Ballast Water Management on special vessel types

DNV recommendations for special vessel types. The possible implementation of the Ballast Water Management Convention (BWM Convention) will require the treatment of long-haul-transported ballast water in the future. For certain types of ships, such as semi-submersible vessels or barges, the capacity of the treatment system can be a challenge. This newsletter describes a possible alternative arrangement.
Common to these types of ships is that they have very large ballast pumps in order to submerge and refloat the vessel rapidly. This kind of ballasting and de-ballasting operation is usually done within the same coastal state zone, and does not therefore require treatment according to the BWM Convention (reference is made to Regulation A-3.5 of the BWM Convention).
Limited ballast water is needed for the transportation legs between port states, giving little reason to have a BWMS which matches the ballast pumps' capacity. This makes shipowners want a treatment system for the vessel which has a smaller capacity than the ballast pumps. In addition to the obvious cost-saving aspect related to a reduced treatment-rated capacity (TRC), a smaller footprint allows easier retrofitting.
A treatment system with reduced capacity to treat water transported between port states can be fitted by means of circulation during the voyage. The circulation pump capacity and treatment system capacity can then be designed such that the ballast water is treated to the D-2 standard and in accordance with the treatment system's type approval certificate by the time the ship has to discharge this water.
DNV will need a detailed design of the solution for a case-by-case approval.
Residual water in tanks that are ballasted and de-ballasted under Regulation A-3.5 (approximately 2-3% of the tank capacity) may contain sediments and organisms from a foreign habitat. This water must be treated to the D-2 standard before a new ballasting/de-ballasting operation in compliance with Regulation A-3.5 can start.
The treatment of residual water to meet the D-2 standard must be done by a typeapproved system and in accordance with the type approval issued for the treatment system used.
DNV recommends the regular flushing of ballast tanks to avoid the build up of sediments from foreign habitats. This can be done by the exchange methods stated in the BWM Convention or, when applicable, by water treated to the D-2 standard. This should be done when the time window is sufficient for such a process.
For vessels with the characteristics described above, DNV recommends stripping the ballast tanks used in operations falling under Regulation A-3.5 using a stripping system. Collect all the water in one tank and then recycle it through the treatment system before the intake of new ballast water.
When choosing a treatment system with filtration it must be considered that: a filter is often based on a system in which the accumulated sludge is back-flushed out of the system on site. This is not acceptable when the ship is on route in waters where the BWM Convention would require otherwise discharged water to be treated to the D-2 standard. The sludge will therefore need to be stored on the vessel and treated by a land-based facility. This is not practical.
Source: DNV