In this thesis, the integration of mooring design with the layout optimisation of floating offshore wind farms is investigated. The objective of the research is to devise a method to unite the two design processes and then evaluate the benefits of this approach. Further, it will be investigated how to model mooring systems in an optimisation problem, the stage of the wind farm design process at which mooring design can be integrated, the couplings between mooring design, turbine placement and cable routing that exist and must be used to arrive at an improved design, and to what extent the approach improves the design of the floating wind farm.
Floating wind farms are still in a developmental phase, but will be key to meet the energy goals of the future. However, floating wind farm design brings with it challenges that do not exist for bottom-fixed offshore wind farms. Chief among these challenges is the mooring system, which not only has a significant impact on the performance of the turbine, but also due to its potentially large footprint can force alterations to the layout of the inter-array electrical cables with which turbines are connected to each other and the substations. Particular focus in this regard is on catenary mooring systems, given they are common in existing demonstrator projects and have a large footprint.
In this thesis, mooring design will be performed through a multi-objective optimisation using the NSGA-II algorithm, where the objectives will be to minimise the anchor radius as well as the system cost, with constraints to ensure adequate performance in terms of handling motions and loads. The mooring system will be described by 5 design variables, the line length ratio, synthetic fraction, anchor radius, synthetic line diameter, and chain line diameter. The Pareto-optimal solutions from this optimization will then be used to optimize the layout and cable routing using an algorithm based on the work of Cazzaro and property of Vattenfall AB.
It is found that the Pareto-optimal designs have anchor radii ranging from 394 to 494 m, with system costs ranging from \$3.352 million to \$6.441 million. However, with current layout design methods, the turbine placement is done independently of mooring system parameters, and the cable routing is not affected by a change in mooring system. Thus, using the cheapest mooring system regardless of footprint is optimal on a farm level, using current layout design methods.

This study presents an innovative algorithm designed to optimise the layout of floating offshore wind farms (FOWFs), addressing inter-array cable length and the number of anchor structures required. This research is particularly noteworthy as no prior work has been conducted on wind farm layout optimisation for FOWFs. The Local-Search-Inspired Layout Optimisation (LSILO) algorithm was developed, incorporating a variety of novel heuristics such as Edge Impact Destroy (EID), diversifying repair, and neighbourhood tabu search, most of which were innovatively designed for this research. The LSILO algorithm is capable of optimising cable routing while facilitating anchor-sharing and minimising cable length. In comparison to the industry standard, LSILO displayed promising results, outperforming the random benchmark by a 24% improvement in cable length. However, the algorithm does plateau at a certain turbine density. Turbine density has a substantial influence on the algorithm’s speed and rationality. Therefore, we recommend utilising LSILO with an EID of three turbines, diverse repair without alternatives, and traditional tabu search, on current floating wind turbine sites.

Wind turbine wakes have been a topic of intense research since the maturation of wind energy. This is justified given the complexity of the physics involved and their crucial impact on the operation of a wind turbine. Only recently though, the advancement in computational resources and the development of models like the Actuator Line Method (ALM), has made it possible to study wind turbine wakes while capturing their multi-scale and multi-physical aspects. In this regard, the Actuator Line - Large Eddy simulation framework has become an academic and industrial standard. Thus, this thesis has been carried out under the joint supervision of Siemens Gamesa Renewable Energy and TU Delft to study wind turbine wake interaction using the coupling between the LES library YALES2 and the servo-structural solver BHawC. The latter extends the framework to include the effects of control and structural deformation. It allows for the simulation of real wind turbines with their industrial controller.

The aim of the project was twofold, involving the development of an automatic workflow for coupled ALM-LES of wind turbine wakes which is then used to gather insights into the impact of inflow conditions on wake properties as well as the structural impact of different partial-wake incidence scenarios.

The workflow integrates the steps of external flow convergence, mesh refinement in the wake region and generation of the converged and time-averaged flow field. It allows parametric studies to be carried out with minimal intervention from the user, while ensuring the reliability of the results. With respect to the external flow, the recycling method was used to obtain fully developed turbulence with sufficient control on the turbulence properties.

An investigation of the inflow conditions showed the sensitivity of the velocity field in the wake to the ambient turbulence and wind speed. An increase in ambient TI from 5\% to 10\% led to 35\% more wake expansion in the lateral direction and 30\% faster wake recovery. The ambient wind speed played an important role in recovery by determining the operating condition of the upstream turbines which in turn affected the wake-added turbulence. An analysis of wake meandering showed that this phenomena is largely dependent on the ambient turbulence.

