16.4.2 Operation and Maintenance Phase

16.4.2.1  Powered Allision

  1. Powered allision risk may be caused by human/navigational error, unfamiliarity with the Array and/or a failure of an aid to navigation.
  2. Experience from previous operational offshore wind farms indicates that Masters regularly choose to transit greater than 1 nm from an array, with it being likely that the deviations established during construction will remain in place during the operation and maintenance phase. In doing so, vessels are unlikely to navigate close enough to a structure to create an allision risk. There is a distance of 27 nm between the Array and the Seagreen 1 Offshore Wind Farm, the closest baseline offshore wind farm, which provides notable sea room for safe navigation.
  3. Based on the modelling (see section 15.4.3), with the main commercial route deviations in place, the base case annual powered vessel to structure allision frequency is estimated to be 6.91×10-3, corresponding to a return period of approximately one in 145 years.
  4. The structures will be lit and marked as directed by the MCA and NLB and in compliance with IALA G1162 (IALA, 2021b) to ensure passing mariner awareness (e.g. lights, sound signals, AIS AtoN). Additionally, commercial vessels are expected to comply with international and Flag State regulations (including the COLREGs and SOLAS) and will be able to passage plan in advance given the promulgation of information relating to the Array, including display of the structure locations on appropriate nautical charts.
  5. NLB raised during consultation (section 4) that contingency of overall lighting and marking would need to be considered, in particular in a scenario where a wind turbine with a key AtoN was towed from the Array for maintenance. Appropriate measures for this scenario will be agreed as part of the LMP process.
  6. RYA Scotland raised during consultation that outage of marine lights should be rectified in a timely manner (section 4). Associated measures and procedures will be detailed in the outline Aids to Navigation Management Plan, provided in volume 4, appendix 26, annex A, noting that IALA Availability targets will be set out in the LMP (outline LMP provided in volume 4, appendix 26).
  7. Based on historical incident data (see section 9.6.1), there have been two reported instances of a third-party vessel alliding with an operational wind farm structure in the UK (one in the Irish Sea and one in the Southern North Sea). Both of these incidents involved a fishing vessel, with a RNLI lifeboat attending on both occasions and a helicopter deployed in one case.
  8. The most likely consequences in the event of a powered allision incident are as per the equivalent construction phase impact, namely minor damage to property. Although considered less likely allision could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Applicant’s MPCP will be implemented to reduce the environmental effects should pollution occur.

16.4.2.2  Drifting Allision

  1. Drifting allision risk may be caused by mechanical or technical failure, adverse weather and/or a navigational system error. A vessel adrift may only develop into an allision situation if in proximity to a structure and this is only the case where the adrift vessel is located in proximity to the Array and the wind and/or tide directs the vessel towards a structure.
  2. Based on the modelling (see section 15.4.4), with the main commercial route deviations in place, the base case annual drifting vessel to structure allision frequency is estimated to be 2.16×10-4, corresponding to a return period of approximately one in 4,619 years.
  3. For drifting allision incidents, the adrift vessel would initiate its emergency response procedures to avoid a CPA with a structure resulting in an allision. This may include emergency anchoring following a check of the relevant nautical charts (thus ensuring that the anchor deployment does not lead to other impacts such as anchor snagging on a subsea cable), noting this would depend on the vessel and water depths. These measures may also include the use of thrusters (depending on availability and power supply). Moreover, under SOLAS obligations (IMO, 1974), other nearby vessels including project vessels associated with operation and maintenance (via marine coordination) may be able to render assistance, depending on the type and size of vessel.
  4. Based on historical incident data (see section 9.6.1), there have been no instances of a third-party vessel alliding with a UK operational wind farm structure whilst Not Under Command (drifting).
  5. The most likely consequences in the event of a drifting allision incident are as per the equivalent construction phase impact, namely minor damage to property. Although considered less likely allision could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Applicant’s MPCP will be implemented to reduce the environmental effects should pollution occur. The consequences are less likely to be severe for a drifting allision incident given that the speed at which the impact occurs (and subsequent energy of the impact) will generally be dictated by the wind and/or tidal speeds.

