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Liquefaction, reaction

The Naval Architect: September 2015

Norwegian and German classification society DNV GL has responded to the frequency and seriousness of related disasters with a new guideline: “Design and operation of vessels with bulk cargo that may liquefy.” In it, they explain the physics behind the phenomena, susceptible materials including bauxite (IMSBC Code Group C), and provide ways to mitigate such dangers for conventional bulk carriers and ore carriers.

Cargo sliding and liquefaction has resulted in the listing, capsize, and structural damage of old and recent bulk carriers. Since 2009, at least six ships of more than 40,000dwt have been lost to suspected liquefaction of cargo. This period includes a six week stretch in which three bulk carriers, Jian Fu Star, Nasco Diamond and Hong Wei, were capsized while transporting nickel ore from Indonesia during the rainy season of 2010, killing 44 seafarers. Recently, the 55,652dwt Alam Manis, owned by Malaysian Bulk Carriers, ran aground after the nickel ore it was carrying shifted in the hull and created a severe list of 20degrees.

Measures are already in place to prevent further vessel losses, but the new DNV GL guideline suggests more should be done; cargo liquefaction is still the “single most significant factor for life lost at sea for bulk carriers,” says DNV GL’s Morten Løvstad, business director of bulk carriers and Håvard Helling, bulk carrier ship type expert. ClassNK has also responded to the ongoing loss of life, putting its weight behind research and change to further safe-guard vessels and personnel in a joint industry R&D project for bulk carrier safety: LiquefAction. The major ore exporting countries Australia and Brazil have also been pro-active in responding to this issue.

DNV GL’s two-tiered strategy identifies the different hazards impacting conventional bulk carriers and ore carriers, and offers individuated responses. Conventional bulk carriers (which have no longitudinal bulkhead) are shown to be more susceptible to stability issues when carrying cargo at risk of liquefaction. This is for two reasons: the transported material is generally quite dense, occupying a small part of the cargo hold volume, and conventional bulk carriers are quite wide, leaving a significant volume of space for the liquefied cargo to move around in.

While it is difficult to design a vessel for general duties that can also transport ore materials, it can be done, according to DNV GL experts. The drawback lies with lost capacity and cost; “According to DNV GL experience, arranging longitudinal bulkheads to narrow the holds is the only feasible way of obtaining sufficient stability to withstand cargo liquefaction for a conventional bulk carrier,” making the vessel less commercially attractive for general trades. Ore carriers are less affected by stability issues when under the free surface effect because their longitudinal bulkheads limit the width of the cargo hold. However, they face increased strength issues when compared with conventional bulk carriers. This is due to the higher filling level of their cargo hold and the fact the cargo hold boundaries are not designed to withstand flooding. The guideline states that the lateral pressure load is significantly increased on the longitudinal bulkheads’ plates and stiffeners in this situation, and normally means the scantlings should be increased.

Similarly, scantlings should need to be increased for the plates and stiffeners on the lower stool in way of the transverse bulkhead. The strengthening of the transverse corrugated bulkheads depends on how a vessel’s loading conditions have been designed. For example, if a vessel is designed for homogenous loading, it must be checked for the worst condition. Ship owners and operators should also be aware that low density cargoes with a higher filling height exert a larger total force on the corrugations, according to the guideline.

DNV GL has suggested strengthening ore carriers by the reinforcement of bulkheads with a higher percentage use of high tensile steel. According to Helling, past concern for high tensile steel fatigue has been quelled due to improved fatigue control, and this build process keeps the steel weight, and therefore costs, lower, making it more viable.

The current International Maritime Solid Bulk Cargoes (IMSBC) Code defines cargoes of particular properties, splitting cargoes that may liquefy into Group A, cargoes with chemical hazards into Group B, and cargoes that have neither of those properties into Group C. In doing so, the organisation has aimed to control the transportation of bulk cargoes and make the process safer.

However, as with any safe system, its success depends upon the rigour of those operating within it. Inaccurate cargo declarations in the form of unreliable moisture content readings and the confusing of cargo identification and the identification of the correct cargo group have shown regulatory shortcomings and why ship design could be a saving grace. In addition, up until 1 January 2015, the IMSBC Code stated that liquefaction did not occur when the cargo contained very small particles, something that has now been removed from the Code.

The new guideline recommends a tightening of operational procedures, especially in relation to conventional bulk carriers. DNV GL suggests a number of measures including: always making sure the cargo is correctly identified and proper documentation of the cargo is received before loading; using an independent surveyor or cargo specialist to assess TML; and carefully considering the metacentric height of the vessel when carrying cargoes that may liquefy. It also suggests that cargoes that may slide should be trimmed reasonably level, thereby improving their stability and weight distribution. However, this may increase the time and cost of loading. 

The nature of bauxite is also demystified in the new guideline, which quotes Intercargo’s secretary general, David Tongue: “What is normally considered a Group ‘C’ cargo may have the potential to behave like a Group ‘A’ cargo when that cargo’s specified characteristics are not maintained, especially when cargoes with higher levels of fines and moisture beyond those specified in the IMSBC Code are presented.” This is the case with a fine-particled bauxite cargo, and is exemplified in the sinking of Bulk Jupiter, an 8-year old Supramax bulk carrier believed to have capsized after the liquefaction of its bauxite cargo on 2 January 2015.

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