Onshore Wind Farm Construction

This blog post is the fifth in a five-part series related to onshore wind energy. The series covers topics including wind turbine and wind farm basics, planning and scheduling considerations for wind farms, the permitting and approval process, and construction considerations. Offshore wind facilities will be discussed in a separate blog post series.

Construction of a large wind farm has both similarities to and differences from other large construction projects. As with any large construction project, a well-planned and developed construction schedule is important for the successful completion of the project. This is best accomplished with the use of a Critical Path Method (CPM) schedule. The development of the construction schedule will utilize data from the wind farm planning and engineering schedule as discussed in the second blog post in this series.

A wind farm consists of wind turbines and other infrastructure, which is referred to as the balance of plant. A large portion of the balance of plant is civil work, which includes site grading, drainage, access roads, crane pads, and a foundation for each wind turbine. Also included in the balance of plant are the electrical transmission system within the wind farm and the interconnection transmission line to the existing electrical transmission lines. A substation (or two) may be part of the scope of work, as well as an operations and maintenance facility.

One of the unique features of wind farm construction is the size of a large wind farm. A large wind farm may include 100 wind turbines, have a material and construction cost of over $1 billion, and be as large as 50,000 acres (approximately 78 square miles). Due to the size of a wind farm, the sequence of procurement of the wind turbine components may be more important than procurement of items in other large projects. As discussed in the fourth blog post in this series, wind turbine procurement is typically done by the owner, and therefore the contractor’s schedule for construction is most likely based on the owner’s sequence of delivery of the wind turbine components.

Turbine components arrive on trucks, and several deliveries are required for each turbine. Typically, rotor blades are transported individually within the wind farm, and the tower components and nacelle may be shipped on several trucks. Before the components arrive at site for a specific turbine, the roads to the individual turbine must be completed, as well as the crane pad for the turbine that is arriving. If components arrive out of sequence, it is possible that the infrastructure may not be available to support delivery. For this reason, management of the procurement and construction schedule is paramount.

The contractor must have crews and equipment (including specialized cranes) ready at each turbine site as the components arrive. For some projects, the wind turbine components will be offloaded and stored at the wind turbine site, or components may be erected as they arrive to avoid double handling.

A brief description of each aspect of the wind farm construction is provided below.

Access Roads and Crane Pads
Typically, access roads are designed and built to facilitate transportation of the large components and to minimize disturbances on leased lands. If a development is in an area with environmental concerns, the roads will be designed and built as far from these areas as possible.

Roads within the wind farm are typically gravel and built for access to the turbines and other infrastructure. These roads must be designed to meet the load requirements specified by the turbine vendor. If there are existing access roads within the area of the wind farm, these roads are likely to be used as much as possible but may require reconstruction to handle delivery truck loads and sizes. Existing roads may be widened for construction and restored to their original condition after construction is complete.

Because the wind development may be on a large site, many miles of access roads may be required. A large wind farm may require building as many as 100 miles of access roads.

As the roads are built, crane pads at each wind turbine site are also constructed. These crane pads are built to provide space for the large crane to install the wind turbine components. After the wind turbine is erected, the crane pad is typically left in place and maintained for operational and maintenance purposes.

Wind Turbine Foundations
Due to the remote locations of most wind farms, mobile batch plants are erected to produce concrete for wind turbine foundations and for other concrete that may be necessary for the wind farm, such as the foundations and floor slabs for the operations and maintenance facility. For these batch plants, aggregate and water for the concrete are typically sourced locally. Because sites are large, batch plants may be relocated as the construction progresses and wind turbines are erected. Depending on the construction schedule, size of site, and crew availability, more than one batch plant may be utilized.

The foundations for a wind turbine are not unlike foundations for other structures and consist of concrete and steel reinforcement. The foundation design is based on the size and weight of the turbine and the loads due to the rotation of the blades. Depending on the load and geotechnical conditions, the foundations may be designed and built with piles or drilled piers.

A unique aspect of wind turbine foundation design and construction is the anchor cage system. This system was designed as a more effective method to transfer loads from the wind turbine to the foundation. The anchor cage consists of an upper and lower anchor plate, which are located at the upper and lower parts of the foundation. The two plates are connected by anchor bolts, which are housed within PVC protective pipe. During the placement of concrete, the PVC sleeve protects the anchor bolt from corrosion from the water and concrete.¹

Wind Turbine Erection
After the foundation has been cured to the requirements specified by the design engineer, the wind turbine can be erected. The tower is erected in pieces, and upon tower erection, the nacelle is installed on the tower. Typically, the nacelle arrives on site with all the interior components installed and ready for installation.

Contractors may employ different methodologies to erect the hub and rotor blades. Some contractors will install the hub and then install the blades one at a time. Other contractors may attach the blades to the hub on the ground and then lift the unit as one piece to mount to the nacelle.

In some wind turbines, the transformer is within the tower, and other turbines require a transformer to be installed at the base of the tower. If the transformer is located outside of the tower, a small concrete pad will be built for the transformer.

Each wind turbine is individually tested and commissioned as power is available. The entire system of wind turbines is further tested when all construction and infrastructure is complete.

Electrical Distribution System
The underground electrical collection lines will be installed in the vicinity of the wind turbines and will most likely be installed adjacent to access roads when possible (to avoid additional impact to the existing terrain). These collection lines may be installed above ground if the location of access roads or topography make underground installation problematic.

If a substation is required, the substation will be installed on a concrete foundation and a security fence installed around the substation.

Operations and Maintenance Facility
The operations and maintenance facility is typically a large building consisting of office space, a large garage (with capacity to perform maintenance on turbine parts), a control and data facility, and kitchen and bathroom facilities. These buildings may also include sleeping quarters for maintenance crews.

¹     “Wind Turbine Anchor Cage System,” Anyang General International Co., Ltd., accessed May 20, 2023. https://www.high-strength-steel.com/wind-turbine-anchor-cage-system.

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