Windfarm Turbine Foundation Construction
There are 187,749 wind turbines with an average generating capacity of greater than or equal to 1,700kW each in one hundred and one countries (mid 2014). The average size of wind turbines is increasing and this places greater demand on the design and construction of the foundation that will support the wind turbine.
Before taking a closer look at the foundation requirements let’s consider the wind turbine generating capacity on a global basis. The relative size of wind generating capacity installed in each country can be considered. China has just over 91GW of wind generating capacity followed by the USA with 61GW of capacity. In Europe, Germany has the greatest installed wind generating capacity with 34.2GW followed by Spain and the UK with 22.9GW and 10.3GW respectively.
However, the nature of wind farms is very different when viewed from a global perspective. The map below shows that Germany has the greatest number of wind farms followed by Denmark. This indicates that these countries have more widely distributed their wind farms and rely to a lesser extent, than the USA, on using the electricity grid to distribute the power generated to the rest of the country.
Taking a closer look at Germany and the USA – shown below:
Bearing in mind the relative scale of Germany and the USA it can be seen that Germany’s wind farms are more evenly distributed around the country than in the USA. And out of interest we can see below how wind turbines are distributed by number around the UK and Ireland.
Between 1994 and 2008 much of the global growth in wind farm generating capacity was driven by Europe. From 2008 onwards China and to a lesser extent the USA started to grow very fast. The growth in Europe will continue at a high rate as European countries attempt to respect the NREAP (National Renewable Energy Action Plan) 2020 requirement for 20% of all energy generated in Europe to be derived from renewable energy sources.
Wind farm turbines are getting bigger this helps reduce the cost per MW generated as long as the weight of the wind turbine does not increase in a linear fashion with the generating capacity. Taller wind turbines access greater wind speeds. However, greater size means that the design of the wind farm turbine’s foundation becomes more critical.
Wind farm foundations
The forces that act upon the foundation are shown in the diagram below.
T = Torsion due to the rotation of the rotor assembly when changing direction, generally small for large wind farm turbines that have automated yaw control
W = Weight of the structure
Mx and My = Bending moments about the X and Y axes
Vx and Vy = Shear forces in the X and Y directions
The choice and design of a foundation depends upon the location and the size of the wind farm turbine. Location can be onshore or offshore.
Onshore the foundation design can vary from a simple reinforced concrete pad to a complex inclined piled foundation into poor quality soil without a solid bedrock. Offshore, consideration has to be given to the effect of moving water, with its much greater density than air, on the tower structure. However, on and below the seabed similar principles are applied for creating a stable foundation. In deep water there are alternative methods such as tension legs and ballast stabilized buoys with suction anchors.
Here we will look at the inclined pile foundation for a land based wind farm turbine.
In the example above the piles will be inclined at 4.47°. You can see that if the piles were extended upward they would intersect at point where the force created by the wind on the rotors meets the vertical tower. By inclining the piles in this way the bending moment experienced at the top of the pile is reduced to a minimum.
The initial hole for the pile can be drilled with either a concentric drill or an eccentric drill. Let’s consider a thick steel casing sleeve that will penetrate the soil and rest on the bed rock. In this case a drill bit with an eccentric reamer will be utilised. The eccentric reamer creates a bore hole with a diameter slightly greater than the external diameter of the steel casing. The steel casing is pulled down by the pilot drill bit. Once the drilling has reached the desired depth the drill reverses direction this retracts the reamer and the drilling tools are removed leaving the steel casing behind.
Now that the pile steel casing is resting on the bed rock it is time to install the reinforcing steel followed by concrete.
Onshore piling of foundations for wind farm turbines is a relatively new development in the UK.
There are numerous international standards governing onshore and offshore wind farms. A sample list is provided below: