the water is distilled down through the hold and pumped out to leave a dry cargo.
It does not remove the free surface
effect altogether but provides a means of removing the water once loaded. However, it is not without its problems as evidenced by the Taharoa Express incident in New Zealand, June 2007 . There are of course, more operational
solutions ranging from the simple (in theory) such as covering up of the cargo on the dockside to protect it from the elements, through to the more involved such as insistence on independent cargo surveys to verify and oversee the moisture testing of appropriate cargo samples. Te tragic loss of life that took place in
late 2010 demonstrates how serious an issue cargo liquefaction is. Mineral ores and slurries can contribute to major stability problems and their transportation needs to be treated with special care. An enhanced regulatory framework is now in place and ship’s crews, ship owners, ship managers and the cargo shippers need to fully understand the risks and implications and each be strong enough to resist the commercial pressure that will, undoubtedly continue but must not be at the expense of seafarers lives. NA
Simon Burnay, BMT Marine & Offshore Surveys.
References: 1. “Bulk cargo liquefaction (iron ore fines and nickel ore)”, Standard Cargo, Charles Taylor & Co. Ltd, February 2011.
2. “Why loading flowing mud ends up in capsizing…the danger of wet iron ore fines ex India” by Willem van Renterghem, IUMI 2010.
http://www.taic.org.nz/MarineReports/ tabid/87/ctl/Detail/mid/484/ InvNumber/2007-207/Page/0/Default. aspx?SkinSrc=[G]skins/taicMarine/skin_ marine
The Naval Architect April 2011
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