Way Ahead for the Underwater Radiated Noise Management

Underwater Radiated Noise (URN) is increasingly becoming a serious environmental concern across the globe, and the international community is aggressively getting into means and measures to contain this problem. The United Nations Sustainable Development Goals – 14 (UN-SDG – 14), clearly highlights the need to contain noise underwater. Shipping noise is recorded to be increasing at an alarming rate of 3.3 dB per decade. It is doubling every decade, as per recordings underwater since 1950, published by Donald Ross. Analytical efforts by G V Frisk have also given similar figures. Shipping noise is the single ubiquitous source of low frequency noise in the ocean. The low frequency means minimal attenuation while propagating underwater, so the impact of URN is over an extended range (over a few thousand kilometres). Since shipping is directly coupled to economic growth, measures to contain shipping traffic are socio-economically and politically unviable.

URN management has been a very critical requirement for naval platforms, due to the acoustic stealth requirements. While acoustic stealth measures have progressed significantly among advanced navies, the classified nature of such measures has ensured that these are not available in open source. Another aspect that merits attention is the aspect of cost. Military platforms prioritise the effectiveness of the stealth measures and cost is not a very major limiting factor. Right from the design stage to construction and operations, acoustic stealth is a major consideration for naval platforms. However, commercial merchant shipping can ill afford such high costs and thus, the solutions need to be viable on all fronts.

"URN management has been a very critical requirement for naval platforms, due to the acoustic stealth requirements."

URN generation has three main components as per the Source-Path-Receiver model:

Source: The noise control measures on-board the platform to contain the noise being emitted from the hull on the platform. The machinery on-board needs to be individually effective in containing noise, then the transmission path from the seat of the equipment to the hull and also the structural aspects that will mitigate the noise transmission. All these aspects require attention to achieve success.
Path: The underwater medium fluctuations are extremely sensitive to the acoustic propagation conditions in the specific region based on the local parameters. The tropical waters have very unique acoustic propagation characteristics and modify the source signal, prior to being recorded at the receiver. The overall impact of noise on the marine species needs to account for the medium impact. The acoustic propagation in the tropical waters could degrade upto 60% and thus, the propagation characteristics are critical in our analysis of the impact.
Receiver: Marine mammals have specific auditory systems and each specific species has a unique characteristic. The signal originating from the hull of the platform travels through the underwater medium and is received at a distant location by the marine species. The receiver sensitivity may vary in amplitude, frequency and phase, affecting the ability to get an exact assessment of the impact. Species specific impact assessment is important.

The true assessment of the impact on the marine species will involve a comprehensive source-path-receiver analysis. The regulatory provision must be able to account for the diversity in the impact across specific species and also the underwater medium distortions in signal propagation. The standards promulgated by the western nations in the temperate and polar regions may be too stringent for us, given the tropical conditions in the IOR.

"It is highly recommended that we in India evolve our own URN standards and then have our own plans and timelines for implementation."

It is highly recommended that we in India evolve our own URN standards and then have our own plans and timelines for implementation. Our ship operators and ship builders need to understand the importance of URN management and participate in the URN management process. Regulatory framework and monitoring mechanism needs to be formulated with deeper understanding of our unique conditions on all the three fronts – socio-economic, political and technological. Accepting the West-driven formulation and being in a denial mode will be detrimental to our strategic interest.

URN management has three stages to contain the noise being emitted from the hull of the platform:

Design Stage: The noise control measures need to be adopted right at the design stage by the ship designer and the ship owner. This will involve choosing the right machinery and the effective structural design to minimise the noise on-board. This can mean high costs to the owner.
Construction Stage: The implementation of the design by the shipyard is another very critical factor. The best of designs, if not implemented well, could mean high URN at the hull of the ship. Even repair of the ships can mean high URN post refits.
Operation Stage: Noise control measures can be implemented during the operation stage as well. The propeller cavitation is the single most important noise source on-board. However, the cavitation inception is the key inflection point, where the URN suddenly increases exponentially. Some major machinery on-board could also have certain deformities, resulting in high noise during operations. Thus, noise control measures could be realized at the hands of ship operators as well.

