Bolt & Beautiful: Development of a cost-effective set of measures to optimize inspection and use of material in monopiles

At a glance

• Flange connections are of great importance in wind energy, as they are used to assemble  important components of wind turbines. In light of the growth in the size of wind turbines, larger components not yet subjected to extensive testing are being employed, and the design, manufacturing, and maintenance costs are also increasing significantly.
• The Bolt & Beautiful project is taking a holistic approach to this problem and developing toolboxes and methods for producing flanges and studs or bolts for new support structures optimized for material use and safety as well as enabling improved service life predictions and maintenance.
• In cooperation with the Leibniz University Hannover, Fraunhofer IWES is performing tests on large-scale full-ring flange models with external loading at the Test Center for Support Structures (TTH).

The challenge

A wide variety of connections are secured with studs or bolts in a wind turbine. For example, the base of the tower and foundation are bolted together, as are the steel tower segments and the blade root to the hub. As wind turbines are reaching ever larger sizes, these connections also need to grow with them and transfer ever greater loads.

Current design methods, manufacturing tolerances, and assembly, operation, and maintenance procedures are no longer sufficient for the optimal use of even larger bolts and ring flanges. This can result, among other things, in bolt damage in existing turbines due to insufficient pretension or material fatigue, leading to expensive repairs. Other connection methods (e.g., grout/slip joints) have their own area of application with corresponding particularities and are therefore not viable as an alternative in the majority of cases.

The solution

In the Bolt & Beautiful project, the project partners are investigating and solving the size problem of large, bolted connections in a holistic approach. The aim is to reduce the costs of the manufacturing and maintenance of bolts, studs, and flanges while simultaneously improving the bolting safety for both existing and future large wind turbines.

The first part of the project involves the development of a probabilistic toolbox which enables a structural safety assessment for  new structures to be built as well as for predicting the remaining service life of existing ring flanges. The second part of the project deals with the development of measuring tools which contribute to the optimization of the preloading process by the assembly of ring flange connections and also can be employed for the evaluation of the bolt pretension of an existing ring flange during maintenance.

Fraunhofer IWES will prepare, monitor, and evaluate testing on full ring flanges with external loading at the Leibniz University Hannover’s Test Center for Support Structures (TTH). In addition, cyclic tests are also planned for determination of possible pretension losses

The added value  

The results of the project will make it possible to produce bolts and flanges for new structures in a manner optimized for material use and safety. At the same time, it will be possible to plan inspection intervals for existing constructions better and more cost-effectively. This will ensure that the technology currently in widespread use will remain cost-efficient even for larger wind turbines.

More info about the project: https://projecten.topsectorenergie.nl/projecten/bolt-and-beautiful-37060