LEFM-Based Evaluation of Structural Stress Approaches for Fatigue Assessment of Welded CHS with Large Geometric Scale Differences

Fatigue assessment of welded offshore structures commonly relies on hot-spot stress (HSS) approaches. However, pronounced geometric scale differences between monopile shells and welded secondary attachments, such as boat landings, may place these joints outside the applicability ranges of established tubular-joint recommendations. This study compares five structural stress approaches for single-sided welded circular hollow section (CHS) joints: IIW plate extrapolation, IIW/CIDECT and DNV-RP-C203 tubular-joint extrapolation, the Xiao–Yamada (XY) 1 mm stress approach, and Dong’s structural stress method. Chord-side stress concentration factors (SCFs) were determined using linear-elastic finite-element analyses within a Design of Experiments (DoE) framework. Response surface models (RSMs) were developed to quantify the influence of geometric parameters and their interactions across loading conditions and evaluation positions. An analytical linear-elastic fracture mechanics (LEFM) model provided an independent computational reference for geometry-dependent fatigue strength. By transforming LEFM-derived nominal fatigue strengths using method-specific SCFs, the ability of each stress definition to account for geometric effects was assessed through correlation and residual scatter. Substantial differences between the approaches depended on joint geometry, loading condition, and evaluation position. Chord loading was mainly influenced by thickness ratio and brace slenderness, whereas brace loading additionally depended on chord slenderness and diameter ratio. The XY method showed the strongest agreement with the LEFM-based reference and the lowest residual scatter, followed by Dong’s method, whereas conventional surface-extrapolation approaches exhibited greater scatter. These findings highlight limitations of directly transferring established CHS-based extrapolation procedures to monopile attachments with pronounced geometric scale differences.