Coupled hydrodynamic-structural modeling of marine growth effects on fatigue life of floating offshore wind turbine pontoons

Marine growth (MG) accumulation on floating offshore wind turbine structures poses a significant challenge to fatigue life estimation because it modifies hydrodynamic loading, increases drag forces, adds biological mass, and accelerates structural degradation at fatigue-sensitive welded joints. To address this problem, this study develops a coupled numerical framework combining time-domain hydrodynamic-structural simulations in BLADED with rainflow-based fatigue analysis to evaluate the representative FLS-based fatigue effects of varying MG thickness and density across six pontoon-column welded connections. Five MG conditions, including a clean-surface case and four progressive MG stages from MG-1 to MG-4, were analyzed using 600 s simulation blocks, equivalent stress ranges, cycle counts, S-N curve evaluation, and Miner’s rule-based damage accumulation. The results show that MG-4 increased the equivalent pontoon diameter from 7.000 m to 7.084 m, the drag coefficient from 0.70 to 1.32, the inertia coefficient from 1.00 to 1.16, and the added MG mass per unit length to 1728.3 kg/m. These changes reduced the average fatigue life of the six welded connections from 19.58 years under the clean condition to 15.73 years under MG-4, while the most critical connection, C6, decreased from 16.90 years to 11.70 years. The 600 s simulation-duration check showed that the normalized fatigue damage-rate remained within approximately ±1% of the 1000 s reference window, supporting its use for comparative MG-induced fatigue assessment. The findings provide a quantitative basis for incorporating MG-modified hydrodynamic parameters, critical-joint fatigue sensitivity, and MG-aware inspection or maintenance planning into floating offshore wind platform design.