Inclined pullout performance of helical anchors for floating offshore wind turbines: A geometry-driven mechanism shift

Helical anchors serve as critical mooring foundations for floating offshore wind turbines (FOWTs) under complex inclined pullout loads. However, the effect of anchor geometric stiffness on the inclined load-bearing mechanism remains poorly understood. This study employs three-dimensional finite element analyses to investigate the inclined uplift behavior of large-diameter single-helix anchors in sand, isolating the helix-to-mandrel diameter ratio ( λ = D h / D c ) as the core geometric variable. Results reveal that λ governs the load-bearing mechanism, driving a transition from lateral-resistance dominance to axial end-bearing dominance. For small diameter ratios (e.g., λ = 1.2 ), the relatively stiff mandrel undergoes near-rigid-body rotation and mobilizes an asymmetrical passive soil wedge. This response produces an “inclination gain,” increasing the ultimate pullout capacity by up to 18% at a loading inclination of 45°. Conversely, large diameter ratios (e.g., λ = 5.0 ) exhibit an “inclination decay” of up to 28% at 60°, associated with pronounced mandrel flexure and localized shallow soil deformation. Based on the observed kinematic responses and contact stress distributions, a favorable diameter-ratio range of λ = 1.2 – 1.5 is identified for mooring foundations subjected to 30°–60° inclined loading. This range promotes the mobilization of deep soil resistance while limiting excessive mandrel flexure, providing a useful design reference for large-diameter helical anchors in offshore mooring applications.