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 HowellBWEA Conference 4/3/04
2 Aim of studyTo 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 acousticsAre 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 farFirst 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 processedTwo top of range desktops processing nonstop (bar failures) for 4 monthsFour 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 shoalsAbout 1000 individual measurements of ambient noise made
11 Monitoring strategyFixed position - Sound level meter in fixed place, gives information about changes in level with time but no spatial informationTransects - 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 regulatorsMay be implemented in later stages of project
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.
17 Diurnal variabilityVariability vs time of day - note influence of shipping during day
18 Variability with windVariability 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 differentVariability 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
38 North Hoyle; Could piling noise cause injury ? Injury range for marine mammals about 30 metres
39 Piling; mitigationAim 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 contractorsUse of periods when target species absent (is enough known?)Caged fish, tagged mammals or AAM/PAM trials to confirm
40 Operational noiseInitial 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 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 levelsVaries significantly more during the daytime than at other times of dayDouglas 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 SummaryPiling: 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 HoyleStrong 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, andPlanning 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.