Trans RINA, Vol 154, Part A2, Intl J Maritime Eng, Apr-Jun 2012
Although there are limited standardised noise measurements across merchant fleets, some quantitative estimates can be made of the likely effects of tackling a proportion
of the noisiest assumptions about noise propagation.
ships based on simple One study of
measured noise from 54 vessels documented relative source levels expressed in dB across the 30–150Hz range [19]. Based on this distribution, vessels that are quieter than average contribute 10% or less to the total area ensonified by vessels to a specified received level. By contrast, the noisiest 10% of vessels (those that are 6.8dB or more above average) may contribute between 48% and 88% of the total acoustic footprint2 [20].
For six
cruise ships between 23 and 77GT, a standard deviation of 3.7dB in overall sound level was reported but up to 24dB differences between the quietest and noisiest in 1/3 octave band levels [21].
in the upper and lower bounds of sound levels across an assemblage of 15 ships
been reported [22],
demonstrating large differences in levels at certain frequencies.
In July 2011, the IMO introduced a mandatory Energy Efficiency Design Index (EEDI) for new ships which will require ship designers and builders to produce energy efficient ships [23]. At the same time, the Ship Energy Efficiency Management Plan (SEEMP) was made mandatory for all ships. The EEDI has been developed for
the largest and most energy intensive
sectors of the global merchant fleet, including tankers, bulk carriers, general cargo and container ships. The intention is to stimulate continued technical and design developments and to separate these from operational measures. The sectors of the fleet for which EEDI applies are also likely to include some of the noisiest vessels. Technological initiatives generated by EEDI and operational measures taken through SEEMPs could play an important role in reducing underwater ship noise. However, to achieve this, it is critical that the implications of all developments for noise are properly evaluated.
While the over-riding consideration of IMO remains on fuel efficiency and reducing CO2 emissions, the noise issue should also be tackled at the same time.
In this
2 In this paper the term ‘acoustic footprint’ is used to denote the
relative area over For the subsequent acoustic which the average,
wideband sound from a ship will exceed a certain level under very simple and approximate assumptions about propagation.
footprint
comparisons in this paper, a spreading loss in dB of 15log(r) where r is the distance from the ship is assumed. For example, a 15dB increase in source level will result in the same received level at a factor of 10 greater range and an area of the acoustic footprint which is 100 times greater.
This is a crude measure that does not take into
account frequency characteristics of the noise or the complexities of propagation, but does provide a simple measure for comparative purposes.
A-80
Similarly, 20-40dB differences have
paper, potential methods for reducing ship noise that may also improve energy efficiency for both existing vessels and new builds are reviewed. In particular we identify the research needed to ensure that the opportunities for noise reduction arising from efficiency measures are not missed.
2.
BACKGROUND TO UNDERWATER NOISE FROM SHIPS
2.1 PRINCIPAL CAUSE OF SHIPPING- RELATED HYDRO-ACOUSTIC NOISE
There are a number of different causes of noise from shipping. These can be subdivided into those caused by the propeller, those caused by machinery and those caused by the movement of the hull through the water. The relative importance of these three
different
categories will depend, amongst other things, on the ship type. For a typical vessel, the ratio of energy emitted as noise to the energy used for propulsion is around 10-6 [24] with the amount of energy generated as noise typically a few hundred watts or less. Therefore small changes in propulsive efficiency can make dramatic differences to noise output.
It should be noted that until recently, there have been no standards for measuring and assessing hydro-acoustic noise propagated into the water. made by different
organisations
techniques, with different methods of extrapolation to determine the source level.
In December 2009, a new
voluntary consensus standard for the measurement of underwater noise from ships was developed by the American National Standards Institute and the Acoustical Society of America (ANSI/ASA 2009). The standard describes measurement
procedures and data analysis
methods in order to quantify a ship’s underwater radiated noise level referenced to a normalised distance of 1m. Three different standards
according to the level of precision required.
The International Standards Organisation (ISO) has been developing a standard3 in close co-operation with the group that developed the ANSI/ASA standards and expects this to be published in 2012. The standard has been developed at the request of the IMO, shipping and shipbuilding industries, who wished to have an easy-to- use and technically sound International Standard for measuring underwater noise radiated from merchant ships.
(A, B, C) are specified
Measurements were using
different
3 International Standard ISO 16554 Protecting marine ecosystem
from underwater irradiated noise –
Measurement and reporting of underwater noise radiating from merchant ships
©2012: The Royal Institution of Naval Architects
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62