Ballast Water Treatment & Vessel Performance – 4 Key Impact Points
Editors note: One of our most popular articles from 2017, we have now updated to include specific considerations with regards to USCG Type Approvals and their impact on vessel performance.
Vessel performance can be impacted by a ballast water treatment system in many ways, some of which could have ongoing commercial and/or contractual implications. In a recent article on the impact of vessel ballast performance on Charter Parties, North P&I Club stressed the importance of ensuring charter party warranties are adjusted to reflect any change in vessel performance as a result of installing a ballast water treatment system:
“…North stresses the importance of thinking about the potential impact on charterparty warranties. The vessel description/warranties may need to be amended to reflect any change in the vessel’s performance… If the charter party remains unamended, a shipowner may be exposed to a charterer’s allegations of breach of warranty if delays are experienced as a result of the longer time needed for ballasting/deballasting.”
These are valid points which, if not addressed correctly, could result in many owners and/or operators being in breach of warranty, unknowingly.
To fully understand the potential contractual and logistical implications, it is important to examine the underlying causes of changes in vessel performance. This article examines 4 key critical impact points a ballast water treatment system may have on vessel performance.
Ballast Pumping Rates
During the course of a vessel design, marine engineers determine the ballast pumping rates and head requirements based on a combination of the cargo requirements of the vessel (i.e. how much cargo is loaded / unloaded and how quickly), as well as the calculated maximum head loss in the ballast system pipework (based in its design). From this data, marine engineers will identify and select suitable ballast pump(s) for this intended duty – ensuring the pump selected will operate within its “efficiency zone” on the performance curve.
With the focus of pump selection being on efficiency, it is seldom the case that pumps selected have ample spare capacity. Most ballast pumps are centrifugal, meaning pressure and flow are directly related via the pump’s performance curve, and any increase in pressure drop of the ballast piping system, will correspond to a reduction in flow.
Ballast water treatment systems, by their very nature, create an increase in pressure drop. The extent of the pressure drop varies depending on technology employed and specific system components, however pressure drops of 0.3 bar – 0.8 bar are not uncommon. Such additional pressure drops will have one of two effects on the existing ballast pumps:
- The pump flow rate will drop due to the increase in pressure drop
- The new system pressure demand will be unachievable by the pump, which will result in zero flow and/or erratic running
Even in the best case scenario, owners should anticipate anywhere from 10% to 50% reduction in ballast flow rate. Such reductions in flow rates, for certain vessel types, will have a drastic impact on cargo loading/unloading rates, and hence time required for cargo operations.
Worst case scenario, which we seldom find in ballast water retrofit projects, is that upgrades to existing pumps are required, or, replacement pumps and/or motors are required.
Power availability onboard existing vessels is an important consideration of any ballast water treatment retrofit project. The problem is not simply a question of what level of spare power capacity is available onboard – but rather what spare capacity is available during vessel operations requiring ballasting / de-ballasting.
Vessel powering is generally optimised, during the design phase, based on the operating conditions of the vessel. The maximum power consuming condition, other than sailing conditions, is analysed and the generators sized and selected to efficiently meet this demand.
With ballast water treatment systems potentially requiring up to 20kW per 100m3/hr of ballast flow rate (for UV based systems), the power requirements can be significant for larger vessels with larger flow rates. Of course, other technologies become more economical at flow rates beyond the 1000m3/hr mark, such as Electro-Chlorination (which requires less than 5kW per 100m3/hr), and chemical systems (which also require less than 0.5kW per 100m3/hr).
For many vessels, even smaller vessels, the additional power requirement from the ballast water treatment system may tip the threshold on the power balance spreadsheet for the number of running generators – and require an additional generator be run. The additional fuel consumption from an additional generator running is not insignificant, and will have an impact on the operating costs of the vessel long term. This is particularly applicable for vessels carrying out ballast operations whilst in Sulphur Emission Control Areas (SECAs). The requirement to run on low sulphur fuel will add an additional premium to the fuel consumption of the additional generator.
For larger vessels, with flow rates of 7000m3/hr and above, power requirements, even for low power consuming EC based ballast water treatment systems, could still be in the region of 350kW. Such a high power demand during critical ballasting/de-ballasting operations, which tend to coincide with cargo operations, can have a serious impact on vessel performance.
Take tankers as an example. During cargo loading/unloading, a tanker is generally operating at its highest power consuming condition, with power consumption close to the maximum available. As cargo is loaded or unloaded, the vessel is ballasting or de-ballasting at significant flow rates to ensure stability is maintained throughout. The addition of another 350kW requirement during this critical condition is potentially unachievable with the existing power plant onboard. Vessel performance is likely to suffer as a result- requiring a drop in cargo flow rates to ensure sufficient power is available on the switchboard for the ballast water treatment system. Alternatively, the ballast pump flow rates could be reduced, requiring a smaller (and hence lower power consuming) ballast water treatment system – but this would likely still result in reduced cargo flow rates.
