Conceptual Design of Floating Wind Turbines

The need for different numerical models with varying degrees of simplification for the conceptual design of a floating offshore wind turbine is the focus of this paper. While parts on the component level can be designed apart from the others the overall dynamics on the system level have to be assessed from the beginning. Starting with very simple models and identifying the significant contributions to the system behavior while going step by step to more detailed ones makes a successful dimensioning possible. The significant effect of the blade pitch controller on the system dynamics is analysed and preliminarily designed with a simple 1-degree of freedom (dof) model. Further on the section forces at tower base and the distributed platform loads are calculated with a 9-dof multibody system with simplified aerodynamics and Morison equation allowing a pre-dimensioning of the structure. INTRODUCTION Along with the intensified construction of offshore wind parks worldwide research seeks solutions for seas with a depth beyond the limits of bottom mounted foundations. Several technologies for floating offshore wind turbines (FOWTs) have already been applied in the oil & gas industry. The concepts differ in the way how stability and a reduction of wave excitation is achieved. Tension-leg platforms, for example, achieve stability through a taut mooring, whereas others, like the barge and sparbuoy, rely on high mass and inertia. These concepts have been repeatedly improved so that abundant experience in their conceptual design exists. There are basic, mostly empirical rules for, e.g., the spacing of the columns of a semi-submersible, the eigenfrequency of a spar or the additions to minimize vortex induced vibrations, see [1]. In the field of wind energy, however, demands and standards are different: Due to the technology that is still very young the readiness to invest is limited and the cost of energy needs to be close to the one of conventional power plants. Thus, it is a main target to conceptual design to keep the material cost low. With the resulting floating geometries that are much lighter as their oil & gas counterparts new challenges are arising that have not been known before. While a common practice is to increase the inertia of a floating structure in order to shift the RAOs to frequencies lower than the wave spectral peak this is not feasible in the same way for FOWTs. It is necessary to ensure a high hydrostatic restoring stiffness in order to decrease the static wind misalignment of the rotor. On the other hand the reduction of costs requires a reduction of used wall and ballast material so that a FOWT platform easily reaches RAOs at higher frequencies, close to the wave spectrum. When looking at the dynamic behavior of FOWTs compared to oil & gas structures it is obvious that the aerodynamic forces play a major role. The turbulent wind field introduces stochastic forces and moments in all directions on the rotor hub. Out of this reason the wind turbine controller is crucial to the design of a floating wind turbine system. It has been observed that conventional controllers that do not take into account the tower or platform motion yield an unstable behavior for wind speeds closely above rated wind speed. This phenomenon is even enhanced for platforms with high eigenperiods resulting from a small metacentric height and a high structural inertia. When designing a new concept of a floating wind turbine the interconnectedness of the mentioned facts has to be considered and therefore numerical models with increasing degree of detail need to be applied. In the following an exemplary process of conceptioning a FOWT on system level with regard to its dynamics is outlined.