ORIGINAL RESEARCH ARTICLE Fragility analysis of an ageing monopile offshore wind turbine subjected to simultaneous wind and seismic load Baran Yeter 1 & Mesut Tekgoz 1 & Yordan Garbatov 1 & Carlos Guedes Soares 1 Received: 6 September 2019 /Revised: 28 January 2020 /Accepted: 31 January 2020 # Springer Nature Switzerland AG 2020 Abstract The loads induced as a result of seismic activities may jeopardize the serviceability of offshore wind turbines or may even lead the structure to reach the ultimate strength. The maximum load-carrying capacity of a support structure can be estimated by performing a structural assessment which accounts for the nonlinear effects arising from the material and geometry. The present work aims to analyze the fragility of a 5 MW monopile offshore wind turbine structure subjected to seismic activities accounting for soil interactions and time-variant structural degradation. The offshore wind turbine structure is subjected to different ground motions with different intensity. The nonlinear full transient dynamic structural analysis is carried out based on the finite element method, and the nonlinear monopile structural response during the different seismic activities is discussed. Finally, the fragility curves associated with the serviceability limit state design and the ultimate strength limit state are developed. Keywords Fragility curve: Nonlinear dynamic analysis: FEM: Offshore wind turbine: Seismic load: Soil-pile interactions Introduction The majority of the fixed offshore wind turbines, currently oper- ating in Europe, have been installed in the regions where no major earthquakes are expected. Therefore, the structural re- sponse and the safety of the offshore wind turbine (OWT) under seismic activities have not been a subject of many studies. It can be stated that performing the structural design based on a rare earthquake can lead to structural overdesign since such major earthquakes occur quite rarely. However, the earthquakes do oc- cur, and the decision-makers need appropriate tools to know how to operate when such hazardous events occur as the serviceability can be at risk under a seismic event. Furthermore, there are several regions in the USA, Japan, and Southeast Asia that are quite resourceful in terms of wind energy and these regions are subjected to a significant seismic hazard. The fragility analysis has been commonly used to evaluate the structural safety of the structures that are subjected to seismic loads. The fragility analysis varies due to the use of different numerical modelling techniques. The detailed numerical models are found to be computationally expensive and unpractical from the probabilistic analysis point of view. The present study acknowledges the gap stated above and aims to perform a comprehensive assessment of a fixed off- shore wind turbine subjected to seismic activities. It is targeted to have a numerical model, which evaluates the nonlinear structural response accurately for ageing OWTs. The maximum load-carrying capacity of a support structure is estimated by performing a nonlinear finite element assessment. Moreover, the developed numerical model needs to account for the soil-structure interactions (SSI) and the time-variant structural degradation. In the present study, the fragility curves are derived based on the nonlinear dynamic finite element (FE) analysis. A sophisticated FE model is developed, taking into account non- uniform corrosion degradation, imperfections and SSI. The FE model of the monopile OWT structure is subjected to many different ground motions with different intensity. Asareh et al. (2016) performed the fragility analysis of a 5- MW NREL wind turbine considering aero-elastic and seismic interaction using FEM. The study emphasized the importance of developing the fragility curves based on the nonlinear dy- namic FE analysis. It is stated that the interaction between the wind and earthquake, and its impact on wind turbines are not yet well understood. Most of the research in this field con- siders multi-body dynamic linear models with limited degrees of freedom without considering the earthquake excitation. * Yordan Garbatov yordan.garbatov@tecnico.ulisboa.pt 1 Centre for Marine Technology and Ocean Engineering (CENTEC), Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal Safety in Extreme Environments https://doi.org/10.1007/s42797-020-00015-9