Install scour protection (likely to be rock dumping). 80.
Ballast works would be undertaken by a trailer suction hopper dredger. The scour protection works would typically be installed by a DP rock dumping vessel equipped with a fall pipe. The scour materials would be placed in one or multiple layers.
5.4.4.2.4 Spoil removal and disposal for gravity base structures 81.
For gravity base sea bed preparation would be required and is dependent on the nature of the ground conditions present underneath the bases (for example, if sand waves are present). In extreme cases, it may be necessary to remove material up to a depth of several metres.
82.
Examples of the volume of materials requiring excavation are given in Table 5.6. In these examples it would be preferable to use some of this material as ballast but in an extreme case all of it may need to be removed if it is unsuitable for ballast. This is an issue that requires further investigation on receipt of the geotechnical information.
5.4.4.2.5 Type of scour protection for gravity base structures 83.
Scour protection may be necessary around the base of gravity base structures to protect against currents and waves that may cause erosion of the sea bed. This would typically consist of quarried rock, well graded with d50=200 to 400, i.e. half the rock would be less than a specified median (200 to 400mm diameter) and half would be greater.
84.
It is possible that scour protection may not be required dependent on the particular gravity base foundation and ground conditions.
85. More scour protection would be needed when the base is buried at depth due to sand waves. Here the scour protection could extend to cover the full dredge area – up a diameter of 180mwith a worst case volume of scour material of up to 25,500m3.
86.
The scour protection works are likely to be installed by a DP stone dumping vessel equipped with a fall pipe. The scour materials would be placed in one or multiple layers. Alternative methods of installing scour protection are also under consideration. For example, Sea bed Scour Control Systems (SSCS) Limited Frond System which comprises continuous lines of overlapping buoyant polypropylene fronds that when activated create a viscous drag barrier that significantly reduces current velocity. The frond lines are secured to a polyester webbing mesh base that is itself secured to the sea bed by anchors pre-attached to the mesh base by polyester webbing lines.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 19
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