Grain is an important commodity which is carried onboard ships since ancient days. Right from the moment the Grain cargo is fixed for the vessel, the Chief Officer, who is usually the Cargo Officer, gets busy! This topic will show you how to draw grain stability calculation curve.
From cleaning of Cargo holds as per “grain clean” standards to hold inspection by the surveyor and loading of grain cargo to discharging the cargo safely is a mammoth task in itself. Not to forget the care for the cargo during the voyage, each step requires thorough monitoring and requires sincere effort. After all, it is the freight of the cargo which pays our salary!
What is Grain?
The term ‘grain’ includes wheat, maize, corn, oats, rye, barley, rice, pulses, seeds and their processed forms, whose behavior is similar to that of grain in its natural state. <International Grain Code’ 91>.
What are the hazards of Grain?
The Grain mainly has two hazards, namely Settling and Shifting. Both these factors affects the stability of the vessel.
Flowchart of Grain Loading
Calculate the maximum quantity that can be loaded into the account the ship’s deadweight, the load line zones she will pass through, the weights on board, draft limitations etc. This process is called the Deadweight Calculation
Distribute this cargo into the holds so as to have the maximum number of filled holds but also considering the vessel’s required draft, trim, stresses etc.
Divide the cargo weight by the stowage factor to obtain the cargo volume/depth of cargo in each hold
From the heeling moment diagrams in the grain loading booklet, find out the volumetric heeling moments for each hold
Multiply the volumetric heeling moments by the appropriate factor to compensate for the vertical shift of G
Divide the corrected Volumetric Heeling Moments by stowage factor to obtain Weight heeling moments
Add up all the heeling moments for each hold to calculate Total heeling moments
Compare the Total Heeling Moments with the Allowable Heeling Moments obtained from the Grain Stability Booklet
If the calculated Total Heeling Moments are lesser than Allowable Heeling Moments. Proceed with the loading else re – distribute the cargo and re-work the calculation
Two crucial things are required to be considered for Carriage of Grain –
The ship should have a valid Document of Authorization
The ship should have an approved Grain Stability Booklet/Grain Loading Manual
Document of Authorization
Document of authorization shall be issued to a ship which is permitted to be loaded in accordance with the regulations of the Grain Code and is authorized to carry grain in bulk
It shall be incorporated in the Grain Stability Booklet/Grain Loading Manual.
A ship can’t load grain without DoA unless Master demonstrates to the satisfaction of the Administration that the ship is under loaded condition complies to the regulation of the Code.
Grain Loading Booklet
Grain loading manual or Grain Stability Booklet are Class Approved information booklets which gives the ship’s officer various information with respect to Grain Loading onboard
Following are the contents of Grain Loading Booklet —
Ship’s particulars, lightship displacement & KG values
Table giving values Free Surface Correction
Capacities and Centre of Gravity of Hold Compartments
KN Curves and angle of flooding tables, where less than 40 degrees, at all degrees of heel
Curves or tables of hydrostatic properties suitable for the range of operating drafts
Cross curves of stability at 12 and 40 degrees
Curves/tables of volume/vertical centre of volumetric and assumed heeling moments for every compartment, including the effects of the temporary fittings.
Tables/curves of maximum permissible heeling for varying displacements
Details of scantlings or any temporary fittings
Loading instructions in the form of notes
A worked example for Master.
Typical service, loaded departure and arrival conditions
Drawing the Grain Stability Curve
The Grain stability can also be calculated, more accurately, if necessary by drawing the stability curve as shown above. We shall discuss the method of drawing the stability curve. As you can see above the meaning of various factors taken into account while drawing the stability curve.
Let us start drawing the Stating Stability Curve (also called the Righting Arm Curve). This curve is drawn using the Cross Curves (also called KN Curves) given in the Stability Booklet.
Step 1: The corrected righting arm (corrected GZ) at any given angle (say θ), is obtained from following equation
Corrected GZ = GZ + (KGa – KGo)Sin θ
GZ = Value of GZ obtained from Cross Curve tables KGa = Value of KG assumed in the Cross Curves KGo = KG +GGo
KG= KG value obtained for loading condition under consideration GGo = Loss of GM due to free surface θ =Angle of inclination
The above formula is modified version of GZ = KN – KGSinθ (Normally read in academics). It is modified above to suit the data given in stability booklet.
Let us look at the Cross curves data given in the stability booklet –
The tables above are self explanatory and GZ values can be easily read off from the table using the draft / displacement and the heel angles. Assumed KG is 0.001 as mentioned on the top of table.
Using the corrected GZ Values for various angles of inclination (including 12 degrees and 40 degrees), a statical stability curve is drawn from the displacement of the vessel as below –
An example calculation from a vessel’s grain stability booklet is given below for you to understand-
The reason I am not drawing the curve step by step is that onboard, practically, this curve is not drawn anytime, anywhere and by anyone. Modern loadicators give this information readily merely on the click of a button. However, what is important to understand is the logic behind it.
In the near future, we will be writing extensively about all aspects of grain loading. Read more on grain loading here.
Write to us in the comment section below. Till then fair seas and Bonn Voyage!