67.
Some gravity base structure solutions may require the injection of a cement grout mix under the foundation to strengthen the sea bed and / or fill voids.
5.4.4.2.2 Sea bed preparation and penetration for gravity base structures 68.
Gravity base structures may require sea bed preparation to level the sea bed, provide a base with adequate bearing capacity and to ensure adequate contact between foundation base and sea bed.
69.
Within the East Anglia THREE site, sand waves of varying magnitude are known to be present. Gravity base structures are not suitable in areas of very large and mobile sand waves but they could be used in areas where smaller sand waves are present. At these locations the sea bed may need to be excavated to the trough of the sand wave and the underside of the base would be placed at this level on a bedding layer.
70. 71.
At some locations, excavation of upper sediments may be required to reach a competent formation level.
Sea bed preparation would consist of dredging works and the installation of a bedding and levelling layer. The dredging works are likely to be carried out using a trailer suction hopper dredger. The bedding and levelling layer installation would be undertaken by a fall pipe vessel.
72.
The dredged sand would be deposited at an agreed disposal area as close as possible to the installation operations. This site will be determined through further consultation, it is anticipated that EATL will seek to designate the East Anglia THREE site as the disposal area for dredged materials. However, for locations requiring significant excavation, it is likely that some of this dredged material would be used later for infill works, and as ballast material.
73.
The preference is that gravity base structure foundations are installed where no or limited ground preparation is required and micro-siting would be used to minimise any dredging requirements. Table 5.6 below shows two scenarios, based on a 7MW and 12MW gravity base structures.
Table 5. 6 Estimated dimensions of excavation for gravity base structures Wind turbine size (MW) 7
Max. overall dredge area (m2)
12
1,257 2,828
Max. volume of excavation (m3)
17,000 26,000
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 17
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