Floating wind farms rely on dynamic power cables

Maxime Toulotte Sep 7, 2020

Maxime Toulotte, Head of Technical Marketing, Nexans Subsea & Land Systems Business Group (SLS), explains how decades of expertise in developing dynamic cable solutions for the oil and gas industry will prove crucial for connecting floating wind turbines to the grid.

Offshore wind is making an important contribution to the global energy transition, with the installed capacity growing by around 30 percent year on year. Most of these wind farms are usually constructed close to shore in shallow waters. Yet for offshore wind to achieve its full potential very large wind turbines must be built far offshore, where winds are both stronger and more consistent.

This poses a number of technical challenges, with one of the most important being that the water far offshore is generally much deeper. However, conventional wind turbines installed directly on the seabed are not really suitable for waters beyond 50 to 70 meters deep. Beyond that depth, their foundations become very expensive to build and install.

The answer is to build wind turbines on floating bases that are held in place by anchoring them to the seabed. Wind farms that could utilize floating turbines fall into two main geographic groups. First, there is a group in water depths of 100 to 200 meters, such as in the North Sea. Second, there could be wind farms in waters that are five to 10 times deeper, such as the Mediterranean or the west coast of the United States.

The challenge of waves and currents

While floating wind farms overcome the difficulties of building permanent foundations in deep water, they present another challenge. This is because waves and currents subject the cables that connect the turbines to the grid to significant and varying dynamic loads. Fortunately, at Nexans we can call on our wealth of experience gathered in developing and manufacturing cables and umbilicals for customers in the deep-water oil and gas industry. In fact, our first dynamic cable dates back to 1983 and we have deployed solutions in many fields from Norway to the Gulf of Mexico and Australia.

This experience told us that further development work was needed to ensure that our cables can operate at the high voltages that will be needed for large floating wind farms. This is because they will feature very large turbines that will eventually reach power levels of around 20 MW – against a current average power output of 7.2 MW. High voltage connections are essential to feed the electricity they will produce to their local collector substation, and then export it to shore. But working at high voltage means we cannot tolerate any water penetrating into the insulation system.

In static subsea cables there is a relatively easy way to provide a water barrier – we extrude a lead sheath on to the cable. But a lead sheath cannot flex to accommodate the movement of a dynamic cable during the lifetime of the windfarm. We have worked therefore to develop alternatives, such as a metallic foil or polymer sandwich, suitable for applying to cables many kilometers in length. This barrier needs to be thick enough to provide reliable protection, but not so thick that it resists the movement of the cable.

Furthermore, when we make this type of structural change to a known design, it will naturally affect other properties. That is why we take a holistic approach to engineering design to cover a wide variety of factors such as strength, flexibility, floatation and temperature regulation. We have a special advantage here, as our design teams can consult the predictive models built up from decades of experience in the oil and gas industry.

Another important factor for dynamic power cables is that we can incorporate optical fibers within their design for a relatively small additional cost. This provides a very useful monitoring system that gives the operator an early warning should any part of the cable
come under excess stress or is subjected to an increase in local temperature.

Cables for the world’s first floating wind farm

We have put our expertise to work in supplying dynamic and static cable solutions for Equinor's Hywind Demo and Hywind Scotland floating wind turbine projects. The demonstration unit was based on a 2.3 MW turbine with 85 m diameter blades. It was followed by Hywind Scotland, the world’s first floating wind farm, which has been producing electricity since 2017. The project features five turbines with 154 meter diameter rotors, with a total installed capacity of 30 MW. It is connected to shore by a 30 kilometer export cable at a transmission voltage of 33 kV.

About the author

Maxime Toulotte

Maxime Toulotte is the Head of Technical Marketing of Subsea and Land systems Business Group in Nexans, where he has the responsibility to develop and maintain relations with technical and engineering departments of clients and partners for subsea high voltage cables.
Maxime has held several positions as Sales & Tender Manager and Lead Engineer for high voltage submarine cable system projects.
Maxime holds a Master's degree in Electrical Engineering from the Grenoble Institute of Technology, France.

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