Underwater noise measurement requirements, methodologies and the benefits of propagation modelling – our experiences

Tanja Pangerc, Pete Theobald, Lian Wang, Paul Lepper, Stephen Robinson

StUKplus Conference , Berlin, 31 October 2013

Outline of presentation
Licence requirement for construction noise monitoring
Measurement methodology employed to satisfy this licence condition
Requirement for numerical propagation modelling and its benefits


Operational phase


Summary

Licence Condition Underwater noise monitoring during construction (Example) The Licence Holder must undertake measurements of the noise generated by the installation of foundation pieces. Measurements will need to be taken at various distances for the first few foundation pieces (minimum of four) including during the ‘soft start’ procedure. The specification for these measurements should be agreed with the Licensing Authority, consultation with Cefas and Natural England at least four months before the construction work commences. The results of these initial measurements should be processed and the report submitted to the Licensing Authority within six weeks of the installation of the first foundation piece. Assessment of this report by the Licensing Authority will determine whether or not any further noise monitoring is required. Should noise levels be significantly in excess of those predicted during the Environmental Impact Assessment process then further pile installation will not occur without the consent of the Licensing Authority. Aim: Ensure that actual levels don not exceed prediction in the Environmental Statement.

What do we measure? Different monitoring has been adopted worldwide:
The UK – Measurements typically undertaken at various ranges from the source
Germany – requirement for measurement at 750 m and 5 km from the source
The Netherlands – requirement for a minimum of two measurement locations (one is recommended to be at 750 m)
US – measurements have typically been made at or referred to 10 m from source
Offshore wind farm construction is a young industry

Measurement methodology
No British Standard
Good Practice Guide; Robinson, 2013, draft. Funded by National Measurement Office (NMO – BIS), with support from Crown Estate and Marine Scotland
No International Standard document (ISO standard, Measurement of noise from marine piling, in prep. – ISO TC43/SC3/WG3)


Measurement carried out by commercial consultants/contractors
General approach is guided by Licence requirements
agreed with the Licensing Authority
Typically good quality measurements, suitable equipment
General approach comparable between sites/contractors

Typical pile driving noise measurement methodology
Measure as a function of range along predetermined transects
Transects chosen based on propagation conditions
Sometimes selected to be towards areas of particular interest etc.


Cabled hydrophones provide flexibility in data acquisition (avoid clipping and poor SNR)

Measured at 1 km Measured at 300 m

Received level with range

Piling sequence profile of the hammer strike energy
Hammer strike energy ramps up with the number of strikes/ time
The acoustic energy in the pulse is expected to increase with the hammer blow energy (Robinson et al. 2007*)

Entire piling sequence measured with a static acoustic recorder *Robinson, S. P., Lepper, P. A., Ablitt, J. (2007) The measurement of the underwater radiated noise from marine piling including characterisation of a “soft start” period, IEEE Oceans – Europe 2007.

Measurement details – pile driving
Fixed underwater noise recorders used to measure entire piling sequence
provide a range independent measurement
necessary because of soft-start and other temporal variations in strike energy


Snap-shot measurements of ambient noise in the area – before and/or after
Log metadata: water depth, hydrophone depth, CTD measurement, wind speed/sea-state, GPS position at each measurement location, other local activity

We have satisfied the requirement for measurement during the construction period
Measurement at various ranges
Monitored the ‘soft-start’ using a fixed recorder (variation in source output) Why do we use propagation modelling in relation to the licence condition? i) Limited number of measurements
Measurements usually only possible along one transect
Length of transect or number of measurement positions can be limited by the piling duration

ii) Measurements may not capture worst case
Maximum hammer blow energy required to drive foundations may not be reached for measured piles
Worst case propagation conditions (e.g. seabed geology, tidal changes, seasonal changes)

Propagation modelling
Propagate our measured transect data back to a source level (as a function of frequency)
Propagate our source level out to provide a received level as a function of position (and depth)
Propagation model should account for variations in the propagation environment (bathymetry, absorption, interaction with the seabed, interaction with the surface)
Suitable propagation models are published in the peer reviewed literature, many are freely available

Energy-flux propagation model (Weston, 1976) using third-octave band source level from Ainslie (2012)

Operational noise Underwater noise monitoring during operational phase (Example) The Licence Holder must make provision during the construction phase of the wind farm to install facilities to enable subsea noise and vibration from the turbines to be assessed and monitored during the operational phase of the wind farm. Before completion of the construction phase the Licence Holder must supply specification to the Licence Authority of how it proposes to measure subsea noise and vibration at various frequencies across the sound spectrum at a selection of locations immediately adjacent to, and between turbines, within the array and outside the array at varying distances.

Measurement details – Operational noise monitoring
Measurements close to turbines and between turbines
Measurement within the array and outside the array at varying distances

Summary
There is currently no international standard for how the measurements should be performed
UK - developing guidance on underwater noise measurement (not specific to piledriving)


Current best practice in the UK includes
range dependent measurements
static measurement positions to monitor variations in the sound output with time (including soft-start)


Modelling is employed to compliment measurement data
Limited number of measurements
Measurements may not capture worst case

Danke Schön