Table 7.4 Worst Case Assumptions for Near-Surface Sediment Disturbance during Construction Foundation Type
Maximum no. wind turbines
Gravity base structure for 7MW wind turbine 172
Gravity base structure for 12MW wind turbine
33. 100
Foundation Dimensions (m)
40 (basal diameter) 60 (basal diameter)
Ave. Volume of Sea Bed Preparation per Foundation (m3)
17,500 26,000
In addition, there is potential that the installation of monopiles and jackets (using 3 or 4 pin piles) may require drilling (although the preference is for driving the piles wherever it is feasible to achieve this). Any drilling of piles into the sea bed would have the greatest potential to release sediments from notable depths (tens of metres) below the sea bed surface, hereafter called sub-surface sediments, into the water column (to depths of up to 40m below the sea bed for monopiles and up to 50m below the sea bed for pin piles) (Table 7.5). These sub-surface sediments have a different physical composition to near-surface sediments and therefore may be more widely dispersed by tidal currents (i.e. the drill arisings may be overall finer than the near-surface sediments).
Table 7.5 Worst Case Assumptions for Sub-Surface Sediment Disturbance during Construction Foundation Type
Drilling Depth (m)
Jackets (with pin piles) 50 Monopiles
40
2.3 (diameter piles, 4 no.) 12 (diameter pile, 1 no.)
831 4,524
34. During the operational phase, there is potential, if no scour protection is provided, for the presence of the foundations to cause scour hole formation in the sea bed adjacent to the foundation due to flow acceleration in the immediate vicinity (tens of metres) of the foundation.
35.
As the need for scour protection would not be determined until the wind turbine locations and the associated foundation types are known, the worst case assessments need to consider both the formation of scour holes in the absence of scour protection (and the associated fate of the scoured sea bed material) and, as a corollary, the extent of scour protection that would be required if it is deemed necessary to limit scour hole development.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 9
Foundation Dimensions (m) Max. Sediment Volume per foundation structure (m3)
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