Floating offshore wind turbines (FOWTs) have the potential to harness wind resources in deepwater, which is so far prohibitive for conventional approaches. This, however, comes at a cost: the platform’s extra degrees of freedom (DoFs) introduce complex aerodynamic and hydrodynamic behaviours. Therefore, FOWTs must be accurately modeled to reduce load uncertainties that ultimately prejudice their economic viability. This project implements a framework for the coupled, high-fidelity simulation of FOWTs in OpenFOAM. The tool is built upon two existing libraries: turbinesFoam—for rotor modeling based on the actuator line method— and waves2Foam—for wave-field generation and absorption based on the relaxation zone method. The multi-phase simulation uses the interFoam solver in combination with a morphing mesh technique and rigid-body model to represent the platform. The mooring restraints are computed with a quasi-steady, catenary model from waves2Foam. The turbinesFoam library, targeted at bottom-fixed turbines, is modified so that it can accommodate arbitrary motions along the rigid-body DoFs. The platform-turbine FSI coupling follows a serial sub-iterating strategy based on the PIMPLE scheme. The presented framework proved capable of modeling the aerodynamic performance of turbines under prescribed motion and produced plausible results for a semi-submersible FOWT under combined wind and wave conditions. Once carefully validated, this tool will have the potential to serve as a reliable technique for the advanced modeling of FOWTs.

Floating Offshore Wind Energy represents an enormous potential for the future of wind energy and will play a pivotal role in the global energy transition. However, important technical challenges still need to be overcome by the industry regarding the standardization of processes like transportation, floating substructure concept, and supply chain optimization. To successfully achieve these milestones, FOWTs must be accurately modeled to minimize uncertainties to ultimately attract investors and capital in the industry. This thesis has the objective of developing and validating a high-fidelity model for the hydrodynamics of FOWTs with the ultimate goal of creating reliable results to be used as a benchmark for faster mid-fidelity software mostly used in the offshore industry. The model expanded throughout this thesis is developed in OpenFOAM C++ framework. The whole numerical model is built upon three main pillars: Waves2Foam used for the generation of relaxation zones in the numerical wave tank for wave-current generation and absorption; MoorDyn/Moody numerical tools employed for the mooring lines spatial discretization for the station keeping of the floating platform; InterFoam applied as the main OpenFOAM solver for the multi-phase solution which, in combination with a mesh morphing technique and a rigid body solver, is capable of deforming the mesh to accommodate the 6DOF motions of the platform. A sequential approach has been undertaken. The first step involves the simulation of second-order Stokes wave propagation in an infinite 2D wave tank, followed by a sensitivity study with an associated uncertainty quantification. The next step involves conducting 3D simulations of a floating box, comparing motion and forces under different mooring models: dynamic FEM, dynamic lumped mass, and quasi-static catenary. In the last step, the results and lessons learned from the previous studies are combined for coupled hydrodynamic/aerodynamic simulations of the OC4 DeepCwind semi-submersible floater with the 5MW NREL offshore wind turbine. Forces, displacements, and tensions are extrapolated and compared with experimental and numerical results, validating the model's accuracy. Simulated across diverse wind and wave conditions, the OpenFOAM model demonstrated remarkable robustness and excellent ability to achieve convergence even under challenging environmental conditions, while maintaining a high level of result accuracy. The framework effectively reproduced the hydrodynamic behavior of a Floating Offshore Wind Turbine (FOWT), establishing itself as a dependable and reliable tool for conducting future high-fidelity FOWT simulations.

As wind turbines get bigger and bigger, the simulation of wind turbines becomes more complex. The increase in size brings about a multitude of intricate challenges that must be addressed in the simulation process. These challenges lie in the different aspects of the simulations such as aerodynamics, structural dynamics, power electronics, hydrodynamics, turbine control etc. The central theme of this thesis is to explore the effect of one of the aspects- structural dynamics, on the wind turbine simulations. Moreover, the thesis also emphasises the development of a structural module and it’s integration in an OpenFOAM-based wind turbine simulation library called TurbinesFoam. TurbinesFoam is an actuator line method-based simulation tool, which enables the study of turbine performance as well as wake dynamics using Computational Fluid Dynamics. The key motivation is also to contribute towards the accurate simulation of wind turbines by performing a successful integration as said above. To fulfil the goals, a structural module was developed in Matlab to simulate the structural dynamics of wind turbines. The module is developed from the theory of spinning elements assuming the blade and tower as Euler-Bernoulli beam. The developed module code is called while running the CFD simulations in OpenFOAM to add deflection and rotation of the blades and towers, using a MatLab pipe class.The accuracy of the developed code was measured using BModes and OpenFAST. The results exhibited satisfactory agreement between the outputs. The NREL 5MW turbine was used to study the effects. The results revealed that, at slightly above the rated condition, the turbine showed 1% decrease in power production due to elasticity when compared with a rigid turbine. The thrust and torque coefficients showed similar trends of reduction in value. Moreover, as the inflow wind velocity increased, the differences in performance got broader, due to increased deflection value. The wake region of the elastic turbine showed a mixed region of both increased and decreased wind velocity when compared with the rigid turbine.

