Abstract
Cape Sharp Tidal (CST), a joint venture of Emera Ltd and OpenHydro, is evaluating the potential for generating electrical power from tidal water flow with OpenHydro’s Open-Centre Turbine technology. A demonstration project is currently being conducted at the Fundy Ocean Research Centre for Energy (FORCE) test site in the Minas Passage, NS. The demonstration project aims to improve the turbine technology for long-term efficient generation of electricity from tidal currents and to understand and assess the potential effects of turbines on the environment. As required under the Environmental Assessment (EA) Approval, CST and FORCE developed Environmental Effects Monitoring Plans (EEMPs) to address the predictions of the EA. A key element of the EEMPs is the potential effects of turbine sound on fish and marine mammals.
The scope for the turbine sound component of the EEMPs is two-fold: 1) investigations to determine the best way to record operational and ambient sounds in the Minas Passage (both short and long term); and 2) subsequent data analysis to characterize the tidal turbine sound relative to the existing environment.
Acoustic data collection at the FORCE test site has been ongoing since 2012. Comprehensive measurements began in fall 2016 when CST deployed the first grid-connected Open-Centre Turbine on 7 Nov 2016, in the FORCE Crown Lease Area (the study area, Figure 1). After a six-month engineering evaluation, the turbine was disconnected from its subsea cable in April 2017, recovered in June 2017, and taken to Saint John, NB, for further design improvements. During the engineering evaluation, longterm acoustic recordings were made with hydrophones mounted on the turbine platform, as well as an autonomous hydrophone housed in a protective flow-shield on the seabed 167 m from the turbine. Shortterm drifting hydrophone measurements were made before the turbine was installed on 18 Oct and 20 Oct 2016, and with the turbine in place on 27 Mar 2017. Two methods of hanging the hydrophone below the drifting float were evaluated: one with an ‘S’-shaped catenary cable and one with an elastic cable and baffles to minimize movement.
This report, jointly funded by CST and FORCE, analyzes the 2016–2017 short- and long-term data to:
- Compare tidal turbine sound to flow noise and how it depends on current speed, turbine state, and measurement method.
- Estimate the possible effects of the turbine sound on marine life.
- Evaluate the relative utility of instrument configurations to be used moving forward when measuring the effects of new turbine configurations on the acoustic environment.
- Provide guidance on methodologies for performing acoustic measurements near tidal turbines and processing of acoustic data to mitigate effects of flow noise.
Based on these measurements, we find that at most frequencies the turbine has a lower source level than vessels that might be typical in the area. For porpoises, the sound amplitude of vessels and the turbine will be similar at similar ranges, and we can expect that pressure-sensing fish will detect and be affected by vessels at 7–10 times the range as the turbine.
From these measurements we observed that autonomous recorders near the seabed provided the best data quality and best characterization of the turbine and ambient sound through all tidal and turbine operating states. Drifter measurements provided useful validation of turbine sound at various ranges but were insufficient to develop a model of the turbine sound in all tidal and operating states. Drifter hydrophone suspensions must include an effective means of isolating the hydrophone from surface wave action, and drifters should have a GPS logger attached to record the location at least twice per minute. Hydrophones on the turbine platform need to be more carefully isolated from flow noise and electrical noise.
Based on the results, we recommend:
- Autonomous recorders in high-flow shielded moorings be considered as the primary method of assessing turbine sound levels.
- Cabled hydrophones on turbine platforms should be located as close as possible to the seabed, and they should be protected by a stream-lined flow-shield.
- The sound signature of the Open-Centre Turbine should be re-assessed during the next deployment.
- At least one lunar cycle of ambient sound should be recorded before or after the next deployment, to quantify the ambient sound levels at all current speeds. An Acoustic Doppler current profiler should be deployed at the same time as the acoustic recorder.
- The detection performance for porpoise should be compared during a simultaneous deployment of the autonomous recorder and the turbine mounted hydrophones.