16.7 Reduction of Underkeel Clearance as a Result of Subsea Infrastructure

16.7.1 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 Construction Management Statement (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 Applicant’s MPCP will be implemented to reduce the environmental impacts should pollution occur.

16.7.1.1  Frequency of Occurrence

  1. The frequency of occurrence is considered to be extremely unlikely based on deep water depths within the site boundary.

16.7.1.2  Severity of Consequence

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

16.7.1.3  Significance of Risk

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

16.7.1.4  Additional Mitigation and Residual Risk

  1. No secondary shipping and navigation mitigation is considered necessary because the likely risk in the absence of further mitigation (beyond the embedded mitigation) is of tolerable significance and ALARP which is not significant in EIA terms.

16.7.2 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, the minimum burial depth anticipated to be 0.4 m, subject to CBRA confirmation. Where cable burial is not possible, alternative 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. Section 15.6 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 to 300 m and that larger fishing vessels, such as 70 to 90 m pelagic vessels, would likely keep a 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.
  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 Applicant’s MPCP will be implemented to reduce the environmental effects should pollution occur.

16.7.2.1  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.

16.7.2.2  Severity of Consequence

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

16.7.2.3  Significance of Risk

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

16.7.2.4  Additional Mitigation and Residual Risk

  1. No secondary shipping and navigation mitigation is considered necessary because the likely risk in the absence of further mitigation (beyond the embedded mitigation) is of tolerable significance and ALARP which is not significant in EIA terms.

16.7.3 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 are unlikely to enter on a regular basis.

16.7.3.1  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.

16.7.3.2  Severity of Consequence

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

16.7.3.3  Significance of Risk

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

16.7.3.4  Additional Mitigation and Residual Risk

  1. No secondary shipping and navigation mitigation is considered necessary because the likely risk in the absence of further mitigation (beyond the embedded mitigation) is of tolerable significance and ALARP which is not significant in EIA terms.