Saturday, 31 October 2015

Ballast Free Ship Design


Ballast water is fresh or seawater, held in tanks and cargo holds of ships to increase stability and manoeuvrability during transit. Ballast water is essential to the safe and efficient operation of modern ships, providing balance and stability to un-laden ships (often returning empty during return voyages) as well as loaded ships. Its superb operational advantages, however come at a cost. 

 It poses serious ecological and health threats due to transfer of a multitude of marine species (non- native species) into an altogether different host environment containing different native species. 

Didn’t quite get that?

e.g.- The ballast water is taken from coastal port areas(source point) and transported inside the ship to the next port of call(destination point) where it may be discharged, along with all the surviving organisms. This way, the ballast water may introduce organisms into the port of discharge which do not naturally belong there. These introduced species are called exotic species. Populations of exotic species may grow very quickly in the absence of natural predators. In this case they are called ‘invasive’. However, most species can’t survive in the new surroundings – temperature, salinity etc. (Remember, Survival of the fittest?) being less than optimal. Thus only a few species are ‘successful invaders’, however those that do survive, establish a population and have the potential to cause major harm! Aquatic invasions are considered the second greatest threat to global bio-diversity after habitat loss, are virtually irreversible, and increase in severity over time. If that is the case, then one can’t even imagine the damage, caused by transfer of 3 to 5 billion tons of ballast water each year.



There are mainly three projects in which the concept of a ship with zero ballast water has been developed:
  • Delft University of Technology (DUT)-‘Monomaran Hull’.
  • Det Norske Veritas(DNV)-‘Volume Cargo Ship’
  • Daewoo Shipbuilding and Marine Engineering(DSME)-‘ Solid Ballast Ship’
  1. ‘The Monomaran Hull’ – An unloaded rolling ship (without ballast water) requires adequate stability. DUT proposed a monomaran hull by adopting a catamaran shape to the base of the broad single hull.
  2. ‘Volume Cargo Ship’ - DNV proposed a design similar to DUT but with a trimaran hull shape thus imparting high level of stability.
  3. ‘Solid ballast ship’ – In this case, the ballast water is replaced by 25 tonne Solid ballast in standard containers. However the application of this method is limited to container ships only. The hull form (size) remains the same.
Another solution to this problem is the Yokohama buoyancy control compartment concept, which converts conventional ballast tanks into a series of buoyancy control compartments. 

(Fig. 1: Comparison between conventional ballasting and ballast free ship design. (Courtesy:"Development of a ballast free ship design" by Avinash Godey, Prof. S.C. Misra, Prof. O.P. Sha)

Each compartment is flooded to provide adequate draught in the unloaded condition then continuously flushed at normal voyage speeds to ensure efficient exchange without the need for pumps. Each compartment is fitted with intake and outlet valves that are optimally designed and positioned for each compartment so as to maximize its flushing rate during normal voyage speeds.

Although ballast water treatment is an effective way of tackling ballast water issues.
The details of it will not be discussed in this article.

The Ballast Free Ship (BFS):

When a ship moves forward it produces regions of increased water pressure near its bow and reduced water pressure at its stern. This pressure differential is utilised to drive water through a set of these below-waterline corridor (trunks) without the need for pumps. Although this leads to slight increase in the resistance of the ship, the discharge of the trunk flow into the upper half of the propeller disc tends to smooth out the inflow to the propeller, allowing the propeller to operate at higher propeller efficiency and thus compensate for the added resistance to some extent.

Fig.2:Rendition of the concept behind ballast free ship design (Copyright: Learn Ship Design)

Fig.2:Rendition of the buoyancy control compartments with provisions for flushing water at normal voyage speeds. (Copyright: Learn Ship Design)

However, it also has to overcome some other challenges like:

1.) Loss of cargo carrying capacity- due to ballast water volume restraint. As it’s quite difficult to sustain the cargo carrying capacity and also the same ballast water volume

2.) Loss of ship strength- There would be a total redesign of the double bottom. As the conventional transverse framing will create difficulty for ballast water to flow through the tunnels. Hence this elimination will enhance the flow of ballast water at the cost of ship’s strength.
Classification societies might not permit the elimination of all of these frames. Moreover, watertight trunk boundaries will be required at transverse locations. Longitudinal stiffeners could be replaced with sandwich panels, thereby improving the flow. Thus compensating for the loss of strength.

3.) Increase in ship’s resistance- due to disturbance from discharge of ballast water into the flow around the propeller – The introduction of a plenum at the bow and stern of the ship, as well as the location of the plenums will affect the resistance of the ship, increasing fuel consumption.

Also, the increased ballast water flow velocity at discharge location will increase resistance as shown experimentally

Equipped with such technology, we can hope to minimize our environmental footprint to the greatest possible extent in the different spheres of conflict with the marine ecosystem.LSD

Article By: Vishal Kumar Jha