The growing interest in the development of offshore wind farms in seismic active areas demands a better insight into the requirements earthquakes impose on the design of wind turbine foundations. The complexity of the soil-structure interaction (SSI) during an earthquake results in large uncertainties in the design process of a monopile foundation. The design codes do not provide a structured framework on how to deal with these uncertainties. On top of this, the design codes are for (offshore) structures in general and do not specify for the large diameter tubulars which are characteristic for the offshore wind energy sector. This thesis presents the investigation into a 1D seismic Winkler foundation that is able to represent the SSI during an earthquake. The model, a beam on nonlinear Winkler foundation (BNWF) coupled with a nonlinear ground response model, is a fast method to determine the structural response for the stochastic seismic and offshore wind load cases. Semi-empirical formulations are used to implement cyclic loading effects into the model. A 3D finite element model of an embedded monopile is developed in parallel with the seismic Winkler foundation for tuning and validation purposes. The resistance of the soil to lateral pile deflections is further investigated to address the uncertainty in the SSI. A 2D horizontal cross-section of the embedded monopile is considered to determine the linear dynamic properties (impedance) of the soil, and a plane strain finite element model is developed to investigate the effect of nonlinear soil behaviour. A combined frequency-time domain extension for the Winkler foundation is developed to incorporate the obtained frequency-dependent foundation properties into a time-domain analysis. It was found that none of the 2D analyses resulted in SSI representation that could be directly applied in a time-domain analysis. Moreover, the large D/L ratio of a monopile activates a global soil response, making an uncoupled 2D analysis inaccurate for the frequency range of interest.

Floating offshore wind turbines can only withstand a limited amount of (heave) motions before the equipment fails. In order to reduce the heave motion, the DeepCwind floater for offshore floating wind turbines makes use of heave plates. This semi-submersible floater consists of three cylindrical columns with a heave plate attached to the bottom of each column. Potential flow models are often used in order to assess the response. However, potential flow theory does not take into account the viscosity and the vorticity of the fluid. Therefore, this thesis examines the effect of a heave plate on a cylindrical column's response in heave direction and subjected to wave loads with both a potential flow model and a fully nonlinear numerical wave tank. Specifically, the difference between a potential flow model and a fully nonlinear numerical wave tank is examined. The simulations with the fully nonlinear numerical wave tank have been carried out using the open source computational fluid dynamics (CFD) software package OpenFOAM (version 1606+). An unresolved direct numerical simulation (DNS) approach is used throughout this work. Best practices for the dimensions of the wave tank, the mesh and settings of the solver where obtained from Bruinsma (2016) and Rivera-Arreba (2017). The OpenFOAM waves2Foam toolbox (developed by Jacobsen et al. (2012)) has been used to generate waves in the wave tank. The two phase solver interDyMFoam for moving bodies was coupled to the waveFoam solver from the waves2Foam toolbox in order to simulate a moving body under wave loads. The simulations in OpenFoam were carried out on a 1:50 scale. The potential flow model WAMIT has been used in order to obtain the response amplitude operator (RAO), added mass, damping and wave excitation forces from potential flow theory. A single cylindrical column has been tested in the numerical wave tank both with and without heave plate. Firstly, a heave decay test has been carried out. As a result, the linear damping ratio and the linear and quadratic damping coefficients have been determined. Secondly, the structure was exposed to incoming waves. The response of the structure has been assessed under three different wave periods, which were selected in order to align with Rivera-Arebba (2017). The response of the structure was measured, filtered on the frequency of the incoming wave and compared with the RAO from the potential flow model. Also, the wave excitation forces of the potential flow model have been compared with wave loads from the numerical wave tank, based on simulations where heave motion of the structure was constrained. It was found that both the wave excitation forces and the RAO of the potential flow model are in agreement with the CFD model results. The viscous effects included in the CFD model affect the response of the structure only very lightly. The largest differences between the potential flow and CFD model were found around the heave cancellation wave frequency. At the heave natural period of the structure, the heave plate increases the linear damping coefficient with ca. 50%. The damping at this period was dominated by viscous effects. In general, the potential flow model produces an accurate RAO, due to the fact that the system is lightly damped and the damping therefore plays a minor role in the structure's response. The outcome of this work contributes to the understanding of the effects of heave plates in general and can be used to assess the added value of computational expensive CFD software in the design process of floating wind systems.

