1. Brian Polagye & Paul Murphy Keith Bethune, Patrick Cross, & Luis Vega
Temporal and Spatial Variations in Sound Produced by a Wave Energy Converter
Brian Polagye & Paul Murphy
Keith Bethune, Patrick Cross, & Luis Vega
Northwest National Marine Renewable Energy Center
University of Washington
Seattle, WA USA
Hawai’i National Marine Renewable Energy Center
University of Hawai’i
Manoa, HI USA

2. Credit: www.uwphotographyguide.com
Motivation
Research: Sound produced by wave energy converters may affect marine mammal behavior and information is limited
Consenting: Ensure sound levels do not exceed agreed thresholds
Humpback whale
Credit:
Source level threshold for WETS is 151 dB (broadband) – exceeded by more than 3 dB more than 5% of operating time

3. US Navy Wave Energy Test Site (WETS)
80 m berth
WETS
60 m berth
30 m berth
Credit: Google Earth
Located on island of Oahu
Construction funded by NAVFAC, building off test site within the boundary of MCBH original used for OPT power buoy
3 berth: 30 m, 60 m, and 80 m – relatively consistent bathymetry in vicinity of each berth
Wave climate monitored by Waverider buoy with forecast

4. Azura Wave Energy Converter
Point absorber design
Northwest Energy Innovations (NWEI)
Operating since late May 2015
1:2 scale
18 kW peak power
Peak power, as limited by inverters

5. Drifting Instrumentation
Met Station: Airmar WX200
1-2 Hz recording rate
GPS: QStarz BT-Q1000eX
10 Hz recording rate
Surface Wave Instrumentation Float with Tracking (SWIFT)
1.15 m
Thomson, J. (2012). Wave breaking dissipation observed with “swift” drifters. Journal of Atmospheric and Oceanic Technology, 29(12),
Hydrophone: OceanSonics icListen HF
Continuous acquisition at 256 kSamples
Also includes IMU – working to reconstruct wave spectra from SWIFT motion – getting there, but not quite

6. Representative Acoustic Data
Generator Tones
SWIFT Self-noise
“Boom”
Chain
- ~ 10 m distance from Azura
Relatively clean drift shown – some require more cleanup than others
Self noise classifier still a work in progress – sometimes classifies “boom” from Azura float as self-noise, sometimes misses self-noise

7. Source Identification
Generalized Self-Noise
Azura WEC
Chain
Snapping shrimp
Anthropogenic Tonal
SWIFT Self-Noise
What to declare a reference? Using close temporal location (similar sea state) and distant spatial
Azura clearly elevates spectra levels from ~ 200 Hz to ~3 kHz
No indication of higher frequency sound
Chain noise hypothesis consistent with invariance with standoff
Imperfect spectral cleaning creates some uncertainty at the tails of the distribution

8. Spatial Variability “WEC Band” Moderate sea state 200 Hz – 1450 Hz
30 s average
Moderate sea state
Hs = 1.7 – 1.8 m
Te = 6.2 s
10 m
50 m
100 m
Low overall sound levels associated with WEC operation
Hints of directionality in sound production
“Chain” sound does not follow any discernable pattern – may be associated with a more distant mooring

9. Stationary Instrumentation
July – October 2015
Anchor Points
Azura WEC
100 m
April – July 2015
200 m
Loggerhead DSG-ST
“Continuous” acquisition at 96 kSamples

10. Temporal Variability 0 – 500 Hz 500 – 1500 Hz Difficult problem:
Restrict to single deployed platform to avoid convolution of range and directionality
Bin all acoustic measurements by sea state
Manually review 30 s samples until 5-8 minutes of recording in bin (exclude periods of significant whale vocals, vessels, etc.)
Consider two bands:
500 – 1500 Hz clear in all sea states – consistent with highest intensity closer to resonance for Azura, but also some breaking wave noise in this zone.
0 – 500 Hz – dominated by flow noise for long-period, high amplitude waves (wavelength > 150 m during swell events)
As for drifting measurements, WEC tonals in Hz range, but flow noise/self noise convolved with these

11. “SLOW” Logger on Azura Mooring Buoy at WETS
Conclusions
Desirable to get closer to WEC and away from seabed
Drifting measurements provide fast information on extent of sound and sources
Stationary measurements show moderate dependence on sea state
“SLOW” Logger on Azura Mooring Buoy at WETS

12. Acknowledgements
This project is supported by the US Department of Energy Award DE-FG36-08GO18180.
Thanks to Terry Lettenmaier for operational metadata from the Azura and to the whole Sea Engineering team for support in the field, especially Patrick Anderson and Tor Harris.