1.5. Summary of the Array EIA

1.5.1 Piling

  1. Pile driving during the construction phase of the Array has the potential to result in injury to marine mammals through increased underwater noise levels. A detailed underwater noise modelling assessment was carried out to investigate these effects (volume 3, appendix 10.1). Injury, in the form of a PTS was investigated with respect to two metrics over the entire piling sequence from hammer initiation to maximum hammer energy (of 3,000 kJ for wind turbines or 4,400 kJ for OSPs). SPLpk was used to determine ranges for instantaneous injury at the highest point over the piling sequence whilst cumulative Sound Exposure Level (SELcum) was modelled to estimate the injury range from cumulative exposure as an animal flees the area ( Table 1.3   Open ▸ ). The instantaneous injury (based on SPLpk metric) could occur out to a maximum range of 1,600 m across all species during single pile installation at OSPs, with the maximum range predicted for harbour porpoise. Considering cumulative exposure using the SELcum metric, the risk of PTS was estimated to occur out to a maximum range of 7,200 m and was predicted for minke whale during single pile installation at OSPs.
  2. The maximum spatial effect was estimated using two different concurrent piling scenarios, at wind turbines with a hammer energy of 3,000 kJ with either a wind turbine with a hammer energy of 3,000 kJ or an OSP with hammer energy of 4,400 kJ, respectively. Considering cumulative exposure using the SELcum metric, the risk of PTS was estimated to occur out to a maximum range of 9,740 m and was predicted for minke whale during concurrent pile installation at wind turbine and OSP. A summary of the injury ranges without ADD and significance of the effect assessed in marine mammal impact assessment is provided in Table 1.3   Open ▸ .
  3. As discussed in paragraph 14, mitigation is based upon SPLpk in this outline MMMP following consultation with NatureScot on Marine Mammal Consultation Note 2 (volume 3, appendix 5.1, annex E). Therefore, across all species, the maximum range without ADD (based on SPLpk) over which injury could occur was predicted to be 1,600 m, which is greater than the standard 500 m mitigation zone proposed by JNCC (2010a). For SELcum, the maximum injury range was up to 9,740 for minke whale, but this was considered to be highly precautionary as the SELcum metric can lead to overestimates in effect ranges due to the assumptions included in the model for cumulative exposure, including:
  • the sound retains its impulsive character at all distances;
  • animals flee from the sound at constant and conservative swim speeds;
  • the same shift in hearing could occur regardless of how energy is distributed over time (equal-energy rule);
  • the soft-start procedure does not allow for short pauses in piling (e.g. for realignment) when exposure would be reduced; and
  • the maximum hammer would be reached and maintained.
  1. The modelled ranges suggest that injury could occur well below the maximum range, particularly with respect to SPLpk ranges. This is typical for underwater noise modelling results for piling and for this reason, mitigation zones have previously been based on SPLpk ranges only. To adopt a conservative approach in the EIA assessment, the risk of injury and subsequently the ability to mitigate for this risk was investigated with respect to the maximum possible ranges; thus, adopting the dual metric approach as recommended in Southall et al., (2019). A maximum mitigation zone of 1,600 m has therefore been presented here based on SPLpk, but final agreement on the appropriate mitigation zone will be agreed with Marine Directorate – Licensing Operations Team (MD-LOT) following consultation with Marine Directorate – Science, Evidence, Data and Digital (MD-SEDD) and NatureScot post-consent.

Table 1.3:
Potential Maximum Ranges of Effect of Piling (without ADD) on Six Marine Mammal Species as Presented in the Volume 2, Chapter 10. Injury Ranges for SELcum are Shown for Single Piling (Lower) to Concurrent Piling (Higher). Designed in Measures Includes up to 30 Minutes ADD

Table 1.3 Potential Maximum Ranges of Effect of Piling (without ADD) on Six Marine Mammal Species as Presented in the Volume 2, Chapter 10. Injury Ranges for SELcum are Shown for Single Piling (Lower) to Concurrent Piling (Higher). Designed in Measures Includes up to 30 Minutes ADD

1 N/E = Threshold not exceeded

 

