Cable laying vessel NKT Victoria during installation work

Skagerrak 2 repair, Norway – Denmark

The national power grids between Denmark and Norway are connected through four DC-links crossing the Skagerrak strait, capable to carrying 1700 MW of renewable energy to the two countries’ energy mix. One of the cables, the Skagerrak 2, was damaged due to external impact.

Key Facts

Client

Statnett

Total cable route length

36,5 km north of Bulbjerg in Denmark

Location of the project

Skagerrak, (Norway-Denmark)

Duration

September - November 3rd, 2017

Quantity of order

Turnkey repair of a 250 kV cable system with a 250 MW power rating

Scope

  • Turnkey repair operation of the 250 kV MI HVDC cable system after deburial made by the client
  • Project management and repair planning
  • Manufacturing of jointing material
  • Jointing of spare cable, cable-laying and testing
Drone shot from NKT Victoria in the harbour / at the dock
Damaged submarine cable at Skagerrak

The project

The Skagerrak 2 high-voltage cable suffered an external damage on the seabed and needed a repair.

The national power grids between Denmark and Norway are connected through four DC-links crossing the Skagerrak strait, capable to carrying 1700 MW of renewable energy to the two countries’ energy mix.

In the summer of 2017, one of the cables, the Skagerrak 2, was damaged due to external impact. The damaged part was located about 36.5 km north of Bulbjerg in Denmark.

The grid owner Statnett carried out an initial fault location and cut and seal operation. In September 2017, Statnett awarded NKT with the turnkey service order for the repair. NKT's scope of work comprised the complete repair operation following deburial of the cable carried out by Statnett's vessel Elektron. The damaged link Skagerrak 2 is a 250 kV MI (mass-impregnated) HVDC (high-voltage direct current) cable system installed in 1977 between Kristiansand in Norway and Tjele in Denmark.

The offshore mission had to be completed as soon as possible to reduce downtime. During the autumn and winter, the sea can be very rough. The exposure to the weather condition combined with the urgency to get the cable back in operation, defined the requirements for the repair.

Map Skagerrak 2

The solution

Time and operational excellence are crucial in a repair operation and we clearly see how our customers benefit from a NKT turnkey service operation.

We performed the repair with support from our marine, engineering and service departments. The state of the art and purpose built cable laying vessel NKT Victoria was used for the operation.

Thorough preparations, involving close cooperation between the customer and the technical, marine and operation experts, were crucial to ensure a successful operation. A detailed repair plan was set up including marine survey and QHSE (quality, health, safety and environment) documentation. NKT engineering department performed cable and joint analyses in order to maintain cable integrity during the repair operation and ensure a safe and efficient operation.

The spare cable was loaded onboard NKT Victoria in Drammen Norway and the vessel set course to the fault location. Well on site, the repair operation commenced without any delay. During the repair the weather conditions got worse, but NKT Victoria managed to keep positions with help of its powerful DP3 (dynamic positioning) system. The highprecision jointing operation was time optimized with the support of the marine advisory system and the cable system was laid in the planned position with the highest accuracy.

NKT finished the turnkey repair project within the planed time schedule. When handling all from planning to execution during a turnkey repair operation, NKT can ensure maximum efficiency, smooth interfaces and lower risk. The repaired Skagerrak 2 cable link was back in operation on November 3rd 2017 – securing the international transmission of green power.

“NKT has carried out this repair project in a professional and efficient manner. Both the vessel NKT Victoria and the NKT team have proved to be well capable of handling offshore cable repairs. We have enjoyed working with NKT during this project.” Espen Kiær, project manager for the repair at Statnett

Welding of a high voltage cable during an offshore service installation

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