Fig. 1: Simulation of the FSI of the 6 MW downwind turbine.
Fluid-structure interaction and optimization of a floating platform for offshore wind turbines – FSIOpt
In recent years, floating offshore wind turbines (OWTs) have evolved as an attractive alternative to conventional fixed-foundation turbines, which are limited to on- or near-shore areas. Floating OWTs, in contrast, can be installed in greater water depths and may harvest the stronger and more steadily blowing wind far away from the coast. However, they may also experience significantly higher hydrodynamic forces and accelerations due to their motion in the seaway. In order to ensure their structural integrity and also assess their performance, it is essential to take the fluid-structure interaction (FSI) into account and analyze the impact of the aero- and hydrodynamic forces on the motion behavior of the structure and the resulting stresses inside.
In the scope of the subproject “Fluid-structure interaction and optimization of a floating platform for offshore wind turbines” (FSIOpt) within the joint project HyStOH, we simulate the FSI of a fully weather-vaning 6 MW downwind turbine as shown in Fig. 1. We follow a partitioned solution approach, where the aero- and hydrodynamic as well as the structural problem are solved separately and are coupled by iteratively exchanging and updating boundary conditions within a time step until all of the subfields are equilibrated with each other. Due to separate treatment of the subproblems, it is possible to employ different spatial and temporal discretization schemes as well as already existing specialized and particularly fast solvers for their numerical solution. Consequently, the partitioned approach is not only very flexible but also allows for an efficient solution of FSI and other strongly-coupled multifield problems. Regarding the simulation of the floating OWT, we first solve the hydro- and then the aerodynamic problem and apply the resulting fluid tractions on the structure to determine its motion response. The components of the structure can be modeled as rigid or elastic, depending on whether only the global motion behavior or the local deformations and internal stresses are of interest. This way, the computational cost can be easily adjusted to match the objective of the simulation.