Effect of scour- and cyclic loading-induced soil-monopile stiffness degradation on long-term dynamic response of support structures for OWT

Offshore wind power has expanded rapidly, making the long-term safety and stability of offshore wind turbine support structures a key concern in design and maintenance. Changes in structural natural frequencies can trigger resonance if they enter the 1P or 3P rotor excitation bands, accelerating fatigue damage. In marine environments, support structures are subjected to seabed scour and cyclic loading from wind, waves, and currents, which alter natural frequencies by degrading foundation stiffness. To evaluate these effects, this study established a soil-structure interaction model coupling scour and cyclic loading to analyze variations in foundation stiffness and natural frequencies. Results indicate that both scour and cyclic loading reduce ultimate soil resistance and foundation stiffness, causing structural natural frequencies to decrease. These effects become more pronounced under coupled effects, especially general scour with cyclic loading. Scour most significantly affects the second natural frequency, whereas cyclic loading has a greater influence on higher-order natural frequencies. The combined action produces a greater effect than either factor acting alone, while direct addition of the two individual effects overestimates the coupled response. This nonlinear response highlights the necessity of coupled modeling and provides a baseline for the design, operation, and maintenance of offshore wind turbine support structures.