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220kV Submarine Cables, Ireland

We undertook various feasibility, design and cable routing studies of subsea cable transmission systems across Ireland for the transmission system operator, EirGrid Plc, in order to upgrade and improve the electricity transmission system in the Republic.

Undertaking geophysical survey
Landing of 220kV submarine cable in Cork, Ireland.
Close
Landing of 220kV submarine cable in Cork, Ireland. Undertaking geophysical survey in the Shannon Estuary, Ireland.

You have brought your experience and skills in extra high voltage cable systems to our teams. We would like to make special mention of the high professional level of Mott MacDonald’s experts involved in the projects.
I would like to highlight the proactive nature of your staff in dealing with the technical issues which arose during both these projects, most especially since the projects resulted in the installation of a new type of technology onto the transmission system. It was the technical assurance provided by Mott MacDonald staff that ensured EirGrid was confident in proceeding with the cable connections.

Senior project manager
EirGrid

Our overall scope included feasibility studies, outline design and foreshore license application for two new 220kV submarine cable circuits, covering around 3km across Cork Harbour and the Shannon Estuary, and introduced a new HV submarine cable technology to the Irish transmission system.

Key facts

We acted as lead consultant engineer on behalf of EirGrid for the projects in Cork Harbour and the Shannon Estuary. In this role, our team:
  • Carried out feasibility studies to identify preferred routes and establish the submarine cable design and installation methodology
  • Prepared outline design specifications based on the preferred option for the new cable circuit
  • Undertook tender assessments for two submarine cable route surveys ensuring that the most appropriate surveying techniques, skills and equipment were used
  • Assisted in the procurement process and applied for the relevant government permissions and foreshore licences required for the investigation
  • Lead and managed detailed marine surveys including marine ecological, hydrographic and geophysical surveys as well as intrusive ground investigations to assess ground conditions for the installation of high voltage electricity cables at 1.5m depth into the seabed. The site investigation included undertaking surveys with an acoustic doppler current profiler, charting the bathymetry of the seabed with multibeam systems, and mapping features of the seabed with side scan sonar systems to identify likely areas of rock outcrops or sand waves indicating potentially mobile sediment systems. Additional surveys gathered information on the geological profile of the near-surface through a ‘pinger’ geophysical investigation and an intrusive investigation looked at sediment cores from sites across the proposed route
  • Managed the preparation and submission of a marine environmental report and foreshore licence application which included stakeholder management and marine and terrestrial environmental surveys

Challenges

  • Inclement weather made the marine survey difficult to undertake in the Shannon Estuary
  • A foreshore licence was required to undertake the marine survey. The licence took some time to be issued and included a number of conditions to considert
  • Results from the investigation in the Shannon Estuary showed that tidal currents of 5-7m/s are common during spring tides. Water depths exceed 60m and areas of the Estuary feature steep side slopes, rocky outcrops and sand waves. All features provided a challenge for cable installation and long-term protection

Solutions

  • Surveys were taken in exposed areas and at the appropriate time of year weather-wise to help to ensure the accuracy of the results. The importance of a detailed and precise marine survey contract document and specification meant that there were few disputes during the survey and the accuracy of the results were of a high calibre
  • Through active engagement with the issuing authority, we were able to anticipate the requirements and ensured no subsequent delays in the licensing
  • Early engagement with the submarine cable manufacturers allowed a clear understanding of the most applicable cable technologies available. Close attention was paid to the availability of proven and tested submarine cable jointing accessories especially for XLPE transmission type cables

Value and benefits

Our expertise in specialist areas, such as marine ecology, allowed our client to have just one point of contact for the full cope of work. We worked closely with EirGrid to introduce new HV submarine cable technology to the Irish transmission system. Issues that could have delayed the project were identified and solved early on, resulting in successfully obtaining a foreshore licence for submarine cables circuits.
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