Reaching new lows in subsea HV cable qualification

On 11 August Nexans’ cable-laying vessel, C/S Nexans Skagerrak, began cable laying for BKK’s record breaking new connection across Fensfjorden, a fjord close to Bergen, Norway, which is about 8 km wide and reaches a depth of some 526 m.

The Fensfjorden project forms part of a broader plan by BKK Nett AS, one of Norway’s largest power companies, to strengthen the electrical grid in western Norway, which will include a new 420 kV connection between Modalen and Mongstad.

The 420 kV XLPE cable being employed for the Fensfjorden project has been qualified by Nexans for use in unprecedented water depths, down to 550 m.

Ivar Rolfstad, Nexans project manager, said: “We have pushed the technological boundaries and our own previous world records!” The new depth achievement follows a previous record, also set by Nexans, with a 20 km 420 kV XLPE cable installed in 390 m of water in Hjeltefjorden, Norway.

Type testing

Nexans developed and produced the Fensfjorden cable at its Halden facility in Norway.

Final type-testing of the cable (following Cigre guidelines) was completed at Halden in May, with third party verification. Halden is suitably certified for such testing and is equipped with a large wheel able to simulate cable laying and retrieval conditions and the mechanical forces that cables will be subjected to.

Bjørn Sanden, Nexans technical director, subsea & land systems, explains that the focus of the type testing is essentially two-fold:

• to ensure that the cable has the mechanical robustness to withstand the kinds of forces it will be subjected to during installation at these depths, without affecting its electrical properties in any way; and
• to demonstrate that the cable is retrievable and repairable in the event of damage (eg, by trawler) and cannot, for example, suffer ingress of water, which is challenging to achieve for large cables at such depths and therefore pressures (55 bar).

During the test sequence, as recommended by Cigre in its technical brochure 490, the cable is subjected to the required mechanical forces (plus a security factor) and then to rigorous inspection/ testing to verify that no damage has occurred, including visual, electrical, and water penetration.

“We think the qualification of 420 kV XLPE cable to these depths is a significant achievement, and it has been far from straightforward”, says Bjørn Sanden. He notes that Nexans subsea cable technology development at Halden has been driven by “specific challenges we see posed by the Norwegian coastline”, with grid connections needed across fjords, for example.

“The fjords have a tendency to be quite deep and challenging when it comes to installation conditions. We can be regarded as fortunate as we have been challenged by our customers to take our high voltage cables deeper and deeper and into more complex locations. This has been instrumental in the kind of product development we have done. Not too many countries in the world have the kind of coastline we have. In a way, it’s good for cable development as it keeps us at the forefront of technology.”

And Fensfjorden is by no means the end of the story. Future projects in Norway will require high voltage transmission cables to go even deeper, to 600-700 m. But, based on experience and test results to date, Nexans’ 420 kV XLPE cable design is well capable of achieving this, Bjørn Sanden, believes.