Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
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Physical change<br />
The assessment concluded that the fine s<strong>and</strong> disturbed during the cabling<br />
could typically be carried a distance <strong>of</strong> 1170m in the 30 minute period it would<br />
remain in suspension (based on peak flows). At other tidal conditions settlement<br />
would occur more rapidly <strong>and</strong> the distance fine s<strong>and</strong> would be carried would<br />
be significantly reduced. It was concluded that with the rapid dispersion <strong>of</strong> the<br />
sediment it was unlikely that concentrations would be measurable above the<br />
ambient conditions. Coarser sediments disturbed during the cabling operations<br />
would fall out <strong>of</strong> suspension in far shorter distances.<br />
The conclusions from the study were supported with reference to monitoring at<br />
Nysted <strong>and</strong> Kentish Flats Offshore Wind Farms.<br />
Nysted Offshore Wind Farm<br />
During the construction <strong>of</strong> the Nysted Offshore Wind Farm in Denmark, strict<br />
requirements were imposed on the release <strong>of</strong> sediment from sea bed operations<br />
including the installation <strong>of</strong> the array cable. To ensure compliance with the<br />
requirements monitoring was undertaken <strong>and</strong> subsequently reported upon by<br />
Seacon (2005).<br />
The wind farm is located in the Femer Belt separating Germany <strong>and</strong> Denmark<br />
<strong>and</strong> is approximately 2km from the coast in water depths between 6 <strong>and</strong> 9.5m.<br />
The surface sediments within the wind farm area are generally medium s<strong>and</strong>s<br />
with very low silt/clay content. The thickness <strong>of</strong> the overlying s<strong>and</strong>s varied<br />
between 0.5 <strong>and</strong> 3m, underlain with clay deposits.<br />
Cable laying operations were undertaken using jetting where the substrates<br />
permitted <strong>and</strong> using pre-trenching <strong>and</strong> backfilling where hard substrates<br />
were encountered. The trenching operations were undertaken using a back<br />
hoe dredger rather than the more specialist systems described earlier in this<br />
section. Measurements <strong>of</strong> turbidity were taken continuously during the cabling<br />
operations <strong>and</strong> daily mean <strong>and</strong> maximum values determined. The jetting<br />
operations resulted in significantly less turbidity than the pre-trenching <strong>and</strong><br />
backfilling operations with mean <strong>and</strong> maximum values at 200m from the various<br />
operations <strong>of</strong>:<br />
Trenching Mean = 14 mg/l<br />
Max = 75 mg/l<br />
Backfilling Mean = 5mg/l<br />
Max = 35mg/l<br />
Jetting Mean = 2mg/l<br />
Max = 18mg/l<br />
These values compare with the restrictions set by the Danish Energy Agency <strong>of</strong><br />
15 mg/l as a mean value <strong>and</strong> 45 mg/l as a maximum value.<br />
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