Seismic performance assessment of monopile-supported offshore wind turbines using unscaled natural earthquake records

Dynamic analyses of operating offshore wind turbines including soil-structure interaction

Seismic considerations in design of offshore wind turbines

Wind, wave and earthquake responses of offshore wind turbine on monopile foundation in clay

Offshore wind turbines in seismic active areas suffer from earthquake impacts. In this study, seismic fragility analysis of a monopile offshore wind turbine considering different operational conditions was performed. A finite element model for a 5 MW monopile offshore wind turbine was developed using the OpenSees platform. The interaction between the monopile and the seabed soil was modeled as a beam-on-nonlinear-winkler-foundation (BNWF). A nonlinear time history truncated incremental dynamic analysis (TIDA) was conducted to obtain seismic responses and engineering demand parameters. Potential damage states (DSs) were defined as excessive displacement at the nacelle, rotation at the tower top, and the allowable and yield stresses at the transition piece. Fragility curves were plotted to assess the probability of exceeding different damage states. It was found that seismic responses of the wind turbine are considerably influenced by environmental wind and wave loads. Subject to earthquake motions, wind turbines in normal operation at the rated wind speed experience higher levels of probability of exceeding damage states than those in other operational conditions, i.e., in idling or operating at higher or lower wind speed conditions.

Seismic fragility analysis of monopile offshore wind turbines under different operational conditions

Summary
The dynamic response of a wind turbine on monopile is studied under horizontal and vertical earthquake excitations. The analyses are carried out using the finite element program SAP2000. The finite element model of the structure is verified against the results of shake table tests, and the earthquake response of the soil model is verified against analytical solutions of the steady-state response of homogeneous strata. The focus of the analyses in this paper is the vertical earthquake response of wind turbines including the soil-structure interaction effects. The analyses are carried out for both a non-homogeneous stratum and a deep soil using the three-step method. In addition, a procedure is implemented which allows one to perform coupled soil-structure interaction analyses by properly tuning the damping in the tower structure. The analyses show amplification of the ground surface acceleration to the top of the tower by a factor of two. These accelerations are capable of causing damage in the turbine and the tower structure, or malfunctioning of the turbine after the earthquake; therefore, vertical earthquake excitation is considered a potential critical loading in design of wind turbines even in low-to-moderate seismic areas. Copyright © 2015 John Wiley & Sons, Ltd.

Remi A. Kjørlaug

Papers

Vertical earthquake response of megawatt‐sized wind turbine with soil‐structure interaction effects