16.8 Anchor Interaction with Subsea Cables (Including Dynamic Cabling)

16.8.1 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 long-term vessel traffic data (see Appendix E), 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 embedded mitigation measures in place including charting and cable burial/protection, this risk will be reduced. 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 the 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.

16.8.1.1  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.

16.8.1.2  Severity of Consequence

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

16.8.1.3  Significance of Risk

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

16.8.1.4  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.

16.8.2 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 given the presence of subsea components including mooring lines and dynamic cables may dissuade vessel entry.
  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 cable (e.g. to avoid drifting into a structure, of 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, 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 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 practicable. 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 practicable, alternative 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/or the vessel’s anchor and the cable could be damaged. However, with the embedded mitigation measures in place, this risk will be reduced. 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 the 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.

16.8.2.1  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.

16.8.2.2  Severity of Consequence

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

16.8.2.3  Significance of Risk

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

16.8.2.4  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.

16.8.3 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 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.

16.8.3.1  Frequency of Occurrence

  1. The frequency of occurrence is considered to be negligible given very low frequency of baseline anchoring, the use of cable burial/protection and charting, and increased familiarity with the project post construction.

16.8.3.2  Severity of Consequence

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

16.8.3.3  Significance of Risk

  1. Overall, the severity of consequence is deemed to be moderate and the frequency of occurrence is considered to be negligible. The risk will, therefore, be of broadly acceptable significance.

16.8.3.4  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.