15.6.2 Mooring Lines

  1. Figure 15.9 presents an illustration of an indicative mooring line arrangement based on worst-case parameters, noting that this is subject to final design and selection of floater concept. This illustration presents the distance of the mooring line below the sea level at each distance within 300 m of the foundation. Also included is the maximum draught value recorded within the shipping and navigation study area during the combined 28-day period.

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Figure 15.9: Mooring Line Illustration

16 Risk Assessment

  1. This section provides a qualitative and quantitative risk assessment (using FSA) for the hazards identified, based on baseline data, expert opinion, outputs of the Hazard Workshop, stakeholder concerns and lessons learnt from existing offshore developments.
  2. At the end of the assessment of each hazard, these frequency of occurrence and severity of consequence rankings are summarised with the resulting significance of risk given in highlighted bold text.
  3. The risk control log (see section 18) summarises the risk assessment and a concluding risk statement is provided (see section 20.5).

16.1 Increased Vessel to Vessel Collision Risk Resulting from Displacement (Third-Party to Third-Party)

16.1.1 Construction Phase

  1. There will be no restrictions on entry to the Array other than through any active safety zones. However, it is considered likely that commercial vessels will deviate to avoid the Array during the construction phase, which will be marked as a buoyed construction area as directed by NLB. This aligns with input received in the Hazard Workshop including from commercial vessel representation, and operational experience of other UK wind farms including the nearby Seagreen 1 Offshore Wind Farm and Neart na Gaoithe Offshore Wind Farm.
  2. Anticipated deviations for the main commercial routes identified from the vessel traffic data have been defined. The full methodology for main route deviations is provided in section 13.4.1 with reasonable worst case deviation assumptions established in line with industry experience and consultation feedback.
  3. Deviations from the pre wind farm scenario (current baseline) would be required for seven out of the 11 main commercial routes identified (Routes 4, 5, 7, 8, 9, 10 and 11). However, it should be noted that the busiest routes (Routes 4 and 5 in Figure 13.1, which have six vessels per week and two to three vessels per week, respectively) would have very low deviation (no more than 0.2 nm) while the routes with larger deviation (Routes 7 and 8 in Figure 13.1, which would have a deviation of 5.7 nm and 4.9 nm, respectively) are quieter routes (with only one to two vessels per week each). Further, worst case assumptions have been made in terms of deviations as set out in section 13.4.1.
  4. It is noted that one regular commercial ferry operator was identified in the area, namely Smyril Line, who run a service between the Faroe Islands, Iceland and Rotterdam.   
  5. With the main commercial route deviations in place, the base case annual vessel to vessel collision frequency for commercial vessels is estimated to be 5.42×10-4, corresponding to a return period of approximately one in 1,845 years. This represents a 31% increase in collision frequency compared to the pre wind farm base case scenario. The return period of one in 1,845 years is reflective of the low volume of vessel traffic in the area.
  6. Smaller vessel types (e.g. fishing, recreation) may still choose to transit through the Array during the construction phase, noting this would be at the discretion of individual vessels. In this regard it should be considered that there is limited experience of deployment of large scale floating offshore wind projects, and as such vessels may be less likely to transit through floating structures than those on fixed foundations (this assumption aligns with consultation input, see section 4). However, there is considered to be sufficient sea room to accommodate any vessels that chose to avoid the Array without notably increasing vessel density around the site boundary given that the nearest baseline offshore wind farm (Seagreen 1 Offshore Wind Farm) is in excess of 25 nm away from the Array.
  7. The impact will be present throughout the construction phase which will last for up to eight years. Given that third-party vessels are expected to be compliant with relevant Flag State regulations including the COLREGs, the likes of collision avoidance action seek to ensure that the likelihood of an encounter developing into a collision incident is low. This is furthered by the promulgation of information and charting of the buoyed construction area which will increase awareness of ongoing construction activities, thus allowing third-party vessels to passage plan in advance (see section 18.1).
  8. The most likely consequences in the event of a collision incident between third-party vessels are minor contact between the vessels resulting in minor damage to property and minor reputational effects on business but no perceptible effect on people. Although considered less likely, collision between third party vessels could involve one of the vessels foundering resulting in Potential Loss of Life (PLL) and the environmental consequence of pollution. Such a scenario would be more likely if one of the vessels involved was a small craft which may have weaker structural integrity than a commercial vessel. The Applicant’s Marine Pollution Contingency Plan (MPCP) will be implemented to reduce the environmental impacts should pollution occur.

16.1.1.1  Frequency of Occurrence

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

16.1.1.2  Severity of Consequence

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

16.1.1.3  Significance of Risk

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

16.1.1.4  Additional Mitigation and Residual Risk

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

16.1.2 Operation and Maintenance Phase

  1. Based on experience at existing operational offshore wind farms and consultation undertaken, it is anticipated that commercial vessels will generally choose not to navigate internally within the Array. Therefore, the anticipated deviations discussed for the construction phase are directly applicable to the operation and maintenance phase, and it is likely that the deviations already established during the construction phase will continue into the operation and maintenance phase. On this basis, the risk of third-party to third-party vessel collision for commercial vessels is considered analogous during the operation and maintenance phase as during the construction phase.
  2. It is anticipated that commercial fishing vessels and recreational vessels may choose to navigate internally within the Array (and this may be more likely during the operation and maintenance phase than in the construction phase given there will be no construction buoys or construction activities) based on experience at existing operational offshore wind farms, particularly in favourable weather conditions. Such navigation may result in an additional encounter and collision risk associated with these small craft exiting the Array. However, with the application of good seamanship and given the high minimum spacing between wind turbines (1,000 m), there is not expected to be a visual obstruction to vessels passing at the edge of the Array. It is also noted that most small vessels this far offshore would be expected to be broadcasting on AIS. This assumption aligns with both consultation input (section 4), and the vessel traffic survey data collected (section 10).
  3. The impact will be present throughout the operation and maintenance phase which will last for up to 35 years. Given that third-party vessels are expected to be compliant with Flag State regulations including the COLREGs, the likes of collision avoidance action will seek to ensure that the likelihood of an encounter developing into a collision incident is low. This is furthered by the promulgation of information and charting of infrastructure associated with the Array which will increase awareness of the Array and any ongoing major maintenance activities, thus allowing third-party vessels to passage plan in advance (see section 18.1).
  4. The most likely consequences of the impact are as per the equivalent construction phase impact, namely minor contact and damage to property and minor reputational effects on business, but no perceptible impact on people. Although considered less likely, collision between third party vessels could involve one of the vessels foundering resulting in PLL and the environmental consequence of pollution. Such a scenario would be more likely if one of the third party vessels involved was a small craft and the other a commercial vessel since the small craft may have a weaker structural integrity than the commercial vessel. The Applicant’s MPCP will be implemented to reduce the environmental impacts should pollution occur.

16.1.2.1  Frequency of Occurrence

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

16.1.2.2  Severity of Consequence

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

16.1.2.3  Significance of the 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.

16.1.2.4  Additional Mitigation and Residual Risk

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