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Suppression Of underwater Noise Induced by Cavitation

Final Report Summary - SONIC (Suppression Of underwater Noise Induced by Cavitation)

Executive Summary:
The first goal of SONIC has been to enhance the understanding of underwater radiated noise from ships and shipping. The project aimed to improve computational and measurement techniques (full scale and model scale) to determine the underwater radiated noise extend to be used in ship design. Subsequently these methods were applied to potential noise mitigation measures to study the effect of these design and operational measures. The resulting knowledge has been summarised in a set of guidelines for underwater radiated noise, developed in close collaboration with the AQUO project.

A shipping noise footprint and mapping tool (NFMT) has been developed, implemented and tested for a number of benchmark scenario’s. Vessel Noise Footprints and shipping noise maps have been generated to compare the effectiveness of different noise mitigation measures. The limited information available from AIS is, at this stage, a major uncertainty in the NMFT output.

Model scale experimental tools have been improved to measure the radiated propeller cavitation noise at partner test facilities. Techniques have been developed to remove the effects of background noise and reflections from the measurements. A suitable scale model test has been performed in each facility for various test conditions. Seven different approaches to the computational prediction of cavitation noise have been developed and applied to the suitable test case.

Measurements of radiated noise from a test ship at full scale have been carried out using on-board sensors and off-board hydrophone arrays. These data have been used to validate the experimental and computational results. The radiated noise from a variety of larger merchant ships have been measured near a shipping lane off the Dutch coast. The measurement data are published in the SONIC public database of ship radiated noise data.

The numerical and experimental techniques to determine radiated noise have been applied to investigate the effectiveness of a number of noise mitigation measures. Technical measures to reduce the cavitation and machinery noise have been studied as well as operational measures. For the operational measures it was shown that spatial planning can significantly reduce the underwater sound levels in specific marine areas.

Project Context and Objectives:
2. Context and Objectives
2.1. Investigation of Radiated noise generated by Shipping
The Marine Strategy Framework Directive (MSFD) identifies underwater noise as a qualitative descriptor for achieving Good Environmental Status.
Computational and experimental techniques for investigating radiated noise at the design stage of ships using model scale facilities (model basins and cavitation tunnels) have been developed. These tools have been used to investigate design aspects to reduce cavitation noise from ships’ propellers and hence assist with implementing the MSFD

2.2. Modelling of cavitation related noise
Cavitation of merchant ship propellers has been identified as a main source of background noise in the oceans. The EU requires member states to demonstrate that human caused underwater noise does not harm marine life.
Accurate tools have been developed and validated to predict underwater radiated noise levels of cavitating merchant ship propellers to allow early and efficient judgement of new builds with respect to their noise emission properties. By means of these tools design aspects and mitigation measures have been investigated in order to minimise the cavitation noise of ships without reducing the propulsion efficiency.
Existing experimental and computational tools had to be investigated and further developed to extend the underwater radiated prediction towards .

For the computational procedures different codes for wake prediction, propeller operation, propeller cavitation dynamics and noise radiation have been adapted and coupled in order to obtain an applicable tool for the prediction of underwater radiated noise.
Experimental and computational procedures have been validated by means of full scale measurements and finally applied to different test cases in order to investigate design changes and to serve as basis for mitigation studies.

2.3. Mitigation measures of cavitation noise, machinery noise and operational mitigation measures
The objectives set to meet this task in SONIC were as follows:
• To develop cavitation noise reduction measures without reducing fuel efficiency.
• To develop machinery noise reduction measures without reducing fuel efficiency.
• To assess effect on sound maps of spatial planning of ship traffic.

Mitigation of cavitation noise
The experimental and numerical techniques that have been developed in WP1 are applied to study the following mitigation measures for reducing cavitation noise:
• Optimisation of propeller design and selection
• Optimisation of control schemes for controllable pitch propeller installations
• Application of wake equalizing devices
• Air injection into the propeller plane

Mitigation of machinery noise
The contribution of noise emission by 4-stoke Diesel engines has been studied in a Source-Path-Receiver model, applying the sub-structuring breakdown of single steps contributing to the mechanical noise chain. Every single step in the chain has been analysed and modelled by means of the most advanced computer techniques. The main steps analysed are: engine combustion forces, engine structures, engine resilient mounts, ship foundation, ship hull and hull radiation efficiency.

