Engineering Approach to Designing New Solutions for FoundationsWith the increase in average turbine size witnessed by the global wind industry over the years, foundations have also become enormous. Therefore, innovation and expertise are crucial to remain competitive in an industry where optimisation is essential in order to reduce the costs associated with foundations.
By Alexander Martin, CEO, CTE Wind, France
Wind Turbine Generator Foundation Design Principles
The engineering approach of designing foundations has greatly evolved since the industrial take-off of the sector. In 20 years, the foundation diameters have increased from 10 to 27 metres. The latest wind turbines transfer colossal loads to the tower, which are then transmitted through the foundation into the ground. The international guidelines and regulations have also progressed, becoming more complex. Foundations are not designed in the same way as they used to be. Another aspect to take into consideration is the local standards. For instance, a foundation for a particular turbine type with similar soil conditions would be different in Argentina compared with France solely due to the regulations in force. Other aspects include seismic requirements, liquefaction and pile solutions, to name a few.
The engineering approach of designing foundations has greatly evolved since the industrial take-off of the sector. In 20 years, the foundation diameters have increased from 10 to 27 metres. The latest wind turbines transfer colossal loads to the tower, which are then transmitted through the foundation into the ground. The international guidelines and regulations have also progressed, becoming more complex. Foundations are not designed in the same way as they used to be. Another aspect to take into consideration is the local standards. For instance, a foundation for a particular turbine type with similar soil conditions would be different in Argentina compared with France solely due to the regulations in force. Other aspects include seismic requirements, liquefaction and pile solutions, to name a few.
Several criteria influence the choice of foundation to be used. Geotechnical investigations reveal the nature of the soil, providing a first set of foundation solutions that can adapt to the site. But the type of wind turbine also has a decisive influence. Firstly, the loads specific to each machine will have an impact on the sizing of the foundation and the type of connection (i.e. anchor cage) between the tower and the foundation. Hybrid concrete and steel towers require, for example, concrete sections to be prestressed by cables. A cavity is then needed in the heart of the foundation to install the anchors.
Foundation Solutions
CTE Wind is constantly developing new solutions to reduce the concrete volume required – Star Foundation and Soft-Spot designs for instance. Both are particularly useful in soils with buoyancy (water pressure), where the foundations are heavier in order to withstand the upward forces and to ensure stability. Star-shape foundations appeared in the market a while ago without major success. This is probably due to the fact that turbines, and subsequently foundations, were too small to make them worthwhile. Nevertheless, the star shape allows a saving of concrete of 40% per foundation. This type of solution consists of a thin concrete base with star-shaped elevation on top, and backfill is compacted between the ribs. However, the formwork to form the ribs of the star is more complicated than that of a conventional shallow foundation. Savings are due to concrete volume reductions but expenditures increase on formwork. It is therefore necessary to perform a comparative analysis at the predesign stage, but this is not always easy in countries where concrete is affordable. In Europe, it is on a case-by-case basis. In Latin America, on the other hand, the calculation is quite different since concrete is usually twice as expensive as in Europe and the cost of labour is lower.
CTE Wind is constantly developing new solutions to reduce the concrete volume required – Star Foundation and Soft-Spot designs for instance. Both are particularly useful in soils with buoyancy (water pressure), where the foundations are heavier in order to withstand the upward forces and to ensure stability. Star-shape foundations appeared in the market a while ago without major success. This is probably due to the fact that turbines, and subsequently foundations, were too small to make them worthwhile. Nevertheless, the star shape allows a saving of concrete of 40% per foundation. This type of solution consists of a thin concrete base with star-shaped elevation on top, and backfill is compacted between the ribs. However, the formwork to form the ribs of the star is more complicated than that of a conventional shallow foundation. Savings are due to concrete volume reductions but expenditures increase on formwork. It is therefore necessary to perform a comparative analysis at the predesign stage, but this is not always easy in countries where concrete is affordable. In Europe, it is on a case-by-case basis. In Latin America, on the other hand, the calculation is quite different since concrete is usually twice as expensive as in Europe and the cost of labour is lower.
Soft-Spot FoundationThis solution was developed by CTE Wind’s engineering team (patent and trademark pending) and implemented recently in Thailand and other countries such as Mexico, Finland, France, Spain and Argentina. One of the requirements for this kind of solution is to have relatively good soil because the foundation will behave in a ring shape. This means that stresses will be higher in the outer area and the stresses in the middle section covered by the Soft-Spot are reduced to zero. For example, Figure 1 shows a comparative analysis of a conventional foundation compressed (in contact) with the soil presenting gapping and the Soft-Spot without gapping, both taking buoyancy into consideration.
Moreover, Figure 2 shows an example where the Soft-Spot has 135kPa (maximum stress – higher loads concentrated on the ring area) as opposed to the 115kPa of the traditional foundation when stresses are distributed in a larger area. This saves several cubic metres of concrete compared with a traditional shallow foundation. The figure shows the finite element method diagram for this solution, with the actions in the middle section set to zero.
In terms of the benefits, the traditional shallow foundation initially proposed in a recent project had a concrete volume of 1,192 cubic metres and a diameter of 27.4 metres. The Soft-Spot solution allowed a significant reduction, leading to a concrete volume of 980 cubic metres and a diameter of 24.5 metres. A central space in the lower area of the foundation is needed for a soft material such as EPS foam. This may have a variable diameter of 7 or 13 metres (depending on the project), but CTE Wind has demonstrated that the ring foundation, due to eccentricity determination, takes off from the ground at a later stage as opposed to a conventional foundation slab.
Other Aspects of the Soft-SpotFollowing OEM requirements, the Soft-Spot complies with the dynamic rotational stiffness stipulated in the specific technical specification according to the wind turbine chosen. In addition, the Soft-Spot material (EPS foam for instance) is available worldwide, which makes it easier for contractors to purchase the material locally. The Soft-Spot is a solution proposed on a case-by-case basis that must be compared with others at the predesign stage. The client will then have all available information to choose the best solution following the company’s criteria. It is important to state that the construction sequence is very similar to that of a shallow foundation. Therefore, time delays in its construction remain the same. Some of the cost savings in choosing the Soft-Spot include the following: excavation, backfill, concrete and steel reductions.
Conclusion
As wind turbine technology advances steadily, constant innovation is required when dealing with foundations. The Soft-Spot is a solution that fits well into this dilemma, with optimised materials and continued simple construction. Developing new foundation solutions is therefore necessary to keep up with the speed of the latest turbine models.
As wind turbine technology advances steadily, constant innovation is required when dealing with foundations. The Soft-Spot is a solution that fits well into this dilemma, with optimised materials and continued simple construction. Developing new foundation solutions is therefore necessary to keep up with the speed of the latest turbine models.
Biography
Alexander Martin is a civil engineer who graduated from the University of Stuttgart with more than 15 years of experience in the wind energy sector. He currently holds the position of CEO at CTE Wind, a structural engineering office specialising in Wind Turbine Generator Foundation Design.
Alexander Martin is a civil engineer who graduated from the University of Stuttgart with more than 15 years of experience in the wind energy sector. He currently holds the position of CEO at CTE Wind, a structural engineering office specialising in Wind Turbine Generator Foundation Design.
