The main objectives of the surveys were as follows:
- Survey of 1200m wide corridor for long term monitoring of sand wave migration. Side scan imagery also to be used to assess seabed material types.
- Close pass survey to assess scour around the base of each turbine; multibeam echo sounder only.
- Side scan sonar and multibeam echo sounder survey of the inter-array cables and export cable.
- Close pass multibeam echo sounder only survey around each turbine base and a meteorological mast.
Data acquisition was undertaken from Bibby HydroMap’s own dedicated shallow draft vessel Bibby Tethra, equipped with a Reson 7125 multibeam echo sounder, Klein 3000H side scan sonar, C-Nav RTG positioning and IxBlue Hydrins/Octans motion and heading sensors; each interfaced along with ancillary equipment into a high specification navigation PC, running QPS QINSy survey software.
At Rhyl Flats, a main survey corridor was covered by multibeam echo sounder and side scan sonar. Survey lines were spaced at 25m intervals and run in a SE/NW direction. The nature of the data collection within the main survey corridor was to provide information on long term sediment migration and changes of the seabed, as this will be repeated every three years. The side scan imagery was also used to provide an indication of seabed material types. The multibeam survey of the individual turbines was used to assess the condition of the seabed around each of the turbine foundations and covered an area 45m radius around each turbine.
At the North Hoyle wind farm the turbines are spaced in a regular grid pattern. Four survey lines were carried out within each corridor with the outermost of these four lines was surveyed using both side scan sonar and multibeam echo sounder, whereas the inner lines were surveyed using the multibeam echo sounder only. Where no inter-array cables were present, all four lines run using multibeam echo sounder only.
Using this technique, the requirement to survey all inter-array cables with side scan sonar together with the requirement to ‘box-in’ each turbine foundation were both fulfilled. The export cable from the North Hoyle wind farm to shore was also surveyed using both side scan sonar and multibeam echo sounder over a single high water period.
Spec Sheet: Bibby Tethra