Mitigation by spatial and operational planning
The NFMT sound mapping tool is used to study the effects of spatial and operational planning of ship traffic on the underwater noise distribution. Sound maps are calculated for different scenarios, to demonstrate the effects of operational measures like a speed limit or a limit to the noise footprint of individual vessels.

2.4. Developed of methodology to calculate the noise footprint of a vessel, and the development of noise and mapping tool for shipping
A methodology has been developed to assess the ‘vessel noise footprint’ of individual ships and of ‘sound maps’ representing the spatial distribution of underwater sound from shipping traffic for a specific part of the sea using AIS datasets. The methodology has been implemented in a Noise Footprint and Mapping Tool (NFMT) that is suitable for EU member states to support the implementation of the Marine Strategy Framework Directive (MSFD; 2008/56/EC). One of the MSFD descriptors of Good Environmental Status requires Member States to ensure that levels of underwater noise do not adversely affect the marine environment. In particular, Descriptor 11 requires Member States to monitor trends in underwater ambient noise, which requires a combination of measurements and modelling.
As a first step, definitions are proposed for basic terminology of underwater acoustics, including shipping sound maps and vessel noise footprints, as geographical representations of the sound pressure level generated by ship(s) in a specified environment, and agreed with AQUO. The methodology to assess these distributions combines AIS data of the locations and characteristics of ships and their operation in the area, to model the sound sources, with environmental data (bathymetry, oceanographic, meteorological and geo-acoustic data), to model the sound propagation loss. Basin wide propagation modelling requires a trade-off between accuracy and computational speed. Various models are implemented and tested and the results compared in a joint SONIC-AQUO benchmark workshop. To be able to evaluate the sound as perceived by marine life, the noise footprint and mapping tool includes options for representing the sound pressure level distribution reflecting the hearing sensitivity (frequency weighting) and swimming behaviour (depth weighting) of different species of marine mammals and fish.

2.5. Development of Guidelines for regulation of under water noise
The developed guidelines summarise the conclusions and recommendations of SONIC as advice to policy makers for the way ahead to develop good environmental status of European marine waters with respect to underwater noise pollution from commercial shipping. The guidelines are furthermore aimed to enhance international standardisation of underwater noise terminology, measurement methods and assessment criteria.
As a base line the guidelines describe how more knowledge on the present status and the future trends of underwater noise pollution and its impact on marine life can be gained. Noise emissions by individual vessels and commercial shipping as a whole is addressed. Furthermore, the guidelines describe solutions to reduce underwater noise. Mitigation measures applied in ship design in retrofit and in ship operation are considered, thus offering solutions from a designer’s as well as an operator’s perspective.

Project Results:
3. S&T Results / Foreground
3.1. Investigation of Radiated noise generated by Shipping
Experimental tools have been developed to improve the measurement of radiated noise from ships at model scale in various hydrodynamic testing facilities including cavitation tunnels, a depressurised wave basin and a towing tank.
At each facility a suitable model scale test vessel and set of test conditions was selected. Measurements were carried out to determine the level of underwater noise in the facility during model scale tests of a cavitating propeller.
Further measurements have also been undertaken to identify and characterise sources of unwanted noise such as facility background noise and reflections from surfaces such as the walls of the test facility and the water surface. Techniques have been investigated to remove or compensate for these unwanted sources of noise to allow better prediction of propeller cavitation noise.
Seven different approaches to the computational prediction of cavitation noise have been applied and developed in the SONIC project. These have each been applied to suitable test cases to allow the results to be compared and validated.

Measurements of radiated noise from a test ship at full scale have been carried out using on-board sensors and off-board hydrophone arrays. These data have been used to validate the experimental and computational procedures and to investigate the separation of noise generated by machinery and by a cavitating propeller.

The validation process has identified where agreement is good and also highlighted areas that require improvement. The process has therefore been a valuable step in assessing where current procedures provide reliable results and where future work needs to be concentrated.

