58. Most types of gravity base structure are similar in form. Usually the base is hexagonal, octagonal or circular. Bases with a cruciform plan shape are also being considered, occupying a similar footprint.


59. 60.


The base is typically 1 to 2m deep. The shaft is usually cone shaped at the bottom, tapering to a cylinder at the top with an outer diameter of approximately 5 to 9m.


Some alternative gravity base concepts do not have a cone transition, just a cylindrical shaft, with an outer diameter of approximately 5 to 9m. The bases of these flat base gravity structures are much deeper, typically up to 10m, but the footprint sizes are similar and within the same envelope.


61. Footprint sizes for the base are outlined in Table 5.5 below:


Table 5.5 Gravity base structure dimensions Wind turbine size (MW)


7 12 Maximumdiameter (m)


40 60


Maximum Footprint (m2)


1600 3600


62.


For conical base gravity structures, the top of the base is usually 1 to 2m above sea bed or slightly more if a bedding layer is used to provide a level formation. However, in areas where sand waves are present, it is possible that the top of the base would be installed below sea bed level.


63.


For flat base gravity structures with only a cylindrical shaft, the top of base could be up to 10m above sea bed level.


5.4.4.2.1 Material requirement for gravity base structures 64.


65.


Gravity base structures are generally concrete with steel reinforcement and pre- stressing strand. There are also hybrid concepts that include a steel tower.


Secondary structures, such as handrails, gratings, fenders and ladders, would be produced using steel (or possibly another metal or composite material). The working platform could also be made from steel.


66.


The ballast material used is commonly sand. Other materials may be considered as an alternative, such as olivine, dolerite, basalt or pig iron. However, it is most likely that sand dredged locally to the site would be used, depending on the suitability of the material.


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm


Chapter 5 Description of the Development Page 16


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