Internal allision
  1. As noted in the discussion on third-party vessel to third-party vessel collision risk, it is likely that only smaller vessels (e.g. fishing, recreation) may choose to transit through the Array during construction. On this basis it is considered very unlikely that a commercial vessel would be involved in an internal allision.
  2. Minimum spacing between structures of 1,000 m is considered sufficient for safe internal navigation, i.e. keeping clear of the structures in the Array. The final layout will be agreed with both NLB and MCA, noting these discussions will include consideration of ensuring safe internal navigation.
  3. As with any passage, any vessel navigating in or near the Array is expected to passage plan in accordance with SOLAS Chapter V (IMO, 1974), and promulgation of information will ensure that such vessels have good awareness of the works being undertaken. Charting of the buoyed construction area will further increase mariner awareness.
  4. The Applicant will apply for safety zones of radius 500 m around structures where construction is underway, with 50 m pre-commissioning safety zones applied for around structures where work is not underway during the construction phase. These safety zones would make clear to passing mariners the areas which should be avoided to minimise allision risk.
  5. Should an internal allision occur, the consequences will be similar to those noted in the discussion for the case of a powered allision, including the unlikely worst case of foundering and pollution. In the highly unlikely scenario of an internal allision incident resulting in pollution, the implementation of the MPCP will minimise the environmental risk.
                        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.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be extremely unlikely. The effect will therefore be of tolerable significance and ALARP, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect, in the absence of mitigation beyond the designed in measures outlined in Table 13.11   Open ▸ , is of tolerable significance and ALARP which is not significant in EIA terms.

                        Operation and maintenance phase

                        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 NRA (volume 3, appendix 13.1) modelling, 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 Aids to Navigation (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 13.5) 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 13.5). Associated measures and procedures will be detailed in the outline AtoN Management Plan, outline 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 as set out in the NRA (volume 3, appendix 13.1), 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 collision between third party vessels  could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Array’s MPCP will be implemented to reduce the environmental effects should pollution occur.
                        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 NRA (volume 3, appendix 13.1) modelling, 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 activities (via marine coordination) may be able to render assistance, depending on the type and size of vessel.
  4. Based on historical incident data as set out in the NRA (volume 3, appendix 13.1), 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 collision between third party vessels  could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Array’s MPCP will be implemented to minimise 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.
                        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 (section 13.7.1) and input from consultees (section 13.5).
  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 13.5) 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 maximise 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 collision between third party vessels  could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Array’s MPCP will be implemented to minimise 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.
                        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.
                        Severity of consequence
  1. The severity of consequence is therefore considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be extremely unlikely. The effect will therefore be of tolerable significance and ALARP, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect, in the absence of mitigation beyond the designed in measures outlined in Table 13.11   Open ▸ , is of tolerable significance and ALARP which is not significant in EIA terms.

                        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.
                        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.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be extremely unlikely. The effect will therefore be of tolerable significance and ALARP, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of tolerable significance and ALARP which is not significant in EIA terms.

Reduced Access to Local Ports and Harbours

                        Construction phase

  1. The closest port or harbour to the Array is the Port of Aberdeen, located approximately 44 nm to the north‑west, on the east coast of Scotland. Given the distance offshore of the Array and the anticipated deviations for the main commercial routes, it is not anticipated that there will be any notable impact on vessel approaches to and from local ports above and beyond the deviations outlined for the vessel displacement impacts associated with the buoyed construction area or the construction activities therein. Regardless, details of construction activities including the presence of safety zones and any advisory safe passing distances, as defined by risk assessment, will be suitably promulgated to increase awareness of ongoing construction activities.
  2. It should be noted that there are also no pilot boarding stations, port authority limits or Vessel Traffic Service (VTS) areas in proximity to the Array given its distance offshore and as such these services will not be impacted.
  3. Up to 7,902 return trips by construction vessels (including site preparation activities) may be made throughout the construction phase and will include vessels which are RAM, noting this will include towing operations. It is not yet known which ports will be used for construction, however, regardless of ports used, all project vessels will be managed by marine coordination, including the use of traffic management procedures. Project vessels will also carry AIS and be compliant with Flag State regulations including the COLREGs. These measures will seek to ensure any impacts on access to ports used are reduced.
  4. The most likely consequences of the impact are increased journey times and distances due to the presence of the buoyed construction area and project vessels, as per the vessel displacement impact. The MDS may include disruption to schedules, but this is considered highly unlikely given the international nature of routeing in the area and the ability to passage plan to reduce timing impacts. No effect is anticipated on port related services such as pilotage.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be remote based on the designed in measures in place to manage project vessel movements.
                        Severity of consequence
  1. The severity of consequence is considered to be minor.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be minor and the frequency of occurrence is considered to be remote. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

