Underwater noise and offshore windfarms Dr Jeremy Nedwell, Mr John Langworthy and Mr Daren Howell...

Underwater noise and offshore windfarms

Dr Jeremy Nedwell, Mr John Langworthy and Mr Daren Howell

BWEA Conference 4/3/04

www.subacoustech.com 544 R 0502

Aim of study

To evaluate the noise from construction and operation of windfarms and to rate it in terms of its potential for environmental effect

“Noise Audit” approach - may include noise during construction, operation and decommissioning


General considerations

General description of locations of windfarm?

Shoals - “A shallow place in a body of water.” “A sandy elevation of the bottom of a body of water, constituting a hazard to navigation; a sandbank or sandbar.”


General considerations

Typical windfarm area (shoals) not previously subject of study for acoustics

Are the sound propagation and underwater noise charecteristics the same as for deep water?


Major questions to answer

What is the prevailing level of background noise?

What noise sources are created by windfarm developments, and how do these vary with range?

What are the dominant sources?

What are the statistics of the noise?


Status of work so far

First 9 months of work has been largely reactive (opportunity based)

Split about 50/50 in time on measurements of background noise and measurements of construction noise

‘04: Emphasis on evaluation of operational noise, continuing opportunity measurements


Statistics

1324 individual measurements made About 60 Gbyte of data processed Two top of range desktops processing

nonstop (bar failures) for 4 months Four hard disks worn out, 1442 cups of

coffee,……..


Status of work so far

Measurements taken at both night and day In addition to acoustical data, GPS log of

position, sea state, instrumentation details, water depth, wind speed, source type, position bearing and distance (if any), acquisition and analysis information and CTD information all recorded on data header for every recording


Background noise & its statistics

Large body of data needed to establish average background levels and statistics in shoals

About 1000 individual measurements of ambient noise made


Why statistics?


Monitoring strategy

Fixed position - Sound level meter in fixed place, gives information about changes in level with time but no spatial information

Transects - Limited information concerning time variation but gives good indication of spatial variation


Fixed position monitoring

“At this position, is the noise causing an effect?”

Mainly applicable to monitoring limits set by regulators

May be implemented in later stages of project


Transect monitoring


e.g. transect at North Hoyle

Mid HoyleChannel

WelshChannel

RHYL

Target Measurement Location

Wind Turbine

Not to Scale

N0

N1

N2

N3

S1

S2

S3

S4

S5

W1W2W3

W4

W6

W5

E1 E2E3

E4

N


Typical noise measurement

27th Octave smoothed PSD of background noise time history, calculated from an ensemble of 30 one second samples. The black line represents the power spectral density, the coloured lines above and below represent the confidence interval.


Background noise in shallows

Wenz curves


Diurnal variability

Variability vs time of day - note influence of shipping during day


Variability with wind

Variability vs wind speed - quieter at low w/s


North Hoyle noise measurements

222 measurements of SPL for background noise at North Hoyle. The plots were produced by

counting the number of occurrences of measured levels that fall into bins separated by 5dB.


Scroby Sands noise measurements

28 measurements of SPL for background noise at Scroby Sands. The plots were produced by

counting the number of occurrences of measured levels that fall into bins separated by 5dB.


Noise measurements in dBhts

Distribution of dBht levels for all measurements of background noise taken at 5m depth at North Hoyle.


Summary: background noise in shallows Levels rather noisier than typical deep

water noise levels (at upper end of Wenz scale)

Slope and detailed shape of spectrum different

Variability great at frequencies below 1000 Hz, low at higher frequencies


Summary: background noise in shallows Influence of shipping causes variability

of noise in the day to be higher than at night


Douglas Platform


Construction noise

Measurements have been taken on an opportunity basis of:

Monopile hammering at North Hoyle and Scroby Sands

Rock socket drilling at North Hoyle

Cable trenching at North Hoyle


North Hoyle; Cable trenching noise

Recorded at a range of 160m with the hydrophone at 2m depth


North Hoyle; Rock socket drilling noise

Recorded at a range of 330m with the hydrophone at 10m depth


North Hoyle; Rock socket drilling noise


Piling noise


North Hoyle; Piling noise

Time history of pile hammering recorded at 3905 m from sound source, 5 m below water surface.



Power Spectral Density of pile hammering noise at distances of 3905m , 1881m, and 955m.



SPL plotted against range for all measurements of pile hammering to show variation of SPL with range and transect.



Typical peak pressure SL and TL Model for measurements of pile hammering noise from North Hoyle at 5m depth


North Hoyle; Behavioural effects of piling noise?

Pile hammering noise measurements at 10m depth



Pile hammering noise measurements at 5m depth



Assuming a “strong reaction” threshold of 90 dBht, the corresponding reaction ranges are:

Salmon 1400 m

Cod 5500 m

Dab 1600 m

Bottlenose Dolphin 4600 m

Harbour Porpoise 7400 m

Harbour Seal 2000 m


North Hoyle; Could piling noise cause injury ?

Summary of effects of different impulses on mammals diving beneath the watersurface (from Yelverton et al., 1972).

Impulse(dB re 1Pa.s)

Impulse(bar.msec)

Effects

169 2.76 No mortality. High incidence of moderately severe blast injuries,including eardrum rupture. Animals should recover on their own.

163 1.38 High incidence of slight blast injuries, including eardrum rupture.Animals should recover on their own.

157 0.69 Low incidence of trivial blast injuries. No eardrum ruptures.

151 0.34 Safe level. No injuries.


North Hoyle; Could piling noise cause injury ?

Injury range for marine mammals about 30 metres


Piling; mitigation

Aim should not be to stop all piling work but to assess effect and minimise impact:

Minimise noise at source (pile diameter?) Bubble curtain/physical screens (enough known?)

Monitoring with RT feedback to contractors

Use of periods when target species absent (is enough known?)

Caged fish, tagged mammals or AAM/PAM trials to confirm


Operational noise

Initial measurements of turbine operational noise taken at Blyth windfarm site on Northeast coast (not reported).

Further measurements are planned at Blyth and North Hoyle (when fully operational in 2004).


SummaryOverall: Good quality set of measurements of

background and construction noise in typical windfarm areas.

Background noise; levels at upper bound of deep water ambient noise levels

Varies significantly more during the daytime than at other times of day

Douglas Platform is pre-existing contributor to background noise level at North Hoyle.


Summary

Cable trenching Source Level of 178 dB re 1 Pa @ 1 metre

Rock socket drilling: Components of the drilling could be identified at ranges of up to 7 km.


Summary

Piling: high Source Level at North Hoyle (260 - 262 dB re 1 Pa @ 1 metre), Transmission Loss of 22 log (R) where R is the range. Measurements of piling at Scroby Sands were similar in level to those at North Hoyle

Strong avoidance reaction by range of species likely up to several kilometres, injury within perhaps a hundred metres.


Summary

(Piling contd.) Should be regarded as capable of causing

significant environmental effect, and Planning of piling operations should take

account of the effects of noise on sensitive species.

If environmental consequences are unacceptable, mitigation measures required to reduce impact to acceptable level.


Sound files Ambient noise

Between turbines, Blyth

Cable trenching, 400m

Freighter 800m

Piling 750m

Piling 6500m

Douglas Platform, 500m

Underwater noise and offshore windfarms Dr Jeremy Nedwell, Mr John Langworthy and Mr Daren Howell...

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