This study investigates the behavior of interacting surface cracks at the circumferential weld toe of monopile-supported offshore wind turbines. Relying on a numerical model that explicitly considers weld profiles, we explore the impact of crack interaction and loading scenarios on crack propagation. Our findings reveal that, initially, surface cracks grow independently, resembling single crack behavior. However, a pronounced interaction effect accelerates their growth as cracks propagate further, potentially leading to crack coalescence, high stress intensity factors, and reduced fatigue life. Consequently, this work highlights the need for integrating specific weld geometry representation in numerical models, as neglecting this can lead to significantly inaccurate fatigue life estimates in typical practical applications. Moreover, this study points out the challenge in accessing representative crack growth material parameters, vital for accurately evaluating the fatigue life of structural connections in offshore wind turbines.
The structural integrity of marine structures and other engineering systems is adversely influenced over their operational lifespan by deterioration mechanisms and mechanical stressors. In harsh marine environments exposed to corrosive agents such as saltwater and atmospheric contaminants, welding defects found in marine structures steadily propagate under cyclic loading induced by waves and other dynamic forces, making fatigue one of the main structural failure modes. In marine and offshore engineering communities, the fatigue assessment of welded structures is usually performed by relying on nominal stress S-N curves (DNVGL, 2016), yet the applicability of such S-N curves is constrained to structural configurations and loading conditions analogous to those in the original experimental setup.
A comprehensive modeling strategy entails the adoption of a linear elastic fracture mechanics (LEFM) approach for predicting the fatigue life of welded structures, particularly in scenarios that deviate from the experimental specimens used to establish conventional S-N curves. The crack growth modeled via LEFM is mainly governed by the stress intensity factor (SIF). This factor represents the stress field near the crack tip, derived from the crack size/geometry and applied loading. The fatigue crack growth of welded offshore steel structures can be modeled by a power law that establishes the relation between crack propagation rate, da/dN, and SIF range, ΔK (DNVGL, 2016; DNVGL, 2019). Naturally, suitable SIF solutions should be computed to yield accurate fatigue life predictions.