URN management needs to be undertaken to manage the acoustic habitat degradation and, more importantly, to comply with the SDG-14 requirements. URN management can be undertaken across all the three stages of design, construction and operations. A nuanced strategy can be evolved based on the deeper understanding of the entire URN generation and management process. The parameters for URN management formulation need to be cost, effectiveness and efficiency. URN management could be a very significant opportunity for young India given our demographic advantage. The significant maritime push by the Government of India can be aligned to the sustainable growth of the shipping industry in the country and the region.

"URN management could be a very significant opportunity for young India given our demographic advantage."

It may be categorically stated that the effective noise control measures will also mean efficient ship operations in terms of better fuel efficiency, low maintenance and minimal failure due to optimum operational deployment of the on-board machinery and, most importantly, enhanced crew comfort on-board, resulting in overall improvement of the ship’s operation. Another important issue is the difference between noise control on-board and URN management. Enhanced noise control will help effective URN management; however, it does not guarantee it. Thus, the costing of URN management measures needs to be looked at from a comprehensive perspective.

Multiple experts and researchers have worked on various aspects of the issue in isolation, and the medium impact has not received the attention it deserves. Canada, EU, UN and some other organizations have given significant attention to this issue and efforts have been made to develop measures to contain the noise and also understand the impact on the marine species. While recognizing the challenges in this entire effort, none of them have been able to comprehensively address this issue. The following aspects remain unresolved:

There is no one to one relation between individual noise control measures on-board and URN outside the hull. Thus, the effectiveness of these measures needs to be analysed from an overall perspective. Prediction and validation tools have a major role to play in this. The uncertainties of the transmission path play a critical role in this analysis.
The underwater medium has a significant impact on the acoustic propagation of URN from the source to the receiver. The impact assessment of URN on multiple species needs to account for this.
The species-specific impact has to be factored before we can make a generalized regulatory framework for containment of URN; this is an extremely challenging aspect.
The multi-disciplinary nature of the entire URN management process is a big challenge in terms of bringing synergy among the stakeholders and evolving a comprehensive solution.
A comprehensive solution that addresses the Source-Path-Receiver model and the cycle of Design-Build-Operate in URN management is also a challenge.
We need a tool that can effectively assess the impact of individual solutions proposed by multiple experts and researchers to the overall URN levels, and aid in bringing about a nuanced policy framework and monitoring mechanism.

India has been growing at an exponential rate on all accounts, be it economically, shipping traffic wise, prioritising maritime activities, governance and more. Economic growth cannot be compromised, but the government is also conscious of its responsibilities to the SDG-14 and other international norms. India’s stand on the issue is as follows:

Most of the big shipyards in the country are defence shipyards with major stakes in naval ship building. Acoustic stealth is an integral part of the entire shipbuilding activities. The GoI has also started encouraging percolation of the best practices of noise control to merchant shipping. Smaller shipyards will also follow in due course.
Inland Water Transport (IWT) is a mega project being driven by the GoI; sizable shipping traffic will be seen in the inland freshwater bodies along with coastal shipping. Noise control will be a critical component of the sustainable growth model.
The tropical littoral waters have unique acoustic propagation characteristics and thus the URN standards need to account for the modification in the radiated noise due to the local site specific propagation in our waters. The URN standards of the West are not directly applicable to our waters.
Species-specific study is being encouraged to build deeper understanding of the impact of URN in our waters. Freshwater dolphins in the river systems in India are known to have acoustic vision and the increasing noise levels could be detrimental to their survival.

The Maritime Research Centre (MRC), Pune has developed a unique tool for real-time assessment of the URN from the ships. This high-tech data analysis and prediction tool based on the AIS data provides a realistic assessment of the changes in the low frequency noise due to shipping traffic captured by the AIS system. The tool will aid in detailed analysis of various noise control measures and also local site-specific assessment accounting for the underwater medium impact. Policy formulation can be undertaken based on real data inputs. It is a unique tool that can be applied across the globe with local data inputs.