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News: Anthropogenic sound - practical considerations

08 October 2019

In a previous Standard Club article we introduced the issue of anthropogenic sound. This form of noise pollution, for which shipping is the principal source, has profound effects on the ability of marine wildlife to migrate, mate and feed. Given that commercial shipping is a cross-border endeavour, a coordinated international industry-wide approach will be required to effect meaningful change.

Fortunately, this problem is now gradually being recognised in the shipping industry. Having first discussed the issue of underwater noise in 2004, the IMO have since issued a set of non-mandatory technical guidelines to minimise noise pollution from commercial ships. 
 
Cavitation
 
Propeller cavitation, which accounts for around 80% of ship noise at those lower frequencies at which marine mammals communicate, is essentially the formation and popping of bubbles as the blades rotate. Cavitation may be increased by uneven wake-fields (which is the boundary layer of water around the hull created by friction) in which the propeller operates.   
 
The IMO recognises that the greatest opportunities for reducing underwater noise will arise during the ship's design phase. The guidelines suggest optimising the design of the ship's propeller and hull to reduce cavitation, although they do acknowledge that this may come at the cost of decreased efficiency, which is a commercial consideration that owners will need to take into account.
 
Onboard machinery
 
Noise pollution is also created by machinery onboard ships, which generate sound and vibrations during their operation. The guidelines suggest that owners should take into consideration the sound and vibration levels of any new equipment when equipping new ships. Owners can also use special engine mountings with vibration isolators to reduce the environmental impact of the machinery.
 
Propeller and hull cleaning
 
The guidelines suggest to undertake proper propeller polishing and hull cleaning to remove marine fouling and reduce the roughness of the ship's surfaces, which will in turn lead to the reduction of cavitation. An additional benefit of maintaining a clean propeller and hull is that the reduction in drag will also lead to improved fuel efficiency, and potential cost savings.
 
Restrictions on shipping operations
 
The IMO is able to restrict maritime activity in so-called 'Particularly Sensitive Areas' (PSA). These are areas considered to require additional protection through action by the IMO due to their recognised ecological, socio-economic or scientific significance. 
 
For example, the IMO has already agreed to the reduction in speeds, rerouting and narrowing of traffic lanes in certain areas to reduce the number of whales struck by commercial ships. Notable PSAs include the Great Barrier Reef, the Florida Keys and the Galapagos Archipelago. 
 
Conclusion
 
In future, we may see further restrictions placed on ships transiting areas which contain local fauna susceptible to the effects of anthropogenic sound.  As it currently stands, however, the IMO suggestions mentioned above are only guidelines and are not, therefore, strictly enforceable on ship owners. 
 
That being said, given the increasing global concerns regarding the environment it is not inconceivable that more substantial international rules may be imposed in the near future. The Standard Club will continue to monitor any developments in this space.