Autonomous underwater noise recorders have been used to characterise the noise field (or noise footprint) around the trial ship. This trial used novel measurement techniques and also provided the data required to validate the noise footprint modelling tool developed for the SONIC project.

An autonomous recorder has also been deployed close to a shipping lane off the Dutch coast to measure underwater noise from a variety of larger merchant ships. The results of the full scale trials have been presented at a number of conferences.

The full scale measurement results are being added to a public database of ship radiated noise data measured during the SONIC project.

A mathematical model to predict the source level of different ship types has been developed. The model predicts a source level for a given ship based on ship speed, type and size using information from AIS. The source level model is being used for noise mapping and mitigation investigations in the SONIC project. The accuracy of the model is affected by the parameters available from AIS being too limited to adequately determine the properties of the ship, and by the lack of good quality published ship radiated noise data to validate the model.

Further Results expected
In order to further validate the experimental and computational procedures, each approach was applied to a number of specific vessel test cases. The SONIC project developed a publically accessible database of radiated noise information, allowing the dissemination of full scale and scale model results.

3.2. Modelling of cavitation related noise
Improved experimental and computational tools have been developed to predict the under-water radiated noise levels of cavitating merchant ship propeller down to the lower frequency range (typical of the blade rate and harmonics) that is important for marine fauna.
For the experimental procedures a low background noise has been established and corrections for the spatial limitations and the influence of the gas content in the different involved facilities have been determined.
For the computational procedures, different codes for wake prediction, propeller performance, propeller cavitation dynamics and the noise radiation had to be adapted and to be coupled in order to produce an applicable tool for the prediction of underwater radiated noise at the blade rate and up to 3rd harmonic. Both of the procedures were accordingly improved, validated by full scale data and applied to several test cases in order to study design changes and mitigation measures. The gathered full scale prediction data for selected ship types is to be included into the data base of noise levels. The results have been summarised in publicly available reports.
Further Results expected
Beside the direct dissemination of knowledge via public reports, it is expected, and in some case experienced, that model basins and ship design companies are inquiring and or ordering to predict the underwater noise emission of ships. With increasing awareness of the importance of this signature and the eventuality of regulation this new type of pollution may become a significant design aspect of a modern vessel.

3.3. Investigation, and mitigation measures of cavitation noise, machinery noise and operation
Mitigation of cavitation noise
The results of systematic model scale tests of the cavitation noise characteristics of a standard set of propeller designs has provided guidance for optimised propeller selection. Numerical and experimental techniques have been developed and tested for optimising propeller design against multiple requirements for efficiency and cavitation noise and for evaluating the effectiveness of technical mitigation measures. These have been applied for a number of specific examples: wake equalising devices, control schemes for controllable pitch propellers and air injection techniques. The latter two showed especially promising results.

Mitigation of machinery noise
The main result achieved has been the precise measurement of the transmitted force to the ship foundation by means of instrumented resilient mount with load cells. This measurement allows eliminating one of the remaining uncertainties in the methodology. Thanks to this information, it has been possible to confirm that the force is precisely correlated to the machinery vibrations and that phase information is maintained up to a certain frequency that depends on the mount characteristics. It has also been proven that calculation of transmitted power is possible, so that it may be utilised by ship builders to better design the ship structure. These results will provide added value to the Wärtsilä product manual, thereby given better integration of the engine into the ship design. Patents will be applied for the instrumented resilient mounts proposed.

Mitigation by spatial and operational planning
Calculated sound maps for a generic shipping lane scenario (based on actual AIS data) demonstrated how the NFMT can be applied to study the effects of spatial planning in specific sea areas. The generic scenario studies indicated that the effectiveness of operational measures (a speed limit or a limit to the noise footprint of individual vessels) strongly depends on the local traffic characteristics (density and variety of ship types and their speed).
Further Results expected
The results of the mitigation studies will be implemented in the guideline document, together with the results of similar studies from the AQUO project

