Multi-fidelity simulation of floating offshore wind turbines: a critical comparison

Floating Offshore Wind Turbines are one of the most promising technologies to increase energy harvested from the wind worldwide. These machines are designed to harvest the abundant wind resource available in deep waters where installation of fixed-bottom offshore turbines would be too expensive. Floating wind turbines are challenging to design and operate as their dynamic behavior depends on the influence of many coupled physics. In fact, in addition to the wind-induced aerodynamic actions, wave-induced hydrodynamic forces and control also influence the response of the system. Moreover, modern multi-MW rotors feature large flexible blades and therefore loading also depends on the elasto-dynamic characteristics of the elements. In comparison to onshore wind turbines, floating offshore turbines introduce additional challenges, since the turbine is now allowed to move as it is supported by a floating foundation. Reliable numerical models are crucial, as these machines have to operate for extended periods of time with minimal maintenance and low costs. Therefore, accurate prediction of extreme and fatigue design loads is pivotal to optimize these machines and lower levelized cost of energy. In this thesis, multi-fidelity numerical models for the simulation of wind turbines are critically compared. Most of the focus is put on aerodynamics. In fact, many state-of-the-art medium fidelity codes, that are widely adopted during the design and certification phases of wind turbines, rely on Blade element Momentum theory. These models need several empirical corrections to reproduce the unsteady behavior of a floating rotor, which is introduced by the additional degrees of freedom afforded by floating installation. In the first phase of this work, multi-fidelity aerodynamic models, ranging from momentum theory to computational fluid dynamics are compared to experiments on a rotor undergoing unsteady pitch and surge motion. All the compared theories behaved well, but differences emerged if rotor speed and blade pitch oscillations were introduced and during operation in low wind speeds. In particular, the dynamic wake engineering correction for blade element momentum theory that was tested performed well, and was able to improve the agreement to higher fidelity models of rotor force predictions. In the second phase, complexity is increased and comparisons on a wave-basin experimental test case are performed. During this phase all of the tested models were able to reproduce the dynamic behavior of the system comparatively well. In the final phase, code-to-code comparisons in realistic inflow conditions are discussed. In these tests, the influence of the multi-fidelity aerodynamic modeling is apparent in fatigue loads, with blade element momentum theory based models consistently predicting higher fatigue loading. The influence of the structural modeling on the other hand is apparent in both fatigue and extreme loads. As discussed in detail within this work, OpenFAST, that features a modal-based structural model, shows large undamped response in absence of aerodynamic damping, impacting both extreme and fatigue loads.