The base of a wind turbine anchors the massive tower and rotor assembly to the earth or seabed. This foundation must resist tremendous forces, especially the overturning moment created by high winds pushing on the tower and blades. The size and depth of the base are not standardized; they are highly variable dimensions determined by rigorous engineering analysis based on the turbine’s performance requirements and the installation site conditions.
Variables Influencing Foundation Requirements
The dimensions of a wind turbine foundation are calculated based on several interrelated factors, beginning with the size and power of the turbine itself. A larger turbine, such as a 5 megawatt model, generates substantially greater loads that must be transferred to the ground, requiring a larger and heavier foundation to maintain stability.
The height of the tower is a major influence because it increases the leverage, or overturning moment, exerted on the base. Taller towers are subject to greater bending forces, necessitating a wider and deeper foundation to counteract the increased leverage from the wind. Engineers also account for the maximum wind speed loads the turbine is designed to withstand, including extreme weather events.
Site-specific geotechnical requirements are equally important, as the foundation relies on the soil beneath it. Detailed surveys assess soil properties like type, density, and load-bearing capacity. Foundations placed on weak or compressible soils, such as clay or loose sand, must be significantly larger or deeper than those installed on stable bedrock to prevent excessive settling. The soil’s shear strength dictates how much the foundation needs to spread the load across the surface area.
Onshore Base Construction and Typical Footprint
The most common foundation type for utility-scale onshore wind turbines is the gravity-based foundation, which is a large, reinforced concrete slab. This design relies on its massive weight and broad footprint to resist the powerful overturning forces generated by the turbine. The foundation acts as a counterweight, distributing vertical and horizontal loads across a wide area of the ground.
For a modern utility-scale turbine, the visible base diameter of this concrete slab typically ranges from 50 to 80 feet, depending on the turbine size and soil conditions. The excavation depth and the slab itself often extend between 8 and 12 feet below the surface. A central pedestal, typically 4.5 to 5.5 meters in diameter, rises from the slab to connect directly to the bottom flange of the tower.
The scale of these foundations requires significant materials for stability. A single gravity base often requires hundreds of cubic meters of concrete, heavily reinforced with steel rebar to provide the structural integrity needed to manage the stresses of the turbine’s movement. In locations with strong soil or shallow bedrock, a rock-anchored foundation may be used. This system ties a smaller concrete cap directly to the rock layer using post-tensioned steel tendons, improving stability without relying entirely on mass.
Offshore Foundation Systems
Offshore wind farms require different foundation systems due to deep water and dynamic marine environments. The choice of foundation depends heavily on the water depth and specific seabed conditions. Fixed-bottom foundations are the primary solution for shallower waters, generally up to 60 meters deep.
The monopile is the most frequently used fixed-bottom foundation, especially in water depths up to about 55 meters. This system consists of a single, large-diameter steel tube driven deep into the seabed, sometimes 30 meters beneath the seafloor. Although the footprint of the steel tube on the seabed is small compared to an onshore gravity base, the overall structure is a massive steel cylinder.
For medium water depths, typically 30 to 100 meters, jacket foundations are often employed. These are lattice-like steel structures secured to the seabed with multiple piles, providing a stable base. Deep waters, generally beyond 60 meters, necessitate the use of floating foundations. These buoyant platforms are anchored to the seabed with mooring lines, and while they lack a fixed base footprint, their mooring systems cover a large area to secure the floating assembly.