Optimized design of cyclically loaded ring flange connections in wind turbine support structures, taking sustainability considerations into account

Short title: Opti Flansch WEA

Funding reference number: FOSTA P 1798 / IGF-Nr. 01IF24923N

Project period: Apr 2026 – Sept 2028

Project budget: 238.672 €

Research focus:

Focus 2 (since 02/2024): Sustainable Objects, Urban Structures, Materials and Processes

Structural unit:

Faculty of Engineering

Project management:

Prof. Dr.-Ing. Ralf Glienke

Project supporter:

German Aerospace Center (DLR)

Funding:

Federal Ministry for Economic Affairs and Energy (BMWE)

Collage zu Windenergieanlagen mit Detailbildern zu Schrauben und Rohrkörpern — From left to right: offshore wind turbine, monopile, welded ring flange, schematic diagram of a ring flange connection, ring flange connection
Sources from left to right: orsted.de/offshore-windenergie/unsere-offshore-windparks-nordsee, eew-group.com/de/projekte-referenzen/erfolgsmeldungen/news-detail/all-monopiles-for-sofia-shipped/, euskalforging.com/en/news/200-euskalforging-breaks-world-record-supplying-10-000mm-80t-input-weight-ring-world-s-most-powerful-quiet-hammer, Gregor Steglich, Hochschule Wismar, J.B.O

The objective of this research project is to investigate the fatigue behavior of ring flange connections subjected to cyclic loading, with a view to enabling optimized design.

As modern onshore and offshore wind turbines continue to grow in size, the SCHMIDT/NEUPER method for fatigue analysis of ring flange connections is increasingly reaching the limits of its applicability when it comes to safely dimensioning the bolts. With the development of powerful FE models, analyses are increasingly being performed for arbitrary flange configurations, taking into account characteristic flange gap imperfections that contribute to the normal and bending stresses acting in the preloaded bolted joint. If the prevailing normal and bending stresses are fully accounted for on the load side during fatigue verification of the bolt’s fatigue strength under pure tension, current findings reveal safety margins that have not been utilized in fatigue verification to date.
Verification based on the normal forces acting in eccentrically loaded bolted joints against a detail category that includes bending effects on the load-bearing side cannot be performed in general, or can only be performed within certain limits. In addition to classic design details, such as bolts, circular, longitudinal, and fillet welds for fastening components to so-called welded sockets, there are other design details that cannot be designed according to existing codes, or can only be designed very conservatively. These are ring flanges made of structural steel (S355N), which are typically normalized after manufacture. For the flange radius, existing regulations do not require a fatigue strength verification, provided geometric boundary conditions are met. Here, too, the increasing size of modern wind turbines is having an impact, so that verification is increasingly being required for this construction detail as well, and cannot be provided—or can only be provided very conservatively—using the current state of the art.
The research project aims to investigate the fatigue behavior of the two structural details (eccentrically loaded bolted joint and flange radius). Furthermore, a comprehensive characterization of the materials used will be conducted. Based on these investigations, a design approach for the structural details is to be developed as an enhancement of the existing standards (Eurocode 3, IEC 61400-6).