Benthic Communities at Horns Rev Before, During and After Con

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Horns Rev. Benthic communities Page 20 100 90 80 70 60 50 40 30 20 10 0 µmol/l Total Nitrogen 1999 2000 2001 2002 2003 2004 3.0 2.5 2.0 1.5 1.0 0.5 0.0 µmol/l Total Phosphorous 1999 2000 2001 2002 2003 2004 Figure 1.2. Dissolved nutrient concentrations near Horns Rev (Blåvand west) 1999-2004 (NERI, 2006). Low transparency due to high amounts of re-suspended material in the water column is characteristic for the Horns Rev area. High temporal variability is found in the water transparency, which is influenced by tidal current, wind induced current, current speed and seasonal plankton dynamics. In general, the water transparency is low in spring, 1.8-6.0 in adjusted Secchi depth [Adjusted Secchi depth = estimated Secchi depth x (1+0.4 x wave height] and higher during autumn, 2.5-8.8. Pronounced diel variability in transparency is found within a few hours and is associated with changes in the prevailing current directions from SSW to NNE (Bech et al., 2004; Bech et al., 2005; Leonhard & Pedersen, 2004; Leonhard & Pedersen, 2005). 1.3. Horns Rev Offshore Wind Farm Horns Rev Wind Farm is situated south of the actual reef approximately 14 km westsouth-west of Blåvands Huk (Figure 1.3). a) b) 524-E Bird area No.113 qq qqq qqq qq q qq q qqq qqq qq qq qqq q qqq qqq qq q qqq qqq qq q q q q q q qq q q q qqq qqq qqq q q qq qqq qqq qqq 524-D 524-C 524-F 524-B 524-I 524-A Bird area No.57 q Wind turbines EU bird and habitat areas Sand and gravel extraction areas Figure 1.3. The offshore wind farm at Horns Rev and the cable trace to land at Hvidbjerg Strand. T marks the transformer platform (a). Areas for raw material extraction and international protected bird and habitat areas (b). The coordinates of the outermost wind turbines and the transformer station are shown in Appendix 1. 1.3.1. Construction and layout The offshore wind farm is comprised of 80 (Vestas V80- 2MV) wind turbines erected in a grid pattern as shown in Figure 1.3. Thus, the total installed energy generating capacity is 160 MW. The distance between the individual wind turbines and rows is 560 m with the Doc. No. 2572-03-005 rev. 4
Horns Rev. Benthic communities Page 21 wind farm covering an area of 27.5 km 2 including a 200 m exclusion zone around the wind farm. The wind turbines are interconnected via a 36 kV cable grid that is connected to a transformer platform in the northeastern corner of the wind farm. The transformer platform is connected to land at Hvidbjerg Strand by a 150 kV cable. The cable is embedded into the seabed by water-jetting. The cable trace passes through an internationally protection area and is 19.5 km long. The wind turbine (WTG) foundations are constructed using the “monopile” concept. The monopile foundation consists of two main components; the pile and the transition piece. The pile is a steel pipe that is rammed into the seabed. The transition piece is also a steel pipe but with a slightly larger diameter than the pile. Pile and transition piece are joined together over a stretch of 6 metres. For the Horns Rev project, the monopile diameter is 4 m. The pile is driven to a depth of up to approx 25 m. The joint between the turbine and the foundation is placed 9 m above mean sea level (MSL). At this level, a platform is placed and the wind turbine tower mounted. The main geometry of the wind turbines is shown in Figure 1.4. Figure 1.4. Wind turbine dimensions. On the seabed at Horns Rev, scour protection was necessary around the foundations to minimise erosion due to the strong current at the site (Figure 1.5). The scour protection has a total diameter of approximately 25 m, but it varies between sites. The scour protection is approximately 1.3 m in height above the original seabed and consists of a protective stone mattress, 0.8 m in thickness, of large stones up to 55 cm in diameter at distances of 0–10 m from the towers with a subjacent layer, 0.5m in thickness, of smaller stones 3- 20 cm in diameter. At the edge of the large protective stones area, there is an area up to 4 Doc. No. 2572-03-005 rev. 4
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Benthic Communities at Horns Rev Before, During and After Con

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