2008, Emu Ltd., Sheringham Shoal Offshore Wind Farm, Seabed and Sub-seabed Mapping Survey

Description

The objective of the survey was to collect detailed bathymetry, shallow geology and seabed features data, to determine the best possible routing for the export power cables and for the construction and installation of the turbine foundations and infield cables within the windfarm site. The survey was carried out using the vessel RV Discovery between 1st March and 5th April 2008, utilising the following equipment:


Multibeam Reson Seabat 8125 multibeam echo sounder Sidescan sonar Edgetech 4200 Sidescan Sonar USBL Sonardyne Scout USBL
Pinger GeoAcoustics GeoPulse Pinger (3.5kHz)
Boomer Applied Acoustics AA 200 boomer plate (2kHz)
Hydrophone C-Products 8 element hydrophone streamer
GPS Fugro Starfix DGPS
GPS Leica 1230 RTK and PPK system

A separate Field Report has been produced containing all operational and processing aspects of the work in Report NH0779 Seabed and Sub-seabed Mapping Survey, Field Report 08/J/1/02/1198/0767. All magnetometer survey work carried out has also been reported separately in Report NH0779 Magnetometer Survey of Geotechnical Borehole Sites at Sheringham Shoal Windfarm, No. 07/J/1/02/1137/0730 ver2. Prior to the seabed and sub-seabed mapping survey EMU Ltd completed a video survey at geotechnical borehole sites BH11, 13, 15, 16, 18 and 20. The objective of the survey was to establish if there was any Sabellaria reef at the above borehole locations. Sabellaria crust was identified at BH15 and BH 20 however this did not constitute a reef habitat. No other occurrences of Sabellaria were found at the remaining four borehole locations surveyed. Full survey results were reported in Report NH0779 Video Survey of Geotechnical Borehole Sites at Sheringham Shoal Windfarm, No. 07/1/02/1137/0731. Sabellaria reef was not identified during the Seabed and Sub-seabed Mapping Survey of the turbine area and cable corridor however this cannot be confirmed as no ground truth data was collected. A survey line spacing of 50m was used throughout the cable corridor with lines spaced at 48.5m throughout the windfarm site in order to tie in with the spacing of the turbines. Multibeam infill was run to achieve 100% data coverage where it was not achieved with the designed line plan. Cross lines in the routing corridor and windfarm site were also run. Water depths range from 0.8-24.5m and 11.5-23.5m below Chart Datum throughout the cable corridor and turbine area respectively. Throughout the turbine area sand waves and associated sand ripples run in a band from southeast to northwest. The remainder of the seabed sediments through site are relatively featureless gravelly, shelly SAND. Numerous contacts, boulders and fishing gear were found within the survey area and care should be taken to avoid these when trenching cables and preparing turbine foundations. No new wrecks or contacts of any significant size were located within the turbine area since the Gardline Survey was undertaken in May 2007 (NH0778).
An area of weathered CHALK outcrops inshore on the cable corridor. Vibrocore data suggests this is “weak” which implies that trenching should be possible unless the presence of FLINT inclusions is found. Further geotechnical investigation would be needed to ascertain this conclusively. The remainder of the cable corridor sediment comprises sandy GRAVEL, shelly SAND and SAND. More sand waves are observed towards the northern extents of the cable corridor; care should be taken both here and within the turbine area if jack up rigs are in use due to steep gradients and the soft nature of this seabed type. In
agreement with discoveries from the previous Gardline report, the presence of Botney Cut channel infills could provide difficulties for foundations of jack-up rigs due to the composition of sediments within these channels.It is recommended that further geotechnical investigations be carried out to ascertain the bearing capacity of such sediments now that the outcrop limits have been mapped. Other potential engineering difficulties may arise due to the presence of pockets of SAND and GRAVEL or BOULDERS within the Bolders Bank Formation.