Aarhus University Seal / Aarhus Universitets segl

No. 366: Effects of larger turbines for the offshore wind farm at Krieger's Flak, Sweden

Jakob Tougaard and Mark Mikaelsen. 2020. Effects of larger turbines for the offshore wind farm at Krieger's Flak, Sweden. Addendum with revised and extended assessment of impact on marine mammals. Aarhus University, DCE – Danish Centre for Environment and Energy, 32 pp. Scientific Report No. 366 http://dce2.au.dk/pub/SR366.pdf

Summary

Construction and operation of an offshore wind farm on the Swedish part of Krieger’s Flak has previously been assessed with respect to impacts on marine mammals (Tougaard & Mikaelsen 2018). This report presents an updated and extended assessment of the previous report, based on a revision of the specification of the pile driving operations, improved information on noise abatement systems and improved methods for assessment of behavioural disturbance.

 

Impact was modelled and assessed for pile driving of steel monopiles, either 11 m or 12 m diameter, at a worst-case location within the wind farm area, bordering the nearby Natura2000 site Sydvästskånes Udstövatten. Impact was assessed for piling with and without use of a noise abatement system, exemplified by the double Big Bubble Curtain. The modelling without noise abatement system was included for reference only.

 

Results of the modelling and conclusions of the assessments were:

 

  • Pile driving without use of an adequate noise abatement system is likely to present a significant impact to marine mammals, both in the form of risk of inflicting permanent damage to their hearing system and disturbance to animals over a wide area, including the nearby Natura2000 area.
  • Use of a noise abatement system with sound attenuation properties comparable to, or better than the double Big Bubble Curtain, will reduce the radiated noise and hence also the impact considerably:
    • There is no risk of inflicting permanent damage to the hearing of seals or porpoises, provided an efficient deterrence of the animals is conducted prior to start of the pile driving.
    • A low number of seals and porpoises are at risk of experiencing low levels of temporary hearing loss (TTS). The affected frequency range means that the TTS does not interfere with porpoise echolocation or communication and full recovery will be within hours after the pile driving. The impact on seals and porpoises is therefore considered minor.
    • Noise from the pile driving is predicted to be capable of affecting the behaviour of porpoises out to a distance of 5-10 km. By combining with modelled data for porpoise abundance, the number of affected porpoises were estimated to be 12 and 4 per foundation for summer and winter, respectively. As the duration of the disturbance is likely to be low, about 6 hours, the impact on porpoises by disturbance is assessed to be minor.
    • Although a quantitative assessment could not be made for seals, it was assessed that the impact on seals is likely to be comparable or smaller than the impact on porpoises. The impact of pile driving on harbour seals and grey seals is thus assessed to be minor.
  • A large part of the impacted area, both with respect to effects on hearing and behaviour, is likely to be within the nearby Natura2000 area. However, the number of animals affected is low, in particular during winter months, and the duration of the impact short-lived. The impact on the Natura2000 area Sydvästskånes Udstjövatten is assessed to be minor. The Natura2000 site Falsterbo-Foteviken is too far away from the construction site to be affected by the pile driving.
  • The impact from underwater noise from the turbines during operation of the wind farm is assessed to be low and constitute a minor impact on seals and porpoises.

 

Finally, it is noted that the currently available noise abatement systems are so efficient that there is reason to caution against the common practice of employing acoustic deterrence devices in the form of seal scarers prior to onset of pile driving, as the seal scarer may constitute a source of disturbance equal to or even exceeding the pile driving itself. Effective mitigation of injury to the hearing of marine mammals rely on such deterrence, but the type and intensity of deterrence devices should be adapted to the conditions.