Review of Cabling Techniques and Environmental Effects Applicable

More documents

Recommendations

Info

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report Dispersion and deposition of coarser sediment was not modelled as sand will only be carried a few metres from the point of disturbance. The sediment distribution of the unconsolidated surface layer in large parts of the area affected by the cabling is predominantly gravely sand with a small amount of silt. The high sand/gravel content of the in situ sediment, together with the relatively small disturbance arising from cable ploughing or trenching to 1m depth, suggests that for most of the cable route the majority of any disturbed sediment will fall immediately to the bed in the immediate location of the cable. Because of the minimal disturbance, fine sand is almost all likely to remain within the bottom 1m -2m of the water column (even this is probably conservative) and typical settling velocities of around 10mm/s will ensure that the sand settles within half an hour (or less) or becomes part of the ambient near bed transport. Medium or coarse sand will settle within minutes. The vast majority of the disturbed sediment will initially resettle within 20m of the cable, with almost no sand being carried more than 100m from the cable except as part of the natural background transport. The presence of surface sand/gravel along much of the cable route restricts the extent of fine sediment dispersion, and the modelled results are only applicable to those limited areas where chalk or other competent beds are exposed or have only a very thin surface layer of mobile sand. The plotted model results are therefore conservative for a 1m depth of burial, but still show that suspended sediment will be quickly dispersed. In the areas where the export cables are proposed to be buried up to 3m (shoals / sand waves) the cable is installed in (mobile) sands only, with no disturbance of the underlying chalk or other beds. London Array Offshore Wind Farm An assessment of the cabling operations for the London Array Offshore Wind Farm has been undertaken by ABP Mer (RPS, 2005). The work assumed that jetting techniques would be adopted and burial depths would be between 1 and 3m. An assessment of the surface sediments was based on a comprehensive grab sampling exercise supported by seismic and side-scan surveys. It was found that the inter-array and export routes were predominately covered with fine sands. In parallel with the assessment of seabed conditions, suspended sediment data was collated. This included data from the Southern North Sea Sediment Transport Study, archive data held at CEFAS and measurements taken specifically for the project. Limited modelling was undertaken to quantify the impacts of the cabling operations. An assessment was completed based on the likely type and volume of sediment that would be disturbed, an assessment of the fall velocity of the disturbed sediment and the ambient tidal flows that would carry suspended sediment. 96
Physical change The assessment concluded that the fine sand disturbed during the cabling could typically be carried a distance of 1170m in the 30 minute period it would remain in suspension (based on peak flows). At other tidal conditions settlement would occur more rapidly and the distance fine sand would be carried would be significantly reduced. It was concluded that with the rapid dispersion of the sediment it was unlikely that concentrations would be measurable above the ambient conditions. Coarser sediments disturbed during the cabling operations would fall out of suspension in far shorter distances. The conclusions from the study were supported with reference to monitoring at Nysted and Kentish Flats Offshore Wind Farms. Nysted Offshore Wind Farm During the construction of the Nysted Offshore Wind Farm in Denmark, strict requirements were imposed on the release of sediment from sea bed operations including the installation of the array cable. To ensure compliance with the requirements monitoring was undertaken and subsequently reported upon by Seacon (2005). The wind farm is located in the Femer Belt separating Germany and Denmark and is approximately 2km from the coast in water depths between 6 and 9.5m. The surface sediments within the wind farm area are generally medium sands with very low silt/clay content. The thickness of the overlying sands varied between 0.5 and 3m, underlain with clay deposits. Cable laying operations were undertaken using jetting where the substrates permitted and using pre-trenching and backfilling where hard substrates were encountered. The trenching operations were undertaken using a back hoe dredger rather than the more specialist systems described earlier in this section. Measurements of turbidity were taken continuously during the cabling operations and daily mean and maximum values determined. The jetting operations resulted in significantly less turbidity than the pre-trenching and backfilling operations with mean and maximum values at 200m from the various operations of: Trenching Mean = 14 mg/l Max = 75 mg/l Backfilling Mean = 5mg/l Max = 35mg/l Jetting Mean = 2mg/l Max = 18mg/l These values compare with the restrictions set by the Danish Energy Agency of 15 mg/l as a mean value and 45 mg/l as a maximum value. 97
Page 1:
REVIEW OF CABLING TECHNIQUES AND EN
Page 4 and 5:
Review of Cabling Techniques and En
Page 6 and 7:
Page 9 and 10:
Abbreviations AIS Automatic Identif
Page 11:
TTS Temporary Threshold Shift TWA T
Page 14 and 15:
Page 16 and 17:
Page 18 and 19:
Page 20 and 21:
3 Cable types and 18 installation t
Page 22 and 23:
Page 24 and 25:
Page 26 and 27:
Page 28 and 29:
Page 30 and 31:
Page 32 and 33:
Page 34 and 35:
Page 36 and 37:
Page 38 and 39:
Page 40 and 41:
Page 42 and 43:
Page 44 and 45:
Page 46 and 47:
Page 48 and 49: Review of Cabling Techniques and En
Page 82 and 83: 4 Physical change 4.1 Introduction
Page 140 and 141: 6 Good practice measures This secti
Page 142 and 143: 7 Gaps in understanding Research fo
Page 144 and 145: References Introduction Sinden, G.
Page 148 and 149:
Page 150 and 151:
Page 152 and 153:
Page 154 and 155:
Page 156 and 157:
Appendix A: Standards and codes of
Page 158 and 159:
Page 160 and 161:
Page 164:
Printed in UK on recycled paper. De
show all

Review of Cabling Techniques and Environmental Effects Applicable

Create successful ePaper yourself

Delete template?

Save as template?