۱- Introduction

۲- Modeling, limit states, and analysis setup

۳- Results

۴- Summary and conclusions

References

Abstract

The number of offshore wind turbine farms in seismic regions has been increasing globally. The seismic performance of steel monopile-supported wind turbines, which are the most popular among viable structural systems, has not been investigated thoroughly and more studies are needed to understand the potential vulnerability of these structures during extreme seismic events and to develop more reliable design and assessment procedures. This study investigates the structural performance assessment of a typical offshore wind turbine subjected to strong ground motions. Finite element models of an offshore wind turbine are developed and subjected to unscaled natural seismic records. For the first time, the sensitivity to earthquake types (i.e. crustal, inslab, and interface) and the influence of soil deformability and modeling details are investigated through cloud-based seismic fragility analysis. It is observed that monopile-supported offshore wind turbines are particularly vulnerable to extreme crustal and interface earthquakes, and the vulnerability increases when the structure is supported by soft soils. Moreover, a refined structural modeling is generally necessary to avoid overestimation of the seismic capacity of offshore wind turbines.

Introduction

Wind energy production from offshore wind farms is a reality nowadays around the world. Fig. 1 shows main countries that are developing and investing in offshore wind power according to the Global Wind Energy Council [1]. The same figure shows a global seismic hazard map in terms of peak ground acceleration (PGA) with probability of exceedance of 10% in 50 years [2]. Several countries are in high seismic regions, including the USA, China, India, and South East Asia, and are adjacent to subduction zones (blue lines in Fig. 1), where magnitude M9-class megathrust earthquakes can occur. This highlights that earthquake risk for newly built offshore wind farms can be potentially high and that reliable design and assessment methods for these structures against intense ground excitations need to be developed. In fact, current international standards and national codes (e.g., GL [3], DNV [4], IEC [5]) suggest considering seismic actions but without explaining in detail how to evaluate the seismic performance (e.g. suitable analysis methods). The lack of basic research that underpins codes’ requirements may be related to the limited number of wind turbines that were actually damaged during major earthquakes [6,7]. Moreover, structural damage to wind turbines was mainly reported for onshore wind turbines, thus contributing to common misperception that seismic loading is not critical for offshore wind turbine structures [8]. On the other hand, the global development of such structures in active seismic regions makes imperative to understand to which extent structural demand on offshore wind turbines is increased by earthquake loads, and whether numerical results may be affected by modeling details or by the implementation of the soil-structure interaction (SSI) that has been recognized playing a very important role on the structural performance.