Originally Posted by mch2qh

I'm not sure if this is the correct forum to ask these questions, but I'm looking for information on the methods currently used to measure the volume of dry bulk cargoes loaded on a ship. How is the volume estimated? What is the accuracy of various methods? What hardware/software is used? Similar questions apply to the dockside stockpiles.

Many thanks.

I think you should consult to one of the shipping registration companies. They would be perfectly able to answer your query. Software based weighting systems are widely in use these days. ■

Dear inshoresystems,

I visited your website, which shows that you are involved in a very interesting and very important business sector, mooring of ships.

Normally, the ship’s first officer knows the cargo content in weight (mass) of each hold.

He has to know this because of the allowable ship’s hull stresses.

How the cargo is divided over the ship influences the moment of inertial, especially against gyration (yaw).

To know the cargo volume, is important to know in advance of a loading, to estimate whether the holds are to be filled 100% (or less) to accommodate the cargo in mass.

Therefore the seamen use the stowage factor, expressed in m3/ton (or different units).

This is in fact the inverse of the bulk density.

There are many lists of stowage factors for various materials available.

Bulk density is not a constant over time and certainly not in a ships hold.

Bulk density starts as poured bulk density during loading.

During the voyage, the density increases by settling, cause by de-aeration and ships vibrations.

Also the density is not constant in relation to the cargo depth in the hold.

This means also that it will be difficult to establish the center of gravity of the cargo in each hold, due to the not measurable uncertainties, even if you know the volume.

Furthermore, after loading, on board, no one is interested anymore in the volume.

I am sure that it is possible with special equipment (sounding, lasers, etc) to measure the top level of the cargo in a hold and calculate the volume from there by using the ship’s drawings.

However, which ship-owner is prepared to install these instruments permanently?

If you really need to know the volume, then your best option is to approximate this volume by dividing the known mass by the estimated stowage factor.

In fact, even if you know the volume, defining a center of gravity is still an approximation, because of the unknown spatial bulk density.

Referring to your website, this information (and more to the point of your needs) should be available from your customers.

Have a nice day

Teus ■

Teus has provided excellent insight for this thread which appeared to have been hammered well in the guise of numerous posts & threads about Draught Survey. Any respite from pure landlubber activity is welcomed. Bulk we are and since most of the bulk is shipped around the globe we appreciate the waterborne issues involved.

Shiploading starts and progresses according to the loading manual which, according to the threads, is held by the Master. One or two draught surveys might determine the cargo etc weight: but as we see, the cargo volume and C of G are not determinate to the same accuracy.

My Boy: are we loading or unloading? Loading: it should be reasonable to assume that during loading a dry material is in a state of 'broken bulk density' which is inversely comparable to the stowage factor mentioned by Teus.

Dry Bulk???? Rain exposure will markedly affect the consistency of broken bulk density. If the material has been windrow stacked outdoors then it probably contains free water which will partially drain out during digging and conveying. If it has been chevron stacked then a lot of rainwater will have run off and the core of the pile, being drier than the skin, will be rarer. So we don't exactly know what is where in the holds volumetrically speaking. I cannot discuss the implications of methane build up in very wet coal because I don't have the expertise. But if its there it will affect density.

When the ship is being onloaded the Volume sort of governs. Consolidated cargo from lower in the hold will produce bigger tonnage extraction since the unloading machines, grabs, chain buckets, bucket wheels and even screws are essentially volumetric removal devices. Volume definitely governs during hold clean up. So the unloading rate fluctuates during unloading, very definitely. Our beloved term 'Cream Digging' could be examined further in the light of this thread. Withdraw the hatches and start cream digging. OK but none of the unloader designers I know have ever taken account of higher densities in the substrata. I hadn't either until now. Aren't these forums great? Thanks for asking. More topics like this will bring the forums back to life.

It should be possible to monitor the unloading operation by using a modified version, add-in, of the Draft Survey software which was mentioned in one of the latest threads on the subject. You might be able to predict, limit or prevent sudden movement of ships at berth. Good luck. ■