What is Grain Stability ?

The term “grain” covers wheat, maize (corn), oats, rye, barley, rice, pulses, seeds and processed forms thereof, whose behavior is similar to that of grain in its natural state.

This „Grain Loading Plan and Stability Booklet“ for a ship is prepared to enable the master to demonstrate the ability of the ship at all stages of voyage carrying bulk grain to comply with the stability requirements of chapter VI, SOLAS 1974.

The International Grain Code specifies how to determine the grain shift moment, and it sets the stability criteria for ships carrying grain in bulk. There are some steps to be followed:
• calculate grain shift moments
• calculate the allowable grain heeling moments
• determine whether a loading condition complies with the grain stability criteria
• create the tables needed for the loading manual

Grain Intact Stability Requirements

The stability of this vessel when bulk grain cargo to be carried should comply with the requirements of:

IMO Reg. MSC.23 (59). (ADOPTION OF THE INTERNATIONAL CODE FOR THE SAFE CARRIAGE OF GRAIN IN BULK)

1) The intact stability characteristics of any ship carrying bulk grain shall be shown to meet, throughout the voyage, at least the following criteria after taking into account of the heeling moments due to grain shift:

• The angle of heel due to the shift of grain shall not be greater than 12 degree or the angle at which the upper deck edge is immersed, whichever is the lesser;
• In the statically stability diagram, the net or residual area between the heeling arm curve and the righting arm curve up to the angle of heel of maximum difference between the ordinates of the two curves, or 40 or the angle of flooding (θf), whichever is the least, shall in all conditions of loading be not less than 0.075m-rad (refer to the following figure);

• The initial metacentric height GM0, after correction for the free surface effects of liquids in tanks, should not be less than 0.30m.

Basics of the grain rule

2.1 Grain shift moment
In most holds, grain cargo cannot be loaded so that it perfectly fills up the compartment and no empty spaces exist; there are usually some voids under the deck after loading. The cargo also settles during the voyage, so that even if the cargo filled up the hold perfectly before the voyage, there may be voids during the voyage.

Figure 1 The cargo shifts when the ship heels

When the ship heels enough, the cargo will slide towards the lower side, and the center of gravity of the cargo will not be in the same place anymore. When the ship comes upright again, the load will not level itself as a liquid would and the center of gravity is still off the original one.

Figure 2 The center of gravity moves sideways and vertically

The center of gravity will also move vertically. Depending on the geometry this movement is up or down.

The actual grain shift moment for the loading condition is the sum of the grain shift moment for each hold. This is complicated by the fact that we do not know exactly how the cargo will behave when loading and shifting. Therefore, the international grain code makes some assumptions on which the calculations are based. There are also some pragmatic “corrections” made in the rule in cases where it has been difficult to quantify the effect of the grain shift on the stability.

Load types

The grain rule separates four different ways you can load grain:

Filled, untrimmed load: you fill the hold from the hatch and leave it just as it is. In the hatch, you
trim the load, i.e. level the surface as much as possible. Filled means you fill it to the maximum
extent possible, in normal circumstances up to the edge of the hatch coaming. The rule assumes
that the load will flow from the hatch end beams and the side girders at an angle of thirty degrees.

Figure 3 Untrimmed grain load forward of the hatch

Filled and trimmed load: fill the compartment to the maximum extent possible and level all the
surfaces.

Figure 4 Filled and trimmed load; the cargo is packed into the hold as well as possible

Figure 5 Filled and trimmed load; the rule makes assumptions about how well the cargo can be loaded and how it will settle during the voyage causing assumed voids which are used for calculation of the grain shift moment

Partial load: load the compartment with a certain amount of grain. Partial loads must always be
trimmed, i.e. the surface of the cargo must be level.

Figure 6 Partial load, always trimmed

Filled, with untrimmed ends: this is a hybrid between the filled and trimmed load and the untrimmed one and is allowed in ‘specially suitable compartments’, as defined in A.2.7. 

An example of a specially suitable compartment is a hold with topside tanks sloped more than 30 degrees, and where the hatch sides coincide with the topside tank.

Initial voids

“For the purpose of calculating the adverse heeling moment due to a shift of cargo surface in ships carrying bulk grain it shall be assumed that:
In filled compartments, which have been trimmed in accordance with A 10.2, a void exists under all boundary surfaces having an inclination to the horizontal less than 30 degrees, and the void is paralleto the boundary surface having an average depth calculated according to the formula…”

Note that these assumed voids are used only for calculating the grain shift moment; they are not used for calculating the weight of the cargo.

The formula for calculating the depth of the assumed voids takes into account the distance from the hatch to the boundary of the compartment and the depth of any beams and girders.

Figure 7 The void depth is affected by the distance from the hatch end beam or side girder to the boundary of the compartment, and the highest girder/beam between the hatch and the void

In partial loading conditions, there will be similar voids underneath any horizontal surfaces, in case the grain reaches up to them.

Grain load after shifting

In order to calculate the grain shifting moment, the grain code makes assumptions about how the cargo will settle after shifting. Instead of calculating how the cargo is moving, the assumptions are made regarding how the voids will move.

First, the initial voids are established. Then the shifting angle is determined, based on the type of the load, 15 degrees for filled and trimmed, 25 degrees for untrimmed and partial loads. Using the shifting angle, the maximum void that can be formed against each girder is determined.

If the grain is shifting to the port side, the calculation starts from the void far left (seen from aft). If the area of the initial void is less than that of the max shifted void that can be formed against the girder, the height of the shifted void is adjusted so that the area is the same as the initial void.

If the initial area is greater than the max void, the excess area will be transferred to the next void to the right. Then the process is repeated for this void, but instead of using the initial area of the void, we will use the initial area plus any area that was transferred from the previous void.

Finally, we will have determined the area and the centre of gravity of each void after shifting, and we can get the center of gravity of the cargo in this section.

Figure 8 Shifted voids and void transfers used for determining the shape of the cargo surface after shifting.

Download Grain Stability Booklet
Here is a class approved Grain Stability Booklet:
http://vessel-sea.com/outline/pdf/4-GRAIN_LOADING.pdf