PSI - Issue 75

Ralf Glienke et al. / Procedia Structural Integrity 75 (2025) 474–488

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Ralf Glienke et al. / Structural Integrity Procedia 00 (2019) 000 – 000

1. Introduction The costs for the tower and foundation of modern wind turbines make up large proportion of the total costs. Due to the increasing dimensions and longer operating lifetimes, the fatigue resistance of all constructional details is becoming an economic factor. The design of towers for onshore and offshore application is carried out based on Eurocode 3 (for fatigue: EN 1993-1-9 (2010)). The first generation of Eurocodes represents a frozen state of the art from the mid-eighties. The second generation of Eurocode 3 (FprEN 1993-1-9 (2024)) is expected to be published very soon, but in terms of ease of use, many regulations represent an unsatisfactory design approach for the wind industry. Post-weld treatments such as TIG-dressing or burr-grinding are not included. Butt welds ground flush to surface are still classified in detail category (DC) 112 (m 1 = 3) and non-welded constructional details are still being assessed independently of the material strength. The fatigue strength verifications for flange connections and bolts, for example, have also been fundamentally revised in IEC 61400-6/AMD1 ED1 (2024). Other standards have been further developed regarding welded constructional details and post-weld treatments (IIW Recommendations (2024), DNV-RP-C203 (2024)). Therefore, the EC 3 verification is supplemented by certified detail categories based on experimental studies and additional analyses based on local approaches (effective notch stress approach, fracture mechanics, or two-stage model to consider crack initiation and crack propagation phase). Towers and foundations, which are usually made of structural steel S355, are blast-cleaned in preparation of the coating work. In addition to welded constructional details (circumferential butt welds, fillet welds on attachments), non-welded constructional details are present. This includes drilled holes in the tower wall, through which hazard lights are installed for night visibility signalling, as well as free edges around door openings or cut outs for cable entries (Fig. 1). For the paper, the question arises as to how the blast-cleaning treatment affects the fatigue strength.

Fig. 1. Welded and non-welded constructional details in wind turbine towers

2. State of the Art 2.1. Fatigue strength verification from several guidelines

In order to ensure efficient design verification, the nominal stress concept applies to most components for wind turbine towers, which assumes that the relevant constructional details are included in the catalogue with classified detail categories. For instance, around door openings or planned offsets of the centre lines of circumferential butt welds, the verification is carried out also using modified nominal (SCF∙Δ  n ) or hot spot stresses (Δ  hs ) . The Δ  - concept is therefore used by the relevant design codes (EN 1993-1-9 (2010), IIW Recommendations (2024), DNV RP-C203 (2024), BS 7608 (2014), EN 13001-3-1 (2019)), whereby the DC is assumed to be independent of mean stress and that scale effects and residual stresses are adequately considered in addition to the weld seam and component geometry. For transverse loaded butt welds (fully penetration), DC 90 (m 1 = 3) generally applies for steel grades S235 to S690 (or S960), whereby an execution for quality level B according to ISO 5817 (2023) is assumed regardless of the welding process (manual or fully mechanised). For butt welds ground flush to surface, the characteristic fatigue