                        Operation and maintenance phase

  1. As noted for the equivalent construction phase impact, the closest port or harbour to the Array is the Port of Aberdeen. Again, given the distance offshore of the Array and the anticipated deviations for the main commercial routes, it is not anticipated that there will be any notable impact on vessel approaches to and from local ports above and beyond the deviations outlined for the vessel displacement impact. Given the distance offshore, there are also no pilot boarding stations, port authority limits or VTS areas in proximity to the Array. Details of major maintenance activities including the presence of safety zones and any advisory safe passing distances, as defined by risk assessment, will be suitably promulgated to maximise awareness of ongoing operation and maintenance activities.
  2. Up to 508 return trips annually from vessels may be made throughout the operation and maintenance phase and will include vessels which are RAM. It is not yet known which ports will be used; regardless as per the construction phase, project vessels will be managed by marine coordination, carry AIS and be compliant with relevant Flag State regulations. These measures will ensure any impacts on access to ports used are reduced as far as practicable.
  3. The most likely consequences of the impact are as per the equivalent construction phase impact, namely increased journey times and distances. The MDS may include disruption to schedules, but this is considered highly unlikely given the international nature of routeing in the area and the ability to passage plan to reduce timing impacts. No effect is anticipated on port related services such as pilotage.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be remote based on the designed in measures in place to manage project vessel movements.
                        Severity of consequence
  1. The severity of consequence is considered to be minor.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be minor and the frequency of occurrence is considered to be remote. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is and not significant in EIA terms.

                        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, the number of return trips per year by decommissioning vessels will be similar and a buoyed decommissioning area analogous to the buoyed construction area will be in place.
  2. The impact will be present throughout the decommissioning phase which is expected to be of similar duration to the construction phase (i.e. maximum of eight years). Since the anticipated deviations associated with the main commercial routes accessing a local port and the volumes of vessel traffic on such routes are the same as for the equivalent construction phase impact, similar impact is likely.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be remote based on the designed in measures in place to manage project vessel movements.
                        Severity of consequence
  1. The severity of consequence is considered to be minor.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be minor and the frequency of occurrence is considered to be remote. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

Loss of Station

                        Construction phase

  1. The MCA require under their Regulatory Expectations on Moorings for Floating Wind and Marine Devices (MCA and HSE, 2017) that developers arrange TPV of the mooring systems by an independent and competent person/body. The Regulatory Expectations state that TPV is a “continuous activity”, and that if any modifications to a system occur or if new information becomes available with regard to its reliability, additional TPV would be required. This TPV will facilitate management of any risk of failure of the mooring lines.
  2. On this basis, the potential for loss of station is considered unlikely, noting that for a total loss of station, all moorings would be required to fail (based on the MDS there may be up to six mooring lines per foundation). There have been no reports to date of loss of stations from floating UK offshore wind farms.
  3. The Regulatory Expectations also require the provision of continuous monitoring either by Global Positioning System or other suitable means. The Applicant will put such a system in place, with each wind turbine continuously monitored, and with capability of being tracked in the event of a loss of station as detailed in MGN 654 (see designed in measures in Table 13.11   Open ▸ ).
  4. The most likely consequences are failure of a single mooring line leading to a larger excursion zone than typical. As a worst case, total mooring line failure could lead to a drifting platform leading to a collision.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be negligible based on the designed in measures in place in terms of TPV, monitoring and tracking.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be negligible. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

