1.5.2 UXO Clearance

  1. Low order clearance techniques will be applied as the intended methodology for UXO clearance, noting the position statement from statutory authorities on UXO clearance that low order alternatives should be prioritised when clearing UXO (UK Government et al., 2022). However, there is a small inherent risk with low order clearance methods that the UXO may result in a high order detonation. In addition, some UXOs may not be stable enough to warrant a low order approach and therefore would need to be cleared using high order methods for safety reasons. This will not be known until a UXO pre-construction survey is carried out, but as a precautionary approach, high order is included as part of the MDS.
  2. High order detonation of UXO has the potential to generate some of the highest peak sound pressures of all anthropogenic underwater noise sources (von Benda-Beckmann et al., 2015). Underwater noise from high order UXO detonation could result in physical and/or auditory injury, or death for marine mammals.
  3. Low order techniques of UXO clearance uses a single charge of up to 80 g Net Explosive Quantity (NEQ) which is placed in close proximity to a UXO to target a specific entry point. When detonated, a shaped charge penetrates the UXO’s casing to introduce a small, clinical plasma jet into the main explosive filling. The intention is to excite the explosive molecules within the main filling to generate enough pressure to burst the UXO casing, producing a deflagration of the main filling and neutralising the UXO. Recent controlled experiments showed low-order clearance using deflagration to result in a substantial reduction in acoustic output over traditional high-order methods, with SPLpk and SELcum being typically significantly lower for the low order techniques of the same size munition, and with the acoustic output being proportional to the size of the shaped charge, rather than the size of the UXO itself (Robinson et al., 2020).
  4. It is estimated that up to 15 UXOs are likely to require clearance within the Array area. Furthermore, it has been assumed that the maximum design scenario will be clearance of UXOs up to 698 kg NEQ, with the most realistic maximum size being 227 kg. The maximum frequency would be up to two detonations within 24 hours. The clearance activities will be tide and weather dependant as detonations will take place during daylight hours and slack water only. The aim is to allow clearance of at least one UXO per tide, during daylight hours only.
  5. For bottlenose dolphin, white-beaked dolphin, minke whale, humpback whale and grey seal, the magnitude of impact was deemed to be low and the sensitivity of receptors to be high. As the estimated number of individuals with the potential to be injured was low (presented in volume 2, chapter 10), the potential effect was assessed as of minor significance, which is not significant in EIA terms ( Table 1.6   Open ▸ ).
  6. For harbour porpoise however, the magnitude of impact was deemed to be medium as the estimated number of animals with the potential to be injured was higher. Additionally, the sensitivity of receptor for harbour porpoise was deemed to be high, resulting in a potential effect of moderate significance, which is significant in EIA terms ( Table 1.6   Open ▸ ). As the predicted injury zone (14,450 m) is too extensive to be effectively mitigated by standard designed in measures (marine mammal observers and PAM operators), the use of mitigation in the form of ADDs (up to 30 minutes) and soft start charges (see paragraph 49), has been proposed as designed in mitigation and is detailed in this outline MMMP. Maximum injury ranges in this outline MMMP are based on SPLpk are based on the dual-metric approach, as detailed in volume 2, chapter 10.
  7. If required, secondary mitigation (i.e. ADD use with a duration over 30 minutes) will be applied to further reduce the potential for injury to harbour porpoise occurring during UXO clearance. Final mitigation required will be addressed post consent, in consultation with stakeholders, following more detailed information such as the size, number and quality of UXOs to be cleared (following site-investigation surveys), noting that it may be possible to reduce the ADD activation period and soft start procedure depending on the size and number of UXOs located within the Array. Volume 2, chapter 10 details a worked example for mitigation based on the most significant predicted effect and focused on harbour porpoise (as this is the species with a potential residual risk of injury), which considers the different timescales that would be required to clear the injury zone if ADD and soft-start is required.
  8. Therefore, prior to the commencement of UXO clearance works, appropriate secondary mitigation measures will be discussed with stakeholders and proposed as a part the final MMMP for UXO clearance works.

 

Table 1.6:
Potential Effect of UXO Clearance on five Marine Mammal Species for High Order Detonation of Maximum case

Table 1.6: Potential Effect of UXO Clearance on five Marine Mammal Species for High Order Detonation of Maximum case

 

