Impact of Viscous Damping Linearization on FOWT Control Design

Abstract

Abstract Control design is traditionally carried out based on simplified linear models and subsequently tested on more realistic non-linear time domain models of a system. Issues with control performance and robustness can be introduced if the linear model is not representative of the real system, requiring additional design and testing with the realistic non-linear time domain models. This can be time consuming, and, in some cases, a satisfactory control design solution may not be achieved. It is therefore paramount to capture, in the linear model, the key physical drivers of the different system responses, which are generally referred to as states in the context of control design. Floating Offshore Wind Turbine (FOWT) numerical models present several non-linearities, which are associated with the different sub-systems they are composed of, including the turbine, the platform and the moorings. In the context of the platform, and to some extent the mooring system, viscous nonlinear hydrodynamic damping effects have a significant impact on the response of the FOWT when subject to different environmental conditions. However, these viscous damping effects are generally not easily captured in the linear FOWT models generated by several aero-hydro-servo-elastic tools, including but not limited to BLADED and OpenFAST. The focus of this paper is to investigate several methodologies to linearize viscous damping forces and to incorporate them in simplified linear models of FOWTs, with a strong emphasis on the initial control design phase in linear space. Two FOWT systems based on the semi-submersible DeepCWind (OC4), and VolturnUS-S platforms are investigated, with 5MW and 15MW ratings respectively. Results show that the impact of linearized viscous damping may depend on system size, and that some degrees of freedom may be more sensitive than others to the methodology used to linearize viscous damping.