IMSBC- 镍矿砂装载法
2010.12.15
IMO 近日推出一份固体散装货装载法其中包括一些可能液化之矿砂如铜 (copper), 镍(nickel) 和Bentonite clay等. 这份新公约名叫IMSBC(International Maritime Solid Bulk Cargo) 它将于2011年1月1日生效. 替代 1980年之Codes of Safe Practice for Solid Bulk Cargoes (BC Codes). 公约中所提之程序和要求,船方必需依法执行.
The mandatory code falls under the provisions of the SOLAS Convention, replacing the Code of Safe Practice for Bulk Cargoes (BC Code). Come 2011, it has to be aboard every bulk carrier, and its requirements must be followed. 否则不能通过 PSC 检查.
鉴于近月来3艘运镍货轮之沉没, 特刊出一份镍矿砂石 (nickel ore) 装载法之英文原本以供参考.
Carriage of Nickel Ore
Introduction
Have you ever turned a bottle of ketchup upside down only to find nothing comes out, put the lid on, shaken the bottle and then swamped your plate with sauce? If so, you have experienced liquefaction. However, what you do not want to experience is cargo liquefaction. The consequences for your ship can be: • delays at the load port from rejecting cargo or problems with certification of cargo already loaded • stability problems on voyage from loss of metacentric height (GM) due to free-surface effect leading to a list, angle of loll or capsize
• delays at the discharge port or port of refuge making the ship safe and discharging a cargo in a fluid state. The definitions, tests and precautions in the International Maritime Solid Bulk Cargoes (IMSBC) Code for cargoes that may liquefy are widely associated only with metal ore concentrates, for which their application is relatively straightforward. But any cargo with fine material and moisture could potentially liquefy and should be queried with the shipper. The high price of minerals recently, has made some trading viable which would otherwise be uneconomic. One such trade is the shipment of unprocessed nickel ore from various remote islands in Indonesia and the Philippines. If the moisture content of the ore is too high then it can liquefy just like concentrates and display the same liquid behaviour. Serious problems have been experienced recently with ocean transport of these cargoes. The IMSBC Code certification requirements apply to nickel ore but the test methods do not always give well-defined results. Several laboratories have obtained widely differing results on samples supposedly representing the same cargo. This briefing describes in more detail the problems associated with liquefaction of nickel ore, and the difficulty in determining its moisture content and flow moisture point, information that is critical when deciding if it is safe to carry.
Liquefaction In its solid state the particles of the material are held together by friction and the cargo has the characteristics of a solid. Cargo on loading appears ‘normal’ – like slightly damp sand (see Figure 1). However, if there is sufficient moisture in the cargo, external agitation can increase the pore water pressure to the ‘flow moisture point’ (FMP), where water pushes the particles apart. The material then undergoes a sudden transition to the flow state where it loses the friction between particles. The cargo begins to behave
like a liquid.
International Maritime Solid Bulk Cargoes (IMSBC) Code Cargoes that may liquefy will contain moisture and at least a proportion of small particles. This includes a wide range of mineral cargoes other than concentrates, with widely differing physical and chemical properties. The IMSBC Code certification requirements apply to all cargoes which may liquefy regardless of whether or not the cargo is specifically identified as posing a liquefaction risk. Never assume there is no risk of liquefaction simply because a cargo is not identified as ‘Group A’ in the IMSBC Code. Transportable Moisture Limit (TML) Sections 4, 7 and 8 of the IMSBC Code deal with assessment of acceptability of consignments for safe shipment and production of test certificates showing the ‘transportable moisture limit’ (TML) and actual moisture content of cargoes. Any ship operator contemplating carrying fine-grained mineral cargoes should carefully read these sections of the IMSBC Code. On voyage the cargo can be agitated by wave impact and engine vibration and, if there is sufficient moisture present, the cargo will reach FMP and liquefy. This may result in loss of GM from free-surface effect, sudden cargo shifts and structural impact damage from sloshing. For this reason the master must be completely satisfied that testing has been carried out strictly according to the procedures set out in Appendix 2 of the IMSBC Code. Because of the severe consequences of exceeding the FMP, the safety margin provided by the lower TML is critical and should not be compromised. The TML is defined as 90% of the FMP. It is a requirement of the International Convention for the Safety of Life at Sea (SOLAS) that the average moisture content of any type of granular cargo in any cargo space must not be higher than the TML. The difference between the TML and the FMP is intended as a safety margin to protect against uncertainties in testing – such as laboratory errors, sampling errors and variations in moisture content in the cargo. Shippers must certify the TML and the moisture content of the cargo before start of loading. No cargo should be accepted for loading without valid certificates. If the actual moisture content at any location in the cargo is greater than the FMP then the cargo can liquefy at any time without warning. There are no ‘safe’ weather conditions or routings for carrying a cargo above its TML. If masters have doubts about the testing procedure and appearance of the cargo then they should conduct a ‘can’ test as described in the IMSBC Code section 8.4.
