A practical approach to rating ship noise
Canadian researchers have developed a rating system that measures vessels’ underwater noise during normal operations and compares each ship with others of its type, giving owners a clearer route to quieter shipping.
Nobody likes a noisy neighbour – not even in the ocean. Noise from vessels can interfere with marine life, disrupting their ability to feed, detect predators, and communicate. To help manage that noise, researchers in Canada have developed a new ship noise rating system that could make it easier to assess vessels.
More practical noise measurements
Rating the amount of noise a vessel produces is not a new endeavour. Classification societies already offer quiet-ship notations to vessels that meet specified underwater-noise requirements.
The trouble is, says Marie-Noël Matthews, Project Scientist at JASCO Applied Sciences, that the “noise ratings of ship classification societies are inconsistent in both measurement approach and the noise limits used. It is also not always clear how some classification societies established the thresholds used in their quiet-ship notations.”
For these notations, vessels generally need to suspend normal operations while noise measurements are taken under prescribed conditions. “That approach helps standardise the measurements, but it is costly and logistically complex for operators and does not necessarily represent the sound a vessel produces during normal operation,” Matthews explains.
Published in the Marine Pollution Bulletin, the system proposed by Matthews and colleagues at JASCO and Transport Canada allows vessels to be measured during normal operations. Such measurements could be gathered, for example, from monitoring stations in busy shipping lanes.
“To make any fleet-wide noise-management goal achievable, we need a large number of vessels to be rated,” says Matthews. “At the moment, that scale is difficult to achieve with existing quiet-ship notation systems.”
Comparing like for like
Measuring noise is only part of the approach. The researchers also developed noise limits based on measurements of the existing fleet, with different limits for different types of vessel.
“With a single limit, the thresholds would only target the noisiest categories like container ships and vehicle carriers; quieter categories would not have an opportunity to improve,” explains Matthews. “A common limit may also be achievable for only a few vessel categories while being unrealistic for others.”
A vessel is rated according to how its noise compares with other vessels of the same type, placing it in one of four groups from quieter to noisier ships.
“Our goal was to develop a system that lets noise-management authorities set realistic targets for the current fleet and make those targets more ambitious over time,” Matthews says.
“The long-term goal is for the worldwide fleet’s noise distribution to become quieter. Then in a few years the noise levels associated with each vessel category can be reduced.”
What can shipowners do?
For shipowners who find their vessel on the noisier side of the rating, there are several ways to reduce its noise.
“Some vessels may receive a noisier rating because of elevated levels in only one or two frequency bands – in other words, because of tonal noise,” says Matthews. Those tones may come, for example, from specific onboard equipment that in some cases could be turned off when not needed, or fouled hulls and propellers.
Keeping the propellers clean also helps with reducing cavitation – the formation and collapse of bubbles around a propeller, which can generate underwater noise.
On an operational level, Matthews says vessels with noisier ratings could be asked to reduce their speed in certain areas. While this would not change the underlying rating, “slowing down is often the easiest operational change for reducing noise emissions.”
Other options include retrofitting existing vessels with quieter equipment, “while designers of new vessels could incorporate more efficient hull forms and propulsion systems from the start,” says Matthews.
Ultimately, Matthews hopes the system can help turn those individual changes into reductions across the fleet. “We hope it provides a common, transparent framework for moving from vessel measurements to practical action.”
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Main image: Aerial view of a pod of 9 humpback whales
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