Anthropogenic Noise from Offshore Wind Farm Construction

www.southampton.ac.uk

BackgroundA key element of the UK plan for the delivery of renewable energy is the development of offshore wind farms. It is envisaged that up to 10,000 additional turbines will be installed by 2020.

The favoured construction method is to drive a monopile (steel tube) into the seabed using a percussive (impact) hydraulic ram. These monopiles can be up to 6 m in diameter and are driven up to 25 m into the seabed. This piling process radiates high levels of impulsive noise into shallow coastal waters.

With a growing awareness of the potential impact of anthropogenic (man made) noise on marine life it is important to gain an improved understanding of the physics of the noise generation process and how the sound propagation into the shallow water environment is affected by the pile, hydraulic piling, seabed and water column characteristics.

ResearchThis research aims to provide an improved understanding of the noise generation process via a combination of numerical modelling and experimental measurement. It will eventually include:

o Finite Element modelling of the impact process in order to determine the acoustic field distribution near to the source;

o Numerical modelling of the sound propagation through the seabed and water column;

o Scaled laboratory experiments to obtain confirmation of the generated sound under controlled conditions, with easily varied system parameters;

o Comparison with data collected at sea to verify the performance of the models developed;

o Calculation of acoustic metrics used in assessing the impact on marine life; these include the peak pulse pressure, and pulse energy - important for calculation of sound exposure level (SEL).

CollaborationThis research is part of an ongoing collaboration with the National Physical Laboratory (NPL) and the Department of Electronic and Electrical Engineering at the University of Loughborough. They have already made measurements of piling noise in situ. Avenues for funding research in the area of anthropogenic noise are being jointly pursued.

ImpactAn improved understanding of the generation of underwater pile-driving noise will have an impact on construction planning, mitigation measures, environmental impact and commercial development.

o An improved knowledge will facilitate impact assessments at the planning stage, and help to develop strategies for minimising environmental impact.

o The development of effective mitigation measures – such as acoustic baffles – will be aided by an understanding of the noise generation process.

o An improved understanding and mitigation measures will enable the environmental effects to be minimised and help to prevent environmental concerns delaying construction.

o An improved understanding will aid the construction industry as the world market in this area grows.

Numerically modelled signal loss as a function of range from a uniform 200 Hz extended source in 40 m of water and 20 m of sediment.

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Experimentally measured acoustic output from an impact generated in a scale model system in the laboratory.

Pile-driving a monopile (on right of picture) into seabed from a platform.

Victor Humphrey, Fluid Dynamics and Acoustics Group, ISVR.(vh@isvr.soton.ac.uk)