1.5.3 Geophysical Surveys

  1. Site investigation surveys during the construction and operation and maintenance phases has the potential to result in direct or indirect effects on marine mammals. The potential impact ranges predicted for injury were the same for both phases. During the construction phase, site investigation geophysical surveys will take place over a period of up to five months within a three-year period. Geophysical surveys are expected to be short-term (up to three months) and occur intermittently over the lifespan of the Array. For example, during the operation and maintenance phase, routine geophysical surveys will take place once every 24 months for wind turbines and OSP foundations as well as wind turbines interior and exterior and annually for the first three years, then every 24 months for inter-array cables and interconnector cables.
  2. A detailed underwater noise modelling assessment was carried out to investigate the potential for auditory injury on marine mammals due to geophysical surveys (volume 3, appendix 10.1). Several sonar-based surveys will potentially be used for the geophysical surveying; these include:
  • Multibeam Echosounder (MBES);
  • 2D Ultra-high Resolution Seismic (UHRS);
  • Site-Scan Sonar (SSS); and
  • Sub-bottom Profiler (SBP).
  1. The equipment used can typically operate at a range of frequencies, depending on the distance to the seabed and the required resolution. Sonar based sources are considered continuous (non-impulsive) as they typically compromise a single frequency instead of a broadband signal with high kurtosis, high peak pressures, and rapid rise times. Unlike the sonar-based surveys, the UHRS survey is likely to use a sparker, which produces an impulsive, broadband source signal.
  2. The noise modelling showed that the ranges within which there is potential for marine mammals to experience PTS as a result of geophysical surveys are relatively low ( Table 1.7   Open ▸ ). For harbour porpoise PTS could occur out to 310 m as a result of the SBP survey.

 

Table 1.7:
Potential PTS Impact Ranges for Marine Mammals During the Geophysical Site Investigation Surveys, Based on Comparison to Southall et al. (2019) SEL Thresholds

Table 1.7 Potential PTS Impact Ranges for Marine Mammals During the Geophysical Site Investigation Surveys, Based on Comparison to Southall et al. (2019) SEL Thresholds

1 N/E = Threshold not exceeded

*Non-impulsive threshold from Southall et al. (2019)

**Impulsive threshold from Southall et al. (2019)

  1. Due to low impact ranges for all species ( Table 1.7   Open ▸ ), there is potential for less than one animal to experience PTS (and none when the threshold is not exceeded) as a result of geophysical surveys. Standard designed in measures to reduce the risk of injury will be implemented following JNCC guidance (section 0; JNCC, 2017). With these measures in place, the risk is deemed to be negligible. It should also be noted that as sonar-based systems have strong directivity, there is only potential for injury when the marine mammal is directly underneath the source.

1.6. Mitigation Methods and Procedures

  1. The mitigation measures presented in subsections 1.6.1 to 1.6.3 below include designed in measures and secondary mitigation in order to reduce the risk of injury to marine mammals as described in the volume 2, chapter 10.

1.6.1 Piling

  1. As per the JNCC (2010a) guidance, a 30 minute pre-piling search will be undertaken using marine mammal observers and a PAM operator to monitor the specified mitigation zone in order to minimise the likelihood of marine mammals being present within this range. In addition to visual and acoustic monitoring, an ADD will be deployed in close proximity to the pile to be installed at the start of the pre-piling search. The ADD will be activated for a maximum period of 30 minutes to allow animals sufficient time to disperse while also minimising the additional noise produced by the device and therefore emitted into the marine environment. Visual and acoustic monitoring will continue throughout the ADD deployment to seek to ensure marine mammals leave the potential impact zone prior to the start of piling.
  2. Piling commencement during periods of low visibility or darkness, where visual monitoring is not possible, will involve the PAM of the mitigation zone over the duration of the pre-piling search, which will be conducted for a minimum of 30 minutes.
  3. After the 30 minute pre-piling search and ADD activation period has elapsed, the piling initiation, soft start and ramp up designed in measures will commence with hammer initiation at the lowest hammer energy and strike rate (as specified in volume 3, appendix 10.1 of the Array EIA Report). The ADD will be turned off immediately after the piling activity has commenced.
  4. The piling soft start and ramp up designed in measures comprise of the three following activity stages:
  • initiation (1 minute for both anchor piles and OSP jacket piles);
  • soft start (20 minutes for both anchor piles and OSP jacket piles); and
  • ramp up (30 minutes for both anchor piles and OSP jacket piles).
  1. The initiation stage is a slow start to allow for alignment and to allow marine mammals to leave the area.
  2. If marine mammals are detected within the mitigation zone during the pre-piling search, piling will not commence until at least 20 minutes after the last visual or acoustic detection of the animal. The marine mammal observers and PAM operative will track any marine mammals detected and ensure that they have left the mitigation zone before piling commences. If a marine mammal is detected in the mitigation zone during the soft-start procedures, the piling operation should cease, whenever possible, or at least not be increased further until the marine mammal clears the mitigation zone and is not detected again for 20 minutes.
  3. If for any reason there is a break in piling activity for over ten minutes, then the pre-piling search and ADD activation should be repeated before piling recommences.
  4. If during piling at full power a marine mammal is detected in the mitigation zone, there will be no requirement to cease piling, as the JNCC guidance (2010a) concludes that the animal is deemed to have entered the mitigation zone voluntarily. It may also not be possible to stop piling at full power due to engineering restrictions.
  5. The development of and adherence to a PS will set out further details on mitigation (such as soft start, ramp up)
  6. The designed in measures detailed in this outline MMMP and the PS reduce the risk of injury to a safe threshold, whereby marine mammals are not at risk of auditory injury, in terms of PTS. With designed in mitigation in place, the potential effect of piling (auditory injury) on marine mammals is considered to be of minor significance, which is not significant in EIA terms.