Offshore wind energy is still a fledgling industry in comparison to onshore wind power, and much of the technology in use today is borrowed from land based wind farms. The challenges and demands of the installation process are, however, markedly different to onshore installations due to the difficulties of operating at sea. The logistical complexity of these installations and the costs involved in terms of construction place heavy constraints on the industry; the design challenge is to make offshore wind farms an economical and viable source of power.
Construction, transportation and installation of foundations can cost up to three times as much as the installation of the turbines, so it forms a critical part of cost analysis. There are several design concepts in research and development and at testing stage that may answer the economic questions through engineering.
Design considerations
Downtime at sea is one of the biggest cost factors involved with
Design of foundations, particularly for deep water, is still in its infancy. The demands placed on turbine foundations vary greatly from those experienced in the offshore oil and gas industries. The technology and the knowledge of offshore installations for oil and gas can be utilized, but there are several differences that present technical and design issues. The design of wind turbine foundations must cater for dynamic response and fatigue under a working load over a long period, rather than the ultimate conditions. Generally, the vertical load is much smaller but the horizontal load is much greater. The design also needs to be considered in terms of multiple foundations as opposed to one large structure.
Deep water design
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| Source: NREL |
The image above shows the generally accepted depths for offshore installations, which are split into three categories; shallow water, transitional water, and deep water. The vast majority of installations worldwide are in shallow water, with only a few test projects in operation at greater depths.
The design challenge is to develop substructures or floating platforms to support high power turbines; moving away from the current designs borrowed from onshore wind farms, and incorporating technology from the oil and gas industries, while adapting it to the specific requirements of wind turbines.
Existing shallow water designs
The designs currently used in shallow water include monopiles and gravity base foundations, while a suction bucket concept is in the later stages of development.
Monopiles are used in the majority of offshore wind farms. The London Array project, which is currently under construction, is set to become the worlds largest offshore wind farm with 177 turbines producing 1000MW supported by monopiles. They are relatively simple by design and have a minimal impact on the seabed; but they are limited by resonance in deeper water. The frequency of the turbine decreases as water deepens. To maintain stiffness the diameter of the monopile must increase; eventually to the point where it far exceeds the necessary size for structural support. The greatest practical depth for a monopile is considered to be around 30 metres.
Gravity base foundations are an alternative and have been used on several sites, including the Thornton Bank project in Belgium. They require in depth seabed analysis as they are sensitive to soil conditions, but do provide an option where pile-driving is not viable.
Suction buckets are not in commercial use as yet, but significant research has been carried out on them. They consist of a wide base that is driven into the seabed by drawing a vacuum and using hydrostatic pressure to seat the bucket. This technology offers an alternative to monopiles and the use of large pile-drivers.
Norwegian company Seatower introduced a crane-free gravity base foundation late in 2010 that is economically viable at depths of up to 50 metres. In an interview with WindEnergyUpdate CEO Petter Karal said, “The crane-free gravity is economical down to about 50 metres water depth. However, we have ways to build cost-efficiently down to as much as 100 metres based on crane-free technology.”
The foundations can be fully manufactured at quayside, before being towed out to sea using standard tugs. There is no need to use specialist vessels or cranes during installation, and the entire installation process takes just 12 hours, which means less downtime due to bad weather conditions. Although these foundations are not yet in use commercially the technology has been proven in the oil and gas fields, and as there is no seabed preparation required there is less impact on the environment and local eco-system.
This technology could provide an economically viable solution for deep water installations due to the low manufacturing and installation costs.
Transitional water concepts
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| Source: NREL |
The guyed steel tube design has yet to be tested in the water, but offers a wide support base. The Spaceframe concept is derived from land based designs, although they were not used extensively onshore due to high costs.
The Talisman, or ‘jacket’, design was first used by Talisman energy in 2006 at the Beatrice Wind Farm Project; with two turbines installed on the lattice like structures at depths of 45 metres.
The design was also used, along with a tripod foundation at the Alpha Ventus Project, north of the island of Borkum, Germany. Completed in 2009 the project incorporated six jacket type foundations alongside six tripod foundations. All twelve turbines are 5 megawatts, and it is the largest transitional wind farm in the world today.
Learn more about about deep water designs here.
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Offshore Foundations for Wind Turbines
Don't miss the "Offshore Foundations for Wind Turbines" Conference, taking place 4-6 July 2011 at the Swissôtel Bremen, Germany.
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