Aarhus Universitets segl

Nr. 409: Stora Middelgrund Offshore Wind Farm

Jakob Tougaard, Mark A. Mikaelsen, Floris M. van Beest, Line A. Kyhn. (2021). Stora Middelgrund Offshore Wind Farm. Effects of underwater noise on marine mammals. Aarhus University, DCE – Danish Centre for Environment and Energy, 142 pp. Scientific Report No. 409.


Construction and operation of a proposed offshore wind farm on the Swedish part of Stora Middelgrund in the Natura 2000 site Stora Middelgrund & Röde Bank has been assessed with respect to impacts on marine mammals and Natura 2000 sites.

Abundance of marine mammals

One cetacean, the harbour porpoise (Phocoena phocoena, tumlare) is common in the waters around Stora Middelgrund. In the southern part of Kattegat these porpoises belong to the Danish Belt Seas population, which is assessed on national red lists as Least Concern.

Two species of seals, harbour seal (Phoca vitulina, knubbsäl) and grey seal (Halichoerus grypus, gråsäl) use the area. The harbour seal is common and red-listed as Least Concern, whereas the grey seal appears in low numbers and is red-listed as Vulnerable.

Sensitivity to impact

Underwater noise is likely to be the main source of impact on marine mammals from windfarm construction, but the impact of sediment spill is also assessed in this report. Percussive pile driving is known to generate very high sound pressures, likely capable of inflicting permanent damage to the hearing of seals and porpoises and has been shown to cause behavioural disturbances at distances of tens of km from the pile driving site.

As none of the three marine mammal species depend on visual cues for foraging or survival, the sensitivity of the extensive sediment spill from construction of the offshore wind farm, is assessed as negligible.

Various mitigation measures are available, including use of deterring devices, soft-start and reduction of radiated noise by means of for example air bubble curtains and other noise abatement systems, or shifting to gravity foundations where possible.

Magnitude of impact on harbour porpoises and seals was assessed for sediment spill, as well as for effects of underwater noise from installation of piles by either gravity or piling with hammers. The assessed effects were from direct damage (acoustic trauma), hearing loss (permanent threshold shift, PTS), disturbance of behaviour and masking of other sounds. Hearing loss was assessed by considering total cumulated sound exposure levels (SELcum) over the duration expected for piling of one foundation (15 m diameter, 6 hours), taking movements of the animals into consideration and applying appropriate auditory frequency weighting to the acoustic measurements. Disturbance of behaviour was evaluated through assessing area and time exposed to levels above the reaction threshold.

Impact was also assessed for the nearby Natura 2000 areas, within one of which the offshore windfarm is intended built. Here, the impact was assessed as the area over which the noise level exceeded the reaction threshold of harbour porpoises. The impact was assessed in accordance with the guidelines recently put forward by the British Joint Nature Conservation Committee (JNCC).

Impact magnitude from construction

For the noise exposure assessment, modelling was performed for two positions (on a shallow part and a deeper part of the proposed windfarm area) and two months (February and May). The two months were picked to represent the most extreme hydrographical conditions with respect to sound propagation, with February being the worst (upward-refracting conditions) and May the most favourable (downward-refracting conditions). Modelling was performed for two different monopile diameters: 12 and 15 m. The 15 m diameter monopile was used in impact modelling, representing the worst-case scenario.

A further dichotomy was introduced, by modelling pile driving with and without best available technology for noise abatement, currently a system of Hydro Sound Dampeners and Double Big Bubble curtain (HSD-DBBC system) deployed around the monopile to reduce radiated noise from the piling. However, because unabated piling is unlikely to be permitted inside the Natura 2000 site, only abated scenarios are assessed with respect to marine mammals. Some calculations for unabated scenarios are however shown for comparison.

