CARGO HETEROGENEITY AND SAMPLING Trying to get a sample from a big pile is a problem. Stockpiles are always highly heterogeneous, especially if made of unprocessed materials like raw ores with a wide range of lump sizes:-


In the photograph above, a small stockpile was formed against a glass plate in our laboratory; it was built using a well mixed stream of salt (white particles mostly 2-3mm diameter) and mung beans (brown material about 6mm diameter). But as the stockpile forms, the larger particles roll more readily down the rough surface of the heap, so they finish up at the bottom and outside, whereas the fine particles do not roll so well, hence accumulating more in the middle. Most real materials have a wider size range than these two materials, so it is obvious that most real stockpiles will be highly segregated internally. There are special methods for building stockpiles in a way that minimises the separation, but we have never seen this done at a port.


Taking a sample from a large pile segregated like this, is impossible. Even if you try to dig into it, you can’t go in far enough to get to the centre; and it is the centre where the material is finest, often wettest, and therefore most dangerous from a liquefaction perspective. Take a sample from the outside and it will be coarser, more free draining and therefore usually drier. Also bear in mind that according to the IMO protocol, the TML is determined from a Flow Moisture Point (FMP) test on a sample and is valid for 6 months for the same cargo flow, whereas the MC is taken from a


sample on the actual pile to be loaded, within 10 days before loading. Due to the inhomogeneity, these two separate samples will have different size gradings, and potentially very different behaviour, so the measured TML can relate to material with a very different behaviour from the MC sample. The limited data we have been able to get on the variation in both values measured through a stockpile, has shown changes much bigger than specified “safety margin” between MC and FMP. Basically the value of any such sample, and any comfort drawn on it in relation to the safety of the ship and crew, may in many cases be simply an illusion.


The only way to get a truly representative sample of a stockpile is to sample the stream that it is built from, over its full width, many times through the entire stacking operation, preferably with a mechanical sampler.


In a few


industries this is done routinely (the guide “Recommended Best Practices for the Sampling of Dry Bulk Fertilizer Shipments” by the International Fertilizer Industry Association is a good guide to proper procedure) but for ores and coals this is almost never done due to the cost, time and difficulty.


Also, it must be recognised that even if true samples are obtained


70 | The Report • September 2017 • Issue 81


For raw ores like nickel ore, the danger of large sampling errors is worse because the material is more likely to vary from different parts of the pit, and has a wider range of sizes and moisture contents; whereas for processed materials (even separated fine iron ore) there is a degree of “standardising” that goes on in processing the cargo, so the FMP will vary less. However both the MC and FMP will still


for both TML and MC tests, these represent the stockpile as a whole. But different parts of the stockpile could be quite different in size, and loaded into different holds.


It’s not


uncommon to find cargo liquefies in only one hold and is secure in the others – which may be partly due to this.


THE NEED FOR AWARENESS AND QUANTIFICATION OF HETEROGENEITY This matter of bulk heterogeneity, sampling errors and the meaning of data from samples, has long been of great importance to us at The Wolfson Centre because we do a lot of work in cargo quality. However, there seems to have been no consideration of what it means for the IMO protocol for group A cargoes. When the sampling error is bigger than the specified margin of safety, which it has been in cases we have analysed, safety is seriously compromised.


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