Skagerrak 2 repair, Norway – Denmark

• 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

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.

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.