2. Methodology

2.1. Maximum Design Scenario

  1. The maximum design scenario (MDS) for piling at the Array has been developed on the basis that pile driving operations would be required for the installation of up to 1,806 piles across:
  • 265 floating wind turbine anchors with hammer energy up to 3,000 kJ; and
  • 15 OSP jacket foundations with hammer energy up to 4,400 kJ.
  1. Full details of the MDS are presented in volume 2, chapter 10.

2.1.1. Maximum Temporal Scenario

  1. The maximum temporal scenario is represented by piling occurring over the greatest number of days, resulting from piling at only one location at a time. Sound introduced into the marine environment over a longer period may increase the risk for disturbance to marine mammals, with potential effects during multiple stages of a species’ annual cycle.
  2. The maximum temporal scenario was assessed on the number of piles that could be installed within one 24-hour period. Piling is expected to take up to eight hours for each pile, meaning that the maximum temporal scenario (i.e. the longest duration of piling) would be represented by up to three piles being installed around a single turbine location or at a single OSP location, in a single day (24 hours), equating to 602 days for the installation of 1,806 piles.
  3. A summary of the maximum temporal scenario for the Array is presented in Table 2.1   Open ▸ .

 

Table 2.1:
Summary of the Maximum Temporal Scenario used in iPCoD for Piling of Offshore Foundations at the Array

Table 2.1: Summary of the Maximum Temporal Scenario used in iPCoD for Piling of Offshore Foundations at the Array

 

2.1.2. Maximum Spatial Scenario

  1. For the maximum spatial scenario, the largest hammer energy and maximum spacing between concurrent piling events would reduce the time required for piling operations overall but would lead to the largest spatial area of ensonification at any one time. Minimum spacing between concurrent piling locations represents the highest risk of injury to marine mammals as sound from adjacent foundations could combine to produce a greater radius of effect compared to a single piling event.
  2. Within the overall piling period, concurrent piling for wind turbine anchors (i.e. at two wind turbines within the same 24 hour period) would occur over 234 days. Concurrent piling of wind turbine anchors and OSP jacket foundations would occur over 63 days of the overall piling period, and single piling for the remaining OSP foundations would occur over nine days of the overall piling period. The maximum duration of piling for the maximum spatial scenario would be 306 days.
  3. A summary of the maximum spatial scenario for the Array is presented in Table 2.2   Open ▸ .

 

Table 2.2:
Summary of the Maximum Spatial Scenario used in iPCoD for Piling of Offshore Foundations at the Array

Table 2.2: Summary of the Maximum Spatial Scenario used in iPCoD for Piling of Offshore Foundations at the Array

 

  1. It is estimated that piling activity at the Array will take place within an eight-year timeframe (2031 to 2038). Piling could potentially take place at any point within the foundation installation phase, however, for the purposes of developing the piling schedule for iPCoD (a requirement of the model) an indicative programme has been developed based on a realistic installation approach with piling spread across the eight years, discussed in section 2.7.

2.2. Key Species

  1. Marine mammal species included in the iPCoD model were those that were determined to be important marine mammal features within the regional marine mammal study area, following a review of the Array Digital Aerial Survey (DAS) data, and available published data sets (see volume 3, appendix 10.2), and for which a population model in iPCoD was available.
  2. The baseline characterisation for the Array identified the following marine mammal species within the regional marine mammal study area (volume 3, appendix 10.2):
  • harbour porpoise Phocoena phocoena;
  • bottlenose dolphin Tursiops truncatus;
  • short-beaked common dolphin Delphinus delphus;
  • minke whale Balaenoptera acutorostrata;
  • grey seal Halichoerus grypus; and
  • harbour seal Phoca vitulina.
  1. Harbour seal occurs in the regional marine mammal study area in numbers considered not high enough to be at risk of a population-level effect and has therefore not been included in iPCoD analysis, with exclusion of harbour seal from further analysis in the EIA agreed with NatureScot (in January 2024) in response to Marine Mammal Consultation Note 1 (volume 3, appendix 5.1, annex D). Similarly, there are currently no parameters available to construct a suitable population model for short-beaked common dolphin in the iPCoD framework and therefore population modelling for short-beaked common dolphin has not been possible for this assessment. In addition, only three individuals were identified from the DAS data therefore it is not considered a key species.
  2. Therefore, the species included for iPCoD modelling were:
  • harbour porpoise;
  • bottlenose dolphin;
  • minke whale; and
  • grey seal.
  1. The piling parameters defined in the MDS (see section 2.1) were subsequently incorporated into an acoustic sound propagation model to predict the potential range of effect (injury and disturbance) for each key species. The assessment considered the efficacy of standard industry mitigation measures (see paragraph 41 for a summary of relevant measures, and volume 1, chapter 3 for full details) to reduce these effects and subsequently the numbers carried forward to this population model were based on any residual effects after accounting for mitigation.
  2. The assessment presented a range of densities for each key species (as listed in paragraph 24), however, for the purpose of undertaking the population modelling the most precautionary densities and relevant reference populations were taken forward (paragraph 26). The total number of animals potentially disturbed for each species was quantified by applying the highest density estimate to the dose-response curve derived by Graham et al. (2019) . This approach considers a proportional response within consecutive mapped contours denoting incremental 5 dB decreases in received single strike sound exposure level (SELss) predicted using the underwater noise model. To this end a 100% disturbance was predicted in all species at received levels >180 dB SELss. The predicted rate of disturbance then decreases proportionally in response to received level, reducing at greater distances from the piling source. The dose-response relationship based on published empirical evidence and further detail is provided in volume 2, chapter 10.

