The impact of shipping’s energy efficiency measures on reduction of underwater radiated noise, and opportunities for co-benefit

Vakili, Seyedvahid, White, Paul and Turnock, Stephen (2024) The impact of shipping’s energy efficiency measures on reduction of underwater radiated noise, and opportunities for co-benefit International Maritime Organization 70pp.

Abstract

This study has been undertaken to assess the interrelationship between measures aimed at enhancing ship energy efficiency and mitigating URN emissions. It commences by conducting a review of the International Maritime Organization's (IMO) initiatives and strategies for decarbonizing the shipping industry. Moreover, it identifies the measures and tools available that can contribute to expediting the transition towards zero-emission shipping by around the year 2050.

In parallel, this study examines ambient underwater noise trends across various regions. This is complemented by projections of seaborne trade growth, enabling an evaluation of the ambient noise trends. These insights are valuable for assessing the feasibility of targets for reducing URN emissions from commercial vessels. Additionally, this report presents cases illustrating the synergies that can be achieved through simultaneously enhancing energy efficiency and reducing URN in commercial vessels. It outlines the potential of various measures and tools in not only improving energy efficiency but also reducing greenhouse gas (GHG) emissions and simultaneously curbing URN emissions.
In pursuit of the IMO’s revised GHG strategy, the maritime industry is endeavouring to define the most effective strategies for curtailing GHG emissions and transitioning toward a zero-emission sector. This necessitates the modernization and revitalization of aging vessel fleets and the adoption of carbon-neutral fuels, all within the context of navigating a complex landscape of green technologies. Further complicating this challenge is the extended lifespan of ships, where some vessels prove too antiquated for retrofitting and yet too young for scrapping. Recognizing the absence of a “silver bullet” and “one-size-fits-all” solution for decarbonizing the maritime sector, a diverse array of measures demonstrates considerable potential for achieving substantial emissions reductions. These measures encompass the adoption of carbon-neutral fuels and energy efficiency enhancements, including speed reduction, and logistics optimization.
The study underscores the pivotal role of scaling up the production and availability of carbon-neutral fuels in achieving the goal of zero-emission shipping by 2050. However, this endeavour is met with various challenges, including the cost of alternative fuels, insufficient infrastructure, logistical constraints, the maturity of technology in both onboard vessel systems and supply-side infrastructure, considerations associated with ship and engine design, crew training, safety protocols, and the significant financial investments required. While it is anticipated that carbon-neutral fuel will contribute to approximately 60% of zero emissions by 2050, the study demonstrates that around 32% of emissions reductions will rely on energy efficiency measures, including roughly 23% attributed to speed reduction, to meet the IMO's GHG emission targets by 2050.
The study anticipates that strategies including speed reduction, wind-assisted propulsion systems, Energy Saving Devices (ESDs), and air lubrication systems will assume important roles in realizing the IMO’s GHG reduction strategy. Moreover, as the maritime sector undergoes a shift towards vessel electrification, the deployment of fuel cells, batteries, or hybrid technologies, particularly in short sea shipping, holds significant promise for enhancing energy efficiency and expediting progress in alignment with the IMO's GHG reduction strategy.
This study delves into the origins of URN in commercial vessels, and highlights cavitation as the primary source, particularly when the propeller Cavitation Inception Speed (CIS) is exceeded. The research also reveals that an increase in propeller blade area whilst reducing URN, can adversely affect the propeller's energy efficiency. Hence, it is important when designing a propeller to carefully weigh the trade-offs between cavitation, noise reduction, and efficiency. Additionally, the challenges associated with mitigating URN through speed reduction in Controllable Pitch Propellers (CPP) is discussed.
Nevertheless, the study highlights the significant synergy between the implementation of energy efficiency strategies and the reduction of URN. The results suggest that reducing vessel speed by 20% can lead to a significant 6 dB decrease in underwater radiated noise (URN) in fixed-pitch propeller vessels. Wind-assisted propulsion systems have the potential to reduce URN by up to 10 dB, while air lubrication systems can achieve even greater reductions, surpassing 10 dB in URN reduction. Considering the aging fleet of vessels and the forthcoming, more stringent Carbon Intensity Indicator (CII) requirements, the study anticipates a growing trend of vessels implementing energy-efficiency measures to align with the IMO’s revised GHG strategy targets. This shift is expected to result in a reduction of URN on individual vessels.
Ross (1976) includes an equation for ambient URN, and this correlates, the most influential factors contributing to deep ocean URN. Applying this equation to worst case scenarios suggests that implementing suitable energy efficiency measures not only reduces the average source level URN from individual ships, but also mitigates ambient noise by an approximately equivalent level. Given that energy efficiency measures, encompassing speed reduction (constituting approximately 23% of these measures), are expected to contribute up to 32% to the reduction of GHG emissions from the shipping industry by 2050, it is conceivable that a 6 dB average reduction in global URN levels can be achieved through a mere 20% speed reduction, with the potential for further reductions through the adoption of other energy-efficient measures.
In consideration of the above, the study underscores that GHG reduction initiatives will exert a substantial influence on diminishing global URN levels. Achieving URN reduction targets, such as the 3 dB per decade Okeanos target, appears to be a feasible outcome. Furthermore, the study reveals that, under a worst-case scenario involving annual growth in transport work of 3.3%, and assuming nothing else changes, ambient noise levels are projected to increase by up to 2.6 dB by the year 2050. In light of these insights, the achievement of the 32% contribution from energy efficiency measures in GHG emission reduction by 2050 could effectively offset the effects of substantial seaborne trade growth and mitigate ambient noise levels, even in the most challenging scenarios.

This record has no associated files available for download.

Contributors

Author: Seyedvahid Vakili

Author: Paul White

Author: Stephen Turnock

Download statistics