1.6.2 UXO Clearance

  1. The primary technique implemented to reduce the risk associated with UXO clearance will be avoiding the need for the use of explosives, either by leaving the confirmed UXO in situ and constructing around it or by relocating it to a safe place and leaving in situ in the new location. However, where this is not possible, it is assumed that up to 15 UXO may require clearance. The controlled explosions of the UXOs will be undertaken by specialist contractors using the minimum size of explosive possible in order to safely dispose of the UXOs. The detonations will take place during daylight hours only and in good visibility.
  2. A pre-detonation search will be carried out. This will involve two dedicated marine mammal observers and one dedicated PAM operator over a minimum 1 km mitigation zone (standard set by JNCC guidance; JNCC, 2010b) and out as far as possible over the predicted injury range. This will be undertaken for a specified duration based on the JNCC (2010b) guidance (paragraph 49 onwards). Since injury could occur beyond the 1 km standard mitigation zone (up to a maximum range of ~14,540 m), an ADD will also be deployed and activated to deter animals from the potential injury zone. The ADD selected will be suitable for the target species (McGarry et al., 2020) and will be placed in the water in close proximity to the UXO. ADD activation will commence at the start of the search period for a specified period ( Table 1.8   Open ▸ ). If marine mammals are observed or acoustically detected within the potential injury range, noting the limitations of visually or acoustically detecting marine mammals at large ranges, they will be tracked until they move out of range. Detonation will not occur until the animal has not been detected again for 20 minutes.
  3. A range of UXO munitions sizes have been considered for the purpose of determining effective secondary mitigation measures, up to a maximum scenario of a UXO size of 698 kg NEQ. This approach follows a similar strategy as per the Seagreen EPS Risk Assessment and MMMP (Seagreen Wind Energy Ltd, 2021). The assumption is that marine mammals swim away from the ADD in a straight line at speeds agreed in consultation with NatureScot for the Array (volume 3, appendix 10.3). These swim speeds are summarised in Table 1.4   Open ▸ . The duration of the activation of the ADD prior to UXO detonation will determine whether animals can move out of the potential injury zone. The potential range of displacement based on these swimming speeds for varying UXO sizes is summarised in Table 1.8   Open ▸ .
Table 1.8:
Recommended ADD Duration for High Order UXO Clearance and Sizes and Associated Displacement Distance

Table 1.8 Recommended ADD Duration for High Order UXO Clearance and Sizes and Associated Displacement Distance

 

  1. The length of the pre-detonation search will depend on the size of UXO to be cleared ( Table 1.8   Open ▸ ). For all species, except harbour porpoise, duration of ADD for 30 minutes (a designed in measure) would be sufficient to deter marine mammals from the potential injury zone. With the inclusion of 20 minute of soft start, in addition to 30 minutes of ADD, all species except for harbour porpoise would be deterred beyond the maximum injury zone. For high order UXO clearance, injury ranges are larger.
  2. Assuming the ADD is activated for an indicative 60 minutes, the displacement distance for harbour porpoise would be 5,400 m, meaning there is a need to deter harbour porpoise from larger ranges that cannot be achieved using an ADD for 60 minutes duration alone (i.e. the injury zone exceeds 5,400 m).
  3. Therefore, for high order UXO, to reduce the risk of PTS, there is a need to deter animals from larger ranges than cannot be achieved using an ADD alone. Therefore, following an ADD activation period of 60 minutes, a ‘soft start’ will be undertaken, using a sequence of small explosive charges, detonated at five minutes intervals, over a total of maximum 20 minutes. It is expected that 80 minutes of combined ADD/soft start procedure (60 minutes of ADD and 20 minutes of soft start) will displace harbour porpoise to ranges of 7,200 m. Whilst this secondary mitigation is considered to be sufficient to deter most animals (noting that use of 30 minutes of ADD alone deterred all other species from the injury zone), there may be a residual effect for harbour porpoise for this largest UXO size, as the maximum predicted PTS impact range for this species was 10,080 for the 227 kg NEQ and 14,580 m for 698 kg NEQ. Therefore, additional secondary mitigation may be required and proposed as part of the final MMMP (see paragraph 53).
  4. Following detonation, the marine mammal observers and PAM operator will undertake a post-detonation search of the mitigation zone for at least 15 minutes after the final detonation. The purpose of this search is to look for evidence of injury to marine life, including fish kills. Any other unusual observation will also be noted.
  5. Prior to the commencement of UXO clearance works, a more detailed update to this outline MMMP will be produced as a part of the EPS licence supporting information. Additionally, tailored secondary mitigation measures will be further developed based on further information on UXO number, sizes and depth of burial. The final secondary mitigation will be agreed with statutory stakeholders as a part of a UXO specific MMMP.

