Information to support the assessment

                        Overview of underwater noise from operational offshore wind farms
  1. Throughout the operation and maintenance phase of the Array, there is a potential for mooring lines as well as wind turbine structures to generate underwater noise.
                        Auditory injury (PTS)
  1. As described in paragraph 717, periods of mooring line slackening and tensioning have the potential to produce transient ‘pinging’ or ‘snapping’ noises during the operation and maintenance phase of the Array (Liu, 1973). As described in volume 3, appendix 10.1 of the Array EIA Report, the presence of snapping transient noise was identified during acoustic underwater noise measurements at the Hywind Demonstrator Project in Norway in 2011 (Martin et al., 2011). The data was subsequently analysed and Stephenson (2015) extrapolated results from a single wind turbine to a theoretical array and it was found that with up to 115 snapping events per day, the resultant potential cumulative SEL over a 24 hour period was 156 dB re 1 µPa2s at 150 m from the wind turbines. This value is below the PTS and TTS onset acoustic thresholds for non-impulsive sources (Southall et al., 2019) ( Table 6.4   Open ▸ ).
  2. Underwater noise measurements were also taken at the completed Hywind Scotland Pilot Park Project (Burns et al., 2022). The study reported three distinct transient sounds characterised as ‘bang’, ‘creak’ and ‘rattle’ (Burns et al., 2022) and their presence was found to be correlated positively with wave height but to a limited extent with wind speed. The sounds were shown to originate from close to the wind turbine as opposed to further down a mooring line. A quantitative analysis of the impulsiveness of the soundscape at Hywind showed that sounds generated by floating offshore wind farms should be considered as non-impulsive (i.e. continuous) (Burns et al., 2022). The underwater noise measurements found little difference in the daily marine mammal weighted SEL between the Hywind Scotland Pilot Park and control site, and no exceedances of the TTS threshold occurred. The maximum distance at which the TTS could occur across all hearing groups was estimated for harbour porpoise at 50 m from a wind turbine assuming that the animal would remain stationary for the 24 hour period (Burns et al., 2022). Potential TTS ranges for all species are presented in Table 6.53   Open ▸ . The study concluded that even at a wind speed of 25 knots, the noise footprint is negligible and in the relatively noisy soundscape of the North Sea, it does not present any realistic threat of auditory injury to marine species.

 

Table 6.53:
Modelled Maximum Distances to Weighted SELcum TTS Threshold for 15 Knots Wind Speed (Burns et al., 2022)

Table 6.53: Modelled Maximum Distances to Weighted SELcum TTS Threshold for 15 Knots Wind Speed (Burns et al., 2022)

 