16.4.2.3  Internal Allision

  1. As per the impact on vessel displacement, it is anticipated that commercial fishing vessels and recreational vessels may choose to navigate internally within the Array, particularly in favourable weather conditions. However, consultation input indicated this may be less likely than within a fixed foundation project. Therefore, an internal allision risk exists for such smaller craft. However, due to the distance offshore of the Array, fishing and recreational vessel traffic volume is expected to be low and this was reflected in the vessel traffic data and input from consultees.
  2. From historical incident data, there has been two reported instances of a third-party vessel alliding with an operational wind farm structure in the UK. Both of these incidents involved a fishing vessel, with a RNLI lifeboat attending on both occasions and a helicopter deployed in one case. Given that the size of the Array and the promulgation of information, there is likely to be a reasonable level of awareness of the Array meaning that such an incident is unlikely to occur at the Array.
  3. The base case annual fishing vessel to structure allision frequency is estimated to be 4.08×10-2, corresponding to a return period of approximately one in 24 years. This is high compared to that estimated for other UK offshore wind farm developments and is reflective of the conservatism of the model, which assumes that fishing vessel activity and volume will not change after installation of the structures. However, it was noted during consultation (see section 4) that fishing vessels may be more likely avoid the Array than a fixed foundation offshore wind farm.
  4. Comfort with internal navigation will likely increase throughout the lifetime of the Array and appropriate lighting and marking (agreed with the NLB and MCA, in compliance with IALA G1162 (IALA, 2021b)) will be in place to increase awareness of the structure locations including internally. The final Array layout will be agreed through the DSLP via consultation with the MCA and NLB, and this will include agreement of a clear identification (ID) marking system on the structures, with each structure clearly displaying its ID visible in all directions, facilitating safe internal navigation. The structure locations will also be displayed on appropriate nautical charts.
  5. The most likely consequences in the event of an allision incident are as per the equivalent construction phase impact, namely minor damage to property. Although considered less likely allision could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Applicant’s MPCP will be implemented to reduce the environmental effects should pollution occur. The consequences are less likely to be severe for an internal allision incident given that the vessel will be likely transiting at lower speeds whilst in the Array, reducing the severity of impact.

16.4.2.4  Frequency of Occurrence

  1. The frequency of occurrence is therefore considered to be extremely unlikely based on the available sea room, outputs of the modelling, and consideration of historical incident data.

16.4.2.5  Severity of Consequence

  1. The severity of consequence is therefore considered to be serious.

16.4.2.6  Significance of Risk

  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be remote. The risk will therefore be of tolerable significance.

16.4.2.7  Additional Mitigation and Residual Risk

  1. No additional shipping and navigation mitigation is considered necessary because the likely risk, in the absence of mitigation beyond the embedded mitigation measures outlined in section 18.1, is ALARP and not significant in EIA terms.

16.4.3 Decommissioning Phase

  1. Since the methods used to remove infrastructure are expected to be similar to those used for installation, this impact is expected to be similar in nature to the equivalent construction phase impact. In particular, a buoyed decommissioning area analogous to the buoyed construction area will be in place and it is anticipated that third-party vessels will be unlikely to enter. Pre-decommissioning or partially removed structures will be similar in nature to pre-commissioning or partially completed structures, and the movement of third-party vessels within and around the buoyed decommissioning area is anticipated to be similar to that within and around the buoyed construction area.

16.4.3.1  Frequency of Occurrence

  1. The frequency of occurrence is considered to be extremely unlikely based on the available sea room, outputs of the modelling, and consideration of historical incident data.

16.4.3.2  Severity of Consequence

  1. The severity of consequence is considered to be serious.

16.4.3.3  Significance of Risk

  1. Overall, the frequency of occurrence is considered to be extremely unlikely and the severity of consequence is deemed to be serious. The risk will therefore be of tolerable significance.

16.4.3.4  Additional Mitigation and Residual Risk

  1. No additional shipping and navigation mitigation is considered necessary because the likely risk in the absence of further mitigation (beyond the embedded mitigation measures outlined in section 18.1) is ALARP and not significant in EIA terms.