Aarhus Universitets segl

No. 447: Kattegatt Syd Offshore Wind Farm. Effects of pile driving, gravity foundations and sediment spill on marine mammals

Kyhn, L.A., Tougaard, J. & Mikaelsen, M.A. (2021). Kattegatt Syd Offshore Wind Farm. Effects of pile driving, gravity foundations and sediment spill on marine mammals. Aarhus University, DCE – Danish Centre for Environment and Energy, 167 pp. Scientific Report No. 447 http://dce2.au.dk/pub/SR447.pdf.


Construction and operation of a proposed offshore wind farm at the Swedish site Kattegatt Syd between the two Natura 2000 sites Lilla Middelgrund and Stora Middelgrund and Röda 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 of the Kattegatt Syd offshore windfarm site. In the southern part of Kattegat, these porpoises belong to the Belt Sea population which is assessed on national red lists as Least Concern (LC).

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 (LC), whereas the grey seal appears in low numbers and red-listed as Vulnerable (VU).

Sensitivity to impact

Underwater noise is likely to be the main source of impact on marine mammals from wind farm construction, but the impact of sediment spill is also assessed. Unabated 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.

Various mitigation measures for pile driving 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.

Magnitude of impact on harbour porpoises and seals was assessed for sediment spill, as well as for effects of underwater noise from installation of foundations by pile driving. The assessed effects are 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 (14 m diameter, app. 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 sites. 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) (JNCC 2020b).

Impact from construction

For the noise exposure assessment, modelling was performed for three positions (north, middle and south) and two seasons (Summer/July and Winter/December). The two seasons were picked to represent the most extreme hydrographical conditions with respect to sound propagation, with December being the worst (upward-refracting conditions) and July the most favourable (downward-refracting conditions) due to a complete mixing of the water column at this time of the year, which leads to less favourable conditions for long-range sound propagation (iso-velocity, or downward-refracting conditions).

Modelling was performed for a monopile diameter of 14 m, which at the time of writing this report was the worst-case scenario for the monopiles to be installed.

Based on experience from similar projects it was assumed that construction will require substantial mitigation of noise impact, in the form of noise abatement systems. Scenarios without noise abatement was therefore judged as unrealistic and therefore not included in the assessment. Modelling was thus performed for pile driving with industry standard noise abatement (Big Bubble Curtains) and with best available technology (BAT) for noise abatement. Currently, the technology identified as best available is 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. The results of modelling using the HSD-DBBC system also pertains to piling with an equivalent, but other abatement system that mat be developed before the windfarm will be constructed.

Assuming use of the assessed noise abatement systems, the following can be stated from the assessment:

  • It is considered unlikely that marine mammals will be exposed to sound pressures able to inflict acute injury (acoustic trauma involving damage to lungs and other air-filled structures and gas-embolism) as noise abatement will be used.
  • Modelling predicts that seals and porpoises would have to be located within 25 m of the bubble curtains surrounding the mono pile during piling, at onset of pile-driving (when soft start begins)  in order to be exposed to sound levels capable of inducing permanent hearing loss (PTS). It is considered unlikely that animals will be this close to the site at onset to pile driving, due to the presence of multiple working vessels and the bubble curtains. The impact on the population of seals and porpoises is 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 abatement with big bubble curtains in December, less than 1 % of the porpoise population in the Southern Kattegat and the Belt Seas was predicted to be exposed to sound pressures above the behavioural reaction threshold, amounting to an exposed area of 208 km2 for a period of about 120 days if piling of the 60 foundations takes place every other day. The impact of pile driving in December with use of big bubble curtains on both seal populations and the Belt Sea harbour porpoise population is therefore assessed as minor. With use of best available technology and best environmental practice, which is pile driving in July, with the HSD-DBBC noise abatement system or equivalent, the impacted area will decrease (predicted to be 19-26 km2) and hence the fraction of the population being affected will decrease as well. 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 pile driving noise will be capable of masking sounds relevant to porpoises 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. This communication is especially important during the mating season and takes place primarily close to breeding and haul-out sites. Given the large distance from the construction site to nearest haul-outs, the overall impact of masking from the pile driving noise on both seal populations is assessed to be negligible.
  • There is no evidence suggesting that the three marine mammal species assessed are affected by low water turbidity, and the impact of the sediment spill from construction of the offshore wind farm, is assessed as negligible.

Impact from operation

There is a lack of long-term studies examining the effect on harbour porpoises of operating offshore windfarms, especially taking service vessels into account, as well as the increase in size of the turbines. The existing studies varies in effect from attraction (likely due to a lack of trawling) to a reduced number of animals as compared with reference stations. The impact is therefore assessed with some uncertainty to negligible for harbour porpoises, and it would be of benefit if the area were to be closed off for all fishing activities. Based on studies of effects from existing offshore wind farms in operation, no negative effects of the wind farm is predicted on seals 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 for the seal population in Kattegat.

Impact on Natura 2000 sites

The Natura 2000 sites Lilla Middelgrund, Stora Middelgrund & Röda Bank, could be affected by construction of the offshore wind farm. Under worst case conditions, which is pile driving abated with Big Bubble Curtains in December, both sites will be affected by exposure to noise levels above the behavioural reaction thresholds for porpoises to an extent of more than 20 %, which is used as the recommended maximum disturbance threshold as put forward by JNCC and used in the absence of national guidelines from Sweden. This is according to the JNCC guidelines assessed as an unacceptable impact.

Application of Best Available Technology for noise abatement during pile driving, which would be Double Big Bubble Curtains in combination with hydro sound dampeners (or similar), and piling during a period with a downward refracting sound speed profile in July (Best Environmental Practice) will reduce the emitted noise considerably. This would reduce the fraction of the Natura 2000 sites Lilla Middelgrund and Stora Middelgrund & Röda Bank exposed to noise levels above the behavioural reaction threshold, to between 3 - 5 % for 14 m monopiles. The impact is according to the JNCC guidelines thus assessed to be acceptable.

Under worst-case conditions vessel noise from installation of gravity based foundations none of the nearby Natura 2000 sites will be affected by the vessel noise above the 20 % threshold and the effect is assessed as acceptable