In this paper, linear frequency-domain hydroelastic analysis is performed for investigating the behavior of a spar-type supporting platform of a floating offshore wind turbine considering the platform's flexibility. A "dry" mode superposition approach is utilized, where the flexible mode shapes are determined through the application of a FEM based structural model. The diffraction/radiation problem is solved using the boundary integral equation method. Focus is given on the flexible modes' generalized hydrodynamic forcing and responses, the spar's hydroelastic response and the coupling effects between the flexible and the rigid-body modes. For irregular waves, the effect of the peak period on the response spectra and the hydroelastic response is analyzed.
The efficient exploitation of the vast offshore wind energy potential can contribute to the satisfaction of the European Union's energy policy targets in terms of greenhouse gas emissions' reduction, global energy climate change impacts' prevention and energy supply security enhancement. Floating Offshore Wind Turbines (FOWTs) had shown great progress in the past decade facilitating access to deeper waters, where stronger winds exist. In 2017, the world's 1st floating offshore wind farm started its operation (Pineda, 2018), while, currently, extensive investments to build floating offshore wind farms to deeper offshore sites have been decided. To make offshore wind more competitive, offshore wind turbines grew in size. The trend shows a continuous increase in the size of wind turbines (Wind Europe, 2017; Pineda, 2018), which, in turn, requires the utilization of proper, large-size floating supporting platforms for them.
For designing cost-efficient large-size supporting platforms for FOWTs, slender load-carrying structural elements should be engineered to decrease the material used. This fact leads to FOWTs that are characterized by substantial structural deformations, i.e. great flexibility, resulting not only from the tower, but, also from the supporting platform. In these cases, the elastic responses of the supporting platform may become important and the platform's flexibility may affect the dynamic response of the whole floating system. Therefore, the application of an appropriate hydroelastic analysis, accounting for a wave-flexible platform interaction, is very vital towards a reliable and realistic design and structural integrity assessment of this type of structures.