Underwater noise and offshore windfarms Dr Jeremy Nedwell, Mr John Langworthy and Mr Daren Howell BWEA Conference 4/3/04 Subacoustech reference: 544R0503, COWRIE Copyright.
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 Target Measurement Location Not to Scale Wind Turbine N3 N2 W3 W2 N1 N0 W1 S1 E1 E2 E3 E4 W4 S2 W5 S3 W6 S4 Mid Hoyle Channel N S5 Welsh Channel RHYL
Typical noise measurement 27 th 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 db ht s 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.
North Hoyle; Piling noise Power Spectral Density of pile hammering noise at distances of 3905m, 1881m, and 955m.
North Hoyle; Piling noise SPL plotted against range for all measurements of pile hammering to show variation of SPL with range and transect.
North Hoyle; Piling noise 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
North Hoyle; Behavioural effects of piling noise? Pile hammering noise measurements at 5m depth
North Hoyle; Behavioural effects of piling noise? Assuming a strong reaction threshold of 90 db ht, 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
Summary of effects of different impulses on mammals diving beneath the water surface (from Yelverton et al., 1972). Impulse (db re 1 Pa.s) North Hoyle; Could piling noise cause injury? 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).
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.
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.