How could operational underwater sound from future offshore wind turbines impact marine life?

Offshore wind farms are part of the transition to a sustainable energy supply and both the total numbers and size of wind turbines are rapidly increasing. While the impact of underwater sound related to construction work has been in the focus of research and regulation, few data exist on the potential impact of underwater sound from operational wind farms. Here, we reviewed published sound levels of underwater sound from operational wind farms and found an increase with size of wind turbines expressed in terms of their nominal power. This trend was identified in both broadband and turbine-specific spectral band sound pressure levels (SPLs). For a nominal power of 10 MW, the trends in broadband SPLs and turbine-specific spectral band SPLs yielded source levels of 170 and 177 dB re 1 µPa m, respectively. The shift from using gear boxes to direct drive technology is expected to reduce the sound level by 10 dB. Using the National Oceanic Atmospheric Administration criterion for behavioral disruption for continuous noise (i.e., level B), a single 10 MW direct drive turbine is expected to cause behavioral response in marine mammals up to 1.4 km distance from the turbine, compared to 6.3 km for a turbine with gear box.

Effect of impact pile driving noise on marine mammals: A comparison of different noise exposure criteria.

Regulators in Europe and in the United States have developed sound exposure criteria. Criteria range from broadband levels to frequency weighted received sound levels. The associated differences in impact assessment results are, however, not yet understood. This uncertainty makes environmental management of transboundary anthropogenic noise challenging and causes confusion for regulators who need to choose appropriate exposure criteria. In the present study, three established exposure criteria frameworks from Germany, Denmark, and the US were used to analyse the effect of impact pile driving at a location in the Baltic Sea on harbor porpoise and harbor seal hearing. The acoustic modeling using MIKE showed that an unmitigated scenario would lead to auditory injury for all three criteria. Despite readily apparent variances in impact ranges among the applied approaches, it was also evident that noise mitigation measures could reduce underwater sound to levels where auditory injuries would be unlikely in most cases. It was concluded that each of the frameworks has its own advantages and disadvantages. Single noise exposure criteria follow the precautionary principle and can be enforced relatively easily, whereas criteria that consider hearing capabilities and animal response movement can improve the accuracy of the assessment if data are available.