Fatigue Assessment of Large-Diameter Stiffened Tubular Welded Joints Using Effective Notch Strain and Structural Strain Approach

Floating offshore wind turbine platforms typically use stiffened tubular joints at the connections between columns and braces. These joints are prone to fatigue due to complex weld geometries and the additional stress concentrations caused by the stiffeners. Existing hot-spot stress approaches may be inadequate for analysing these joints because they do not simultaneously address weld-toe and weld-root failures. To address these limitations, this study evaluates the fatigue strength of stiffened tubular joints using the effective notch strain approach and the structural strain approach. Both methods account for fatigue at the weld toe and weld root and can be applied to both low-cycle fatigue (LCF) and high-cycle fatigue (HCF) regimes. Reanalyzes of a series of fatigue-tested specimens confirm the effectiveness of both approaches. The stiffener-shell fillet weld root is identified as the most critical fatigue location, which is consistent with fractographic observations. Although the brace-to-shell weld root exhibits lower stress levels in finite element (FE) models, weld quality was determined to be a crucial factor in fatigue failure. Furthermore, the results emphasise the importance of material plasticity in the LCF regime and demonstrate that full weld penetration significantly enhances fatigue strength. These findings provide valuable insights for the fatigue design of stiffened tubular joints in floating offshore wind turbine platforms.