Floating wind turbines (FWTs) are proposed as a method to harness the significant wind energy resource in deep water. International research efforts have led to the development of coupled numerical global analysis tools for FWTs in order to understand their behavior under wind and wave actions. The hydrodynamic models in such tools are typically based on first-(and second-)order potential flow theory, sometimes also including Morison’s equation. In order to accurately model highly nonlinear waves, their interaction with a floating platform, and obtain estimates of the resulting loads on the structure, a numerical wave tank approach is generally needed. In this thesis, the response of floating structures to nonlinear wave loading is investigated by means of two different numerical approaches: a fully nonlinear Navier-Stokes/VOF solver and a second-order potential flow theory solver. Firstly, the fully nonlinear Navier-Stokes/VOF numerical wave tank, developed within the open-source CFD toolbox OpenFOAM, is validated against experimental data for two cases. These comprise the response of a 2D floating box and a 3D floating vertical cylinder. In order to model the motions of the floating structures, together with the generation and absorption of the waves, the interDyMFoam solver, provided by the OpenFOAM library, is extended with the waves2Foam package, developed by Jacobsen et al. (2012). Furthermore, a simple catenary mooring line is implemented for the moored cases. Secondly, a potential flow theory based model of the OC5-semisubmersible floating platform is generated. The frequency-domain analysis is done with the Wadam software and the time-domain simulations with SIMO. This model is validated against measurement data from a 1:50 scale test campaign performed at the MARIN offshore wave basin and the fully nonlinear validated CFD model. Lastly, both numerical models of the OC5-semisubmersible are compared in order to assess the suitability of the diffraction model in two different conditions where nonlinearities are of relevance. The first one involves very long waves, with periods close to 20s, which are likely to take place under swell wave conditions. These may excite the OC5-semisubmersible platform in heave at its natural frequency. The second situation deals with regular waves with increasing steepness. In principle, the fully nonlinear CFD model, if no experimental data is available, is needed to calibrate the diffraction model. However, once the latter is adjusted and validated, results are given at a much lower computational cost. According to the computations throughout this work, when dealing with waves with high steepness and high excitation frequencies, the motions, as well as the peak forces, are properly captured by the diffraction model. However, other local effects of importance for offshore structures, such as wave run-up, or the different components in frequency of the loading, require the use of a fully nonlinear CFD solver. Therefore for preliminary design stages a diffraction model is able to give suitable results regarding the motions and peak inline and vertical forces at a much lower computational cost; however, for detailed design, or optimisation phase stages, where local effects are of relevance, a fully nonlinear CFD model is required.

Climate change, as a result from global warming, requires an energy transition: the reduction of greenhouse gas emissions from fossil fuels and a radical innovation of the global energy system to proceed apace. Offshore wind is an important source of clean, renewable energy, and it plays a key role in the transition. 80% of the worldwide offshore wind is to be produced on locations in deep waters; here floating foundations are required, that to date are far more expensive than their bottom-fixed counterparts. To reduce costs of floating wind energy, reliable, detailed predictions of the system’s loads and motion response are crucial. Floating offshore wind turbine structures are designed using ’aero-hydro-servo-elastic’ software codes that simulate the dynamic response of a floating offshore wind turbine system to the offshore environment. Predictive accuracy can be improved by comparing simulation results from a model of a known system against measurements taken from the real-world system, a so-called model validation. One promising state-of-the-art aero-hydro-servo-elastic software code is BHawC/OrcaFlex, developed by Siemens Gamesa Renewable Energy (SGRE). Due to its novelty, however, validation of the code has only been carried out to a limited extend, giving rise to uncertainty about the interpretation of simulation results. The purpose of this MSc. thesis project is to validate the performance of BHawC/OrcaFlex by comparing its simulated load and motion results to measurements on a real-world floating turbine from the Hywind Scotland floating offshore wind farm (Hywind). Measurement data and a description of the ’as-built’ system were made available by the wind farm owner Equinor ASA. In order to establish an achievable level of modelling accuracy and predictive value of BHawC/OrcaFlex, the code was verified against another aero-hydro-servo-elastic software code: OrcaFlex, by setting up a similar model of the Hywind system in both codes. Limited information is available on the performance of OrcaFlex in floating wind load and motion predictions. Therefore, it was in turn verified against a wide range of industry-standard aero-servo-hydro-elastic software codes, using a modeled system that closely resembled the Hywind turbine and load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. OrcaFlex predictions matched very well across all load cases. The main differences were attributed to differently modeled additional linear hydrodynamic damping, as the official damping prescription resulted in prediction errors. In the BHawC/OrcaFlex verification against OrcaFlex, both models were subjected to multiple load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. Simulation results from running both models appeared to be nearly identical, though some discrepancy was observed from due to the simplified aero-servo-elastic OrcaFlex code. The final validation of BHawC/OrcaFlex to full-scale Hywind measurements is performed at below-rated, rated and above cut-out wind speeds with multi-directional wave and current components. In general, BHawC/OrcaFlex motion frequency domain predictions appeared to correspond well to the actual Hywind measurements. Most phenomena in the low-frequency, wave-frequency and high-frequency region were captured by the simulations. However, large errors were observed in the mean surge, sway and bridle line tensions predictions. Most discrepancies were found originating from errors in the model set-up, e.g. lack of hydrodynamic damping, simplifications in the wave model or errors in the mooring system set-up. Tuning of the mooring system showed improvement of the results, but further improvements could be made. Several sensitivity studies were added on parameters, such as hydrodynamic drag, tower damping and mooring drag. This showed overprediction of the surge/sway and roll/pitch frequency responses can be mitigated by both additional linear and viscous hydrodynamic damping. The main recommendations for further research are to further analyse errors identified in the model set-up. In addition, some yet unexplained phenomena that are not captured by BHawC/OrcaFlex in the current model, are to be addressed. Finally, a the development of a standardized approach to relate model validation studies in the field of floating wind to cost improvements could further quantify the value of future comparison studies.