Key milestones in wind farm development

The development of a wind farm project is characterized by several milestones linked to contractual obligations of the parties.

The most relevant milestones are marked below. They are in chronological order, and some of them could not appear in a specific project (for instance, not all projects have a limited notice to proceed or the payment milestones can vary a lot from project to project).

Limited notice to proceed (LNTP): in this milestone an agreement is reached between the party to perform some works (for instance, to purchase some long lead time item, such as the substation main transformer). It makes sense when it’s necessary to accelerate the project for some reason.

Advance payment: this is a down payment paid by the customer before the start of the works. It is usually done to secure the production slot of the turbines

Commencement date: in this milestones, all condition precedents are met and the contract is activated.

Payment milestones: every contract has different payment milestones (and different percentage of payment associated with each milestone). However, some “standard” milestones are marked below to give you an idea of how they could look like:

  1.                WTG ex works
  2.                WTG shipped
  3.                WTG erected
  4.                WTG commissioned

Mechanical completion: in this milestone, a certificate is issue stating that the wind turbine has been erected following the relevant technical specifications and it’s ready to start Commissioning

Commission certificate: at the end of commissioning (a set of test done to confirm that the turbine is ready for production) the turbine is ready to start trial operations. A certificate is issued to formalize this fact.

Taking over certificate (TOC): this milestone is usually linked to transfer of risk and beginning defects liability period for a specific turbine (the “defects notification period”).

Provisional acceptance: from this point in time, usually an Operation and Maintenance contract for the wind farm start.

Final acceptance: in this milestone the customer formally accept that the wind farm is complete, fully operational and compliant with the relevant technical specifications.

Strength in Numbers: the multi rotor, 12 blades turbine concept

Picture courtesy of

The idea is not new: for instance around 1800 some “twin” mills have been used to pump water in Denmark and there are quite a few similar concepts and international patents dating the first half of 1900 - see for instance the drawing  below.

What is interesting here is the size of the company testing the idea – Vestas has installed approximately 1 year ago a multi rotor wind turbine, using old refurbished V29 nacelles equipped with new sensors and electronics. Blades tip distance is less than 2 meters.

I’m sure that people who think that wind turbines are ugly will think that this solution is atrocious. For me it’s a quite interesting “out of the box” exercise in a business where rotor diameter has been relentlessly growing in the last decades (V29 is from the ’90… twenty-something years later we have V112 and bigger models).

The benefits are self-evident: greater swept area, saving in tower and foundation, less land use, etc.

The challenges are equally impressive: a whole new set of loads to consider, a system that can easily become unbalanced, increased turbulence and so on. It's early to say if it will ever become commercially viable but it is for sure an interesting experiment.