Lastly, the structural impact of two different partial-wake interaction scenarios was studied to highlight the importance of accounting for the wake position on the rotor of downstream turbines while carrying out load assessments. In one scenario, the turbine operated in half-wake and half free-stream while in the other, it operated under two half-wakes. It was observed that at above-rated wind speed, the first scenario led to a 35\% increase in the flap-wise damage accumulated by the blade. On the other hand, the edge-wise damage changed by 4\%. These results emphasized the need for considering the spatial distribution of the wake on the downstream turbines.

The development of offshore wind technology has progressed rapidly since the first offshore wind farm came to birth in 1991. During the last three decades, offshore wind has become an economically attractive option of renewable energy source for many countries to fulfill their commitment to combat the climate crisis. However, an open challenge remains in offshore wind, namely the aerodynamic interactions between multiple turbines, referred to as wake effects. When the upstream turbines extract energy from wind, they generate the wake with low velocity and high turbulence level on the downstream turbines, which substantially reduces the overall wind farm performance. The annual power production loss due to wake effects is generally 10 - 40%, which limits the reduction of the levelized cost of electricity for offshore wind energy.

Therefore, developing effective wind farm control strategies which could mitigate the wake effect has become an emerging research area in the past decade. Recently, a novel wind farm control strategy called the Helix approach is proposed. The Helix approach adopts the individual pitch control technique to dynamically deform the wake into the helical shape, which induces wake instability and thereby stimulates wake recovery. This control strategy has demonstrated promising potential to mitigate the wake effect and to enhance the wind farm performance. To date, the Helix approach employs single harmonic pitch signals. However, more complex and higher-harmonic signals to potentially improve the effectiveness of the Helix approach have never been explored. Therefore, the purpose of this master thesis is to explore the potential of using multi-sine pitch profiles to further enhance the wake mixing.

The high-fidelity aeroelastic simulator OpenFAST with its recent free vortex wake code is adopted to simulate the dynamic wake evolution. A Fourrier stability analysis is used to quantitatively identify the wake breakdown position. According to the simulation results for the multi-sine pitch profiles, the wake breaks down at 1.75 D, which is earlier than the one at 2.50 D for the original single-sine Helix strategy. The earlier wake breakdown indicates the potential of faster wake recovery, which is required to be validated by the higher-fidelity model in the future studies.

Reducing the levelised cost of energy is crucial to accelerating the energy transition. To develop offshore wind solutions in greater water depths, a floating solution is required. The time-domain simulations of these Floating Offshore Wind Turbines (FOWTs) under wind-wave misalignment used in research and industry projects are computationally intensive and limits researchers and industry in their developments. To better understand the sensitivities of the fatigue loads of FOWTs to different parameters and environmental conditions, a computationally efficient method is needed. The aim of this research is to develop a frequency-domain method to quantify the effects of misaligned wind and waves on the response of a semi-submersible floating offshore wind turbine. Therefore, the following research question is defined: What is the effect of misaligned wind, windsea waves, and swells on the loads at the tower base of a semi-submersible type floating offshore wind turbine?

Several sensitivity studies are conducted to quantify the contribution of yaw-roll coupling effects and aerodynamic damping to the responses and loads. From these studies, it appears that the yaw-roll coupling can increase the response when excited at wind/wave directions in which the structure is asymmetric.
The magnitude of this effect is related to the wave peak period (and the resulting wavelength), the angle of misalignment with respect to the structure, and the apparent length of the structure. Also, the lack of aerodynamic damping in the direction of the rotor plane (side-side direction) leads to a noticeable
increase in the response, directly or through coupling effects. Finally, the frequency-domain method is compared with the time-domain simulations (BHawC-OrcaFlex) carried out by Siemens Gamesa. Although reasonable agreement is found for the load driving rigid body modes, significant differences in the tower bottom loads are found for the lowest and highest production wind speeds.

These results show that misaligned wind and waves can increase the response for headings where the structure is asymmetric due to coupling effects. Wind-wave misalignment leads to an increased response in the direction of the rotor plane due to the lack of aerodynamic damping. In general, the wind-wave misalignment can also have a mitigating effect on the maximum equivalent moment at the tower base, as the aerodynamic damping also reduces the response in the wave frequency range. Furthermore, the comparison shows the need to extend the frequency-domain method with the first tower bending modes and improvement of aerodynamic/mooring property estimation. Based on the findings and the conclusions, the recommendation is to investigate the floater specific sensitivities at an early stage of the design. Future research should focus on: the implementation of tower flexibility, improvement of the quasi-static estimation of mooring stiffness, frequency dependent aerodynamic properties, and implementation of second-order wave forcing.

