Report of the Round Table Session
Audoly, C.1*, de Jong, C.2*, Baudin, E.3, Brooker, A.4, Coomber, F.5, Gervaise, C.6, MacGillivray, A.7, Salinas, R.8, Širović A.9, and Wittekind, D.10
1 DCNS, France
2 TNO, The Netherlands
3 Bureau Veritas
4 Institute of Sound and Vibration Research, University of Southampton, UK
5 CIMA research foundation, Italy
6 GIPSA-lab, France
7 JASCO Applied Sciences, Canada
8 TSI, Spain
9 Scripps Institution of Oceanography, UCSD, USA
10 DW-ShipConsult, Germany
This report can be referenced as:
Audoly, C., de Jong, C., Baudin, E., Brooker, A., Coomber, F., Gervaise, C., MacGillivray, A., Salinas, R., Širović A., and Wittekind, D. (2015). Report of the Shipping Session, Oceanoise2015, Vilanova i la Geltrú, Barcelona, Spain, 10-15 May. (Editors Michel André & Peter Sigray). Retrieved from http://oceanoise2015.com
Sound radiated by ships provides a relevant contribution to the underwater sound in oceans and seas. This ‘shipping noise’ is part of the soundscape in which marine species live. It may mask other sounds relevant to these species, like the communication sounds from their conspecifics or sounds that help them to orientate and to avoid predators, but to what extent this masking occurs is largely unknown. Current research towards assessment of the risks that shipping noise poses to marine life aims at quantifying the sound levels to which marine species are exposed as well as establishing threshold levels above which the noise has a significant impact on marine species.
In the ‘shipping’ session of OCEANOISE 2015 the focus was on the exposure assessment. How well do we know how much underwater sound is produced by ships and how it is distributed in the underwater environment? Researchers from various institutions presented the state-of-the art in measuring and modelling radiated noise from ships. Several presentations were from two current EU research projects on shipping noise modelling and measurement: ‘AQUO’ and ‘SONIC’, that will be finalized at the end of 2015. In a panel discussion the main conclusions of the session were summarized and discussed, revealing what remains to be done.
Ship noise measurements
The first part of the session was devoted to ship noise measurements. Various approaches are being used to quantify the radiated noise output of ships, encompassing dedicated trials on a cooperating ship as well as measurements of opportunity of ships passing acoustic sensors that are installed close to a shipping lane. Considering the fact that it is unlikely that all vessels in the world would be submitted to dedicated radiated noise trials, these two approaches are complementary. However, comparison of the results of these approaches is generally difficult, due to the lack of standardization of measurement and analysis procedures. Though international ship noise measurement standards have been available in the military domain for many years, the translation of these to the civil domain is very recent. In 2009, the American organization ANSI issued a standard for ship noise measurements in deep water . Starting from this, the International Organization for Standardization (ISO) is now developing ship noise measurement standards, the first of which has become available in 2011 as a temporary ‘publicly available specification’ . These are for dedicated ship trials in which the ship cooperates by carrying out stable runs at fixed distance to the recording system. The result is presented as a ‘radiated noise level’ which quantifies the measured mean square sound pressure when the ship is around its closest point of approach to the measurement system, normalized with the square of the distance between the hydrophone and a fixed reference position on the ship. It does not take into account the actual sound propagation between ship and hydrophone during the measurements. Correction for the actual propagation loss would result in a ship ‘source level’ for the specified ship reference position, of which in particular the depth below the water surface has a large influence. This source level and source depth are the input parameters required for calculation of the distribution of sound in the environment, e.g. to generate sound maps. Procedures to determine source level from measurements have been proposed, and the development of a standard procedure (for dedicated ship trials) is undertaken by ISO, but will take some time.
A particular topic of interest during the session was the uncertainty in the reported radiated noise levels. Variations in the operational parameters (speed, engine settings) leads to variations in radiated noise, but even at fixed nominal settings the measurement results show a significant spread. This uncertainty can be partly understood from variations in measurement geometry and properties of the environment, but shows a substantial random component as well. Only a proper control of the measurement parameters and statistical information from multiple runs and multiple hydrophones can reduce the uncertainty in the reported levels to within a few decibel.
Ship noise modelling
Ship noise modelling was discussed in the second part of the session. Various institutions propose production of underwater sound maps of oceans and seas in relation with shipping, on the basis of appropriate underwater sound propagation models and the estimated source levels of vessels sailing in the area. Regarding the environment, some databases provide the parameters that are relevant for the propagation (bathymetry, sound speed, sediment properties). Regarding the description of the traffic, many sea areas are nowadays covered by vessel tracking systems (Automatic Identification System; AIS) that provide the identification, position, course and speed of ships that can be used as input for calculating shipping sound maps. Regarding the ship acoustic source levels, an ideal situation would be to have an extended database of radiated noise levels of all existing ships, taking into account their operational parameters (speed, load condition,…), which is not realistic. As a consequence, there is need for modelling the underwater radiated noise of vessels, according to their type, size, speed and other parameters, in order to feed the models that produce underwater sound maps of oceans and seas.
There are, however, several problems that need to be solved. Not all vessels carry AIS equipment and not all parts of the seas are covered by the AIS receivers. The AIS messages contain a list of parameters that characterize the ship and its operation, but not all parameters relevant for radiated sound are included and the reliability is not always sufficient for all parameters. Besides, the radiated noise from a particular ship depends generally more on its specific architecture (type of propulsion, hull, and propeller) than on its size or type. The development of models that predict the acoustic source level of ships on the basis of the AIS parameters is still in its infancy. In particular the lack of data of standardized radiated noise measurements in combination with the relevant AIS parameters hinders the development of reliable ship noise models.
Remaining open issues
Although significant progress has been made in recent years in the understanding of radiated noise from individual ships and shipping noise in general, assessment of the risks for the environment and of the need for mitigation needs further development. Open issues are:
– Standardized procedures for radiated noise measurements
– Standardized procedures for determining ship source levels from measurements
– Reliable AIS information, including the parameters relevant for radiated noise
– Reliable and validated models for the radiated noise of ships on the basis of AIS parameters, and how to get inputs related to ship architecture or technology
– Dose-response studies: What are relevant indicators to quantify the effects of ship noise on marine life? At what value of these indicators does ship noise cause a risk?
- ANSI/ASA S12.64-2009/Part 1: Quantities and Procedures for Description and Measurement of Underwater Sound from Ships – Part 1: General Requirements.
- PAS/17208-1: Acoustics — Quantities and procedures for description and measurement of underwater sound from ships — Part 1: General requirements for measurements in deep water, 2011.