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Towards standardization of the measurement of underwater noise radiated from marine pile driving.

Theobald, P D; Robinson, S P; Pangerc, T; Lepper, P A* (2014) Towards standardization of the measurement of underwater noise radiated from marine pile driving. In: OCEANS 2014, 14-19 September 2014, St. John's, Canada.

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Abstract

Foundations for offshore structures, wind turbines for example, are often installed using an impact pile driving technique with hammer strike energies which can be up to a few thousand kJ. This construction technique can radiate substantial levels of low-frequency impulsive noise into the water column, which given the right acoustic propagation environment conditions can propagate large distances. This is often a concern for developers planning an offshore construction project as there is a potential for an impact on marine mammals and fish.

This often results in a regulatory requirement to measure the radiated noise level from such infrastructure construction activities to either validate a prediction made in an environmental impact assessment or to ensure the level does not exceed a defined threshold. The required measurement distances may extend over tens of kilometres, and if the transmission loss is to be established or validated it would be necessary to measure as a function of range at a number of positions. Because of the variation in hammer energy during the pile driving activity, particularly if a soft-start is employed, it is also desirable to perform the measurement independent of distance for the duration of the pile driving operation. A number of others factors will also influence the noise radiated into the water column, including the water depth (exposing a different amount of the surface area of the pile), the seabed properties, the penetration depth into the seabed by the pile, the pile dimensions, and the hammer energy. Importantly, some of these factors may change during pile driving of an individual pile, for example tidal variation and sediment penetration, and this means that the acoustic output is likely to change during the driving of a specific pile. A method for the measurement of this underwater noise, that is generally in relatively shallow water, which considers both the spatial and temporal variation of the sound field is described. The method includes a combination of fixed autonomous recorders and vessel based hydrophone deployments.

The method also aims to establish a parameter which describes the acoustic output of the source. However, it is not yet clear how to provide an entirely robust definition of source level for marine pile driving due to the complexity of the source, which penetrates both the water surface and the seabed, and where the source is not self-contained but is intimately connected to the environment. This means that if the environment changes, so does the source output. A method for establishing a type of energy source level reported in one-third octave bands, obtained from measurements made sufficiently far away from the source, is proposed.

Furthermore, this paper describes the activities within the International Standards Organisation (ISO), Technical Committee 43, Sub-Committee 3, Working Group 3 to produce an international standards document to define the appropriate measurement methodology which should be followed when measuring underwater noise radiated from marine pile-driving. The aim of the standard is to harmonise both the measurement methodology and will inform the hydrophone specification and calibration requirements, the required frequency range, the hydrophone measurement depth, the measurement distances, the required metadata etc. Importantly the standard will also aim to harmonise the sound level metrics which are used and the way in which the data is reported. The metrics and their definitions will primarily be driven by work in ISO, Technical Committee 43, Sub-Committee 3, Working Group 2 to produce a standards document on underwater acoustical terminology.

Item Type: Conference or Workshop Item (UNSPECIFIED)
Subjects: Acoustics
Acoustics > Underwater Acoustics
Identification number/DOI: 10.1109/OCEANS.2014.7003200
Last Modified: 02 Feb 2018 13:13
URI: http://eprintspublications.npl.co.uk/id/eprint/6499

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