3D Numerical Modelling of Non-Linear and Breaking Wave Forces On Offshore Wind Turbine Substructures
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Offshore wind energy technology has experienced a remarkable growth in the last few decades due to an increasing focus towards renewable energy research. The objective of the research towards offshore wind energy is to develop methods for design and construction which will make offshore wind turbines safe, efficient, functional, economical and able to resist the severe environmental loads over longer durations. Among the environmental loads, the breaking wave loads are one of the most important loads to be accounted for in the design of offshore wind turbines. During the last few decades, considerable efforts have been made to study wave breaking in coastal regions. Wave breaking primarily governs the various hydrodynamic processes in the surf zone like destabilization of the sea bed, wave setup and wave energy dissipation. When waves approach the shore, the sea bottom starts to affect the propagating waves and they undergo wave transformation. In this wave shoaling process, the wave height increases, the wavelength decreases and consequently the waves break and energy is dissipated. This process complicates further for breaking irregular waves. The main aim of the present thesis is to investigate regular and irregular wave breaking in shallow waters and breaking wave forces on offshore wind turbine substructures like monopiles and jackets. The computational fluid dynamics (CFD) module of the open-source hydrodynamics model REEF3D has been used for modelling wave breaking and computing wave breaking forces on slender cylinders in shallow waters. The model is based on the Reynolds-Averaged Navier-Stokes (RANS) equations together with the level set method for the free surface modelling and the k-ω model for turbulence closure. Numerical simulations to investigate wave breaking on sloping sea beds and submerged structures are performed in a numerical wave tank (NWT) and breaking wave forces on monopile and jacket substructures are evaluated. Moreover, the numerical model is thoroughly validated against experimental data. The breaking wave statistics and the changes in spectral characteristics such as skewness and bandwidth parameters during the breaking process are studied in detail. An extensive analysis to explore the breaking wave characteristics and the geometric properties for the different cases is performed. The wave crest steepness and asymmetry parameters are examined in order to understand the influence of the water depth and incident wave steepness in determining the deformation of the wave crest. The statistics of breaking wave characteristics and geometric properties of the breakers are also analysed and discussed to quantify these parameters in a thorough manner. Further, breaking irregular wave forces on monopiles are investigated in the frequency-domain and the effect of the higher-harmonics on the peak wave forces is discussed. The quantitative wave spectral parameters are related to the irregular wave forces to identify the role of the shape and bandwidth of the wave spectrum in determining the irregular breaking wave forces. The wave velocity is also an important factor in determining the breaking wave forces. Therefore, the transformations of the velocity components are investigated in the frequency-domain to understand the spectral evolution of the wave particle velocity during the wave breaking process. The effect of the breaker location is investigated for breaking regular waves on a jacket substructure. The present study also provides the best fit cumulative distribution functions (CDFs) for the breaking wave forces and the horizontal wave velocity components. The numerical results for the different cases are consistent with previous studies. The analysis of breaking characteristics and wave profile geometric properties shows that breaking irregular waves behave differently compared to breaking regular waves due to the presence of multiple harmonic wave components. The higher-order components in the pressure and velocity spectra play an important role in determining the higher-order force components, when breaking irregular waves interact with the structures. Most of the wave breaking features including the overturning wave crest and its reunion with the free surface and the generation of a secondary water jet due to the splash up phenomenon are well represented in the numerical model.