The advent of global warming has brought an increased interest in non-conventional sources of energy, one of which is nuclear energy. Threatening the almost year-round functioning of nuclear power plants are Flow-Induced Vibrations (FIV). One such mechanism, Vortex-Induced Vibration (VIV), holds importance in areas of cross flow in nuclear power plants. To make fail-safe
designs, computational analysis in the domain of Fluid Structure Interactions (FSI) has been increasing over the past two decades. The thesis work aims to add to the body of knowledge by making predictions for an in-line two-cylinder configuration, set up as part of a benchmark proposed by the Organization for Economic Co-operation and Development (OECD), using the commercial code Simcenter STARCCM+ (v2020.3.1).

The main objective of this study is to test the efficacy of the URANS framework in predicting VIV which is strongly correlated with the objective of the OECD to propose recommendations for the Best Practice Guidelines. To do so, it is desired to shortlist the most appropriate turbulence model for the final application and point out gaps in the prediction of the same. The thesis work is thus carried out in three phases: code validation, turbulence model selection and final application. Key results from this study reveal the ‘Standard K-Epsilon
Low Re: Cubic’ model to be the most apt for the final application. Furthermore, gaps are also identified in the application of URANS to predict VIV resonance conditions, the primary of which is the overprediction of the vortex shedding frequency.

Simulations of a spar-type floating wind turbine supporting the DTU 10 MW reference turbine under fitted atmospheric conditions are performed. Atmospheric conditions are modelled using the turbulent models recommended by the IEC: the Mann model and the IEC Kaimal and exponential coherence model. Model parameters are fitted to measurements at the FINO-1 platform to generate turbulent boxes that are as close as possible to measurements. The simulations are carried out using OpenFAST for three mean wind speeds: 7.5, 12 and 16 m/s and the power law wind profile is used with fitted shear exponent. The motions, wind, loads spectra are quantified using the WAFO toolbox and the loads are assessed with short-term damage-equivalent loads using MLife. Depending on the wind model, global motions differ by up to 52%. Generally, Kaimal resulted in higher surge and pitch motions, and the opposite was found for the yaw motions. The influence of atmospheric stability on global motions was found to be important as well, unstable conditions giving the largest motions and stable conditions the lowest. It was observed that the wind model influenced the fairlead tension and tower top loads. Mann resulted in up to 35% higher loads on the tower top yaw moment and Kaimal resulted in up to 30% higher fairlead tension loads. Tower base fore-aft bending moment and blade root out-of-plane moment were not very sensitive to the choice of wind model. Atmospheric stability had an influence on all loads by up to 30%. Unstable conditions led to the most damage while stable conditions generally led to the least. Whether the choice of the wind model or the change in atmospheric
stability has more influence on the global motions of the platform and the loads depended on the wind speed, the degree of freedom or the load considered. It was found that wind models should always consider atmospheric conditions, not only neutral. Finally, impact of Mann turbulence model parameter variations on the wind, global motions and loads was investigated. It was found that turbulence intensity is influenced by the three parameters and has a bigger impact than the differences in coherence.

One of the emerging technologies in sustainable energy is Floating Offshore Wind. GustoMSC designs a structure for Floating Offshore Wind, the Tri-Floater. One of the problems that need to be investigated before building the unit is wave impact loading. Wave impact occurs when a large wave hits the structure, placing large forces on the structure.

In this research project, a model is developed to find the wave impact loads in 3-hour sea states in which the significant wave height and period can be set. The model will work using CFD solver ComFLOW and time-domain solver aNySIM.

In the first part of the thesis, a CFD model and an aNySIM model are created and verified with model test data using decay test analysis. Decent results were found for the period and damping of the motion by both the CFD and aNySIM model. When more time is spent on the CFD model, it could closely match the model test results.

In the second part of the thesis, waves are included in the simulation. A grid sensitivity study is done with 2 m, 1m and 0.5 m grid sizes. The difference between these grids is slight regarding wave height dissipation.
Irregular waves are used to do longer simulations. Two tests are done, a full 3D CFD simulation and a combination of a 2D CFD simulation with time-domain solver aNySIM. When comparing the computational time, the 2D CFD simulation proved to be the most practical solution, saving much computational time.