3.4. Development of methodology to calculate the noise footprint of a vessel, and the development of noise and mapping tool for shipping
A shipping noise footprint and mapping tool (NFMT) has been developed, implemented and tested. It is coupled to various external databases to define the environment and includes various acoustic propagation models, including the recently developed efficient hybrid model SOPRANO.
The tool is tested and compared with the models developed in AQUO for a number of benchmark scenario’s in a joint workshop and experimentally validated against ‘footprint’ measurements on a small research vessel.
Marine mammals and fish are distinguished in ecological groups with characteristic hearing sensitivity and swimming depth, as input for species specific sound maps.
The NFMT development has been presented at several European conferences.
Footprints and sound maps have been generated to compare the effectiveness of different technical and operational noise mitigation solutions studied in SONIC.
The main uncertainties in the NFMT output are currently associated with the available AIS data, because not all ships are equipped with AIS transmitters, the AIS receivers do not cover the complete sea area, and the transmitted AIS information appears to be insufficient for reliable ship source level modelling on its own.
Further Results expected
In the final phase of the SONIC project, the NFMT has been applied to generate sound maps for the Dutch and German parts of the North Sea, based on historical AIS shipping density data for a single year, to demonstrate the capability and to illustrate the various maps that can be made as input for the MSFD monitoring and for studies of the impact of underwater sound on marine life.
The NFMT is developed such that it can be easily extended to model other anthropogenic underwater sound sources such as pile driving, air-guns and explosives and natural sound sources such as wind, rain and lightning.

3.5. Development of guidelines for regulation of under water noise
The developed guidelines represent a summary of the individual SONIC work packages condensed into recommendations for potential future regulation. The main topics are:
• Standardised and systematic measurement, monitoring and documentation of status and trends of underwater noise pollution by individual vessels, vessel types and commercial shipping as a whole and its impact on marine life.
• Relative effectiveness of technical underwater noise mitigation measures under consideration of the impact on fuel efficiency and building cost, and implications for practical realisation in ship design or retrofit.
• Relative effectiveness of operational underwater noise mitigation measures considering their impact on fuel efficiency, operating and maintenance cost.
The guidelines are jointly developed with the AQUO project. A dissemination event of the common draft guideline document to EU, IMO and interested stakeholders was held on the 21st September 2015, with the final guideline document to be finalised and published in November 2015.
Although the guidelines primarily aim at supporting policy makers in defining the way ahead, the findings and recommendations are also of interest for the ship building and shipping industry, since a variety of design and operational solutions for underwater noise mitigation are described and assessed.

Further Results expected
The guideline document may serve as a reference document for EU and IMO policy makers, national authorities, classification societies, ship designers ship yards and operators. Furthermore it provides a sound basis for further research on the impact of underwater noise on marine species.

Potential Impact:
4. Impact
4.1. Investigation of Radiated noise generated by Shipping
Experimental and computational techniques have been developed within SONIC and validated against full scale radiated noise data. These improved techniques can be used to predict the likely levels of radiated noise from new-build ships and hence investigate approaches to reducing noise levels at an early stage
4.2. Modelling of cavitation related noise
The developed procedures for prediction of the underwater radiated noise of cavitating merchant ship propellers enables ship owners, yards and ship operators to take into consideration the underwater noise emissions of an individual ship at the design stage. In this phase design changes and mitigation measures can be investigated most cost-efficient manner. In the end it is expected that merchant shipping will reduce noise emission while keeping the high propulsion efficiency.
4.3. Investigation, and mitigation measures of cavitation noise, machinery noise and operational measures
This study has provided an overview of the state of the art for mitigating shipping noise and of the tools and techniques available to quantify the effectiveness of the different available mitigation measures, both for the ship building industry and for regulators.

4.4. Development of methodology to calculate the noise footprint of a vessel, and the development of noise and mapping tool for shipping
An underwater noise footprint and mapping tool (NFMT) has been developed and tested, that can be used in the implementation of the monitoring for descriptor 11 of the European Marine Strategy Framework Directive.

4.5. Development of Guidelines for regulation of under water noise
In future the guideline document may serve as a reference document for EU and IMO policy makers, national authorities, classification societies, ship designers ship yards and operators. Further it provides a direction for further research on the impact of underwater noise on marine species.

List of Websites:
http://www.sonic-project.eu/