Reducing underwater noise pollution from large commercial vessels
There is increasing concern about the effects of underwater noise on marine life. A major contributor to this is the noise generated by shipping.
The International Fund for Animal Welfare (IFAW) has identified that significant reductions in ambient noise can be made by reducing the noise output from the noisiest vessels. Resulting from this, IFAW commissioned Renilson Marine Consulting Pty Ltd (RMC) to undertake a brief desk top study into technologies that may be used to reduce the underwater noise output from the loudest commercial vessels.
This report is the primary output of the study, and is intended to inform discussions of technical measures and future research needs that can be implemented by governments and industry.
The report is arranged in four parts. Part I is the introduction and background, where some of the general issues are discussed. Part II covers some of the possible technologies that can be used to reduce noise for merchant ships, and Part III gives some examples for different ship types, discussing the practicalities and likely costs involved. Part IV is the recommendations and concluding comments.
It appears that there is considerable difference in the noise propagated by the noisiest and the quietest conventional merchant ships (excluding those designed specifically for low noise). Based on the current desk top study it is reasonable to develop a cautious note of optimism that the noisiest ships can be quietened using existing technology without reducing their propulsive efficiency.
There is little doubt that the dominant feature of these noisiest merchant ships is cavitation associated with the propeller. The two major aspects that influence the level of cavitation are:
1. propeller design; and
2. wake flow into the propeller.
Improvements in propeller design, either by modifying the existing propellers, or by fitting new propellers designed with noise reduction in mind, have the potential to reduce hydro-acoustic noise for the noisiest merchant ships, and increase propulsive efficiency.
In addition, there is the potential to improve the wake flow into the propeller for existing ships by fitting appropriately designed appendages such as wake equalising ducts, vortex generators or spoilers. The technology exists to do this, and although there is some understanding of the improvement that these devices will have on propulsive efficiency, there is little knowledge about how they will reduce the hydro-acoustic noise – however available data suggests that they will do so.
For new ships the wake flow can be improved by more careful design, which will require an increased design effort, including careful model testing and computational fluid dynamics analysis.
For ships which spend time in ballast this work should be extended to include optimisation of the propeller design and wake flow in that condition. This extra effort will cost more, however on the basis of the data available it is likely to result in improved propulsive efficiency as well as in reduced hydro-acoustic noise.