Numerical Modeling and Dynamic Analysis of Two-Rotor Floating Offshore Wind Turbines

Abstract

The exploitation of offshore wind in deep waters by means of floating wind turbines is steadily gaining traction as a suitable option to produce renewable energy. Among the innovative technologies recently proposed, two-rotor floating wind turbines offer significant advantages in terms of smaller blades deployed offshore, cheaper operations, fewer installations, and sharing of the floating platform. Although examples of commercial prototypes currently under development are many, the scientific literature lacks thorough studies on the dynamic performance of such systems. As a consequence, a better understanding of the major design drivers of such systems leading toward the definition of a baseline design is required. Floating wind turbines are highly dynamic systems subjected to environmental loads from waves, currents, and wind. Moreover, the dynamic response of wind turbines is heavily influenced by the nonlinear behavior of the servo systems, such as cutin, cutout, and failure conditions. As such, dynamic analysis is often carried out by means of fully-coupled tools able to consider all factors in an integrated environment. To date, there is a lack of an open-source fully-coupled tool able to easily analyze the dynamics of two-rotor floating wind turbines. This PhD thesis presents the development of such a tool. Development work was mostly carried out in Modelica through the opensource environment OpenModelica and the freelyavailable Modelica Standard Library. The dynamics of the system and structural dynamics of tower and blades were implemented by means of a multibody approach. Linear hydrodynamics was solved in DNV Wadam and the associated hydrodynamic loads were imported into the tool as time realizations. Moreover, the well-known aerodynamic module AeroDyn within the NREL package FAST was integrated into the environment for the computation of aerodynamic loads based on the blade-element momentum approach. Benchmark studies showed good performance and accuracy compared with standard single-rotor packages. The tool was also employed to examine various dynamic aspects of two-rotor floating wind turbine concepts. Global dynamic analysis of a two-rotor wind turbine mounted on a spartype floating platform showed significant platform yaw response due to wind turbulence intensity and the distribution of thrust loads on the structure. Modification of the bladepitch control system was found to be beneficial to reduce platform yaw response. As a consequence, the design of an optimal control strategy based on a linear quadratic regulator was carried out, showing better performance than reference control strategies without the need for large usage of the actuation systems. Furthermore, the analysis of the longterm extreme response of the same two-rotor floating system was carried out by means of a variety of methods. A modification of the 50 year environmental contour method considering the cutoff condition of the system was assessed, showing a small underestimation of wind-induced responses as compared with a full longterm analysis. This method may thus be considered a fast alternative for the longterm extreme assessment of two-rotor floating wind turbines. The global response of the two-rotor wind turbine system mounted on different platform types was also assessed. Results showed the largest platform yaw response for the spar type, while the greatest structural loads were obtained for the tensionleg type. The semi-submersible type showed the greatest response in extreme conditions, but the most balanced response in operational conditions. The analysis of bladepitch actuation faults on the dynamics and loads of a two-rotor system was also carried out. Results showed significant dynamic loads on the tower structure which can be detrimental in terms of fatigue life. Shutdown delay between rotors implies greater torsional loads on the structure, while loads on the faulty blades are not affected by the choice of platform employed or two-rotor application.