Laure Tavernier outlines some of the challenges in creating the power export cable connection for the Beatrice Offshore Windfarm and the role it will play in future developments in the renewable energy industry

The Beatrice Offshore Windfarm, scheduled to be fully operational by 2019, is a partnership between SSE, Copenhagen Infrastructure Partners and Red Rock Power – the UK subsidiary of China’s SDIC Power Holding.

It is a 588 MW windfarm, located 13.5 km from Caithness in the outer Moray Firth of the North Sea. The project has been bringing benefits to the Scottish and UK economies throughout its development and construction and will continue to deliver further profits throughout the operational phase.

With 84 turbines, each rated at 7 MW, Beatrice will be the largest offshore windfarm in Scotland, producing enough electricity to power approximately 450,000 homes. The wind power will be transmitted through subsea and land cables to the Blackhillock Substation, close to Keith, from where it will be distributed through the British transmission system.

Nexans, in a consortium with Siemens Transmission and Distribution, part of the group’s Energy Management division, is supplying, installing and protecting two 220 kV subsea and land cable circuits to connect Beatrice to the Blackhillock substation, and 400 kV land cables to connect to the UK transmission system.

This is a huge collaborative project between teams from Nexans Norway, France, UK, Belgium, Sweden and Switzerland.

Nexans Subsea and Land System Business Group manages the overall project, being responsible for the design, supply, testing and delivery of a total of 265 km of cable and associated accessories.

This includes 145 km of 220 kV subsea cables, 115 km of 220 kV land cable and 5 km of 400 kV onshore cable links, which will transmit the electricity to the UK transmission system and also provide control to the windfarm through integrated fibre optic cable elements.

The onshore cables were designed by Nexans France, produced at Nexans’ facility in Charleroi, Belgium and installed by the UK branch of Nexans France. The offshore cables were made in Halden in Norway, and laid and protected by Nexans Norway. All of the high-voltage accessories associated with the cables were delivered by Nexans Switzerland. The fibre optic cables were supplied by Nexans Rognan, Norway for the offshore cables and by Nexans Grimsås, Sweden for the onshore cables.

To bring power ashore from the Beatrice offshore wind farm, the electricity will be transmitted through two 220 kV cable circuits that will travel approximately 70 km along the sea floor to a landfall point to the west of Portgordon. The cable transmission system is expected to have a 25-year operating life.

The offshore export cables enable the monitoring and control of the wind farm and the Offshore Transformer Modules (OTM). Each offshore export cable carries a number of fibre optic cores sufficient to provide the necessary functionality for the Control and Instrumentation Systems.

At the landfall, a pre-installed pipe enables the offshore cables to reach the onshore transition joint bay and connect to the onshore cable system which will take a 20 km route to the Blackhillock substation.

The onshore cable circuit comprises three separate phases, each 20 km in length, installed in ducts around 1.5 metres deep. They also form a double circuit, so Nexans has produced
120 km of this cable in total. From here, the
400 kV land cables will transmit the electricity to the GB transmission system.

The specification for the cables was developed during the FEED (Front-End Engineering Design) stage of the project to establish the optimum combination of performance, reliability and cost.

It was therefore agreed with Beatrice Offshore Windfarm Limited (BOWL) to use a cross-linked polyethylene (XLPE)-insulated 1200 mm2 aluminium conductor for the main section of the subsea cables.

However, due to the power rating and the need to respect the environment at the landfall location and pre-installed ducts at approximately 10 metres depth, it was necessary to change to 1600 mm2 copper conductor for the nearshore/landfall part of the subsea
cable. This required the development and testing of a factory transition joint enabling a connection of the aluminium conductor section to the copper conductor section in the plant, prior to the Factory Acceptance Test. The onshore cable is a 1600 mm2 aluminium cross section for the 220 kV and 800 mm2 for the
400 kV.

Before installation, the complete system (subsea cables, land cables, all accessories) must have a successful record or pass a new prequalification test and rigorous type-testing regime as per International Standards to ensure that the system fully complies with all specifications. During the Beatrice project, the offshore systems have been type tested and passed successfully, which was a significant milestone in the project.

The project gained consent for both the onshore and offshore works from the Moray Council and the Scottish government respectively. Both the offshore and onshore consents have mandated certain commitments and conditions on the project, as well as obligations contained within third party agreements. Nexans has ensured that its designs and works complied with the conditions, obligations and commitments of these consents.

At the windfarm, Nexans cables are connected to two OTMs that Siemens Transmission and Distribution has developed and supplied. An inter-connector cable, also part of Nexans scope, between the OTMs, will provide redundancy should one of the export cables or OTM transformers fail.

The subsea export cables were then installed at water depths of up to 100 metres by Nexans’ C/S Skagerrak vessel with a 7000-tonne capacity turntable, multiple cranes, and dynamic positioning (DP2) systems. The DP2 system enables the vessel to maintain a stationary position during operations to safely and precisely lay cable at predetermined coordinates on the seabed.

The Skagerrak uses touch-down monitoring with the Remotely Operated Vehicle. The installation of the Beatrice project was carried out in four campaigns of approximately 35 km each.

To protect the subsea cables, they are either buried into the seabed by Nexans’ specialized trenching system, Capjet, developed in-house, using a water-jet system both to create a trench and to propel the trenching machine, or protected by rock installation.

For the Beatrice project two versions of the Capjet were used. The 1 MW Capjet system has been used along most of the route. The trenching process is carried out with a first pass from the Capjet, followed by a survey. When required, a second pass is carried out to ensure the acceptable depth is achieved. The recently-developed Capjet Mini allows for access in difficult and shallow areas and has been used nearshore. Laying, offshore jointing and burial for both circuits are successfully completed.

At the landfall point, and as a result of the land designation as a Site of Special Scientific Interest, shallow water and geology, it was necessary to pre-install two pipes, one per cable, underneath the beach and near shore areas in order to land the cables. These pipes were installed using the Direct Pipe technique, a combination of micro-tunnelling and using a horizontal drilling machine, which was the best solution for the type of soil expected beneath the seabed. The exit points are 420 metres offshore.

For the onshore installation, the cables were installed mainly in ducts and in HDDs, which required 24 joint bays and 12 specialized high voltage joints. The 400 kV outdoor sealing ends were installed horizontally prior to connecting them vertically at the Blackhillock substation. The installation of all onshore cables is also completed at this stage.

Beatrice is an extremely important renewable energy project that will help meet the UK’s targets to decarbonize electricity generation.

One of the major challenges of the project was to minimize its impact on the environment, which has been a critical focus for all project partners.

To ensure the success of the project, environmental advisors’ services were used both for onshore and offshore aspects, to advise on compliance with environmental law and best practices to respect flora and fauna.

Beatrice will serve as a strategic renewable energy source for Scotland and will lead the way in encouraging further developments in the renewable energy industry in the future. Nexans is continuing to look forward to further projects in clean energy, currently executing the Dolwin 6 windfarm project in Germany, as well as recently completing the cables for the Hywind Scotland Pilot Park windfarm, the world’s first floating windfarm.

Laure Tavernier is Nexans Project Manager for the Beatrice Offshore Windfarm project