BESST Project & LR-ODS: Underwater Noise Emission From Ships

Kevin Cunningham Consultant - Marine

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Agenda

• The BESST project and LR-ODS • Background: Underwater noise from ship to ocean dwellers

• Sources & quieting • Transmission

• Proposal for BESST • An Onboard Monitoring System – Could it work, How do we find out?

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What is BESST?

• BESST – Breakthrough in European Ship and Shipbuilding Technologies • EC funded R&D project initiated by Euroyards • Main aim: Secure and improve the competitive position of European shipyards through decreased life cost, reduced environmental impact and improved safety. • Focus on passenger ships, ferries, mega-yachts (product & process) • 65 partners • 29 M€ budget, 17 M€ EU funding • 8 defined key areas • 4 years running time, 2009-2013

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BESST, Underwater Noise and LR-ODS

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BESST, Underwater Noise and LR-ODS

• Task leader: Fincantieri

• 10 partners (including LR-ODS, Fincantieri, STX France, Marin, Wärtsilä Finland, UoS-ISVR, TSI)

• Background: Trend in international rules and regulations towards

increased environmental protection. • LR-ODS contribution: participating in 3 sub tasks

1. Definition of appropriate measurable targets and acceptance criteria for outdoor

noise emissions from cruise ships in harbour proximity 2. Review of significant noise sources for underwater noise emission, and damping

technologies 3. Prototype application of underwater noise reduction, control systems and

technologies on a cruise ship

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Underwater noise generated by the ships

BESST Task 2: ODS main contribution: Identification and compilation of significant noise sources of in- water emissions. Definition and specification of damping technologies to improve underwater noise reduction.

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So how to regulate underwater noise levels?

BESST Task 3: LR-ODS main contribution: Prototype application of underwater noise reduction, control systems and technologies on a cruise ship. Proposed practical solution is monitoring of ship radiated noise with either: • Fixed monitoring station • Onboard monitoring system Which gives feedback for operational adjustments for noise sensitive areas

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Underwater Noise: From Ship to Ocean Dwellers

• Noise at underwater receiver (e.g. Marine mammal) depends on: • Noise source: generation and radiation • Transmission: geographic locationand conditions • Ambient (background) noise

Criteria for what is acceptable noise levels for aquatic life are not well defined! Public interests concentrate on marine mammals. Besides noise level (sound pressure level) other metrics for evaluating noise include the frequency range, duration and if it is continuos or intermittent. Shipping noise is dominated by low frequency continous noise from a travelling source

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Ship Generated Underwater Noise

• Each ship has an individual signature depending on: • Ship and machinery design • State of maintenance • Operational settings (e.g. speed/load, draft & trim)

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Noise sources in sailing condition, typical cruise vessel:

Propellers

DG sets

Propulsion motor

Gearbox

Secondary machinery

Flow

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Noise Sources: Machinery

• Noise Generation: • Structure-borne vibrations, radiated via hull • Can be dominating, if no cavitation occurs or poor machinery design with

regard to noise, for example rigidly mounted engines

• Reduction: • Same measures as for reducing internal noise and vibration can usually

be used, a large wealth of experience and technologies exist in this area

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Noise Sources: Propeller

• Basic hydrodynamics of propeller: • Water, periodically displaced by propeller blade profile • Fluctuation pressure difference between suction and pressure surfaces due to

wake field • Advanced hydrodynamics of propeller:

• Non-cavitating vortices • Singing propeller

• Cavitation related phenomena: • Cavitation bubbles grow with reduced pressure and suddenly collapse releasing

acoustic energy • Can occur at blade surfaces or downstream such as tip or hub vortices

• Transmission Path: • Emanated to water directly • Scattering of sound from hull

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Reducing the Propeller Noise Source

Design choices:

• Blade thickness, skew, loading • Blade numbers, rpm • Design to minimise or shift cavitation noise • Positioning and/or improvements in wake field • Etc...

However, even with design choices to reduce noise, the noise source level will still vary with operation/loading.

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Transmission: Ship to receiver

A sound field can be classified into 2 regimes:

• Near field, close to the source (close = distances on the order of a few wavelengths or comparable with the size of the source itself, i.e. propeller diameters or hull length)

• Far field, where the source can be simplified to a point source. Directivity is usually ignored which is fair for long wavelengths/low frequencies, e.g at 10 Hz wavelength is about 150 metres

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Sound in the far field: Propagation from source to receiver

Contributions from: • Direct radiation from propeller, scattering from hull, radiation from hull

plating • Reflections from surface and bottom, scattering from surface/bottom

roughness Other considerations: • Sound speed profile, bottom losses, absorption

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Sound in the near field

Complex field including:

Sources at blades: rotating pressure field + transient cavitation

Downstream of propeller, i.e. tip vortices

Radiation from hull

In practice it is not possible to measure the full scale 3-D pressure field and measurement options are limited to hull surface pressures, vibration and viewings.

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Application Case: Prototype applications of noise reduction solutions

The critical question to be answered: Can measurements in the near field (hull pressure and/vibration) be used to characterise the underwater noise source strength of a ship, and thus used to estimate the far field noise? If yes, what needs to be measured and where? Codes exist for calculating hull surface pressures from the 3-D velocity potential calculations of the propeller such as

Precal-Excalibur‘ (Co-operative Research Ships‘ community including LR, Marin amongst others)- need detailed information on flow, geometry. But does not answer the basic question if far field pressures can estimated from a limited number of measurements at the ship? At least one commercial implementaton exists (Noise Control Engineering), but is it feasible?

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Is it realistic to predict far field radiation based on a few measurements on board?

Proposed way forward: 1. Calculation of radiated noise, near and far field, with given multipole

descriptions of the propeller source 2. Calculation of radiated noise, near and far field, with given hull plate

vibrations 3. Evaluate underwater noise measurements. Specifically, how the noise

levels measured using standard methods relate to the actual radiated sound power of the vessel and the noise radiated to the far field.

4. The monitoring application, which as well as (arguably) having some commercial interest in its own right, provides a legitimate motivation for pursuing the other three areas.

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Modelling

Modelling to be performed in Actran: Finite/Infinite element solver Sources to be considered: • Dipole at propeller hub in the propeller plane, at blade rate first couple of

harmonics. To mimic dipole component of propeller noise coming from the fluctuating blade pressures.

• Fluctuating monopole located at propeller hub at the same location and frequencies to model sheet cavitation on the propeller.

• Fluctuating monopole at the same location, but over a range of frequencies corresponding to broadband noise generated by cavitation.

• Fluctuating monopoles distributed at various locations behind the propeller to model the collapse of tip and hub vortices. The frequencies of this mechanism are typically higher, but are also broadband in nature.

• Hull plate vibration field with wavelengths and frequencies corresponding to primary motor frequencies.

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Assumptions & limitations

• Flat sea surface modelled as pressure release surface • Flat sea bed for shallow water applications • Infinite depth for deep water applications • No layering, salinity profile, etc.

A 2 phase approach can be used for investigating into the far field, i.e. source

characterization using Actran in an infinite uniform sea as input to long range propagation calculation.

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Actran – first steps in the model

Achieved so far: • Simple models which can be verified analytically, no hull, pressure release surface, and monopole source •Preparation of basic model with hull Still to come: • Assess configurations with regard to measurements on the hull and measurements In the far field • Assess true sound power vs. measured sound power using standard methodologies