EGS completes Scottish cable route survey

Projects and Operations - August 29, 2012

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EGS (International) Ltd (EGSi) was recently contracted by Scottish and Southern Energy (SSE), to carry out a pre-installation cable route survey between the Orkney mainland and the Scottish north coast at Caithness.

The scope of work involved a geophysical survey of a 70km long 500m wide cable route corridor, a geotechnical survey along the route comprising cone penetration tests (CPTs), and vibrocores, and an environmental survey comprising grab sampling and seabed imagery.

For the inshore geophysical survey and the environmental survey EGSi utilised one of its in-house survey vessels, Wessex Explorer.

For the offshore geophysical survey and the geotechnical survey Neptune was chartered from Neptune EHF.

The geophysical survey comprised the acquisition of multibeam (MBES) bathymetry, side scan sonar (SSS), sub bottom profiler (SBP) and magnetometer data.

The geophysical survey was carried out first in order to confirm the proposed corridor was suitable, and to define the locations for the follow on geotechnical and environmental surveys.

Preliminary processing of MBES, SSS, SBP and magnetometer data was carried out offshore to allow the Client Representative to review results on site and decide if any route deviations were required.

From the results ofr the geophysical survey, the seabed along the route mainly comprised sand and diamicton, with bedrock at the landfall locations.

Diamicton deposits are frequently very variable in nature, with sediment characteristics varying rapidly horizontally and vertically. The majority of sediment types present within the diamicton range from sand to gravel as well as coarser sediment such as cobbles and boulders.

Diamicton is of particular concern for cable installers as by its nature, burial is very difficult to predict. Therefore, it was crucial to determine the exact extents of the diamicton areas so the Client could quantify the potential risk and cost of burying the cable.

Upon completion of the geophysical operations Neptune returned to port to re-configure for geotechnical operations. This involved mobilising a dedicated geotechnical winch for the deployment and recovery of both the CPT and vibrocore. A total of 35 CPT and 35 vibrocore locations were investigated.

After the geophysical landfall surveys Wessex Explorer was reconfigured for the environmental scope. This required the mobilisation of seabed grab sampling equipment and a drop down video/stills system.

Once the sites were chosen the drop down video system was deployed to confirm the geophysical interpretation. All sites were videoed and still images captured, and based on the results of the video grab samples were taken if the site was suitable.

All offshore operations were carried out without any incidents or delays. The project is now in the final reporting stages, compiling factual reports for the geophysical, geotechnical and environmental surveys along with the supporting charts and data deliverables.

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