  1. A recent project by Risch et al. (2023a) collected acoustic data from two floating offshore wind farms, currently deployed off the Scottish east coast: Kincardine and Hywind Scotland. At Kincardine, five wind turbines rated at 9.5 MW were deployed on semi-submersible foundations, while at Hywind Scotland five 6 MW rated wind turbines were deployed on spar-buoys. The study found noise emissions from floating turbines were concentrated in the frequencies below 200 Hz, similar to the operational noise of fixed offshore wind turbines, and showed distinct tonal features likely related to rotational speed (between 50 Hz and 80 Hz at Kincardine and 25 Hz and 75 Hz at Hywind Scotland). The median one-third-octave band levels below 200 Hz were between 95 dB re 1 μPa and 100 dB re 1 μPa at about 600 m from the closest wind turbine for both wind farms, well below the level of mild disturbance for cetaceans. The study found the biggest difference between fixed and floating offshore wind turbines in relation to underwater noise generation is mooring-related noise, rather the operational wind turbine noise. Risch et al. (2023a), (Risch et al., 2023b) found that during higher wind speeds the number of impulsive sounds or transients from mooring-related structures increased at both Kincardine and Hywind Scotland. Source levels for turbine operational noise (25 Hz to 20 kHz) increased with wind speed at both recording locations, with levels ~3 dB higher at Kincardine than Hywind Scotland which may be due to power ratings or difference in mooring structure (semi-submersible versus spar-buoy). The study predicted noise fields for unweighted sound pressure levels were above median ambient noise levels in the North Sea for maximum distances of 3.5 km to 4.0 km from the Kincardine five wind turbine array, and 3.0 km to 3.7 km for the five wind turbine array at Hywind Scotland. At both floating offshore wind farm locations, recorded harbour porpoise detections were reduced at the recording site closest to the wind turbine compared to the site further away, but Risch et al. (2023a) does highlight these floating offshore wind farms have only been operational for a short period and these observed occurrence patterns may change over time as floating offshore wind farms become more mature.
  2. While operational noise is continuous, some studies have suggested that it is unlikely that these noise levels would result in physiological damage (Madsen et al., 2006, Marmo et al., 2013, Tougaard et al., 2009a). Early measurements of underwater noise due to operational wind turbines concluded that the underwater noise from operating wind turbines is limited to low frequencies (below 1 kHz) and of low intensity and would therefore be unlikely to affect marine mammals with main hearing sensitivities at higher frequencies (i.e. VHF and HF cetaceans and PCW) (Madsen et al., 2006). Even so, behavioural responses by marine species to operational wind turbine noise appears to be minimal. Modelled predictions by Marmo et al. (2013) suggested that only a small proportion (<10%) of harbour porpoises would display behavioural responses up to ~18 km away from an offshore wind farm, and the majority of animals studied would not show a behavioural response, indicating low potential for displacement.
  3. Monitoring using acoustic recordings at Horns Rev Offshore Wind Farm in the North Sea revealed, whilst there was a weak adverse effect on harbour porpoise from the construction, no detectable effects were observed on abundance from the operating wind farm (Tougaard et al., 2006). It must be noted however there was a significant difference between when intensive maintenance work took place (termed ‘semi-operation’) in the study, and operation. Acoustic and ship survey data indicated more porpoises in the area as a whole during the operational period than for any other of the periods, baseline included.
  4. However, field measurements and modelling efforts to estimate operational noise levels have predominantly focused on fixed-bottom offshore wind farms in shallow, near-shore environments. Analysis of noise measurements from two Danish (Middelgrunden and Vindeby) and one Swedish (Bockstigen-Valar) fixed-bottom offshore wind farms, concluded that operational noise levels are unlikely to harm or mask acoustic communication in harbour porpoises and harbour seals (Tougaard et al., 2009b). Tougaard et al. (2009a) reported at 100 m distance from 1.5 MW wind turbines, underwater sound would be audible to both harbour porpoise and harbour seal. However, at a greater distance of 1,000 m, the signal to ambient sound ratio is too low for detection in harbour porpoise as a VHF cetacean (detection by harbour seal might be possible). Furthermore, the authors caveat these results, as ambient sound values used in this study were extrapolated from measurements obtained in the Baltic and the ambient sound in most parts of the North Sea is much higher and will decrease the radius of detection significantly. The study concluded that the sound is unlikely to exceed injury thresholds at any distance from the wind turbines and was considered incapable of masking acoustic communication by harbour porpoise.
  5. Given the information presented in paragraphs 740 et seq., injury in terms of PTS and TTS are unlikely to occur as a result of this impact. Further, the noise modelling presented in Stephenson (2015) and Burns et al. (2022) was conducted with the assumption that the marine mammals would remain stationary for 24 hours, which is highly unlikely to occur. Therefore, population-level effects are unlikely to occur for the Annex II marine mammal features of the SACs.
                        Behavioural disturbance
  1. Although the underwater noise study carried out at the completed Hywind Scotland Pilot Park makes no attempt to quantify the disturbance (Burns et al., 2022), the semi-qualitative assessment provided in volume 3, appendix 10.1 of the Array EIA Report concluded that the areas of disturbance are unlikely to extend further than those for fixed wind turbine foundations.
  2. The underwater noise from operational offshore wind turbines comes from vibration in the gear box and generator, which is transmitted down the tower and radiated from the tower wall. Given that there is a paucity of qualitative data on sound radiation from the floating offshore wind towers, qualitative assessment is presented with respect to fixed wind turbines (considered as maximum design case when compared to floating). The desktop review carried out in volume 3, appendix 10.1 of the Array EIA Report suggests that although sound levels are likely to be audible within the hundreds of metres from the wind turbine, these will not be at levels sufficient to cause behavioural changes in marine mammals. However, these findings are based on data collected for wind turbines with capacity between 2 MW to 5 MW and a hub height of up to 95 m (see Table 8.27 in volume 3, appendix 10.1 of the Array EIA Report). Recent developments in turbine technology has resulted in larger turbine capacities up to 15 MW now being commercially available from Original Equipment Manufacturers (OEMs). Further developments are anticipated and may result in greater capacities being available to Ossian in early 2030’s. The maximum design scenario has been developed to take account of future technological developments with the maximum hub height set at 224 m. Given that the maximum capacity and hub height of wind turbines at the Array may be larger than previously monitored at the Hywind Scotland Pilot Park, it is likely that there will be an increase of a few dB compared to smaller wind turbines. However, considering that the Array will be located in the North Sea with relatively high shipping traffic, the difference in ambient sounds is anticipated to be minimal.
  3. Studies using long-term frequency data from wind farms with 5 MW wind turbines (Alpha Ventus, Germany) found that whilst operational sound can be identified, levels hardly exceed beyond ambient sound levels in areas near main shipping traffic routes negligible (Stober et al., 2021). Therefore, marine mammals in high traffic areas may not be able to discern operational wind turbine sound from background levels. Analysis of individual frequencies predicted a correlation between SPLs and the operational status of the wind turbines as well as the wind speed, but the total impact of the operational sound was mostly negligible (Stober et al., 2021). Nedwell et al. (2007) analysed measurements of underwater sound inside and outside of four different offshore wind farms in British waters and found operational sound levels were low and only exceeded background levels close to the wind turbines (<1 km).
  4. The potential impact of operational noise and the effect of behavioural disturbance are of high reversibility. Although noise levels are likely to be audible to marine mammals, individuals are unlikely to experience significant behavioural disturbance including displacement as a result of the increased underwater noise during operational phase. Therefore, population-level effects are unlikely to occur for the Annex II marine mammal features of the SACs.

