Marine Surveys: Understanding and Managing the Liquefaction of Bulk Ores

Phil Thompson outlines the problem of liquefaction and discusses how better understanding of the problem has led to improved risk mitigation strategies.

II_Phil ThompsonLiquefaction of bulk ores has been identified as the root cause behind the loss at sea of five vessels and 80 lives in recent years. While all these cases have involved cargos of nickel ore, there is a growing concern that other bulk cargos including coal have the properties to cause liquefaction-related incidents.  There is therefore a pressing need for better understanding of how different cargos influence the phenomenon.

Phil Thompson, BMT Group Sector Director for Transport outlines the problem of liquefaction and discusses how better understanding of the problem has led to improved risk mitigation strategies that can help deliver far greater levels of confidence and safety.

In recent years there has been a spate of bulk carrier related casualties that have involved vessels either sinking or experiencing significant stability problems. There were 35 bulk carrier losses between 2010 and 2013 with the loss of 100 crew members. At least seven of the losses were due to liquefaction of the cargo. Within an infamous period of 39 days in 2010, three bulk carriers sunk and 44 seafarers lost their lives. All three vessels involved were carrying bulk mineral ore and the liquefaction of the cargoes with the resulting free surface effects and loss of stability was identified as the root cause. So far this year there have been two losses but they were unrelated to cargo matters and mercifully, no crew members were killed.

Table 1 summarises a selection of incidents since 2010:

Vessel

Vessel Details

Date of Loss

Loss of Life

Voyage

Jian Fu Star

1983

27/10/2010

12

Sank In The South China Sea 90 Miles Southwest Of Cape Eluanbi, Taiwan

Nasco

Diamond

2009

10/11/2010

20

Developed list to port,

took water and sank

In The Pacific Ocean.

Hong Wei

2001

03/12/2010

10

Capsized and sank in the South China Sea.

n/a

n/a

2011

7

n/a

Vinalines

Queen

2005

25/12/2011

22

Developed 18 degree list and eventually

sank In the Philippine

Sea.

Harita Bauxite

1983

17/02/2013

15

Sank off Cape Balinao in the South China Sea carrying 47,450 metric tons of nickel ore

Trans Summer

2012

14/08/2013

n/a

Sank en-route from Indonesia to China with a cargo of nickel ore

 

 

Jian Fu Star, Nasco Diamond, Hong Wei, Harita Bauxite and Trans Summer were reportedly carrying nickel ore, loaded in the same country and were bound for China to deliver the cargo 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.

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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 be an apparently safe cargo into something that can have a significantly detrimental effect on a vessel’s stability. The mechanism that causes liquefaction is often 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. This causes the loss of inter-particle frictional force to the point where the cargo can behave like a liquid. The resultant cargo shift and free surface effect 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.

The cause of cargo liquefaction is not a new problem. Current regulations therefore reflect this in terms of transport, loading and shipping. The International Convention for the Safety of Life at Sea (SOLAS) provides general guidance on the carriage of all cargo types including dangerous cargoes. The International Maritime Solid Bulk Cargoes (IMSBC) Code which became mandatory on January 1, 2011 and was subsequently amended on January 1, 2013 provides specific requirements for the carriage of a range of bulk cargoes.

The International Group of P&I clubs and the Association of Dry Cargo Ship owners (INTERCARGO) were both instrumental in ensuring that the International Maritime Organisation (IMO)Maritime Safety Committee (MSC) was made aware of the risks associated with liquefaction. The result is that the shipper is 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). The 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.

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. There 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.  The 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 anecdotal stories of crew’s being threatened if they did not accept the cargo.

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 there is an incumbent responsibility to mitigate the risk whenever reasonably possible on order to ensure safe ship operations. Although many of these ships are well into their service lives, there is a responsibility from a naval architecture perspective to implement operational improvements as well as validating the initial vessel design.

Other risk mitigation strategies include:

A better understanding of cargo characteristics: Focused by a desire to maintain Australia’s excellent safety record in exporting coal, the Australian Coal Industry’s Research Program (ACARP) and the Minerals Council of Australia has funded a substantial R&D effort through Project C24001 Transportable Moisture Limit for Coal. According to the Bureau of Resources and Energy Economics, Australia exported approximately 1.5 billion tonnes of black coal in the five years leading to 2012-2013 using the appropriate schedule previously contained in the BC Code and, from 1 January 2011, using the appropriate schedule contained in the IMSBC Code. This new project has been designed to develop fundamental understandings of coal cargo stability during shipping, including the potential for cargo liquefaction, and to use this knowledge to determine how best to assess the behaviour of coal cargoes during shipping. The ACARP project is investigating the behaviour of minus 50 mm coal cargoes because this is the material sizing of typical Australian exported black coals. In cases where Australian shipped black coal products are shown to have the potential to liquefy, the ACARP project will identify opportunities to modify the three existing Transportable Moisture Limit (TML) test methods specified in the IMSBC code, to ensure applicability to minus 50 mm coal products. The project will also investigate the potential for a new TML test. The outcomes will be compared to measurements using the cyclic tri-axial test used in soil mechanics studies to assess applicability.

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. This 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 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.  The 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[1].

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.

The 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 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 seafarer’s lives.



[1] New Zealand Transport Accident Investigation Commission - Investigation 07-207

 

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Contact Information

Phil Thompson

Goodrich House
1 Waldegrave Road
Teddington
TW11 8LZ
UK

Tel: 0208 943 5544
Email: enquiries@bmtmail.com