Power consumption and availability onboard is a key consideration when analysing a potential ballast water treatment system retrofit and owners/operators should ensure close attention is paid to the power consumption of the system.
Ballast Treatment Technology & Approvals
Vessel performance can also be significantly impacted by the ballast water treatment technology itself – and it is crucially important to understand the limitations of the ballast water treatment system being retrofitted.
UV based ballast water treatment systems, for example, are particularly sensitive to waters with low clarity. UV systems achieve treatment by irradiating the bacteria with UV light – however, in lower clarity waters, this UV light does not transmit through the water with the same efficiency. Manufacturers of UV based ballast water treatment systems often quote the systems UV-Transmittance (UV-T), which is a measure of a UV lamp’s energy which is transmitted through the water, at a given frequency or wavelength. The lower the UV-T percentage, the more energy is transmitted through the water. Theoretically, the lower the UV-T of a ballast water treatment system, the more capable it is treating in waters with low clarity.
Alternatively, many UV based ballast water treatment systems overcome the issue of low clarity waters by including a UV intensity sensor, or similar, which gauges the intensity of the UV light in the water, and often adjusts the flow to ensure sufficient treatment. This can, of course, result in a drastically reduced flow rate causing significant impact on vessel performance.
Either way, vessel trading patterns and locations could have an impact on the suitability of a UV based ballast water treatment system, and hence vessel performance.
EC based ballast water treatment systems also have their own limitations. With electro-chlorination technology utilising the salinity of sea-water to generate hypochlorite through electrolysis, it is immediately apparent that EC based systems are sensitive to water salinity.
Vessels trading in brackish waters, or fresh waters, may therefore struggle to successfully operate an EC system onboard.
Manufacturers of EC based ballast water treatment systems present work-around operational concepts to minimise the impact of sailing into fresh or brackish waters. Concepts such as filling the aft peak tank with sea-water prior to entering the fresh or brackish water, and hence using aft peak tank as the supply of sea-water to the EC system do work. However, these concepts require a change in operation of the vessel – and effectively remove the aft peak tank from the vessel’s useable ballast tanks whilst in fresh or brackish waters.
Vessel owners and operators should pay close attention to vessel trading patterns and hence the suitability of EC based ballast water treatment systems – and the stability and/or longitudinal strength capabilities of the vessel without access to the aft peak tank for ballasting / de-ballasting.
United States Coast Guard (USCG) Type Approval has long been considered the “Gold Standard” of ballast water treatment system approvals. Since its inception in 2012, the USCG Type Approval stamp on any ballast water treatment system has been viewed by many owners and operators as a must have, and one of very few ways to de-risk the investment decision.
However, the onerous testing requirements under USCG Type Approval testing have created some significantly limiting factors, particularly with UV based systems, which could have significant impact on vessel performance.
Perhaps the most significant impact is the holding time onboard. At the time of writing, all UV based ballast water treatment systems with USCG Type Approval require a minimum of 3 days holding time onboard, between ballasting and de-ballasting procedures. Whilst not an issue for vessels with long ballast voyages, for those trading frequently at ports throughout the US, this is a significant logistical challenge.
It is understood that some UV manufacturers are re-testing under USCG for zero days hold time, but this is likely to come at a cost – predominantly flow rate. In order to reduce the hold time onboard, it is suspected that UV manufacturers will have to increase the UV dose that the organisms are being exposed to. Achieving this in practice leaves two options – lower the flow rate, or increase the power. It is strongly suspected that manufacturers will use a combination of both.
Regardless, whether it be a drop in flow rate or an increase in power consumption, operating a UV system under its USCG Type Approval will have a profound impact on the vessel’s operation and efficiency.
Gravity Ballasting Operations
Gravity ballasting operations are very attractive to vessel owners and operators. The ability to de-ballast significant capacities of ballast water (or similarly ballast significant capacities via bottom-flooding valves), without running ballast pumps and hence consuming fuel, is an operational cost saving dream. However, with most ballast water treatment technologies requiring some form of treatment during both ballasting and de-ballasting, gravity ballasting operations are unlikely to be suitable.
Vessels that rely on gravity ballasting operations for efficient cargo operations, or to minimise fuel consumption costs, will be impacted by the inability to utilise gravity ballasting going forwards. Owners/operators should remain conscious of the potential increase in ballasting / de-ballasting times, and the resultant increase in fuel consumption as a result of running ballast pumps.
Ultimately, vessel performance will be impacted by the installation of a ballast water treatment system. The extent of the impact can minimised, through careful planning during the ballast water treatment system feasibility study and selection stage. Close cooperation between the vessel owner, operators and engineering consultancy can help ensure that reductions in vessel performance are minimised, and any potential contractual or logistical issues are avoided.