Neptune Atlas

Grain

Grain cargoes and stability

·

Grain is deliberately kept out of the solid bulk cargoes code and given a code of its own, and 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 inside her deadweight, well inside her cubic, correctly trimmed, and still be a ship that will lie over and not come back.

Why grain gets its own code

The IMSBC Code covers solid bulk cargoes and explicitly does not cover grain. Grain sits 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 any size, including those under 500 gross tonnage.

The Code’s definition of grain is wider than the word suggests. Wheat, maize, oats, rye, barley, rice, pulses, seeds and processed forms of them, meaning anything whose behaviour is similar to grain in its natural state. A cargo of soyabean meal is grain for this purpose whatever the cargo book calls it.

The mechanism: a surface that moves

Grain settles during a voyage. A compartment loaded full to the hatch coamings will, after a few days of working in a seaway, have a void under the deckhead, and once there is a void the surface can move. When the ship rolls, grain runs downhill to the low side and does not run back. The centre of gravity of the cargo moves permanently off the centreline, the ship takes a list, and the next roll shifts more.

The danger is that it is progressive rather than instantaneous. A ship does not capsize on the first roll; she takes a list, her range of stability shrinks on the listed side, and she is then a ship with less reserve meeting the same weather. This is a different failure from the free surface of a liquid, and different again from the liquefaction of a Group A concentrate, though all three end in the same place.

What the Code actually requires

The Code answers the problem by assuming a shift will happen and requiring the ship to survive it. The grain heeling moments for each compartment, calculated on stated assumptions about how far the surface can move, are worked into the stability calculation, and the resulting condition has to meet three criteria at once.

CriterionThe requirement
Angle of heel from an assumed grain shiftNot more than 12 degrees, or the angle at which the deck edge immerses if that is less
Residual dynamical stabilityThe residual area between the righting arm and heeling arm curves, out to 40 degrees or the flooding angle, at least 0.075 metre-radians
Initial metacentric heightAt least 0.30 metres, corrected for free surfaces

Read those together and the shape of the trade appears. Grain is a light cargo with a stowage factor typically between about 1.25 and 1.45 cubic metres per tonne, so a ship carrying it fills up before she goes down to her marks. That means her cargo sits high, her centre of gravity is high, and her metacentric height is under pressure from the same fact that stops her deadweighting out. Grain is the cargo where cubing out and losing stability are the same phenomenon seen twice.

The document of authorisation is the gate

The Code requires a document of authorisation, issued or approved by the flag State administration, for every ship loaded in accordance with it. It sets out the loading conditions under which the ship may carry grain, and it comes with the grain stability data the master works from: the grain heeling moments for each compartment at each depth of loading, filled and partly filled.

This is where a grain enquiry differs from every other dry cargo enquiry. Deadweight, cubic and draft are all necessary and none of them is sufficient. A ship with a perfectly suitable size and capacity that has no document of authorisation cannot load bulk grain, and the alternative route, loading to the satisfaction of the authority at the port of loading under the Code’s provisions for ships without a document, is slower, discretionary and not something to promise a charterer on a Friday afternoon.

The document is a flag State paper about a specific ship. It is not in any commercial vessel register, it is not derivable from her particulars, and the only reliable way to know is to ask the owner for it. Ask early. It is the one question on a grain fixture whose answer cannot be worked around.

Filled, filled untrimmed, partly filled

The Code treats compartments differently according to how they are loaded, and the vocabulary is worth having straight because it appears in the loading condition.

Where the numbers do not work, the historical answer was to secure the surface, with shifting boards or by overstowing with bagged grain. Modern practice on a purpose-built bulk carrier is usually to arrange the loading so that securing is unnecessary, which is another way of saying the stability calculation dictates which holds are used and how full each one is.

What a broker does with this

Three things, and none of them requires being a naval architect. Establish that the ship holds a document of authorisation before quoting her. Expect the intake to be set by cubic capacity rather than deadweight, and take the grain capacity figure rather than the bale one. And expect the loading plan to be constrained: if the charterer wants a part cargo, or a two-port load with a long gap, the partly filled compartment is a stability question and not merely an operational preference.

Everything else is the master’s, and the master’s answer is a calculation with a document behind it.

References

Dry bulk cargo handlingThe ship arrives, and then five separate things have to go right before a tonne of cargo is aboard. Stowage factor and broken stowageTwo numbers that both turn space into tonnes, and 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 she may not lift 52,000. The arithmetic nobody does in their head. Reading a ship's particularsWhat each figure on a vessel description constrains, why GRT is a volume, and why draught excludes a ship long before deadweight does.

Neptune Atlas

Neptune Atlas holds the register particulars a grain enquiry starts from and is explicit about what it does not hold. A document of authorisation is a flag State paper on the ship, not a field in any commercial register, so it stays 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.

Try 7 days free Ship chartering software for brokers What each plan includes