Cargo and ballast managament
This Section concerns issues relating to ship loading, use of ballast water, use of loading or unloading equipment and their impacts on ship energy efficiency.
The Plimsoll line or Load line is placed mid-way between the forward and after perpendiculars of the ship and give the draft of the ship that is the legal limit to which a ship may be loaded for specific water density and temperature. Temperature can influence the draft of a ship because warm water provides less buoyancy as it is less dense than cold water but this factor is not really taken into account in cargo calculations except by the use of Load line Zones of areas that have been defined in the International Load line Convention (MariEMS 2017).
International Load lines Convention applies to all commercial vessels of over 24m length and requires that every ship is surveyed and issued with a Load line certificate every 5 years. If the ship does not have its certificate up to date, then it can be detained by the Flag State or Port State inspectors. The survey mainly consists of checking the vessel to ensure that the watertight integrity of the structure as a whole has been maintained (MariEMS 2017).
It is pertinent to note that as regard to loading aspects, trim and ballasting, ships such as bulk carriers do not have much scope for changing trim without shutting out cargo and reducing the load factor as the holds are often full (MariEMS 2017).
Ships are designed to carry a certain amount of cargo at a certain speed for a certain fuel consumption that generally results in a particular trim for the vessel when fully loaded and in ballast. Trim has a significant influence on the resistance of the ship through water and of the effectiveness of the rudder and propeller. Optimized trim can give significant fuel savings and for any draft there is a trim condition which will give minimum resistance and increase the efficiency of the engine (MariEMS 2017).
One of the key tasks for ship’s master is to pre-calculate weight of the cargo for stability and trim. In this regard the ships master must rely on the ship loading computers and ships final drafts to ensure that stability is maintained throughout the intended voyage, taking into account the consumption of fuel oil and any international load-line requirements. The overall efficiency of the ship is a function of the ship's size. As the ship grows, a lower fuel consumption and lower CO2 production per cargo will be achieved. Operationally, energy efficiency can be increased by concentrating the transportation of cargo on larger ships that can reduce the energy consumption of the shipping industry as a whole. These smaller feeder ships will be less efficient anyway than the large ships and there will also be some extra GHG emissions penalties in the additional discharging and loading operation for trans-shipment. So, the use of economies of scale is as effective as it can be, but it is a good idea to make a lot of money. This means that the overall energy efficiency may also be improved for smaller vessels with access to more ports and cargo types and able to fill cargo holds to full capacity (MariEMS 2017).
Another key factor is the Ballast water (BW) Management. BW is essential to control trim, list, draught, stability and stresses of a ship. Ballast water activities are largely regulated not only because of the above ship’s safety implications but also since they have been recognized to be a pathway for the movement of undesirable and alien bio-species from their natural habitat to other ecosystems. The impact of Ballast Water Management (BWM) on a ship’s fuel consumptions is not normally considered despite the evidence that, regardless of the management method established, the overall energy efficiency of a ship is affected by ballast water because the ballast exchange requires the additional use of the ballast water handling equipment and in particular pumps. Ballast water impacts the ship’s energy efficiency in two additional ways: The change in ship displacement; thus wetted surfaces and ship resistance and change in ship trim (MariEMS 2017).