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Underwater noise and offshore windfarms

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Presentation on theme: "Underwater noise and offshore windfarms"— Presentation transcript:

1 Underwater noise and offshore windfarms
Dr Jeremy Nedwell, Mr John Langworthy and Mr Daren Howell BWEA Conference 4/3/04

2 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

3 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.”

4 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?

5 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?

6 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

7 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,……..

8 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

9 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

10 Why statistics?

11 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

12 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

13 Transect monitoring

14 e.g. transect at North Hoyle

15 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.

16 Background noise in shallows
Wenz curves

17 Diurnal variability Variability vs time of day - note influence of shipping during day

18 Variability with wind Variability vs wind speed - quieter at low w/s

19 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.

20 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.

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

22 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

23 Summary: background noise in shallows
Influence of shipping causes variability of noise in the day to be higher than at night

24 Douglas Platform

25 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

26 North Hoyle; Cable trenching noise
Recorded at a range of 160m with the hydrophone at 2m depth

27 North Hoyle; Rock socket drilling noise
Recorded at a range of 330m with the hydrophone at 10m depth

28 North Hoyle; Rock socket drilling noise

29 Piling noise

30 North Hoyle; Piling noise
Time history of pile hammering recorded at 3905 m from sound source, 5 m below water surface.

31 North Hoyle; Piling noise
Power Spectral Density of pile hammering noise at distances of 3905m , 1881m, and 955m.

32 North Hoyle; Piling noise
SPL plotted against range for all measurements of pile hammering to show variation of SPL with range and transect.

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

34 North Hoyle; Behavioural effects of piling noise?
Pile hammering noise measurements at 10m depth

35 North Hoyle; Behavioural effects of piling noise?
Pile hammering noise measurements at 5m depth

36 North Hoyle; Behavioural effects of piling noise?
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

37 North Hoyle; Could piling noise cause injury ?

38 North Hoyle; Could piling noise cause injury ?
Injury range for marine mammals about 30 metres

39 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

40 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).

41 Summary Overall: 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.

42 Summary Cable trenching Source Level of 178 dB re 1 mPa @ 1 metre
Rock socket drilling: Components of the drilling could be identified at ranges of up to 7 km.

43 Summary Piling: high Source Level at North Hoyle ( dB re 1 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.

44 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.

45 Sound files Ambient noise Between turbines, Blyth
Cable trenching, 400m Freighter 800m Piling 750m Piling 6500m Douglas Platform, 500m

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