  1. Although the potential for injury in terms of PTS were predicted to be minor in EIA terms (volume 2, chapter 10), there is the potential for auditory injury to marine mammals (which are all international IEFs). Therefore, designed in mitigation in the form of ADDs (up to 30 minutes, with any duration longer than 30 minutes considered secondary mitigation), in addition to other designed in protocols, is included (i.e. use of marine mammal observers and passive acoustic monitoring operators) (JNCC, 2010a). Mitigation will be applied by deploying an ADD to deter marine mammals from the area of impact prior to commencement of construction-related “noisy” activities. The JNCC (2010a) guidance for mitigating the effects of piling recommends usage of ADDs in addition to MMO2 and PAM operators, particularly in low visibility or at night. This is because there are inherent uncertainties in these visual and acoustic techniques, as they can be unreliable in detecting animals in high sea state and/or in low visibility. More details about required duration of ADD activation is provided in paragraph 20 et seq.
  2. There are numerous ADDs with different sound characteristics available (McGarry et al., 2020) and a suitable device will be selected based on the key species requiring mitigation, following discussion with relevant stakeholders. It is expected that key species to consider will be minke whale and harbour porpoise, however this will be agreed with stakeholders’ post-application. The sound emitted by the ADD will not injure marine mammals but will be loud enough to deter them from the sound source (hence their effectiveness as an additional mitigation measure).
  3. Results from the Array underwater noise modelling (volume 3, appendix 10.1) suggest that use of an ADD for 30 minutes prior to commencement of piling would further reduce the potential to experience injury to marine mammal receptors ( Table 1.4   Open ▸ and Table 1.5   Open ▸ ). The maximum injury zones for species predicted using the SPLpk metric for piling (used to define the mitigation zone) at a maximum hammer energy of 4,400 kJ (as presented in the volume 2, chapter 10), are illustrated in Table 1.4   Open ▸ . Assuming conservative swimming speeds (agreed via consultation with key stakeholders on the Marine Mammal Methodology Note, volume 3, appendix 5.1, annex B), the Array marine mammal impact assessment demonstrated that use of an ADD for 30 minutes prior to commencement of piling would deter all animals beyond the maximum modelled injury zones. This is in line with previous studies which reported that ADDs were able to deter marine mammal species over several kilometres (McGarry et al., 2020).

 

Table 1.4:
Summary of Maximum Potential PTS Ranges due to Single Pile Installation (at OSPs, Hammer Energy 4,400 kJ) Using SPLpk Metric, Indicating Whether the Individual Can Move Beyond the Injury Range During the 30 minutes of ADD Activation

Table 1.4: Summary of Maximum Potential PTS Ranges due to Single Pile Installation (at OSPs, Hammer Energy 4,400 kJ) Using SPLpk Metric, Indicating Whether the Individual Can Move Beyond the Injury Range During the 30 minutes of ADD Activation

 

  1. Similarly, modelling using the SELcum metric demonstrated that the use of an ADD is useful for reducing PTS injury ranges for minke whale and harbour porpoise. The activation of an ADD 30 minutes prior to commencement of piling (a designed in measure) effectively reduced PTS ( Table 1.5   Open ▸ ). As discussed in paragraph 14, in response to Marine Mammal Consultation Note 2 (volume 3, appendix 5.1, annex E), NatureScot clarified that the mitigation should be based on the SPLpk metric, but the assessment of significance in the EIA chapter should consider the dual metric approach (i.e. both SPLpk and SELcum). It is recognised that additional noise sources, including the use of ADDs, should be minimised and therefore, subject to final agreement on the mitigation zone post-consent, the duration of ADD activation will be agreed as part of the final MMMP.

Table 1.5:
Injury Ranges for Minke Whale due to Concurrent Piling at Wind Turbine and OSP, Hammer Energies of 3,000 kJ and 4,400 kJ Using SELcum Metric with and without 30 Minutes of ADD

Table 1.5: Injury Ranges for Minke Whale due to Concurrent Piling at Wind Turbine and OSP, Hammer Energies of 3,000 kJ and 4,400 kJ Using SELcum Metric with and without 30 Minutes of ADD

1 N/E = Threshold not exceeded