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15.6 Subsea Interaction

15.6.1 Vessel Draughts

There is the potential that vessels could interact with the subsea infrastructure associated with the Array. To assess this, vessel draughts recorded per vessel type during the vessel traffic surveys has been assessed.

Table 15.2 and Table 15.3 present the average and maximum vessel draughts recorded during the combined 28-day survey period, broken down by vessel type, within the shipping and navigation study area and site boundary respectively.

Commercial vessels are considered likely to deviate to avoid the site boundary based on operational experience of existing wind farms and consultation undertaken for the Array (see section 4). Any vessels that do choose to pass through the Array are more likely to be small fishing vessels and recreational vessels compared to commercial vessels.
It was noted during the Hazard Workshop that fishing vessels up to 24 m would keep a clearance of around 250 - 300 m and that larger fishing vessels, such as 70 - 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.
Fishing vessels will typically have larger draughts than recreational vessels, and are slightly more prevalent in the area in and around the Array. The distribution of fishing vessel draughts recorded within the site boundary during the 12 months of fishing vessel AIS (see section E.4.2.4) is presented in Figure 15.8.

As shown, the large majority (78%) of fishing vessels within the site boundary had a draught between 6 m and 8 m. The maximum fishing vessel draught recorded during the 12 months of AIS was 8.6 m.
It is noted that recreational vessels were recorded in limited numbers within the vessel traffic data (see section 10.3.5) and that those that do choose to transit within the site boundary would likely be experienced given the distance offshore. It should also be noted that recreational vessels tend to have shallower draughts compared to fishing vessels.
The final design of the floating foundation, mooring line arrangements and dynamic cable arrangement is not known at the stage. Ossian OWFL are currently considering a number of floating foundation concepts. A realistic worst case mooring arrangement has been derived following engagement with suppliers to understand current approaches to mooring of floating foundations. For the purposes of the NRA, section 15.6.2 provides an illustration of an example mooring line arrangement based on worst-case parameters. As the final mooring line and dynamic cable design has not been confirmed, consultation on the design will be discussed and agreed with MCA and NLB post consent as part of the DSLP process. The principles of the design will ensure appropriate underkeel clearances.