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74.


Gravity base structures might also use a skirt at their base which penetrates the sea bed giving more stability to the structure. The penetration could vary from 0.1 to 5m depending on ground conditions. Underbase grouting may also be used to strengthen the soil beneath the foundation and fill small voids.


5.4.4.2.3 Installation method for gravity base structures 75. A gravity base structure would be delivered to site via one of two methods, depending on the foundation design:


 Floating , towed to site and sunk via ballasting; or  Transported to site by barge and installed by heavy lift crane.


76.


For the first solution, it is possible that a bespoke barge would be used to support the foundation during its journey to site. For the second solution, it is likely that a heavy lift vessel would be required to perform the installation. This could be a jack- up or floating vessel.


77.


The installation of gravity base structure solutions is dependent on design and fabrication methods. Definitive methodology for installation would be finalised following the completion of post-consent commercial and technical discussions.


78.


Irrespective of the type of gravity base structure, the sea bed may require some element of preparation prior to receiving the foundation. This may involve removal of some material up to a depth of several metres and / or placement of rock in order to form an adequate level platform.


79. The overall installation methodology would typically be as follows:


 Confirmation investigation of sea bed to ensure no obstructions are present (this would be on-going throughout the installation process);


 Prepare sea bed (if necessary);


 Gravity base structure transported to site via barge or floated to site, hauled by tugs;


 Mobilise heavy lift floating crane (if foundation is non-buoyant solution);


 Lift foundation from barge and lower to prepared area of sea bed, or adjust buoyancy of floating foundation and sink to prepared area of sea bed;


 Install ballast as necessary; and


Preliminary Environmental Information May 2014


East Anglia THREE Offshore Windfarm


Chapter 5 Description of the Development Page 18


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