A wind turbine is a mechanical device that turns wind energy, by a generator, into electric energy. It consists of three parts: the tower, the nacelle, and the rotor. The rotor consists of three propeller-like blades and is attached to the top of the tower. Inside the nacelle there is a generator connected with the rotor blades through a shaft. As blades spin around due to wind, they transfer their kinetic energy to the generator which also stars spinning. The generator then converts its kinetic energy into electric energy.
There are many types of wind turbines. The height of the wind turbines may vary from between 60m and 210 meters while the length of the blades varies between 50m to 160m. Offshore wind turbines are often much larger than their onshore counterparts in order to withstand the stronger winds met offshore. Their stronger structure enables offshore turbines to operate at optimum efficiency exporting the power ashore through subsea cables. For offshore turbines, there many types of mooring and foundation depending on the depth of the installation areas as well as sea floor geomorphology.
Foundation technology is designed according both to geometrical characteristics of the turbines and to site conditions. Maximum wind speed, wave heights and currents recorded in the area, water depth, and surf properties are main aspects that affect the foundation type and design. Within a wind farm, each foundation is customized to the water depth at its particular location.
Four basic types of foundations have been used in offshore wind farms. They are the monopiles, jackets, tripods, and gravity foundation structures. Additionally turbines may be installed on a floating foundation. Foundations are built onshore, carried offshore by barge or other vessel and set on the seafloor by a crane or derrick barge.
Monopiles are large diameter, thick walled, steel tubulars that are driven or drilled (or both) into the seabed and are currently the most common foundation in shallow water (less than 20 m depth) development.
Tripods consist of a central steel shaft connected to three cylindrical steel tubes through which piles are driven into the seabed. Tripods often used in deep water developments.
Jacket foundations are an open lattice steel truss template consisting of a welded frame extending from the midline to above the water surface. Jackets are robust and heavy structures and require expensive equipment to transport and lift. Jackets are used for developments at depths greater than 50m and can be effective in very deep water (100m+) Gravity foundations are concrete structures that use their weight to resist wind and wave loading. Gravity foundations are less expensive to build than monopiles, but the installation costs are higher, largely due to the need for dredging and subsurface preparation and the use of specialized heavy-lift vessels. Gravity foundations are most likely to be used where piles cannot be driven and the region has dry-dock facilities for concrete construction. They may also have an advantage in ice-prone regions.
For developments at depths greater than 60m, the use of a floating structure is preferable. Floating
structures consist of a floating platform and an anchoring system. There are several alternative
designs for floating turbine foundations all of which are variations on the spar and tension-leg
concepts in the oil and gas industry.
Demand for electric energy has increased rapidly in the last 30 years and today two types of energy resources are used: fossil fuels and renewable energy (file: Tips Coconet Renewable). Fossil fuels come from coal, oil and gas whilst renewable energy comes from wind, sun, water and biomass. Fossil fuels resources are decreasing while their consumption has negative impacts on climate, environment and human health. Renewable energy comes from sustainable sources and can be used to produce electric energy (file: Tips Coconet PowerStation).
Wind energy is considered as a competitive and sustainable form of renewable energy is now widely used. Large amounts of electric power coming from wind energy are produced in wind farms. A wind farm consists of a grid of wind turbines and it is installed either in land or offshore.
To choose an appropriate location for a wind farm involves considering many criteria. One of the main aspects to be considered is the wind climate of the area. These can be assessed using a wind atlas but the most detailed information is obtained by collecting wind data for a specific site. Over a period of at least one year, duration of the wind blow, the wind speed and direction as well as the wind gusts are assessed and the wind power potential of the area is calculated. At an ideal potential site, wind should be blowing almost constantly during the year while the wind speed should always be above a certain value. This value is dependent on the type of turbine to be used. This ensures that the amount of the energy produced will satisfy the demands in energy consumption during the year. Strong gusts of wind are also recorded as these may cause severe damage to the blades of the turbine due to overloading and as well as to the rotor due to over-speeding. Similar damage may also be caused also due to sudden changes in wind direction.
When sitting and designing a wind farm, special attention is always paid to the geomorphology of the area. This relates especially to offshore wind farms where sea depth, sea floor geology, waves, currents and environmental aspects concerning wildlife and habitats must also be taken into account.
The first wind farm was installed at Crotched Mountain in Southern New Hampshire in December 1980. It consisted of 20 wind turbines and each of them produced 30 kilowatts (KW) (file: Tips Coconet Watt). The first offshore wind farm was installed in Denmark in 1991.
Land-based wind farms were widely used at first but strong arguments in favour of offshore wind farms have developed in recent years. One reason is that there is stronger and more uniform wind blowing over the sea surface than on land. Often, the wind speed over the land either decreases rapidly as the wind moves towards the ground or its flow becomes less homogeneous. The former is a result of the varying land morphology and ground friction. The latter is caused mainly due to the presence of land surface obstacles, such as trees, buildings, cliffs or mountains. However, over the sea, no obstacles are present and the sea surface is smooth (except of in storm conditions). As a result, the wind neither decelerates nor changes its flow over the sea surface and therefore offshore wind farms are more energy-efficient. There are also no complaints relating to noise and visual pollution with offshore windfarms as they are sited far out to sea. One disadvantage of the offshore wind farms is their cost. Offshore wind farm installation costs are higher than for land-based wind farms but technological developments are expected to reduce costs in the future. Besides the advances in the installation technology, research concerning the impacts of the installation of offshore wind farms on wildlife and the marine environment is ongoing.