The LiquefAction project is expected to provide a better understanding of cargo liquefaction, and develop recommendations for ship design and operations that avert or mitigate future casualties
Over the past decade liquefaction has been linked to the loss of more than a dozen bulk carriers and over[ds_preview] 90 seafarers. The Allianz Global Safety and Shipping Review 2015 identifies the topic as one of the »key risks to the future safety of shipping«.
The priority being given to the phenomenon by shipping is demonstrated by the joint R&D Project for Bulk Carrier Safety – LiquefAction, whose partners include Hamburgische Schiffbau-Versuchsanstalt (HSVA), Hamburg University of Technology (TUHH) and key German owner Oldendorff Carriers. ClassNK, the world’s leading classification society in the dry bulk sector, is also taking part, alongside Ecole Central de Nantes (ECN), and the Institute of Science and Technology for Transport, Development and Networks (IFSTTAR).
The project’s outcomes are expected to benefit the bulk carrier sector as a whole, and are expected to feed into relevant safety guidelines such as ClassNK’s Guidelines for the Safe Carriage of Nickel Ore.
Although Intercargo has termed nickel ore »the world’s most dangerous cargo«, the liquefaction project examines the assumptions that lie unchallenged behind such statements. For example, ships go down quickly and catastrophically as a result of liquefaction, but do existing regulations really describe the conditions that cause cargoes to act under liquefaction? And would their full application avoid such circumstances?
Cargo profiles may differ regionally and even from within the same mine. Sampling methods may also be due for review. Thus, the condition of cargo on loading and the loading process warrants close attention.
Captain Paul Jeffrey, of Lübeck-headquartered Oldendorff Carriers comments that typical geared vessel will see about 4,000-4,500 grab loads, dropping 10-15t of cargo from a height of 10-15m above the cargo. This has the negative effect of imparting energy into the cargo, liberating water, and driving out air.
To help mitigate these effects, best practices for loading such cargoes includes using a ›soft drop‹ method, according to Oldendorff. Further research may help define the real consequences of such compaction, and the safety benefit of implementing ›soft drop‹ loading techniques.
Ship motions and the frequency range and amplitude that cause liquefaction of a given cargo with given moisture content over specific time periods are central areas of study within LiquefAction. Modelling the phenomenon’s effect on stability by taking into account dynamic behaviour is also critical. Both factors will contribute to qualifying and quantifying preventive and mitigating measures in ship design and operation.
Captain Jeffrey of Oldendorff Carriers comments: »We need to look at the puzzle from a forensic viewpoint, working backwards and involving not just academic science but also empirical science of natural moisture redistribution downward within the cargo column. Empirical evidence would therefore suggest there are factors at play not being considered in our general understanding of liquefaction.« He hopes the ship and cargo modelling that the LiquefAction research will define can be used to help simulate and explain the empirical observations reported by survivors of real casualties.
The first and second editions of ClassNK’s guidelines on nickel ore include the precondition that cargo should not be loaded with moisture content over the defined transportable moisture limit (TML). However, the guidelines also include warnings on measurement errors, environmental conditions during a voyage and other factors that could cause liquefaction even if the moisture content is less than the TML.
Amendments to Section 4 of the International Maritime Solid Bulk Cargoes (IMSBC) Code, which became mandatory from 1 January 2015, require the shipper to provide a certificate, signed by a port state-recognized organization which is clearly stating the TML of a cargo provided that the cargo is categorized as Group A, which is defined as »cargoes that may liquefy«.
Vessels can carry cargoes subject to liquefaction even if the moisture content is above the TML, as long as the ship is »specially constructed or fitted to carry the cargo, and if evidence of approval by the administration is stored on board the ship«.
The second edition of ClassNK’s guidelines on nickel ore include the world’s first hull structure and stability requirements for »Specially Constructed Cargo Vessels«, released as part of revised guidelines in February 2012. The requirements have since been approved by Panama, Japan, the Marshall Islands and Liberia, being also recognized by Intercargo.
However, in the interest of safety, some in the industry have called for re-evaluation of low risk cargoes, such as bauxite, which remains a ›Category C‹ cargo as defined under the IMSBC Code – the least dangerous category from a liquefaction point of view. Allianz also raises the point »whether the list of cargoes in the A, B and C categories in the IMSBC Code needs to be reassessed.«
Action by regulators to sharpen definitions on or reclassify cargoes within the IMSBC Code will surely be welcome. However, the LiquefAction project is based on the premise that they do not provide an exhaustive response to the root causes of the phenomenon. It is surely worthy of note that accidents have occurred with vessels carrying screened bauxite products, effectively outside the standard run-of-mill grade envisaged when bauxite was originally introduced into the BC Code – the predecessor of IMSBC Code.
»Despite the positive steps that have been taken towards prevention of shipping accidents, there is still much more to be done,« says Yasushi Nakamura, Representative Director and Executive Vice President of ClassNK. »A greater level of support and guidance is needed across the board to ensure safety, and ClassNK is convinced there are issues that need to be addressed in a holistic and comprehensive manner.«
Captain Jeffrey says that the various mechanisms which can lead to cargo instability, which are currently »lumped together as liquefaction«, may not necessarily be the same. Slope failure or free water, for example, have specific dynamics. »We cannot conclude necessarily that ores with moisture content below TML will not, in fact liquefy; there being other engineering principles to consider.«
The phenomenon of ›expressed water‹ at the cargo’s surface, whether it be from pore pressure alone and/or some other mechanism, has been reported since the 1960s, but is still not fully understood, Captain Jeffrey says. »Should we be able to predict the propensity for expressed water over a wide range of cargoes, then perhaps we’d have a better understanding of the ›liquefaction‹ risks that may in turn help explain why we see variances.«
According to HSVA, LiquefAction is addressing both design and operational vessel perspectives, »based on extensive experience and accident data, numerical modelling and simulation concerning the behaviour of granular cargoes in various modes of motion«.
Captain Jeffrey is also hopeful that the LiquefAction project might provide further insight into the consequence of a dense surface level slurry occurring with or without a substantial sea state.
One plausible theory might suggest an instability that is initiated by a surface level slurry (liquefaction) sufficient to cause free surface and ensuing »angle of loll« to be developed. The subsequent regaining of neutral or even positive stability at this large angle may be supported by observed accounts. At these large »angles of loll« the free surface effect is significantly reduced with the slurry now finding itself nestled within the »v« shaped wedge between the ship side boundary and the cargo’s surface. The weight of the slurry is now to one side and although the free surface is drastically reduced, the resulting displaced weight maintains the permanent list. This comprises the first of a two-part rolling motion that may merit further study.
»Similarly, considering conventional models of liquefaction, we also need to investigate how a dense viscous medium may have sufficient mass and momentum to cause a large angle of list, the angle of loll, yet not enough to fully roll the vessel in one continuous motion,« says Captain Jeffrey. Continuing this theory and its perceived »two-motion roll scenario«, is that the initial large 45 degree list subsequently will eventually cause the cargo to »avalanche« at some point in time. This mass cargo movement is suspected of dealing the »death blow«, rapidly capsizing the vessel in mere seconds, seemingly corresponding with reports by survivors.« This »secondary roll« is known to occur within minutes or up to several hours after the initial instability.
»The proposition that cargo liquefies top-to-bottom or perhaps due to a wet base is itself questionable,« Capt. Jeffrey says. »The true value of the project will be seen when science based on empirical evidence questions the assumptions used in the standard liquefaction model. There’s just simply a lot we don’t understand as yet.«
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