                        Operation and Maintenance Phase

                        Berwickshire and North Northumberland Coast SAC
                        Grey seal
  1. Given the background information presented in paragraphs 740 et seq., injury in terms of PTS and TTS and behavioural disturbance are unlikely to occur as a result of this impact. Further, the noise modelling presented in Stephenson (2015) and Burns et al. (2022) was conducted with the assumption that the marine mammals would remain stationary for 24 hours, which is highly unlikely to occur given their life history and requirement to surface periodically for air. Therefore, population-level effects are unlikely to occur for grey seal. 
                        Conclusion
  1. Adverse effects on the qualifying Annex II marine mammal features of the Berwickshire and North Northumberland Coast SAC which undermine the conservation objectives of the SAC will not occur as a result of operational noise during the operation and maintenance phase. Potential effects from this activity on the relevant conservation objectives (as presented in in section 6.2.1) are discussed in turn below in Table 6.54   Open ▸ .

Table 6.54:
Conclusions Against the Conservation Objectives of the Berwickshire and North Northumberland Coast SAC from Operational Noise during the Operation and Maintenance Phase of the Array Alone

Table 6.54: Conclusions Against the Conservation Objectives of the Berwickshire and North Northumberland Coast SAC from Operational Noise during the Operation and Maintenance Phase of the Array Alone

 

  1. It can be concluded, beyond reasonable scientific doubt, that there is no risk of an adverse effect on the integrity of the Berwickshire and North Northumberland Coast SAC as a result of operational noise in the operation and maintenance phase of the Array alone.
                        Southern North Sea SAC
                        Harbour porpoise
  1. Given the background information presented in paragraphs 740 et seq., injury in terms of PTS and TTS and behavioural disturbance are unlikely to occur as a result of this impact. Further, the noise modelling presented in Stephenson (2015) and Burns et al. (2022) was conducted with the assumption that the marine mammals would remain stationary for 24 hours, which is highly unlikely to occur given their life history and requirement to surface periodically for air. Therefore, population-level effects are unlikely to occur for harbour porpoise.
                        Conclusion
  1. Adverse effects on the qualifying Annex II marine mammal features of the Southern North Sea SAC which undermine the conservation objectives of the SAC will not occur as a result of operational noise during the operation and maintenance phase. Potential effects from this activity on the relevant conservation objectives (as presented in section 6.2.2) are discussed in turn below in Table 6.55   Open ▸ .

 

Table 6.55:
Conclusions Against the Conservation Objectives of the Southern North Sea SAC from Operational Noise during the Operation and Maintenance Phase of the Array Alone

Table 6.55: Conclusions Against the Conservation Objectives of the Southern North Sea SAC from Operational Noise during the Operation and Maintenance Phase of the Array Alone

 

  1. It can be concluded, beyond reasonable scientific doubt, that there is no risk of an adverse effect on the integrity of the Southern North Sea SAC as a result of operational noise in the operation and maintenance phase of the Array alone.
                        Moray Firth SAC
                        Bottlenose dolphin
  1. Given the background information presented in paragraphs 740 et seq., injury in terms of PTS and TTS and behavioural disturbance are unlikely to occur as a result of this impact. Further, the noise modelling presented in Stephenson (2015) and Burns et al. (2022) was conducted with the assumption that the marine mammals would remain stationary for 24 hours, which is highly unlikely to occur given their life history and requirement to periodically surface for air. Therefore, population-level effects are unlikely to occur for bottlenose dolphin. 
                        Conclusion
  1. Adverse effects on the qualifying Annex II marine mammal features of the Moray Firth SAC which undermine the conservation objectives of the SAC will not occur as a result of operational noise during the operation and maintenance phase. Potential effects from this activity on the relevant conservation objectives (as presented in section 6.2.3) are discussed in turn below in Table 6.56   Open ▸ .

Table 6.56:
Conclusions Against the Conservation Objectives of the Moray Firth SAC from Operational Noise during the Operation and Maintenance Phase of the Array Alone

Table 6.56: Conclusions Against the Conservation Objectives of the Moray Firth SAC from Operational Noise during the Operation and Maintenance Phase of the Array Alone

 

  1. It can be concluded, beyond reasonable scientific doubt, that there is no risk of an adverse effect on the integrity of the Moray Firth SAC as a result of operational noise in the operation and maintenance phase of the Array alone.