ABS Highlights Scaling & Regrowth Issues
The American Bureau for Shipping (ABS) has released two official documents highlighting the issues surrounding scaling of BWM systems and the problem of organism regrowth on longer ballast journeys [1, 2].
These highly authoritative documents confirm the views long held by Coldharbour regarding the fact that most in-line BWM systems have only tested their smallest system in order to achieve Type Approval, and that the potential for regrowth has generally been ignored.
Scaling of BWMS represents a major issue when choosing the correct system, particularly for owners of large ships that require high pumping rates (e.g. VLCCs). Publically-available data show that all land-based and most shipboard testing for type-approved systems has been carried out at low treatment rated capacities, yet about two-thirds of the BWMS installed have a capacity greater than or equal to 1000 m3/h.
For many type-approved systems, it appears that scaling to significantly greater capacities were either not conducted following IMO scaling guidance, or not described in the final test reports. A recent survey conducted among 15 ship-owners concluded that the majority of systems already fitted to their vessels experienced severe operational and reliability problems. Furthermore, BWM systems with capacities greater than 1000 m3/h were found to have very limited operational success. Problems with filters, piping leakage, sensors and major components failures, among others, resulted in downtime which lead to serious operational delays.
To address this issue, ABS makes a series of suggestions to improve BWMS scaling, emphasising the need to clarify IMO’s guidance, to determine the sensitivity of the equipment to “worst case scenario” conditions using models, and to evaluate the weaknesses of the technologies used, among others. Full-scale shipboard validation of scaling should be required if the responses of the system to changes in performance parameters (e.g. temperature, UV transmittance, organic carbon) cannot be validated using a pilot plant and/or bench tests. Additionally, shipboard testing should include the extremes of the treatment rated capacities, so that scaling can be done by interpolating rather than extrapolating tests results. This should address the ship-owners main concern of getting the equipment to operate consistently. Sea trials for the Coldharbour BWMS for IMO Type Approval were carried out on board the VLCC Alfa Glory confirming that this system has been tested at full scale under regular commercial operating conditions.
On the subject of regrowth, the documents published by ABS highlight the inadequacies of the IMO and USCG existing protocols with regard to ballast holding time (5 and 1 days, respectively), as they are not representative of operational use of BWMS. Voyages durations are longer for larger systems and this is not being considered in current protocols. Independent studies show that regrowth of ballast water organisms can occur in treated ballast water in as little as 3 days and will almost certainly be present after 10 or 15 days, which are typical journeys for larger ships.
To address the issue of regrowth, ABS suggests that both, the differences between tested holding times and in-service ballast voyages, as well as accelerated growth should be modelled.
Read the reference documents here.