Renewable Energy

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Wind Farms – Design of Structures Installed

Offshore wind farms consist of a number of connected elements. These include the turbines, foundations, array cables, offshore substation, export cable and onshore substation and infrastructure. These items are installed using a range of different vessels and then serviced by either smaller crew transfer vessels or helicopters.

Turbines convert the kinetic energy of the wind into three phase AC power. They can be up to 150m high above sea level and generally have three blades. Traditionally offshore turbines have been marinised versions of onshore wind turbines at around 3MW but the new generations of turbines being introduced at the moment are much bigger (5-7MW) and are specially designed for the offshore environment. The blades themselves can be up to 75m long.

Wind Turbine Installation
Wind Turbine Installation

The foundations provide support for the wind turbines and facilitate personnel access. The vast majority of wind farms constructed to date have been built using a large circular steel tube termed a monopile which is piled into the seabed. However as wind turbines get bigger other foundation types are starting to be used. These include jackets, lattice structure with three or more legs, tripods structures and gravity base foundations which are big concrete bases. There are also more innovative foundation concepts being developed including suction buckets and floating foundations.

Cables transmit the power from the turbines to the shore. Array cables connect strings of offshore wind turbines to the offshore substation and are typically at 32kV. The power is then stepped up to 120kV in the offshore substation, before the export cables transmit the power to the shore. Historically export cables have been AC, but in the future driven by increasing distances from shore, DC cables will begin being used.

Offshore substations are the single heaviest item of plant, weighing up to 2,000 tonnes. The platform level is about 25m above the sea and has an area of typically 800m2. The platform sits on a foundation, typically a jacket design.

Substations at the Robin Rigg OWF

The onshore substation is on land and transforms the power from the export cable (132kV) to grid voltage (400kV).

Another important structure is the met mast. This provides vital meterological and oceanographic data for the site and is often installed a number of years before the wind farm itself is built. The data collected then feeds into the design and planning of construction and operation. Although the sensors to measure the wind speeds are relatively small, they need to be supported by a stable platform and so met masts typically have a fixed foundation similar to ones used on wind turbines themselves.

A Met Mast at the Kentish Flats OWF