Nickel ore
The high price of minerals recently has made some trading viable which would otherwise be uneconomic. One such trade is the shipment of unprocessed nickel ore from various remote islands in Indonesia and the Philippines on long ocean voyages. These ores have relatively low nickel content and have been shipped on shorter voyages to Australia and Japan for many years. As with many finely particulate minerals, including mineral ore concentrates, these ores have the propensity to liquefy and shift if their inherent moisture level is too high. There have been several serious instances of cargo liquefaction of nickel ore, including total losses and near-misses.
Background
Nickel laterite is an inhomogeneous low-grade ore consisting of very fine clay-like particles and larger rock-like particles. There are two different types, limonite and saprolite, which differ in their chemistry and their physical appearance, but present similar problems in bulk shipping due to their high moisture contents.
The nickel ore in question is simply dug out of the ground, sorted for size, stored in stockpiles and then shipped. Apart from the drying effect of the sun – which is of unquantifiable benefit – there is no further processing involved.
Because of the way the ore is mined the composition and physical behaviour can differ greatly from mine to mine, from shipment to shipment from the same mine, and even within a single cargo. Assessing whether cargo is safe As described earlier, assessing whether a cargo is safe to ship requires the flow moisture point (FMP) to be measured and the transportable moisture limit (TML) to be calculated (90% of FMP). The TML is then compared to the moisture content of the cargo, and provided the TML is the higher figure, the cargo should be safe to load. There are problems with both the determination of TML (which for nickel ore needs to be determined by a competent laboratory separately for every single cargo) and moisture content (which must be of the cargo offered for shipment), which the IMSBC Code requires shippers to provide prior to commencement of loading. The ore is not found in a homogeneous form. Much of the material is very fine clay-like particles but there are also larger rock-like particles, some of which can be very large indeed. The FMP testing methods in the IMSBC Code have been developed with concentrates in mind and rely on uniform physical and chemical properties throughout the cargo. For cargoes that consist of a wider range of particle sizes – from rocks through pebbles to sand or soil-like material – the IMSBC Code tests become less reliable. It may not always be possible to certify the FMP of these types of cargo using the test procedures in the Code. It may also be difficult to find qualified laboratories that are willing to certify the FMP of materials other than concentrates.
Sampling of nickel ore
Various problems arise with sampling for moisture content and FMP testing – both of which are required to enable a reliable TML to be determined. Some problems stem from the actual manner in which the stockpiles are physically sampled. In a recent case, it was found that the mine did not routinely sample the stockpiles prior to shipment, but rather sampling was conducted during the course of loading.
As this was too late to comply with the requirements of the IMSBC Code, their practice was to present the master with information relating to the cargo loaded onto a previous unrelated vessel.
In turn, the results of the analysis of the cargo loaded onboard the subject vessel would then be presented to the next ship and so on. By the time the subject consignment had actually been characterised in terms of its suitability for carriage, it had already been loaded, making it more difficult to resolve any issues arising. The master would have been totally unaware of the fact that he was carrying a potentially dangerous cargo.
The shippers in this case (which is not exceptional in our experience) were in breach of the requirements of the IMSBC Code for a number of reasons. Firstly, the moisture content data on the cargo certificates related to a different cargo and not the actual one due to be carried. Secondly, the stockpiles intended for loading onboard the subject vessel had not been sampled in accordance with the requirements of the IMSBC Code. This details the frequency and extent of sampling for a given stockpile size, and states that sampling should be conducted no more than one week prior to shipment if the ore is stored uncovered - as most nickel laterite stockpiles are. Should there be significant rain between the time of testing and loading check tests should be conducted to ensure that the material is still in a safe state to load.
Moisture content determination The in-homogeneity of lateritic nickel ore means that the proportion of the fine clay-like and larger stone-like fractions in different samples can vary significantly. As the clay material typically has higher moisture content (30 – 50%) compared to the larger stony fraction (about 20%), the actual moisture content determined will be an average. As a consequence, the actual moisture content of the clay-like fraction, which is the one prone to liquefaction, will typically be higher than the declared value.