1.6.3 Geophysical Surveys

  1. Standard JNCC (2017) guidance will be adhered to in order to mitigate any injurious effects to marine mammals. This will involve the use of marine mammal observers and PAM within a standard 500 m mitigation zone, as well as soft-starts where the power is built up gradually from a low-energy. The duration of the soft-start will be survey-specific. As the maximum predicted ranges for PTS are lower than 500 m across all species and geophysical survey techniques ( Table 1.7   Open ▸ ), the standard 500 m mitigation zone will be sufficient to mitigate against injury (in terms of PTS) for all species.
  2. Marine mammal observers and PAM will be used for a pre-shooting search of at least 30 minutes prior to commencement of geophysical surveys. The pre-shooting search is a period of visual and acoustic monitoring of the 500 m mitigation zone for the presence of marine mammals.
  3. If geophysical survey activities are conducted during periods of low visibility or darkness, where visual monitoring is not possible, only the PAM operator will monitor the mitigation zone for a pre-shooting search of at least 30 minutes.
  4. The JNCC (2017) guidance will be adhered to if marine mammals are detected within the mitigation zone. Firstly, if marine mammals are detected in the mitigation zone during the 30 minute pre-shooting search, the soft-start to geophysical activities must be delayed until the passage of the marine mammal(s), or transit of the vessel, results in the animals being outside of the mitigation zone. There will be a minimum 20 minute delay from the time of last detection and the commencement of the soft-start to allow marine mammals to move out of the mitigation zone. Secondly, if seal(s) are congregating around a fixed platform within the survey area, the soft start should commence at least 500 m from the platform. If marine mammals are detected within the mitigation zone whilst the airguns are firing (whether during the soft start or when at full power), there is no requirement to stop firing.
  5. Overall, the implementation of designed in and secondary mitigation measures detailed above ensure the potential risk of injury from geophysical surveys is minimised.
  6. If multi-beam surveys (such as MBES) are conducted in shallow waters (<200 m), the secondary mitigation (in form of pre-shooting search and soft-starts) will not be required (based on the JNCC guidance; JNCC, 2017).

1.7. Roles and Responsibilites

  1. As per the JNCC guidance (2010a, 2010b, 2017), persons involved in implementing, and ensuring compliance with this outline MMMP include:
  • the Applicant’s Environmental Manager;
  • independent Environmental Clerk of Work (ECoW)
  • MMO2;
  • PAM Operator;
  • ADD Operator; and
  • piling, Explosive Ordinance Disposal (EOD) or geophysical survey supervisor.
  1. They will be equipped with the appropriate means of communication between each other in order to ensure that the correct mitigation protocols are undertaken and to allow timely communication if a marine mammal is detected.

1.7.1 The Applicant’s Environmental Manager

  1. The Applicant’s Environmental Manager is responsible for ensuring all compliance documents, such as the outline MMMP, are included in construction contract documents. They will report marine mammal monitoring and activities related to piling, UXO clearance, and geophysical surveying.

1.7.2 Independent Environmental Clerk of Work

  1. The independent ECoW will be responsible for completing inductions and toolbox talks to onsite construction teams (including piling and UXO detonation) on requirements of the outline MMMP and monitoring that all piling and UXO detonation activities are being completed in accordance with the outline MMMP, other related consent management plans and all relevant regulations and legislation. The independent ECoW is also responsible for stopping operations e.g. piling in the event of a non-compliance with the outline MMMP and/or consent conditions and reporting all non-compliances to MD-LOT.