                        Operation and maintenance phase

  1. The same designed in measures in terms of TPV and monitoring details for the construction phase will apply during the operation and maintenance phase, based on the requirements of the Regulatory Expectations on Moorings for Floating Wind and Marine Devices (MCA and HSE, 2017) and MGN 654 (MCA, 2021a).
  2. On this basis, the potential for loss of station is considered unlikely, noting that for a total loss of station, all moorings would be required to fail (based on the MDS there may be up to six mooring lines per foundation), and in the event that mooring lines did fail, monitoring and tracking procedures will be in place.
  3. The most likely consequences are failure of a single mooring line leading to a larger excursion zone than typical. As a worst case, total mooring line failure could lead to a drifting platform leading to a collision.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be negligible based on the designed in measures in place such as TPV of project infrastructure, and implementation of a continuous discrete monitoring system.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be negligible. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

                        Decommissioning phase

  1. The decommissioning phase is considered to be generally analogous to the construction phase in reverse and therefore the likelihood of loss of station during the decommissioning phase is considered to be the same as for the construction phase.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be negligible based on the designed in measures in place in terms of TPV, monitoring and tracking.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be negligible. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

Reduction of UnderKeel Clearance As a Result of Subsea Infrastructure

                        Construction phase

  1. During the construction phase, there may be a need to wet store subsea components including the mooring lines and subsea cables within the Array. During this time, the components would be left on or tethered to the seabed. It is not expected that any components will be an underkeel risk during this period given it is likely that they will be close to the seabed. However, final plans will be confirmed via the CMS which will be approved by MD-LOT in consultation with the MCA and NLB (i.e. it will be confirmed via the CMS that suitable underkeel clearance will be available during the construction phase).
  2. It is noted that the buoyed construction area in place during the construction phase means it is anticipated that third-party vessels will be unlikely to enter on a regular basis based on consultation input and experience of other UK offshore wind farms. This includes the physical marking of the buoys themselves, and the display of the buoyed construction area on appropriate nautical charts.
  3. Should an underwater allision occur, the most likely consequences are minor damage to property and minor reputational effects on business but no perceptible effect on people. Although considered less likely, a more serious interaction could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Array’s MPCP will be implemented to reduce the environmental impacts should pollution occur.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be extremely unlikely based on deep water depths within the site boundary.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be extremely unlikely. The effect will, therefore, be of tolerable significance and ALARP, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in Table 13.11   Open ▸ ) is of tolerable significance and ALARP which is not significant in EIA terms.

