Legislative Basis

BEK73, §9:

[..] a wind turbine which has been in operation for longer than its design lifetime, as stated in the manufacturer's documentation or in the certificate issued pursuant to section 5(1) and (2), shall be subject to extended service, cf. Annex 2, point 4.

BEK73, Bilag 2, pkt 4:

Service inspection of wind turbines that have been in operation for longer than their design lifetime must, in addition to the service inspection performed in accordance with the service manual, as a minimum, cover an inspection and assessment of the wind turbine's structural parts in relation to the turbine's continued operation.

The extended inspection is explained in more detail in the guidelines to this Executive Order, including examples of the scope of the service inspection, as service inspection on wind turbines older than their design lifetime depends on the condition and design of the individual wind turbine

Instruction to BEK73:

Special service inspections of wind turbines that have been in operation for longer than their design lifetime must as a minimum include the following (the items on the list must be added to the turbine's service instructions/report):


  • Inspect the machine frame for cracks in areas subject to strong loads as well as in all welds. Inspect all bolted joints.
  • Inspect the main shaft for dents and rust. The area in front of the foremost trunnion bearing is important. This area must not have dents and rust, i.e. stress raisers.
  • Inspect the yaw bearing for wear and measure the amount of play in the bearing. Inspect important parts of the yaw system.
  • Inspect the tower for cracks in all welds.
  • Tighten bolts in joints (in particular to blades) as previously in accordance with the manual.
  • Inspect the foundations for cracks in the concrete. Inspect and if necessary repair the sealing to keep water out of the foundations.
  • Inspect foundation bolts for rust and wear.

Every three years:

  • Close visual inspection of blades, possibly using camera or a photo drone/UAV and subsequent evaluation.

The extended service inspection with associated check list must be forwarded to the owner and registered in the service report. The Secretariat for Approvals will regularly update an example (which could be turbine-specific). This example can be downloaded from the website of the Energy Agency's Secretariat for the Danish Wind Turbine Certification Scheme. See www.vindmoellegodkendelse.dk for updates.

Implementation and Compliance

The above points are the legislation's minimum requirements. Service companies must, based on own experiences and, among others, information from the manufacturer's, security announcements, etc., develop and maintain their own expanded checklists for the wind turbine types they are certified to service.

A sample sheet is available under Forms.