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Grain

Grain cargoes and stability

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Grain is deliberately left out of the code for solid bulk cargoes and given a code of its own. The reason is that grain is not really a weight problem or a volume problem; it is a stability problem. A ship can be well within its deadweight and its cargo space, correctly trimmed, and still lean over and fail to come back upright.

Why grain gets its own code

The IMSBC Code covers solid bulk cargoes and expressly excludes grain. Grain falls under the International Code for the Safe Carriage of Grain in Bulk, adopted by resolution MSC.23(59) in May 1991 and mandatory under SOLAS chapter VI since 1 January 1994. It applies to ships of every size, including those under 500 gross tonnage.

The Code's definition of grain is wider than the word suggests. It covers wheat, maize, oats, rye, barley, rice, pulses and seeds, and also processed forms of them that behave like grain in its natural state. A cargo of soyabean meal counts as grain for this purpose, whatever the cargo papers call it.

The mechanism: a surface that moves

Grain settles during a voyage. A hold loaded right up to the hatch coamings (the raised edges of the hatch) will, after a few days of the ship moving in the waves, have an empty space under the deck above it. Once that space exists, the surface of the grain can move. When the ship rolls, grain slides down to the low side and does not slide back. The cargo's centre of gravity moves permanently away from the centre line, the ship takes on a list (a permanent lean), and the next roll moves even more grain.

The danger is that this builds up step by step rather than all at once. A ship does not capsize on the first roll. It takes on a list, its reserve of stability on the low side gets smaller, and it then meets the same weather with less safety margin. This failure is different from the free surface of a liquid, and different again from the liquefaction of a Group A mineral concentrate (an ore cargo that can turn to liquid), but all three can end in the same way.

What the Code actually requires

The Code deals with the problem by assuming the grain will shift and requiring the ship to survive it. For each hold, it calculates a grain heeling moment, the turning force from the shift, based on stated assumptions about how far the surface can move. These moments go into the stability calculation, and the result must meet three criteria at the same time.

CriterionThe requirement
Angle of heel from an assumed grain shiftNo more than 12 degrees, or the angle at which the edge of the deck goes under water if that is smaller
Residual dynamical stabilityThe remaining area between the righting arm and heeling arm curves, up to 40 degrees or the angle at which water floods in, must be at least 0.075 metre-radians
Initial metacentric heightAt least 0.30 metres, corrected for free surfaces

Taken together, these criteria explain how the trade works. Grain is a light cargo, with a stowage factor usually between about 1.25 and 1.45 cubic metres per tonne, so a ship carrying it runs out of space before it is loaded down to its load line. As a result, the cargo sits high in the ship, the centre of gravity is high, and the metacentric height (a measure of how strongly the ship returns to upright) is reduced. The same fact that stops the ship reaching its full deadweight also reduces its stability. With grain, running out of space and losing stability are two results of the same cause.

The document of authorisation is the gate

The Code requires a document of authorisation, issued or approved by the flag State, for every ship that loads grain under it. The document sets out the loading conditions under which the ship may carry grain. It comes with the grain stability data the master uses: the grain heeling moments for each hold at each loading depth, both full and partly full.

This is what makes a grain enquiry different from every other dry cargo enquiry. Deadweight, cargo space and draft are all necessary, but none of them is enough on its own. A ship with exactly the right size and capacity but no document of authorisation cannot load bulk grain. The Code does offer another route for ships without a document: loading to the satisfaction of the authority at the loading port. That route is slower, depends on the authority's judgement, and is not something to promise a charterer at short notice.

The document is a flag State document about one particular ship. It is not in any commercial ship register and cannot be worked out from the ship's particulars. The only reliable way to know is to ask the owner for it, and to ask early. On a grain fixture, it is the one requirement that has no way around it.

Filled, filled untrimmed, partly filled

The Code treats holds differently depending on how they are loaded. The terms are worth knowing because they appear in the loading condition.

When the numbers did not work, the traditional answer was to fix the grain surface in place, using shifting boards or by placing bagged grain on top. On a modern bulk carrier built for the trade, the usual approach is to plan the loading so that securing is not needed. In other words, the stability calculation decides which holds are used and how full each one is.

What a broker does with this

A broker needs to do three things, and none of them requires being a naval architect. First, confirm the ship has a document of authorisation before offering it. Second, expect the cargo quantity to be limited by cargo space rather than deadweight, and use the grain capacity figure (the space bulk cargo can fill), not the bale capacity (the smaller space available to bagged cargo). Third, expect the loading plan to be restricted. If the charterer wants a part cargo, or loading at two ports with a long gap between them, the partly filled hold is a stability question and not simply a preference about operations.

Everything else is for the master, who answers with a calculation backed by a document.

References

Dry bulk cargo handlingThe ship arrives, and then five separate things have to go right before a single tonne of cargo is on board. Stowage factor and broken stowageTwo numbers that both turn space into tonnes. Only one of them is printed in the cargo book. What a ship can actually liftA 58,000 deadweight ship does not lift 58,000 tonnes, and at a shallow berth it may not lift 52,000. The sums, written out step by step. Reading a ship's particularsWhat each figure on a vessel description limits, why GRT measures volume, and why draught rules a ship out long before deadweight does.

Neptune Atlas

Neptune Atlas holds the register details a grain enquiry starts from, and it is clear about what it does not hold. A document of authorisation is a flag State document kept on the ship, not a field in any commercial register, so it remains a question to ask rather than an answer to assume. Every paid plan starts with 7 free days. A card is needed to start them, and cancelling before they end costs nothing.

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