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.

Summary and conclusions

This paper presented an analytical procedure to evaluate the seismic assessment of steel monopile-supported offshore wind turbines, which is a structural typology commonly adopted in seismic-prone countries investing in offshore wind power farms. Modeling details about the structure, foundation, material, inertia, and loading are provided and a finite element model was developed through the open-source structural software OpenSees. Important aspects in the modeling, such as different soil structure interaction modeling approaches, different material behavior and the influence of door opening at the tower base, were also investigated. Models were analyzed through non-linear dynamic analyses using five record sets of input ground motions, that facilitated the comprehensive assessments of the influence of the earthquake types and the soil deformability. Two limit states were considered for the assessment: the serviceability limit state, reached when the chord rotation exceeded 0.5 degrees, and the ultimate limit state, reached when either yielding or local buckling occurs. Based on the thorough analyses of the seismic performance, represented in the paper using seismic fragility functions, the following conclusions can be drawn:

1. The analyzed structural typology is particularly sensitive to extreme crustal and interface records;

2. Higher modes are not negligible, especially if the SSI is explicitly modeled;

3. Frequency content of records associated to deformable soil induces an increased seismic fragility with respect to stiffer soil;