Investment allocation for offshore wind turbines (OWT) as an important class of structures is typically carried out through supporting decision-making approaches utilizing some fragility functions. This study attempts to deliver fragility functions for OWTs on monopile foundations accounting for soil-structure interaction (SSI) effects. Simultaneous wind, wave, and earthquake loads were considered probabilistically by adjusting their occurrence hazard levels for predefined damage states in diverse performance levels. The designated damage states in this study are defined based on collapse probability and some targeted performance levels which could be very straightforward to distinguish. The damage state detection is based on rotation in the connection of the tower's transition part to the foundation, which perceptibly reveals the effects of SSI on fragility functions. The expected results comprise modified fragility functions accounting for SSI effects contributing to less median spectral acceleration, more evidently rotational demands, further dispersions, and a subsequent dominant increase in the probability of exceeding performance limit states. Considering operational performance level, the most applied design performance level for turbines as an important class of structures, not considering the SSI effects could noticeably underestimate the demands and lead to high-risk decisions.

Offshore wind turbines (OWTs) often operate in complex marine environments, where they are not only subjected to wind and wave loads, but also adversely affected by scour. Therefore, it is of great significance to explore the effect of scour on the dynamic responses of OWTs under external loads to ensure structural safety, improve performance, and extend service life. In this study, a comprehensive numerical model of a 5-MW OWT, including tower, monopile, and soil-structure interaction (SSI) systems, is established by using ABAQUS platform. Aerodynamic loads is generated using blade element momentum, while wave loads is generated using the P-M spectral. The depth of scour is obtained based on on-site measured data. A comparative analysis is conducted between fixed foundations and SSI systems when conducting dynamic response analysis of OWTs under wave loads. Subsequently, the effect of scour on dynamic responses of OWTs under aerodynamic and wave loads is investigated. Results demonstrate that SSI can significantly influence the natural frequency and dynamic responses of the OWT. Therefore, it is essential to thoroughly consider SSI when evaluating the dynamic response of the OWT with local scour. The tower-top displacement and acceleration of the OWT with show a significant increasing trend compared to the non-scoured OWT. An increase in scour depth leads to higher maximum stress and stress amplitude in the steel monopile, which could potentially cause fatigue issues and should be given due attention.

The safety and reliability of wind turbines subjected to multiple loads has recently attracted great attention. To investigate how soil-structure interaction (SSI) affects the seismic performance of operating wind turbines, a wind tunnel-shaking table test platform has been built which can realize applying wind load and seismic load simultaneously. A 1:100-scaled wind turbine model with two different kinds of foundations (soil-structure interactive foundation and rigid foundation) has been tested under four seismic excitations (El-Centro and Taft, input in two directions) when it keeps operating in wind excitations. Nacelle displacement with the soil-structure interactive foundation was significantly larger than that of the rigid foundation, reaching 3-5 times at a peak ground acceleration (PGA) of 0.8 g. The maximum nacelle displacement of the scaled model with soil-structure interactive foundation always occurs in the direction of incoming wind, unlike the rigid one occurring in the direction of seismic input direction. The rigid foundation model presents a strong whipping effect with an acceleration amplification factor of the tower top (APF = 3). In contrast, the model with soil-structure interactive foundation shows mild whipping effect due to smaller foundation stiffness (APF of the tower top = 1.5). The study demonstrates SSI could weaken dynamic responses, reducing bending moments but inducing excessive nacelle displacement, risking structural damage. This study underscores the importance of SSI in evaluating the safety and reliability of wind turbines subjected to the wind and seismic loads and provides experimental results for future designs.