Upload
lexie-basham
View
220
Download
0
Embed Size (px)
Citation preview
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
www.subacoustech.com 544 R 0502
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.”
www.subacoustech.com 544 R 0502
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?
www.subacoustech.com 544 R 0502
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?
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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,……..
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
Diurnal variability
Variability vs time of day - note influence of shipping during day
www.subacoustech.com 544 R 0502
Variability with wind
Variability vs wind speed - quieter at low w/s
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
Noise measurements in dBhts
Distribution of dBht levels for all measurements of background noise taken at 5m depth at North Hoyle.
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
Summary: background noise in shallows Influence of shipping causes variability
of noise in the day to be higher than at night
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
North Hoyle; Cable trenching noise
Recorded at a range of 160m with the hydrophone at 2m depth
www.subacoustech.com 544 R 0502
North Hoyle; Rock socket drilling noise
Recorded at a range of 330m with the hydrophone at 10m depth
www.subacoustech.com 544 R 0502
North Hoyle; Piling noise
Time history of pile hammering recorded at 3905 m from sound source, 5 m below water surface.
www.subacoustech.com 544 R 0502
North Hoyle; Piling noise
Power Spectral Density of pile hammering noise at distances of 3905m , 1881m, and 955m.
www.subacoustech.com 544 R 0502
North Hoyle; Piling noise
SPL plotted against range for all measurements of pile hammering to show variation of SPL with range and transect.
www.subacoustech.com 544 R 0502
North Hoyle; Piling noise
Typical peak pressure SL and TL Model for measurements of pile hammering noise from North Hoyle at 5m depth
www.subacoustech.com 544 R 0502
North Hoyle; Behavioural effects of piling noise?
Pile hammering noise measurements at 10m depth
www.subacoustech.com 544 R 0502
North Hoyle; Behavioural effects of piling noise?
Pile hammering noise measurements at 5m depth
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
North Hoyle; Could piling noise cause injury ?
Injury range for marine mammals about 30 metres
www.subacoustech.com 544 R 0502
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
www.subacoustech.com 544 R 0502
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).
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
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.
www.subacoustech.com 544 R 0502
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.