Page 378 - Practical Ship Design

Stability and Trim - General 335

Grain cargo stability is dealt with in $ 12.2, whilst the stability of dredgers is the
subject of $12.3.
It is known that some other cargoes such as coal and iron ore may ‘‘liquefy’’ in a
similar way and that this may have contributed to the loss of some bulk carriers.
Whilst this has not been proven a wise designer should pay attention to minimising
the likelihood of this happening and the consequences if it does.
Guidance on the carriage of cargoes which may liquefy is given in the I.M.O.
B.C. Code.

12.2 GRAIN STABILITY

12.2. I Introduction
Grain was formerly carried in general cargo “tramp” ships. In these ships extensive
measures had to be taken with shifting boards and feeders to limit any shift of
grain. Nowadays grain is almost entirely carried in bulk carriers and no special
preparation is required for a grain cargo except for completely filling as many
holds as possible and trimming level any partly filled compartments -provided
the ship meets the stability requirements given in Chapter VI of the 1974 SOLAS
Convention, which has been slightly revised and issued in 1991 by I.M.O. as the
International Code for the Safe Carriage of Grain in Bulk.

12.2.2 I.M.O. rule requirements
These rules require heeling curves to be calculated based on an assumed shift of
grain. Obviously the greatest heeling moment will be generated in any partly filled
compartments, so the first objective is to have as many compartments as possible
completely filled and the minimum number, only one if possible, with a complete
free surface. It is also helpful if the partially filled compartment is as small as
possible, but with the need for a satisfactory trim, it may not be possible to use the
same compartment partially filled for all specific gravities of cargo.
Even for full compartments the calculations must take account of settlement of
the cargo and the voids which exist between hatch coamings and at the sides of the
hatches. As the vessel heels the voids shift from one side to the other thus causing a
heeling moment as the grain moves to fill the first void.
After taking into account the heeling moments due to grain shift the ship must
meet the following criteria at all times:
(i) the angle of heel due to grain shift shall not exceed 12” or a lesser figure if
required by an Administration;
(ii) the residual dynamic stability, as shown in Fig. 12.1, up to 40” or the angle
of flooding if this is less, shall not be less than 0.075 mad;
(iii) the GM after allowance for liquid free surface shall not be less than 0.30 m.