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.

Floating wind turbines despite the the potential to harness energy from deep offshore areas where higher average wind speeds face challenges in terms of competitiveness. One approach to raising the competitiveness of a wind farm is to mitigate efficiency losses resulting from the wake effect. This report focuses on the combination of three notable wake effect mitigation strategies: layout optimization, yaw-based wake redirection, and turbine repositioning. A preliminary analysis of the combined effect of wind turbine repositioning and yaw-based wake redirection on power performance for the case of two turbines only is performed. Above rated wind speeds, upstream turbine yawing reduces downstream turbine movement, with reductions of around 1 to 4 rotor diameters longitudinally and 0.2 to 0.5 times the rotor diameter laterally, keeping the same level of power efficiency. Nextly, an optimization problem that integrate layout optimization with yaw-based wake steering and turbine repositioning for power maximization across an extended wind farm is formulated. The optimization frame followed a sequential approach. The results on a case study confirmed that the effect of adding yaw-based wake redirection to turbine repositioning remains significant for multiple turbines, with several percent-point efficiency improvements for small movable ranges. For larger ranges, the contribution of yaw control diminishes rapidly to one percent-point or less. Yaw control enables movable range reductions of 10% to 50%, preserving wind farm efficiency. Yet, reductions are more pronounced in smaller, less effective movable ranges. Below rated conditions, the effectiveness of yaw control diminishes swiftly. Furthermore, the study delves into the implications of integrating position mooring for turbine repositioning and yaw-based wake mitigation strategies on mooring system performance. This examination employs a proposed methodology aimed at minimizing the position error across most points within the movable range Both the tension of the mooring lines and the static stiffness of the floater showed to be sensitive to the position of floater, the direction of the wind load and the yaw of the wind turbine, with percentual changes ranging from 0.5% up to 50%. It results that the orientation of the mooring lines with correspondence of the prevailing wind direction, as well as that restrictive constraints on the tension and stiffness may be taken should be taken into account when designing a mooring system for turbine repositioning. Overall, combining , yaw-based wake redirection, and turbine repositioning allows for greater wind farm AEP, with gains contingent on turbine movable range and upcoming wind speeds. Designing position mooring systems must factor in the influence of yaw-based wake redirection and turbine repositioning on mooring system tension and stiffness. More advanced analyses, including dynamic assessments, are essential for comprehending the mooring lines system's response to position mooring for turbine repositioning, and yaw-based wake redirection.

A method is developed to automatically analyze over 200000 images of a single mature tree. This method is assessed and applied to a tree located at Risø in Roskilde, Denmark, next to two meteorological masts. Image aspects such as the surface area and the center of area are extracted from the images and they are related to the wind statistics. Concurrence is found between expected behaviors and observations to validate the method and unexpected behaviors are documented. Expected behaviors include the decrease in surface area with an increase in wind speed, swaying directional dependency with wind direction and swaying magnitude dependency with wind speed. New behaviors include a non-linear relationship between porosity and wind deficit and an increase in surface area with wind speed from infrequent wind directions. Furthermore, a specific period is analyzed which determined that for this tree, roughly half of the Vogel exponent is attributed to the decrease in surface area.

