Offshore wind support structures – Evaluation of size effects and influencing factors on the fatigue performance of butt-welded joints using an updated database

Offshore wind turbine support structures, such as monopiles, require high fatigue resistance at welded connections, particularly around the circumferential welds. This paper reviews the influence of plate thickness on the fatigue performance of welded butt joints and presents a statistical evaluation of relevant fatigue test data. The analysis combines the European fatigue database (DASt Database) with newly available data related to welded offshore wind structures to ensure representativeness for modern monopile structures. The unfiltered European fatigue database contains more than 4700 data points for butt welds, but its heterogeneity in materials, welding processes, and specimen geometries produces a large scatter. After filtering for offshore wind-relevant conditions, by including thick plates ( t ≥ 25 mm), various sub-grades of S355 structural steel, double-sided butt welds, submerged arc welding, axial loading condition, and load ratios of R ≥ 0 the dataset reduces drastically but yields consistent results based on relevant test data. In this subset, the characteristic fatigue resistance corresponds to FAT 90, while the inverse slope of the S–N curve increased to m ≈ 3.45, which is higher than the EN 1993-1-9 standard assumption of m = 3 but in excellent agreement with the monopile-specific D curve in DNV-RP-C203 standard. The newly added offshore wind related datasets, particularly for plate thicknesses of 40 mm and 50 mm, confirm these findings. They demonstrate a fatigue life reduction trend with increasing the plate thickness for t ≥ 25 mm, while the thickness correction formulation in EN 1993-1-9 standard has been found to produce a trend that falls at the lower bound of the scatter. Moreover, the variations in the load ratios of R ≥ 0 and the yield strength in various sub-grades of S355 steel were found to have no significant influence on fatigue resistance. Overall, the results demonstrate that targeted filtering and integration of offshore wind-specific data provide a more realistic basis for fatigue design and life assessment of monopile support structures. • An updated international database was used to assess fatigue behaviour in monopiles. • EN 1993-1-9 thickness correction is found conservative for butt-welded joints. • Filtered data for t ≥ 25 mm and R ≥ 0 in air yield an inverse slope of m ≈ 3.45. • This result aligns with DNV-RP-C203 monopile-specific D curve for air environment. • More data for t > 50 mm are needed to further refine fatigue design for monopiles.