wind power
The Special Cabling Challenges of Floating Offshore Wind Farms by Maxime Toulotte
OFFSHORE WIND FARMS ARE
recognized globally as an increasingly popular source of cost-effective and dependable renewable energy. In the United States, while turbines have already started to generate 30 MW from the strong winds off the east coast, additional legislation - in line with state carbon emission mandates - will bring this to a total of 25 GW within the next decade. Current offshore wind farm technology is limited to regions that have strong and steady winds with extended outer continental shelves, such as the eastern seaboard. For these projects, fixed-foundation offshore wind turbines arrays are anchored to the sea floor (around 150- to 200-feet deep). The power generated by the turbines flows through buried medium voltage submarine cables (called array or inter-array cables), to one or more offshore substations. From there, the pooled green energy is sent to shore via high voltage ‘export’ power cables. For offshore wind farms in less hospitable environments, such as where the ocean floor drops steeply close to shore and/or the required strong wind is very distant, an alternative approach is required. Enter “floating” wind farms. This technology involves wind turbines and offshore substations that are not bottom-fixed to the ocean floor, but rather placed on floating structures that are anchored to the seabed. The good news is that the development of floating offshore wind farms in the U.S., like that for fixedfoundation offshore wind, will benefit greatly from projects already underway in UK, Portugal, Norway, Spain, and Japan.
Advanced Dynamic Cables
Exporting energy from floating offshore wind farms requires the development of a new generation of export cables - the unseen, yet indispensable vehicles for the delivery of energy to shore. Unlike those for bottom-fixed offshore wind farms, these cables will be subject to additional variables such as hydrodynamic effects induced by waves and currents, the movements of the floating platforms in relation to the bottom of the ocean, and the effect of their own weight. So far, the needs have been met with dynamic cables rated at 34.5kV and 66kV using wet insulation designs that are similar to array
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cables and other elements used in umbilicals for the oil and gas industry. However, these designs will not work for the export cables needed for utility scale, floating offshore wind farms, will need to carry far more power at a much higher voltage. The mechanical strength and fatigue property requirements for large floating wind farm cables far exceed those for export cables used with bottom-fixed offshore wind farms. One challenge lies in the use of lead sheaths. While they protect the export cables’ insulation from humidity and water penetration, they have limited metal fatigue properties when subject to constant movements. Another challenge for floating wind farm export cables is their weight. For a given weight per foot, the cable will break beyond a certain depth at the point where it is hanging. Different technological improvements are being developed for extending the range of the wet insulation design to the 115kV voltage class. Researchers are also working on finding ways to create water barriers (such as using alternative metallic alloy foil between the cables’ insulation and the jacket for higher voltage classes).