                        Operation and maintenance phase

  1. During the operational phase, vessels navigating in proximity to the floating substructures associated with the Array may be at risk of interaction with either the mooring lines, or any underwater elements of the floating substructures not visible from the surface including the subsea cables. The level of risk will depend on the clearance available above the subsea elements of the substructures (in particular the mooring lines and buoyant sections of dynamic cables).
  2. Up to 681 nm (1,261 km) of inter-array cables and 127 nm (236 km) of interconnector cables may be in place during the operation and maintenance phase. For both the static portion of the inter-array cables and the interconnector cables, with the minimum burial depth anticipated to be 0.4 m, subject to CBRA confirmation. Where cable burial is not possible, external cable protection methods may be deployed which will again be determined within the CBRA. Charted water depths within the site boundary range from 62 m to 84 m below CD and are therefore sufficiently deep that the reduction of underkeel clearance resulting from the presence of cables on the seabed is not of concern to vessel keels.
  3. The inter-array cables may utilise buoyancy modules, which can be used to maintain the lazy-S configuration of the dynamic portion of the inter-array cable to allow extension of the cables in response to the floating foundation movements (see volume 1, chapter 3). The requirement for these buoyancy modules and their final design, including their depth below the waterline, are yet to be confirmed. Final design will be confirmed via the DSLP which will be approved by MD-LOT in consultation with the MCA and NLB (i.e. the DSLP will confirm that the final design of the dynamic cables will maintain suitable underkeel clearances).
  4. Each foundation may utilise up to six mooring lines. There are two substructure types under consideration, namely semi-submersible and Tension Leg Platform (TLP). For semi-submersible substructures, there are three types of mooring configurations: taut, semi-taut and catenary. The NRA (volume 3, appendix 13.1) has considered an example mooring line arrangement based on worst case parameters. On the basis of the example considered, the vessel with the largest draught recorded within the vessel traffic datasets (16.3 m) would need to transit closer than 100 m to the floating wind turbines to risk interaction with the mooring lines. Based on consultation such a passing distance is very unlikely for any third party vessel, particularly for larger vessels. Final design of the mooring lines will be confirmed via the DSLP which will be approved by MD-LOT in consultation with the MCA and NLB (i.e. the DSLP will confirm that the final design of the mooring lines will maintain suitable underkeel clearances).
  5. General consultation input has been that commercial vessels are likely to avoid the Array. This aligns with operational experience of other UK wind farms. Any commercial vessels that does access the Array would be unlikely to transit within close proximity to the floating foundations. Smaller vessels may be more comfortable transiting through the Array, however these will have smaller draughts. It was noted during the Hazard Workshop that fishing vessels up to 24 m would likely keep a clearance of around 250 m to 300 m from the floating foundations, and that larger fishing vessels, such as 70 m to 90 m pelagic vessels, would likely keep a minimum 500 m clearance and would be unlikely to transit through the Array. It was also noted that recreational vessels would likely keep a minimum of 50 m from wind turbines and that even this distance would be unusual, with larger clearance distances typically used.
  6. Details of the infrastructure including the floating foundations, mooring lines and subsea cables will be promulgated to increase awareness of the Array and any potential underkeel interaction risk. The locations of the floating foundations would be clearly shown on appropriate nautical charts, and the locations of the anchors and mooring lines will also be provided to the UKHO for charting purposes.
  7. Should an underwater allision occur, the most likely consequences are minor damage to property and minor reputational effects on business but no perceptible effect on people. Although considered less likely, a more serious interaction could involve the vessel foundering resulting in PLL and the environmental consequence of pollution. The Array’s MPCP will be implemented to reduce the environmental effects should pollution occur.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be extremely unlikely based on deep water depths within the site boundary and the consultation input indicating vessels will not pass in close proximity to the structures.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be extremely unlikely. The effect will, therefore, be of tolerable significance and ALARP, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in Table 13.11   Open ▸ ) is of tolerable significance and ALARP which is not significant in EIA terms.

                        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 are unlikely to enter on a regular basis.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be extremely unlikely based on deep water depths within the site boundary and the consultation input indicating vessels will not pass in close proximity to the structures.
                        Severity of consequence
  1. The severity of consequence is considered to be serious.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be serious and the frequency of occurrence is considered to be extremely unlikely. The effect will, therefore, be of tolerable significance and ALARP, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in Table 13.11   Open ▸ ) is of tolerable significance and ALARP which is not significant in EIA terms.

Anchor Interaction with Subsea Cables (Including Dynamic Cabling)

                        Construction phase

  1. As all cables associated with the Array will be located within the site boundary, anchor interaction with a subsea cables only applies to vessels within the site boundary. However, a buoyed construction area will be in place during the construction phase and it is anticipated that third-party vessels will be unlikely to enter on a regular basis.
  2. It is also considered unlikely that a vessel would drop anchor in the Array unless it was an emergency (e.g. a drifting incident), given water depths are in excess of 60 m. This aligned with the vessel traffic assessment (see section 13.7.1), with no vessels identified as being at anchor over the 12 months assessed in proximity to the Array based on navigational status information broadcast via AIS. In addition, no designated anchorage areas or preferred anchorage locations in proximity to the Array were identified.
  3. Should an anchor interaction incident occur with the cables, the most likely consequences will be low based on historical anchor interaction incidents, with no damage incurred to the cable or the vessel. As an unlikely worst case, a snagging incident could occur and/or the vessel’s anchor and the cable could be damaged. However, with the designed in measures in place including charting and cable burial/protection, this risk will be managed For commercial fishing vessels or recreational vessels the consequences may also include compromised stability of the vessel, however, water depths are such that small vessels are very unlikely to attempt dropping anchor.
  4. As for vessel anchors, there is a risk that fishing gear may interact with any cables. It is the responsibility of fishers to dynamically risk assess whether it is safe to undertake fishing activities within the Array and to make a decision as to whether or not to fish. This decision will be informed by a number of factors, which will include the charted locations of infrastructure within the Array. Further assessment of impacts associated with fishing gear is provided in volume 2, chapter 12.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be extremely unlikely given very low frequency of baseline anchoring and the use of cable burial/external cable protection and charting.
                        Severity of consequence
  1. The severity of consequence is considered to be moderate.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be moderate and the frequency of occurrence is considered to be extremely unlikely. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in Table 13.11   Open ▸ ) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