2.3. Reference Populations

  1. Key species population estimates based upon Management Units (MUs) were specified in the iPCoD models as the reference populations against which any effects (i.e. number of animals experiencing disturbance or PTS) were assessed. Relevant MUs were determined by their coincidence with the location of the Array. This section details these reference populations.
  2. For harbour porpoise and minke whale, only one MU for each species occurs in the vicinity of the Array (IAMMWG, 2022), and the respective population estimates for these MUs have been used for iPCoD modelling: the North Sea MU for harbour porpoise ( Figure 2.1   Open ▸ ) and the Celtic and Greater North Seas MU for minke whale ( Figure 2.2   Open ▸ ).
  3. The site boundary coincides with the boundary between two seal MUs (SMU) ( Figure 2.3   Open ▸ ), so for grey seal the reference population comprises the sum of the population estimates for the East Scotland seal MU and the Northeast England seal MU (SCOS, 2023) as agreed with NatureScot in March 2024, in response to Marine Mammal Consultation Note 2 (volume 3, appendix 5.1, annex E).
  4. For bottlenose dolphin, the Coastal East Scotland MU ( Figure 2.4   Open ▸ ) was used as the relevant reference population. Given the importance of the Moray Firth Special Area of Conservation (SAC) for bottlenose dolphin within the regional marine mammal study area, the sensitivity of this population and its known ranging behaviour further south towards St Andrews Bay and the Tay Estuary, and inshore in north-east English waters, it is important to capture the potential impact to this important coastal ecotype population which may experience potential barrier effects as a result of increase noise in the marine environment.
  5. Whilst there is an abundance estimate for bottlenose dolphin for the Greater North Sea MU (2,022 animals (IAMMWG, 2022)) this large MU extends the entire length of the east coast of the UK and east to Scandinavia, so apportioning numbers of the offshore ecotype to the east coast of Scotland is not possible. It is also unlikely that the Array will create significant barrier effects for this offshore ecotype. Therefore, the assessment has focused on the impacts for the bottlenose dolphin population within the Coastal East Scotland MU, which includes the Moray Firth SAC. This approach has been agreed in consultation with MD-LOT and NatureScot during the initial scoping process (MD-LOT, 2023, Ossian OWFL, 2023).
  6. The population estimates used to parameterise iPCoD models were taken from IAMMWG (2022) for cetacean species and from SCOS (2023) for grey seal, and are summarised in Table 2.3   Open ▸ .

 

Table 2.3:
Reference Populations Used in the iPCoD Modelling

Table 2.3: Reference Populations Used in the iPCoD Modelling


Figure 2.1:
North Sea Management Unit for Harbour Porpoise (IAMMWG, 2022)

Figure 2.1: North Sea Management Unit for Harbour Porpoise (IAMMWG, 2022)


Figure 2.2:
Celtic and Greater North Seas Management Unit for Minke Whale (IAMMWG, 2022)

Figure 2.2: Celtic and Greater North Seas Management Unit for Minke Whale (IAMMWG, 2022)


Figure 2.3:
East Scotland and Northeast England Seal Management Units (SCOS, 2023)

Figure 2.3: East Scotland and Northeast England Seal Management Units (SCOS, 2023)


Figure 2.4:
Coastal East Scotland Management Unit for Bottlenose Dolphin (IAMMWG, 2022)

Figure 2.4: Coastal East Scotland Management Unit for Bottlenose Dolphin (IAMMWG, 2022)