Action is accelerating to tackle the noise caused by ship equipment and vessel structures, writes Dr. Qing Yu, director, technology – structures and hydrodynamics, ABS
Underwater radiated noise (URN) emitted from commercial ships has attracted increased scrutiny in recent years, with studies indicating that sound generated by shipping contains high energy in the frequency range below 1,000 Hertz.
This frequency range overlaps with the band that is critically important for marine species and ecosystems and can cause the masking of acoustic communications crucial for foraging, navigation, ultimately even threatening their survival.
To encourage the reduction of the negative effects of URN on marine species, International Maritime Organization (IMO) published MEPC.1/Circ.833 Guidelines for the Reduction of Underwater Noise from Commercial Shipping to Address Adverse Impacts on Marine Life in 2014.
The European Union (EU) Marine Strategic Framework Directive, 2008/56/EC, which came into force in 2008, requires EU Member States to achieve a ‘Good Environmental Status’ in all the EU waters by 2020, including the reduction of URN.
In January 2019, a technical workshop entitled Quieting Ships to Protect the Marine Environment was held at IMO headquarters in London, with over 140 participants from around the world. The workshop conducted a thorough review of the state of knowledge on ‘quieter ship’ design and contributed to several papers submitted to IMO’s Marine Environment Protection Committee (MEPC) in 2019 and 2020.
Classification societies have also stepped up their efforts to support the maritime industry to address this emerging demand, with optional classification notations offered to those ships designed and operated with consideration of the impact of underwater radiated noise.
Sources and Standards
The main source of URN is from the rotating propeller. Machinery equipment such as the main engine can also be an important contributor, especially at low vessel speeds; a less prominent, yet still relevant source, is water flow around ship. Among various technical challenges for building quieter ships for commercial operation, research and technology development in recent years have been primarily focused on:
To establish a consistent metric for the underwater noise emitted from commercial ships, significant efforts have been made to develop standards for underwater noise measurement, including ISO 17208 and ANSI/ASA S12.64. Due consideration is given to the effect of site selection, environmental conditions, requirements of measurement instrumentation, test procedures and measurement data analysis and interpretation during sea trials.
“URN mitigation measures must be implemented in a way that does not undermine energy efficiency”
Additional requirements are provided in classification society rules, and the further development of ISO standards specifically for sea trial measurements in shallow water is currently underway.
Compliance with classification society requirements for underwater noise is currently to be verified through dedicated sea trials. Analysis methods and tools for prediction of URN can assist in evaluating the noise reduction solutions at the design stage. However, the maturity of these analysis method is still evolving and needs further development before the classification approval can be based on analysis alone.
Active research and development has, in recent years, improved the accuracy of analysis tools capable of modeling the broadband noise emitted from the cavitating propeller and the tonal noise at the frequency of the propeller rotation. The URN analysis typically requires detailed modeling of the effect of hull form, propeller, the interaction of hull and propeller, and machinery configuration.
Simplified methods suitable for fast comparative evaluation of potential design solutions at the early design stage are also available and under continuous improvement. With an increasing market demand for reliable analysis of URN, as well as the accumulation of in situ measurement data for validation, the development of the analysis methods and tools has been greatly accelerated.
There are a variety of mitigation methods for different noise sources. These methods vary greatly in terms of cost, effectiveness, and impact on vessel construction and operation. In general, it is desirable to perform a detailed underwater noise assessment and choose the most appropriate solutions during the design stage because fixing underwater noise issues after construction could be difficult and expensive.
Underwater noise generated by propeller is mainly caused by cavitation, which is basically ‘boiling’ of water and then the collapse of water bubbles as they move away from the low-pressure zone near the propeller blade. To lower propeller-radiated noise, cavitation needs to be reduced. This can be achieved by various methods such as choosing a propeller with high cavitation inception speed, optimising the propeller design, improving the inflow to the propeller by wake optimization, or improving the propeller rudder interaction. In addition, propeller polishing can remove marine fouling, repair erosions, and reduce surface roughness, which helps to reduce cavitation.
Machinery-induced underwater noise is mainly generated by structural vibration. The machinery vibration can first transmit to the foundations, then propagate to the hull structures, and finally result in the radiation of underwater noise. Reducing this vibration and isolating the vibration source from the ship’s hull are effective ways to mitigate machinery-induced underwater noise.
Hull treatment solutions can enhance the ship’s hydrodynamic performance and therefore improve the wake flow into the propeller and reduce power requirement. Commonly used methods include hull form optimisation, installing hull and propeller appendages as flow equalisers and regular cleaning of the hull. Acoustic decoupling coatings and structural damping tiles can also be applied to reduce the radiation efficiency of the hull vibration.
In addition to appropriate ship design and maintenance focusing on those major noise contributors, operational measures can also play a big role in reducing URN. Several classification societies offer optional class notations, recognising the operational measures taken by those ships whose trading routes pass through environmentally sensitive areas where URN needs to be controlled within a given limit.
A commonly applied operational mitigation measure is to reduce or eliminate propeller cavitation by reducing the speed for ships equipped with fixed pitch propellers. For ships with controllable pitch propellers, reducing ship speed may not be effective in reducing propeller cavitation, unless the pitch controller is specifically designed for controlling noise emissions.
“There are a variety of mitigation methods which vary greatly in terms of cost, effectiveness, and impact on vessel construction and operation”
In addition to technical viability and cost considerations, an important backdrop amid growing interest in quieter ship is the maritime industry’s quest for decarbonization, with challenging goals of achieving IMO’s greenhouse gas (GHG) emission reduction targets. Underwater radiated noise mitigation measures must therefore be implemented in such a way that they do not undermine energy efficiency.
Efforts are being made to better understand the impact of various underwater noise mitigation measures on ship energy efficiency and to explore potential co-benefits or trade-offs. The current findings are encouraging in that various measures, such as improving propulsion efficiency and reducing ship speed can in general reduce fuel consumption and URN at the same time.
Clearly, there is an emerging demand to build quieter commercial ships with lower URN emissions, a trend which aligns with the overall goal of developing a more sustainable global maritime industry.
In addition to IMO/EU regulatory developments, local port authorities in environmentally sensitive areas, albeit of a small number at present, have started providing port fee reductions for ships equipped with underwater noise reduction measures.
The recognition of the importance of the issue also led to a recent call for revisiting the 2014 IMO Guideline for the reduction of underwater noise from commercial shipping (MEPC.1/Circ.833). The proposal in MEPC 75/14 submitted to IMO in 2020 recommends the review of the IMO Guideline through identifying barriers for its implementation, promoting the development of technological innovations, leveraging synergies with energy efficiency requirements and developing action plans.
ABS is actively supporting the maritime industry’s efforts to address rising demand for quieter ships by providing classification notations and criteria to signify the application of underwater noise mitigation measures, through the ABS Guide for the Classification Notation Underwater Noise.
For example, vessels entering the Canadian ports of Vancouver and Prince Rupert with the ABS UWN notation are now eligible to receive such a reduction in fees. Further efforts to develop more capable tools and techniques to better predict and mitigate URN from commercial ships are currently underway.