                        Operation and maintenance phase

  1. During the operation and maintenance phase, vessels may be more likely to enter into the Array following removal of the buoyed construction area, however, consultation input indicated entry may be less frequent than at fixed foundation offshore wind farm developments (see section 13.5).
  2. Scenarios which may lead to a vessel dropping anchor include the following (noting that water depths in the Array are in excess of 60 m, meaning the latter two scenarios are considered particularly unlikely):
  • vessel anchoring in an emergency over subsea cable (e.g. to avoid drifting into a structure, or into an area of busy traffic);
  • vessel dropping anchor inadvertently (e.g. mechanical failure);
  • planned anchoring where vessel is unaware of presence of infrastructure; and
  • vessel dragging anchor over subsea cable following anchor failure.
  1. Due to the distance offshore of the Array and local water depths, anchoring activity is expected to be very limited. This aligned with the vessel traffic assessment (section 13.7.1), with no vessels identified as being at anchor over the 12 months assessed in proximity to the Array based on navigational status information broadcast via AIS. In addition, no designated anchorage areas or preferred anchorage locations in proximity to the Array were identified.
  2. In line with Regulation 34 of SOLAS (IMO, 1974), the charted location of any hazards should be taken into consideration as part of the decision making process of where to anchor. The locations of subsea cables, structure locations and mooring lines will be provided to the UKHO for charting purposes, and as such mariners will be able to include the infrastructure within their decision making processes.
  3. Cable protection will primarily be by seabed burial where possible. The extent and method by which the static portion of the inter-array cables and the interconnector cables will be buried will depend on the results of a detailed seabed survey of the final inter-array and interconnector cable routes and associated CBRA. Where cable burial is not possible, external cable protection methods may be deployed which will again be determined within the CBRA.
  4. It is noted that there will be sections of cables between the seabed and the floating substructures. Interaction with these sections is considered an unlikely event given water depths and the presence of infrastructure means anchoring is unlikely to be attempted in the vicinity of the foundations (outside of an emergency).
  5. Should an anchor interaction incident occur with the cables, the most likely consequences will be low based on historical anchor interaction incidents, with no damage incurred to the cable or the vessel. As an unlikely worst case, a snagging incident could occur and the vessel’s anchor and/or the cable could be damaged. However, with the designed in measures in place, this risk will be managed. For commercial fishing vessels or recreational vessels the consequences may also include compromised stability of the vessel, however, water depths are such that small vessels are very unlikely to attempt dropping anchor.
  6. As for vessel anchors, there is a risk that fishing gear may interact with any cables. It is the responsibility of fishers to dynamically risk assess whether it is safe to undertake fishing activities within the Array and to make a decision as to whether or not to fish. This decision will be informed by a number of factors, which will include the charted locations of infrastructure within the Array (e.g. on UKHO charts, and other electronic charts as appropriate). Further assessment of impacts associated with fishing gear is provided in volume 2, chapter 12.
                        Frequency of occurrence
  1. The frequency of occurrence is considered to be extremely unlikely given very low frequency of baseline anchoring and the use of cable burial/protection and charting.
                        Severity of consequence
  1. The severity of consequence is considered to be moderate.
                        Significance of the effect
  1. Overall, the severity of consequence is deemed to be moderate and the frequency of occurrence is considered to be extremely unlikely. The effect will, therefore, be of broadly acceptable significance, which is not significant in EIA terms.
                        Secondary mitigation and residual effect
  1. No secondary shipping and navigation mitigation is considered necessary because the likely effect in the absence of further mitigation (beyond the designed in measures outlined in section 13.10) is of broadly acceptable significance and ALARP which is not significant in EIA terms.

                        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 on a regular basis.
  2. Static cable sections may be left in situ, noting dynamic cable sections will be removed. Cables left in situ will remain charted and will be located in the site boundary where water depths mean that deliberate anchoring is unlikely.