Abstract
This study addresses the interactions between the collective forces of wind, waves, and currents, and floating offshore wind turbines (FOWTs), combining experimental studies with advanced computational simulations. A specific FOWT platform designed to support multiple wind turbines is examined, alongside two cylindrical platforms with square and circular cross-sections, typical of floating platform designs. The FOWT features a single-point mooring configuration that allows for rotation in response to environmental loads, with turbines placed at each vertex of its triangular structure.To investigate the dynamic behaviour of FOWTs under various environmental conditions, with a particular emphasis on the interactions under wind-waves and wave-currents conditions, two theories, Computational Fluid Dynamics (CFD) and Linear Diffraction Model (LDM), are employed. The Navier-Stokes equations, which governs fluid motion through mass and momentum conservation, are solved using the CFD methodology with the assumption of that the fluid is considered Newtonian, homogeneous, and incompressible. For this, the Finite Volume Method is used for discretization within the CFD framework, and the Volume of Fluid method is implemented to accurately track the interface between multiphase fluids. In LDM, the fluid is assumed as inviscid, incompressible, and the flow is irrotational, with small-amplitude wave behaviour. In this model, the hydrodynamic and aerodynamic forces are calculated using the Green function method and the Blade Element Momentum method, respectively. With the assumption of small current speeds, the wave-current interaction with FOWTs is analysed using a boundary integral method combined with a Green function suitable for such conditions.
OpenFOAM, an open-source CFD software package, is utilized to address the problem of wave-current interaction with floating structures in time domain. HYDRAN-WCW, a solver for potential flow integrated with the finite-element method for the hydroelastic analysis of floating structures in frequency domain, is adapted in this study, which is able to capture both the wave-current-induced motions and the aerodynamic loads on FOWTs.
On the other hand, to evaluate the dynamic behaviour of FOWTs in different environmental settings, two series of physical experiments were carried out. The first set of experiments took place at Harbin Engineering University's Deep Wave Basin, focusing on a scaled model of a FOWT at a 1/50 scaling ratio, which was affected seriously by COVID-19. These experiments examined the model's performance and stability to a variety of conditions including wave-only, wind-only, and combined wind and wave conditions. The subsequent experimental series was conducted at the University of Dundee's Fluid Lab. This involved smaller models of two different cylindrical FOWT platforms in circular and square shape, and the multi-unit triangular FOWTs platform. These models were tested in a wave flume capable of generating both regular wave and steady current. The response of the models to these conditions was recorded by a motion tracking system, providing valuable data to compare with the results from numerical models and simulations.
Key findings from these tests include the significant influence of wave-current interaction on the motion responses of FOWTs, highlighting the need to consider these interactions in design and analysis. Additionally, the study uncovers the critical role of mooring lines in influencing motion responses under varied environmental conditions, and the distinct responses between freely-floating and moored conditions. These insights are crucial for the development of effective mooring systems, ensuring the stability and operational efficiency of FOWTs in real ocean environments.
In conclusion, this thesis contributes to ocean engineering and renewable energy fields. It offers insights into the dynamics of floating wind turbines and platforms under wind-wave and wave-current interactions, laying a foundation for more efficient and reliable designs. The findings from this study not only enhance the current understanding of FOWT dynamics but also provide practical guidelines for their design and operation, setting the stage for future research into more complex interactions and non-linear effects.
| Date of Award | 2024 |
|---|---|
| Original language | English |
| Supervisor | Masoud Hayatdavoodi (Supervisor) & Ping Lin (Supervisor) |