About the East West Interconnector
The East West Interconnector is a high voltage DC submarine and underground power cable linking the transmission grids of Great Britain and Ireland. The system is owned and operated by EirGrid Interconnector DAC (EIDAC) which is part of the EirGrid Group. With a power rating of 500MW it is one of the largest High Voltage Direct Current schemes in the world to use Voltage Source Converter technology. The submarine section of the cable consists of a bundled pair of HVDC cables manufactured by ABB. The interconnector also has a number of fibre optic cores.
2015 Marine Survey
In May 2015, Bibby HydroMap Ltd. were commissioned by EirGrid to undertakea baseline geophysical survey of the EWIC cable between Rush Bay in Ireland and Prestatyn in North Wales. The marine segment of the EWIC cable is buried for its entire length of approximately 187 km.
The objectives of the survey were to establish high accuracy baseline measurements of:
- Seabed bathymetry, morphology and features along the cable route.
- Topographic levels at the beach landings in both Ireland and Wales.
- Depth of burial of the EWIC cable bundle measured from the seabed surface.
The HVDC EWIC cable was to remain in service throughout the survey operations and therefore traditional methods of measuring depth of burial using techniques that require the application of a tone to the cable were not possible. Being a DC cable there was also negligible tone emanating from the cable itself.
Despite the testing tidal currents along the cable route, both the swath bathymetric survey and the mapping of seabed features did not present a significant challenge for Bibby HydroMap in terms of survey design. Bibby Athena’s hull mounted, dual head, full rate, Reson 7125 MBES system was used to collect swath bathymetric data, and seabed features were mapped using a dual frequency Edgetech 4200 Multipulse SSS.
The main challenge came in the form of how to accurately map the depth of burial of the HVDC cable whilst it remained in service. More traditional methods of cable depth of burial measurement include a requirement to either interrogate the tone of a tone carrying cable or to induce a tone into the cable. Since the EWIC cable is a HVDC cable, it presents very negligible tone of its own, this minute tone being in the form of a very low amplitude ripple current. With the cable remaining in service throughout the survey period, it was also impossible to apply any external tone to the cable.
The main challenges therefore associated with how to accurately measure the depth of burial of the cable were:
- Which survey sensor to use in order to measure the depth of burial?
- How to deploy such a sensor in the water column in order to achieve measurement?
Cable Depth of Burial Equipment Choice
In order to accurately measure the depth of burial of the EWIC cable, Bibby HydroMap opted to use an innovative and relatively new 3D sub seabed imaging technique in the form of the Sub-Bottom-Imager from Canadian Geoscience technology supplier, ‘Pangeo Subsea’. In order to be able to operate the sub- bottom-imager, Bibby HydroMap had to be able to deploy the unit and carry out acquisition within very tight motion and altitude tolerances. This normally means that the sub-bottom-imager needs to be mounted to an expensive work-class ROV, operated from an equally costly ROV support vessel. This is where Bibby HydroMap’s dynamic Remotely Operated Survey Platform; the ‘d’ROP’ came in.
The d’ROP, deployed from the MV Bibby Athena allowed for the acquisition of depth of burial data using the sub- bottom-imager at a constant optimal altitude of 3.5m above the seabed and with negligible vehicle pitch, roll or heave motion. The d’ROP was also able to maintain its position along the length of the route in order to be able to image the cable.
Armoured fibre optic umbilical allowing full control of the dynamic Remotely Operated Survey Platform from the MV Bibby Athena
Equipment Choice for Cable Depth of Burial in Shallow Water
A further challenge came due to the requirement to measure depth of burial of the cable in the shallow water depths approaching 0m LAT at the two landfall points on the Irish and Welsh ends of the submarine cable. The solution to this requirement was provided by deploying the SBI from the shallow draft survey vessel MV Proteus. This was to be performed in two separate towing configurations; one subsea towing configuration to be used in water depths of between 10.0 - 3.5m LAT and one surface towed, floating configuration to be used in water depths of 3.5m or lower.
Depth of Burial System Verification
In order to prove the suitability of both the d’ROP and the sub-bottom- imager, EirGrid and their consultants Intertek developed a set of acceptance trials. These acceptance trials were to be carried out along a 1km section of the EWIC cable route. The 1km section was chosen on the basis of it including a number of different cable features, including sections that had been trenched and backfilled, areas where the cable had been protected by rock dumping and an omega cable repair joint.