Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012 A REVIEW OF PRACTICAL METHODS FOR REDUCING UNDERWATER NOISE


POLLUTION FROM LARGE COMMERCIAL VESSELS (DOI No: 10.3940/rina.ijme.2012.a2.227)


R C Leaper, International Fund for Animal Welfare, UK M R Renilson, Renilson Marine Consulting Pty Ltd, Australia SUMMARY


Underwater noise pollution from shipping is of considerable concern for marine life, particularly due to the potential for raised ambient noise levels in the 10-300Hz frequency range to mask biological sounds. There is widespread agreement that reducing shipping noise is both necessary and feasible, and the International Maritime Organization is actively working on the issue. The main source of noise is associated with propeller cavitation, and measures to improve propeller design and wake flow may also reduce noise. It is likely that the noisiest 10% of ships generate the majority of the noise impact, and it may be possible to quieten these vessels through measures that also improve efficiency. However, an extensive data set of full scale noise measurements of ships under operating conditions is required to fully understand how different factors relate to noise output and how noise reduction can be achieved alongside energy saving measures.


1. INTRODUCTION


Concerns that shipping noise could be affecting marine mammals were first


observations of considerable overlap between the main frequencies


used by


raised in the 1970s, based on large


baleen whales and the


dominant components of noise from propeller driven ships [1].


fish causing avoidance behaviours [2, 3], stress [4] and masking communication [5].


Shipping noise also affects many species of Increases


shipping have been associated with documented increases in ocean ambient noise levels [6, 7].


increases amount to around 20dB from pre-industrial conditions to the present day in the northern hemisphere with deep water shipping noise up to 10dB higher than wind-related Knudsen noise at sea state 6 at frequencies below 100Hz [8]. In areas of highest shipping density, increases in noise can be much greater [9, 10, and 11].


While there is still considerable uncertainty about the full impacts of noise on marine life, noise from shipping will mask sounds associated with communication, breeding and feeding for many species, with potentially serious consequences for individuals and at a population level. The primary concern regarding potential adverse impacts of


exposures, but rather to the general increase in ambient noise [12].


incidental shipping noise is not related to acute The effects of acoustic masking have been


quantified in terms of loss of acoustic habitat, with shipping noise in some cases contributing to an order of magnitude loss in the spatial area over which large baleen whales can communicate [11].


An International Workshop on Shipping Noise and Marine Mammals held in Hamburg in April 2008 [13] agreed as targets a reduction in the contribution of shipping to ambient noise levels in the 10-300Hz range of 3dB in 10 years and 10dB in 30 years, relative to current levels. These targets have been widely endorsed, including


by the Scientific Committee of the International Whaling Commission [14]. ©2012: The Royal Institution of Naval Architects


in global These


In 2008, based on a proposal by the USA, the


International Maritime Organization (IMO) added “Noise from commercial shipping and its adverse impact on marine life” as a high priority item to the work of its Marine Environment Protection Committee (MEPC) and established a correspondence group to develop non- mandatory technical guidelines for ship-quieting technologies as well as potential navigation and operational practices [15].


The European Union has adopted an indicator for Good Environment Status (GES) for underwater noise in the context of the Marine Strategy Framework Directive1 based on trends in ambient noise levels within the 1/3 octave bands centred at 63 and 125 Hz. These bands are dominated by noise from ships, and achieving GES may require reductions in shipping noise.


In the US, the


National Oceanic and Atmospheric administration has held two symposia on vessel noise [16, 17]. One conclusion was that substantial reductions (5-20dB) in noise emissions could be achieved for most


types of


vessel at relatively little cost without major technical innovation.


There is thus a wide agreement that reducing shipping noise is both necessary


and feasible. The IMO


correspondence group has been developing technical guidelines for how noise reductions may be achieved, bearing in mind the relatively little attention given to underwater radiated noise in ship design and construction to date.


In particular, the group noted that quieting a


relatively few of the loudest ships is a potential way to efficiently reduce the overall contribution of shipping noise to the global ocean noise budget [18].


1 The European Marine Strategy Framework Directive (Directive 2008/56/EC) adopted in 2008 requires Member States to prepare national strategies to manage their seas to achieve or maintain Good Environment Status by 2020.


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