Selection, Design and Construction of Offshore Wind Turbine Foundations

In the past twenty five years, European nations have led the way in the development of offshore wind farms. However, development in offshore wind energy is picking speed in other continents as well. More recently, there has been explosive growth in investment in the clean energy sector, with onshore and offshore wind power taking by far the largest share of that investment. About 50 billion US dollars were invested each year since 2007. Although economic crises may have impeded investment in 2010. In the last few years nearly 30 to 40 percent of all new installed power generation capacity in Europe and the United States is attributed to wind energy. The European Wind Energy Association estimates that between 20 GW and 40 GW of offshore wind energy capacity will be operating in the European Union by 2020. The US Department of Energy predicts that 50 GW of installed offshore wind energy will be developed in the next 20 years (NWTC, 2006). This means at least US$100 billion of capital investment with about US$50 billion going to offshore design and construction contracts. In the United States, offshore wind power development has not been a focus area because there is great potential for wind power on land. However, high quality onshore wind resources are mostly located in the Midwest and Central United States while the demand centers are located along the coasts, thereby making the cost of transmission high. On the northeast coast of the United States, offshore development is an attractive alternative because electricity costs are high and transmission line construction from the mid-west faces many obstacles. Higher quality wind resources, proximity to coastal population centers, potential for reducing land use, aesthetic concerns, and ease of transportation and installation are a few of the compelling reasons why power companies are turning their attention to offshore development. Offshore turbines are being made larger to economize in the foundation and power collection costs. As the technology for wind turbines improves, the industry has developed wind turbines with rotor diameters as large as 150 m and power ratings of over 7.5 MW to 10 MW. As increasing number of wind farms are being planned 15 to 50 km from shore in water depths of over 50 m, the combination of water depth, the increasing wind tower heights and rotor blade diameters create loads that complicate the foundation design and consequently place a greater burden on the engineer to develop more innovative and cost-effective foundations and support structures. Moreover, offshore foundations are exposed to additional loads such as ocean currents, storm wave loading, ice loads and potential ship impact loads. All of these factors pose significant challenges in the