Finally, a methodology has been designed in which a long term wave impact study can be done.
A 2D CFD simulation is performed to simulate the wavefield in the domain. This 2D CFD simulation is used as input for aNySIM, in which the motion of the Tri-Floater is simulated. From this motion, the air gap is calculated, and events are selected that will be simulated in a full 3D CFD simulation. The 3D CFD simulation uses initial and boundary conditions from the 2D wave field. It also uses the motions that are calculated in aNySIM for stability.
In this 3D CFD simulation, the wave impact pressures can be obtained.

The methodology results are hard to judge for value, as there are still some problems that need to be solved. When these problems are solved and more time is spent on improving the individual parts of the simulation, it would be interesting to compare the results to model test data if this is to be done in the future.

In offshore engineering complex simulation models are constructed for design optimization using Monte Carlo methods. These models incur large computational costs. Multi-Level Multi-Fidelity Monte Carlo is proposed as a method to reduce the computational cost of these simulations. In addition, research is conducted on the use of porous media as passive damping systems. Hence, an analysis on the effect of porosity on the vortex shedding frequency is conducted. This thesis is an exploratory investigation on the application of Multi-Level Multi-Fidelity Monte Carlo in fluid dynamics topics and its particular use for analysis of the effect of porosity on the vortex shedding frequency on a porous circular cylinder. Three case studies are conducted. Firstly, applying Multi-Level Multi-Fidelity Monte Carlo on a solid circular cylinder case, which is deemed as a successful application, based on the estimated quantity of interest, variance reduction and computational cost reduction. Furthermore, two parametric studies are conducted: 1) to discover empirical relationships (low-fidelity models) and 2) forward uncertainty propagation with Multi-Level Multi-Fidelity Monte Carlo using a uniform input distribution. Both parametric studies consist of a number of equally distributed points of porosity on a case setup of flow past a porous circular cylinder. The parametric studies use a frequency detection algorithm, which approximates the vortex shedding frequency using the frequency of lift force oscillation. The results of the first parametric study indicate there is a drop in vortex shedding frequency as experienced by the cylinder for increasing porosity. The hypothesis is that for increasing porosity the formation length of vortex shedding increases. Two empirical relationships are derived from the results by curve fitting the Strouhal number (dimensionless form of the vortex shedding frequency) versus porosity. These empirical relationships are incorporated in the Multi-Level Multi-Fidelity Monte Carlo method and applied to a similar parametric study on the effect of porosity on the vortex shedding frequency. The results indicate the presence of systemic errors in the high-fidelity model. The conjecture is that the major influence on these errors is due to the resolution of the frequency detection algorithm being too low. For this reason, no clear conclusion on the validity of the empirical relationships is obtained and further research 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.

Aeroelastic stability must be maintained for the entire flight duration. Of the vast phenomena that fall under the field of aeroelasticity, flutter is the most studied and influential in terms of driving aircraft design. As such, it is the main topic of analysis of this Master Thesis. For flutter analysis, it is of critical importance to accurately model the system unsteady aerodynamics, particularly for harmonic oscillatory motion. Therefore, the aim of this Master Thesis is to study the aerodynamic accuracy of a method that uses high fidelity unsteady CFD simulations to build a linearized aerodynamic model, applicable to flows of linear nature. The unsteady aerodynamic loads obtained with the linearized aerodynamic CFD model are validated by comparing them to that of Theodorsen and unsteady OpenFOAM simulations of a harmonically oscillating 2D airfoil.
The results obtained by the linearized aerodynamic CFD model are unable to predict the unsteady air loads for oscillatory harmonic airfoil motion and therefore cannot be used to accurately predict the flutter boundary. The underlying reason for this is the fact that the current formulation of the model does not take into account the wake effects on the unsteady air loads.

The modelling of gust generation by an array of oscillating vanes at Delft's University of Technology Open Jet Facility is proposed. Focus is placed on identifying relevant in-field gusts that horizontal axis wind turbines are exposed to and reproduction via numerical methods in the wind tunnel setting. A computationally efficient model is developed based on potential flow theory. The model validation showed good results overall, preserving general trends. Several vane configurations are proposed. Veer profiles representative of all atmospheric stability conditions are simulated as well as a maximum profile with veer angle difference between the top and bottom of the rotor of up to 20°. A coupling between the trailing vorticity of the vanes and achieved veer profile is identified. Temporal variation of the streamwise velocity component is achieved, however, not following the intended gust profile. Peak to trough velocity variation of up to 28% of the free stream value is observed. Furthermore, gust length scale was linked to vane spacing, limiting the possibility of small geometric scale testing.