Rapid assessment of seismic performance of large monopile-supported offshore wind turbines under scour

This study focuses on investigating the seismic performance of a reference 15 MW offshore wind turbine (OWT) supported by a large-diameter monopile, under various scour conditions. Fully coupled nonlinear finite element models considering 3D soil continuum for seismic response analysis of monopile-supported OWTs are very complicated and computationally expensive. Therefore, it is important to have good insight on the dynamic behaviour of monopile-supported OWTs subjected to earthquake excitations before performing high-fidelity fully nonlinear 3D simulations, which will not generally be practical in cases considering various seismic loading and scouring scenarios. The purpose of this study is to provide a fast and efficient tool to understand the interaction between the seismic behaviour of emerging large-capacity OWTs when scour erosion affects the dynamic properties of the structure. The dynamic stiffness method (DSM) is applied for the free vibration analysis of the structure. Modal superposition is used to calculate the seismic responses of the OWT by using the exact mode shapes obtained from the dynamic stiffness formulations. The rotor-nacelle-assembly (RNA) is idealized as a lumped mass at the tower top. The tower and monopile are modelled as Timoshenko beam-columns under axial compressive loading. The soil-structure interaction (SSI) is modelled using the Winkler framework, whereby the soil contribution is assumed as elastic to take advantage of the modal superposition method. Two different approaches are considered for the soil stiffness; a constant profile, and a linearly-varying subgrade modulus along the embedded depth of the pile, and both are appraised in this paper. Firstly, natural frequencies obtained from dynamic stiffness formulations are validated using experimental data from literature for the case of controlled dynamic SSI experiments, and for operating OWTs. Additionally, the natural frequency provided by a technical document for the reference 15 MW OWT is compared to the result from the DSM, where a very good agreement is observed. The seismic response time-histories of the OWT model are obtained under different input earthquake conditions, and subsequently verified by comparison with the results of finite element simulations. The effects of different scour scenarios and foundation modelling approaches on the free vibration and seismic responses are subsequently investigated. A parametric analysis is also performed using different soil properties to observe the interaction between the soil stiffness, scour condition, and seismic response. Finally, the influence of the frequency content of seismic records on the dynamic response of the OWT, specifically the tower top acceleration, the mudline rotation, and the mudline stress, is investigated considering various scour scenarios. The specific numerical case study undertaken for a reference 15 MW OWT, which is supported by a large monopile in very dense sand, shows that the seismic responses are very limited and that the elastic soil behaviour assumption is reasonable. Additionally, considering the computational efficiency, the proposed combination of DSM and modal superposition can be used for rapid seismic performance assessment of large diameter monopile-supported OWTs at the preliminary design stage to gain insight to the global dynamic response of the whole vibrating system

The site effects in Izmir Bay of October 30 2020, M7.0 Samos Earthquake

Due to the unique soil and morphological conditions prevailing in Izmir Bay basin, structural damage has been governed by site effects. Consistently, during October 30, 2020 M7.0 Samos Earthquake, which took place offshore of Samos Island, structural damage and life losses were observed to be concentrated in Bayrakli region of Izmir Bay, despite the fact that the fault rupture was at a distance of 65–75 km from the city of Izmir. Additionally, strong ground motions recorded in Izmir Bay showed unique site amplifications that were observed surprisingly at both rock and soil sites. Soil amplifications and duration elongations were mostly due to site effects governed by the response of very deep alluvial deposits of low plasticity. Similarly, due to very extensive faulting-induced fracturing and unusually stratified nature of rock sub-layers, unexpected long period amplifications were also observed at rock sites. These earthquake and site resonance effects were more pronounced in the period range of 0.5–1.5 s. When they were superposed with relatively coinciding natural period of 7–9 story residential buildings of Izmir City, it was concluded that the triple resonance effects among incoming rock ground motions, soil deposits, and the damaged buildings, amplified and prolonged the overall system response. Within the confines of this manuscript, the governing role of site effects leading to increased seismic demand was assessed, through a series of 1D equivalent linear, total stress-based site response assessments, the results of which clearly highlighted the variation of seismic demand in Izmir Bay