Growth through Research, development & demonstration in Offshore Wind


Bolt & Beautiful

To reduce costs and increase bolting safety for future large wind turbines.


In a wind turbine, various parts in the tower, the connection to the foundation and the blade root connection to the hub are secured with large bolts. The size of the offshore wind turbine, monopile and blades are expected to grow strongly in the coming years. This makes it also necessary to use larger ring flange connections and possibly larger bolts. Currently, the size of the most used bolts is M72, i.e. the nominal thickness is 72 mm. However, there is reluctance within the industry to increase the bolt size to M80 or higher.

A GROW workshop on bolted connections has learned that current design procedures, manufacturing tolerances and installation, operation and maintenance procedures are insufficient to apply larger ring flanges optimally. Misalignments in the ring flanges potentially cause too large stress ranges in the bolts. There have been cases of bolt damage in existing structures due to insufficient preload and material degradation, leading to expensive repairs and rigorous inspection regimes. Several actions are required to minimise material usage in bolts and flanges and to optimise inspection intervals. However, this requires an innovative package of measures rather than individual incremental steps.

The industry has indicated a strong need to improve the assessment methods for ring flange connections, considering design, manufacturing and maintenance aspects. Optimisation means, on the one hand, using less steel and, on the other hand, leading to greater tolerances in the production of flanges and/ or less installation and maintenance effort.


In the Bolt & Beautiful project, we will reduce the costs of bolts and flanges manufacturing and maintenance and increase bolting safety both for existing and future large wind turbines. We will ensure that the installation procedures are in line with the design and production of the bolts and flanges. This will lead to the lowest possible total cost while ensuring the structural integrity of the critical components. We ensure that the chosen solutions are in consultation with and supported by relevant stakeholders.


In this project, we aim for the following results:

Contact Details


Joke Luyten
+31 88 866 1411


Richard Pijpers
+31 6 1113 5934

Technology Readiness Level

Maturity level: 5.
        5 6 7

Project duration


Foundation Installation O&M Turbine
© Gemini Windpark magnify © Gemini Windpark

© Gemini Windpark


© Gemini Windpark

Other information

This project is supported with a subsidy by the Dutch Ministry of Economic Affairs and Climate Policy. Find more project information at the TKI Offshore Energy website.


Associate project partners