In 2023, the IMO MEPC approved the Revised guidelines for the reduction of underwater radiated noise from shipping to address adverse impacts on marine life. These guidelines, which took effect from 1 October 2023, aim to reduce underwater radiated noise from ships and concern particular ship and equipment designers, shipbuilders and shipowners and operators, classification societies, suppliers, manufacturers and other stakeholders.
IMO has highlighted that the effective mitigation of underwater radiated noise (URN) impact from ships on marine life requires international collaboration and action at various levels, involving multiple stakeholders including, but not limited to, seafarers, designers, shipbuilders, shipowners and ship operators, maritime authorities, suppliers, manufacturers and classification societies.
It is important to recognize that for both new and existing ships, the technical feasibility and cost-effectiveness of URN reduction measures, considered either individually or in combination, will be strongly dependent on the design, operational parameters and requirements relevant to a particular ship. A successful strategy to reduce URN should be a process that includes, to the extent possible, the design stage, the baselining of URN measurements (predicted or actual), the development of URN targets, and the implementation, monitoring and assessment of technical and operational measures to achieve those targets.
The interactions between the implementation of URN reduction measures and other objectives such as, but not limited to, energy efficiency, biofouling reduction and ship safety, and the resulting contributions should be carefully considered.
Design, technical, operational and maintenance URN reduction approaches applicable to new and/or existing ships
- Optimize ship hull form (and appendages) design for hydrodynamic performance and homogeneous wake field to reduce cavitation
- Optimizing propeller design to reduce cavitation, optimizing load, ensuring a uniform water flow and hull-propeller interaction and careful selection of the propeller characteristics such as diameter, blade number, blade area, pitch, skew, rake, and sections and innovation material
- Emerging technologies like wind-assist technologies to reduce propeller loading and cavitation noise
- Air injection to propeller
- Wake flow improvement
- Careful selection of onboard machinery and installation with appropriate structure-borne noise levels control measures, proper location of equipment in the hull, and optimization of foundation structures
- Machinery installation and isolation for instance resilient mount and flexible coupling in four-stroke engines with a reduction gear, vibration isolation mounts and improved dynamic balancing for reciprocating machinery
- Optimizing the ship’s trim to reduce the required power and therefore propeller cavitation noise
- Improving voyage planning (e.g. optimum route, coordinated across fleets, national and international designated protected areas/sea-ice covered region, including well-known habitats or migratory pathways)
- Decreasing propellor RPM by reducing the shaft RPM (and/or engine output) for ships equipped with fixed pitch propellers
- Ships routeing measures to avoid national and international designated protected areas including well-known habitats or migratory pathways
- Propeller maintenance (and cleaning/coating)
- Hull maintenance (coating and in-water hull maintenance and cleaning, except acoustic anti-fouling systems where possible in national and international designated protected areas)
