46.


For the worst case assessments, the scour predictions from modelling point 3 will be used in all areas with water depths of up to 35m and the scour predictions in the ‘deeper water’ test will be used for all other areas. Since both of these predictions use the upper limit of the gravity base structure size ranges being considered for that water depth, it is reasonable to assume that their use would be associated with the greatest wind turbine ratings, namely 12MW. Consequently, there would be 100 such foundations across the East Anglia THREE site.


47.


In keeping with the assessments of scour hole formation, it has been estimated that the footprint of scour protection material that may be required associated with the different foundation types being considered is also greatest for the conical gravity base structures (Table 7.7).


Table 7.7 Worst Case Assumptions for Scour Protection Footprints Foundation Type


Water Depth (m)


<35


Gravity base structures


Jackets (pin piles)


Jackets (suction buckets)


Suction caisson Monopiles


35 - 45 45 - 55


Up to 55 Up to 55


Up to 50 35 – 45


Foundation Dimensions (m)


Up to 50 (basal diameter) Up to 55 (basal diameter) Up to 60 (basal diameter)


2.3 (diameter pin piles at each leg of jacket)


10 (diameter suction bucket at each leg of jacket)


30 (diameter) Up to 12 (diameter)


Scour Protection Footprint (m2)


17,700 21,500 25,500 1,790


2,121


6,362 2,828


48. During the decommissioning phase, worst case assumptions involve activities that are similar to those that would be encountered during the construction phase.


49. Due to the above considerations, conical gravity base structures of the upper bound diameter for each of the water depth ranges are considered to be the worst case for near-surface sediment disturbance effects during construction or decommissioning, blockage effects during operation and scour hole formation or, as a corollary, scour protection footprints during operation.


50. Drilling of 12m diameter monopiles to depths of 40m is considered to be the worst case for sub-surface sediment disturbance during construction.


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 12


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