Preparation of samples for moisture content and FMP determination can be a lengthy process involving samples being spread out on a floor in hot environments. One can therefore expect moisture loss due to evaporation and contact with a dry surface. Although this is not critical for FMP determination (providing testing is carried out correctly), it will result in an underestimation of moisture contained in the actual cargo to be loaded, from which there will be no such moisture loss.
Flow moisture point testing Appendix 2 of the IMSBC Code provides three methods for determining the FMP of commodities. One of these methods, the flow table test (FTT), is the method of choice of the nickel ore mines. However, FTT was developed for measuring the FMP of relatively homogeneous mineral concentrates. The BC Code states that the method is primarily for materials with grain sizes up to 1mm, but “may also be applicable to materials with a maximum grain size up to 7mm”. The IMSBC Code also warns that the method may “not give satisfactory results for some materials with high clay content”. Lateritic nickel ore is inhomogeneous, comprising a mixture of fine and larger particles (> 7mm), and has a high clay content. This does not preclude the application of the method to nickel ore, but it does mean that great care is required in performing the test.
The FTT method involves preparing a sample on a flow table in the form of a truncated cone. The flow table top is then raised and allowed to fall sharply through a defined vertical distance. This simple procedure is repeated up to 50 times and the behaviour of the sample cone observed to see if “plastic deformation” has occurred. The construction of the flow table and the test methodology is described in great detail in the IMSBC Code. However, it is the experience of the authors that neither the set-up or test method described is being adhered to by the nickel ore mines, with the potential for inaccurate FMP and TML information being declared to the vessel. Some preliminary experiments carried out by MTD (Singapore) on limonite ore, and information gathered by both MTD and Brookes Bell on site, support these concerns.
Before we can address these issues, we first need to discuss another area of debate, the identification of plastic deformation.
Identification of a flow state
The IMSBC Code does not provide any definite criteria for identifying a flow state (Appendix 2, Section 1.1.4.2.3), but instead lists a number of physical observations that indicate plastic deformation, and suggests procedures for measuring this deformation. The physical signs include: ”moulded sides of the sample may deform”; “cracks may develop on the top surface” of the sample cone; “the sample cone begins to show a tendency to stick to the mould”; and there may be “tracks of moisture on the table” following the test. As regards measuring the extent of deformation, “an increase in diameter of up to 3mm in any part of the cone is a useful guide”. An
alternativr approach is to measure the increase in diameter (if any) following additions of water to the sample. If in the first instance there is 1 – 5mm increase, followed by 5 – 10mm increase, a flow state is indicated.
While some of the Philippine mines rely solely on identifying a subjective change in shape of the sample cone, the Indonesian mines tend to rely only on measuring the extent of the deformation (typically 3mm). These vastly different approaches can lead to a great variance in the declared FMP. No consideration is given to the overall behaviour of the sample, and the key indicators referred to in the IMSBC Code may simply be ignored.
Construction of the flow table
According to the IMSBC Code the metal frame of the flow table is to be attached to a metal base plate, which in turn is securely fixed to a concrete plinth that is isolated from the floor by cork matting. This arrangement is designed to provide a known constant force to the sample during testing.
Typically, the mines do not comply with the IMSBC Code, and frequently utilize a free standing table on various surfaces. Figure 1 shows the FMP determination at a Philippine mine for a limonite ore when the table was (a) loosely fixed to a wooden desk, and (b) when securely fixed to a concrete plinth.
A much smaller deformation was obtained with the flow table mounted on a flimsy wooden desk, due to dissipation of energy into the structure of the support, compared to the deformation observed with a similar sample when the flow table was securely fixed to a rigid platform. This would result in a higher FMP being declared for an incorrectly fixed table.
Before the FMP of nickel ore can be determined it needs to be prepared in the form of a truncated sample.
The sample mould is filled in three distinct phases, each layer being compacted by a defined number of actions with a tamper. This is to simulate the packing of the material in the cargo hold. The tamping pressure used is calculated from the bulk density of the cargo (at loaded moisture content) and maximum depth of the cargo in the hold. In the case of nickel ore such tamping pressures can be difficult to apply, and as a consequence, the mines apply incorrect technique and reduced tamping pressure. In effect, the sample is simply spread around to fill the mould, rather than compacted.
Figure 2 shows the significance of this failure, where cone expansion on the flow table is plotted against tamping pressure for a limonite sample in the MTD Laboratory.
The sample contained 35% moisture. Using the 3mm cone expansion used by a number of the mines as indicating a flow state, this sample would only fail the FTT if a tamping pressure >5.2KgF had been applied. This tamping pressure would correspond to a cargo depth of only 3 - 4m. In reality, the depth of cargo would be greater, requiring a correspondingly greater tamping pressure. By using a lower tamping pressure you are underestimating the FMP. You can effectively control whether a samples passes or fails the test.
