Wind Farms construction contracts

Wind farms can generally be broken down into 2 separate areas of construction:

  • Turbines: supply, installation and commissioning of wind turbines and towers
  • Balance of Plant (BoP) works: everything else needed except the turbines (foundations, substations, cabling, etc.)

Several option are available to the developers:

BoP contracts types

The first option is that the wind farm Special Purpose Vehicle signs separate contracts for BoP and Supply and Installation:

If two or more separate contracts are signed, the interface risk is taken by developer.

Warranties are backed by the balance sheet of each contractor, clearly only for the works undertaken. In exchange for this risk taken by the developer, substantial savings are possible and a broader range of participant to the tender is possible.

Moreover, the project management will be more complicated and sometimes it is difficult to attribute the source of a delay or to define de interface between obligations.

Another option is an EPC structure:

In this case, we will have a fixed price and fixed delivery date, with no interface risk and a single point of contact for the developer.

On the downside, the price will increase as the wrapping entity will add his margin for taking the risk. Moreover, they are more complex and time consuming with an heavy legal cost.

The third option is an unincorporated joint venture:

In this case the subcontractors will delivery a project without the requirement to form a separate legal entity. There is a single contract between the wind farm developer and the consortium of suppliers.

This structure has the same advantage of the EPC (fixed price and delivery date, reduced interface risk) but additionally there is a greater security thanks to the exposure to balance sheets of all the company in the consortium.

Disadvantages: increased price, heavy documentation costs and complex coordination work.

What is this BoP?

BoP is an English acronym that stands for Balance of Plants. In the wind farms sector, it means everything but the wind turbines.

Balance of Plant

Basically there are 3 types of Wind Farm contracts commonly used:

  • Supply only. It include WTGs, SCADA, Installation supervision and Commissioning.
  • Supply and Installation, including all the items in Supply only plus WTGs transport and cranes for the installation (basically, it adds the assembly of the machine).
  • Turnkey (full EPC), including all the above plus civil and electrical works.

The sum of wind farm civil works and electrical works are usually called Balance of Plant (BoP).

Often it is done by a company different from the wind turbine supplier, and sometimes even 2 separate contractors are used, one for the civil works and one for the electrical works.

BOP civil engineering scopes of work include roads and drainage, crane pads, turbine foundation, meteorological mast foundations, cable trenches and buildings for electrical switch gear, SCADA equipment, and a maintenance/spare part facility.

BOP electrical work scopes include underground cable networks (medium voltage cables, copper cables and optical fibre cables) and sometimes even an overhead transmission lines, electrical switch gear to protect and/or disconnect turbines or other equipment from the system, and grounding and connections for control rooms, maintenance facilities and point of connection equipment to feed the wind farm’s power generation into the electrical grid. Transformers and switches for individual turbines are normally located within the turbine and they are provided by the turbine supplier.

In the EPC wind farm contract, three phases can be distinguished:

Engineering: A detailed engineering project is developed to fit the machines to the actual conditions of the site. Conditions can vary dramatically, depending on topography, geotechnical conditions, grid connection requirements, permits, local regulations an so on. In this phase technical choices are made, and drawings are produced together with the bill of quantities.

Then comes the Procurement phase, where civil and electrical works are subcontracted to one or more companies. Potential subcontractors are contacted, their quotations are received and compared.  Short listed contractors are screened and the work is finally awarded to the company with the best price/quality ratio.

Finally, the phase of Construction begin, where the works of the subcontractors are monitored to see if they comply with the technical specification and if there are delays respect to the signed chronogram.



San Anton - a Wind Farm from cradle to completion

San Anton is a small wind farm (4 WTGs) located in Lillo, Toledo (Spain).

I had the opportunity to follow this project from the beginning visiting the site very often. I collected several picture to show the main phases of a wind farm construction.

First of all a met mast is installed, and wind data is collected. If there is enought wind in the area and the investment is profitable, an authorizative project is developed and transmitted to the authorities. Normally this project is developed using existing information, that is without developing a new cartography or other field works.

When this project is approved and it start to become reality, than the first work on site is the geotechnical report, done to define the subsoil conditions trought several in situ and laboratory test: here in the picture a trial pit is open to see the materials in the subsoil

Trial pit

A trial pit opened using a backhoe

With this information and a document provided by the seller of the wind turbine with the load transmitted to the base (shear, moment and vertical loads), a preliminary (or sometimes directly a constructive project) of the foundation is developed.

Afterwards, a topographical survey is developed. This can be done using standard field topography, a flight or LIDAR (a laser system that can provide a very dense cloud of point).

The following picture show the first step of the construction phase: topsoil stripping, an operation that consist in the removal of the first layer of fertile vegetal terrain that will be stocked and piled separately and used at the end of the works to revegetate the site area.

Topsoil stripping

Topsoil stripping

The next step is what is called earthworks: the construction of embankments and cuts. For instance in the image below a crane pad is under construction. Several thousands of cubic meters can be moved during this operation.


Crane pad earthworks

In parallel, the foundation for the met mast is constructed. The mat foundation is smaller than a WTG foundations: it is around 7 x 7 meters. It has 3 pedestal where the met mast will be connected.

Met mast foundation

Met mast foundation

After, the circular holes for the turbine foundation are opened: here you can see the topographer checking the dimension and horizontality of the hole. Several days of work are necessary to open the hole, depending on the hardness of the excavated material.

Foundation hole

The internal roads are continuously washed with water, to avoid accumulation of dust in the air due to the traffic of heavy trucks and other machinery.

Roads watering

The following step is to pour the blinding concrete, to assembly the anchor cage and to position the steel reinforcement bars (rebars). More info here.

This is how it looks a completed foundation:

Meanwhile, the operation and maintenance building is built nearby. It will host the SCADA equipment and the medium voltage switchgear:

O&M Building