Assessment of impact:

  • It is considered unlikely that marine mammals will be exposed to sound pressures able to inflict acute injury (acoustic trauma).
  • The risk of inflicting permanent hearing loss can be eliminated by using best available technology and best environmental practice, which in this case is pile driving in May with the HSD-DBBC system. The impact on seals and porpoises through hearing loss is then assessed to be negligible.
  • Noise from pile driving will cause disturbances to the natural behaviour of both seals and porpoises. Under worst-case conditions for sound propagation, which is unabated pile driving in February, 61 % of the porpoise population in the South-Eastern Kattegat is predicted to be exposed to sound pressures above the behavioural reaction threshold, amounting to an exposed area of 10,647 km2, and this impact is assessed as major.
  • With use of best available technology and best environmental practice, which is pile driving in May (due to larger transmission loss), with the HSD-DBBC noise abatement system, it is assessed that only <1 % of the porpoise population, equal to an area of max 37 km2, depending on pile diameter, will be exposed to noise levels above the behavioural reaction threshold. Under these conditions, the impact on seal and porpoise populations is assessed to be minor.
  • The main noise from installation of gravity foundations is  considered to be from vessels. It is assessed likely that under worst-case conditions vessel noise from installation of gravity foundation will affect a total area of 24 km2, which is assessed as a minor impact.
  • It is considered unlikely that abated pile driving noise or noise from installation of gravity foundations will be capable of masking sounds relevant to porpoises e.g. during prey capture or intraspecific communication to any noticeable degree and the magnitude of this impact on porpoises was thus assessed as negligible
  • There is a possibility that communication sounds from both grey seals and harbour seals can be masked by pile driving noise and noise from gravity foundations. The communication is especially important during the mating season near haul-outs. Given the favourable conservation status of the population of harbour seals, the overall impact of masking from the abated pile driving noise is assessed to be negligible
  • As none of the three marine mammal species depend on visual cues for foraging or survival, the impact of the extensive sediment spill from construction of the offshore wind farm, is assessed as negligible.

Impact from operation

Based on studies of effects from existing offshore wind farms in operation, no negative effects of the wind farm is predicted on seals and porpoises once in operation and the effect is thus assessed as negligible, The cumulative effect of adding an additional offshore wind farm to already existing offshore wind farms in the area is likewise considered negligible.

Impact on Natura 2000 sites

As no national guidelines exists on restriction of underwater noise inside Natura 2000 sites, the impact is evaluated on the basis on the JNCC guidelines from 2020.

The Natura 2000 sites Stora Middelgrund & Röde Bank, together with adjacent or nearby seven sites: Skånes Nordvästvatten, Lilla Middelgrund, Store Middelgrund  (DK), Fladen, Balgö, Anholt, and Gilleleje Flak and Tragten could be affected by construction of the offshore windfarm. Under worst case conditions, which is unabated pile driving in February, all eight areas would be affected by exposure to noise levels well above the behavioural reaction thresholds for porpoises.

Application of Best Available Technology to the pile driving, which would be noise abatement, such as double Big Bubble Curtains and hydro sound dampeners, and piling during a period with a downward refracting sound speed profile (Best Environmental Practice) will reduce the emitted noise considerably. This would reduce the fraction of the Natura 2000 site Stora Middelgrund & Röde Bank (in which the wind farm is located) exposed above the behavioural reaction threshold to between 18-27 % for 15 m piles. The impact is assessed to be unacceptable according to the JNCC guidelines, because the impacted area exceeds 20 % of the Natura 200 site. The same applies to the neighbouring Natura 2000 site Store Middelgrund (DK).

The impact on the combined Store Middelgrund and Stora Middegrund & Röde Bank Natura 2000 sites under application of Best Available Technology and Best Environmental Practice and with an additional mitigation amounting to up to 11 dB extra abatement, depending on location, will reduce affected area to below the 20 % threshold put forward by JNCC and is therefore assessed to be acceptable. The same applies to the six more distant Natura 2000 sites.

It is assessed likely that under worst-case conditions vessel noise from installation of gravity foundations will affect up to 17 % of the combined area of the two Store Middelgrund and Stora Middelgrund & Röde Bank Natura 2000 sites, which is assessed as acceptable.