Developing Capabilities for Tidal Hydrokinetic Blade Strike Monitoring

Tidal hydrokinetic power generation involves harvesting the kinetic power from fast moving tidal currents and converting this power to electricity. In most, but not all, cases this requires a rotating blade, operating in a manner analogous to wind turbines. While many possible stressor-receptor interactions have been identified for tidal hydrokinetics, direct contact between the rotating blade and marine animals is one of the few with the potential for direct injury or mortality. To evaluate this risk, information is required on the frequency and occurrence of blade strike. However, monitoring for blade strike is non-trivial. Ideally, monitoring should distinguish between direct contact and a near miss, as well as identify the species involved. Further complicating observations, evidence collected to date suggests that blade strike will be an infrequent event, requiring a high degree of effort per detection.

A blade strike monitoring system is being developed to conduct post-installation monitoring at a pilot tidal energy project in northern Admiralty Inlet, Puget Sound, Washington. A high definition stereo camera system has been selected for primary detection. However, at the project depth of 55 m, artificial lighting is required, which may attract or repel particular species. Tidal currents at the site exceed 3.5 m/s and it is anticipated that most marine animals at risk for strike will be oriented with the flow direction. Because the turbine rotor is aligned perpendicular to the flow, a camera angle that maximizes species identification (perpendicular to the direction of animal motion) does not provide effective coverage of the turbine rotor. The converse is true for observing blade strike, where a camera angle perpendicular to the rotor (and parallel to the direction of animal motion) is preferred. For this reason, the initial design employs two pairs of orthogonally-mounted stereo cameras. During testing, a stereo camera pair and strobe lighting in modular pressure housings are mounted to an aluminum frame in a downward-looking orientation. A test frame is rigidly connected to the camera frame at a fixed range (2-5 m) and equipped with targets to evaluate the possibility of species identification at different angles relative to the camera, as well as the potential to identify strike events. The approach for integrating lighted cameras into a broader post-installation monitoring system is described and preliminary results from freshwater testing are presented.