24 July 2023 (Lloyd's List) - REVISED guidelines for the reduction of underwater radiated noise from shipping, as approved earlier this month by International Maritime Organization’s Marine Environment Protection Committee (MEPC80), will take effect on August 1, 2023.
There has been a growing focus on URN from commercial shipping because the adverse impact on marine life has been identified as relevant to the UN Sustainable Development Goal number 14 about life below water.
Draft guidelines were agreed by the IMO Sub-Committee on Ship Design and Construction (SDC 9) in January this year. They revised guidelines that had been issued in 2014 and drew on latest developments in ship design and technology.
The guidelines recognise that commercial shipping is one of the main contributors to URN, which has adverse effects on critical life functions for a wide range of marine life, including marine mammals, fish and invertebrate species upon which many coastal indigenous communities depend for their food, livelihoods and cultures.
They provided an overview of approaches applicable to designers, shipbuilders, and ship operators to reduce the noise generated by any given ship. Further, they are intended to assist relevant stakeholders in establishing mechanisms and programmes through which noise reduction efforts can be realised.
The correspondence group that developed the guidelines has been instructed to report back to SDC 10 in January 2024 to address the remaining work under the agenda item.
In particular, it was tasked with revising a flowchart on the URN Noise Management Planning process to reflect the revised guidelines, with the noise management plan to be used as a tool for raising awareness.
The group will also finalise the provisional list of suggested next steps to further prevent and reduce URN from ships.
Later this year, IMO’s Department of Partnerships and Projects will launch a two-year project called the Global Partnership for Mitigation of Underwater Noise from Shipping (GloNoise Partnership).
The initiative will include a strong developing countries’ element as well as public-private engagement.
The specific objective of the GloNoise Partnership is to assist developing countries and regions to raise awareness, to build capacity, and to collect information to assist the policy dialogue on anthropogenic underwater noise from shipping.
In a white paper published on July 17 focusing on ship hull air lubrication technology, Prof John Carlton, a Fellow of the Royal Academy of Engineering, concludes that the use of an air-bubble mixture close to the ship’s hull reduces the radiated noise emissions from the hull.
This is “because of the large difference in characteristic impedance between the air bubbles and the surrounding water which make the bubbles effective reflectors of acoustic energy”.
The degree of noise attenuation mainly depends on the bubble diameter. Air emissions from the leading edges of propeller blades are also helpful in reducing radiated noise emissions.
“The presence of air exhausting into the propeller disc attenuates the higher harmonics of the radiated pressure spectrum emitted by a cavitating propeller,” Carlton adds.