Determining moisture content at flow point
The FMP is determined by adding water to a stock sample of nickel ore until a flow state is determined. At the mines there is no control of the laboratories’ environment, and moisture loss can be expected.
The IMSBC Code is specific in requiring that “the whole moulded sample should be placed in a container, weighed immediately and retained for moisture determination”. This is not done at many of the mines. Instead, they start with a known weight of sample that is fully utilized in the sample mould, and use the declared moisture content of the cargo as the baseline moisture content. If the sample passes the FTT the whole of the sample is removed and water added, with the test being repeated. The new moisture content is then calculated based on the original sample weight and the volume of water added. We have witnessed both moisture and sample loss during this procedure. The failure to determine the moisture content of the samples experimentally will result in an overestimation of the moisture content, and consequently, FMP.
Advice to shipowners
In all recent instances that we are aware of, shippers of nickel ore have issued certificates based on sampling and testing carried out by the respective mine’s in-house laboratory.
Regrettably, extensive audits of the sampling and testing methods used by these mines have in every instance so far revealed serious deficiencies, which have rendered the values certified by shippers effectively meaningless.
This presents shipowners with a serious dilemma. They are faced with a choice of either accepting the values certified by shippers at face value, despite the high probability of these certificates being flawed, or of becoming actively involved in an (inevitably acrimonious and time-consuming) investigation of the safety of the cargo being offered for shipment.
Responsibility of crew
It is imperative for the safe operation of the vessel that officers involved in cargo operations understand the characteristics of the cargo to be loaded. Owners and managers have a responsibility to ensure that prior to presenting the vessel for loading the master is fully informed of the characteristics of the intended cargo. This will enable him to take action in a timely manner, especially if concerns require the appointment of a cargo specialist, many load ports are isolated and the appointment and arrival of a cargo specialist can be time consuming.
Accuracy of information provided
Cargo documentation provided for this cargo is often inadequate and does not alert the crew to the potential of the cargo to liquefy. There are many examples of insufficient information being presented by shippers to the master and a few cases of documentation for cargo loaded on an entirely different vessel being handed over. Ore cargo characteristics and moisture content in particular can change during seasonal climate variations, shippers have a responsibility to ensure cargo information provided is recent, relevant and accurately determined. For nickel ore cargoes that may liquefy this must include the moisture content and the transportable moisture limit (TML).
The composition of some nickel ore cargoes can often be of an inhomogeneous nature. Testing methods for
cargoes that are prone to liquefy described in Appendix 2 of the IMSBC Code were designed to assess the suitability of ore cargoes with a maximum granular size of 7mm and are described as not necessarily giving satisfactory results for material with a high clay content. Nickel ore and lateritic nickel ore in particular have both. Testing can still be carried out, however the process and results often give values of moisture content below the actual value of the intended cargo. Values of TML and moisture content should therefore be treated with caution.
Shipowners should be aware that in recent cases in the Philippines, we have come across certificates similar to those encountered during our first involvements with nickel laterite ores being shipped from Indonesia. These certificates state simply that the material has been tested in accordance with the IMSBC Code flow table test method and found to pass. No figures for the FMP and TML are stated although average moisture content, which is valueless without a TML, is provided. Needless to say, it is not possible to assess the safety and suitability for carriage of a material based on such an incomplete declaration.
Inspection of cargo prior to shipment
This can be very difficult for the master if cargo is being transhipped by barge as is usually the case. If the cargo is stored in uncovered stockpiles and has standing water, moisture content may be high, especially during the wet season. If possible a comparison of appearance between stockpiles and cargo loaded on barges should be conducted to determine if they are from the same stockpile.
The IMSBC Code describes a shipboard method (the “can test”) for checking whether a cargo may be suitable for shipment. This involves filling a small can with the material and repeatedly banging it on a hard surface. The appearance of the material at the end of the test can be used to determine form an opinion about the suitability of the material for shipment. This test should not be a substitute for proper laboratory testing using an appropriate methodology. However, if can tests carried out on a cargo presented for loading indicate a propensity for liquefaction, this is a major warning sign that the cargo as a whole is unsafe for carriage.
Expert advice should then be sought. If shippers present significant amounts of ore that fails the can test, this is an indication that the cargo as a whole is unsafe, and that any certification to the contrary is flawed.
During loading nickel ore cargoes with high moisture content can often be detected by the way the cargo splatters and liquefies when it enters the hold, evidence of free water in the hold would reinforce the masters suspicion.