The climate actions defined by United Nations require a rapid transition to low environmental footprint technologies. The energy sector is the major emitter of carbon dioxide emissions and a significant contributor to extracting resources for fuel and power plant construction materials. Wind energy is projected to produce a significant share of electricity and energy in the following decades. The wind turbines have a small footprint during the operation, but the turbine with its foundation is a massive structure with a significant material footprint. Airborne wind energy uses tethered devices to harness high-altitude wind energy, substantially reducing bulk material use. However, better models are required to make the systems reliable and efficient. This thesis focuses on membrane traction kites that harness wind energy by flying fast crosswind maneuvers. A high-fidelity aeroelastic model for the kites is developed to predict the aerodynamic loads and the structural deformations of real systems. The aeroelastic model assumes that the membrane kite flight can be modeled as multiple steady-states without memory from the past. The steady-state aerodynamics are simulated by solving the incompressible Reynolds-averaged Navier-Stokes equations numerically. High-quality numerical grid generation strategies are developed for the unconventional wing shape of the membrane kites. The membrane kites are tensile structures, and therefore a finite element model with cable and membrane elements without rotational degrees of freedom is used to calculate the deformed shape. The solver calculates the average surface without wrinkles and applies an additional model when an element is under compression. The steady-state response of the structure is calculated with a dynamic relaxation technique. The two solvers are coupled in a partitioned manner, and during each iteration, both solvers compute a steady state. The staggered approach requires several coupling iterations to converge. The fluid mesh needs to be altered to the deformed geometry during each iteration, and therefore, the mesh is deformed with radial basis function with greedy point selection. This thesis presents three computational studies with the framework. The first two studies focus on the aerodynamics of rigidized LEI kite airfoil and wing. The aerodynamic model is validated with an already existing wind tunnel experiment on a similar airfoil. Generally, the largest model uncertainty in CFD is the mesh and therefore, the uncertainty is assessed by mesh refinement studies. A range of flight conditions is simulated by varying the inflow angle of attack, sideslip angle and Reynolds number. The flow around the wing is characterized by a recirculation zone behind the leading edge tube due to the lack of second skin. The zone is highly influenced by the inflow conditions. The effect of the chordwise inflatable tubes on aerodynamics is assessed by creating a model with and without them. The results show that the chordwise tubes have an almost negligible impact on the aerodynamic forces, which suggests they could be left out of the aerodynamic model in future work, simplifying the mesh generation and mesh deformation. The third study shows the aeroelasticity of a ram-air kite for several power configurations by changing the trim of the bridle lines. The kite forms a typical ram-air shape with ballooning in between ribs, and the nose of the wing is flattened at the stagnation region. The aerodynamics of the flexible kite is compared to a rigidized version of it. The wing is fixed at the symmetry plane and fixed to the pre-inflated shape with stagnation pressure. The results show that the flexible kite is aerodynamically more efficient than the rigidized version. The morphing wing adapts itself to the incoming flow in a way that extends the range of feasible flight conditions and improves efficiency. The aeroelastic framework converges satisfactorily with all the power setups, and it is computationally relatively inexpensive for fidelity. Consequently, the framework could be integrated into a membrane kite design process and could be a valuable asset in evaluating kite designs. @en

Offshore wind is a rapidly maturing renewable energy technology and is expected to play a central role in future energy systems. It has the potential to generate more than 420,000 TWh per year worldwide, an amount approximately equal to eighteen times the global electricity demand today. While many of the most abundantly resourceful sites are at water depths too extreme for fixed-base solutions, current floating turbine technologies suffer with high-costs and inefficiencies. Seawind Ocean Technology B.V., a Netherlands based company, is a manufacturer of fully integrated floating wind turbine systems. They have designed an innovative set of low-cost, low-weight, upwind two-bladed wind turbines, with teetering hinge and yawed power control. This teetering hinge decouples the shaft from rotor, by adding an extra degree of motion, and protects the turbine from harmful aerodynamic and gyroscopic loads, as well as the rotor from hydrodynamic loads. The innovative active yaw control eliminates the need for complex pitching systems to regulate power output, in turn reducing turbine head weight and turbine costs. There are a number of aims and objectives for this thesis project. The first, and most fundamental, was to investigate two-bladed and floating offshore turbine technology and share these findings with the hope that they make their way into the hands of change-makers who can promote the technology and consequently accelerate the green transition. The second was to optimise the loading analysis models of the Seawind 6 turbine using DNV GL’s wind turbine design software, Bladed. This is a 6 MW two-bladed floating offshore turbine and is intended for commercialisation by 2024. Seawind have extensive operational data from the Gamma 60’s deployment (the world’s first variable speed, two-bladed, wind turbine with a teetering hinge) in the 1990s. The Gamma 60 was modelled and compared to this operational data, with the calibrations in turn applied to the Seawind 6 models to improve its accuracy. Once achieved, the third goal was to carry out a thorough ultimate and fatigue load analysis of the optimised Seawind 6 model for various design load cases at extreme water depths. This investigation proved that the various investigated turbine components have been adequately sized and that suitable construction materials have been selected considering the loads they are expected to withstand. Following this large body of work, the fourth objective was to verify the theory that teeter motion is aerodynamically damped when the blades of a two-bladed, teetered turbine, are vertically oriented due to differences in angles of attack of the oncoming and retreating blades when yawed out of the wind. The fifth and final goal of this thesis work was to pull everything together and compare the floating offshore two-bladed Seawind 6 to a conventional floating three-bladed state-of-the-art turbine competitor. This was in terms of ultimate and fatigue loads experienced, capital and operational expenses (CAPEX and OPEX), lifetime carbon abatement, levelised cost of energy (LCOE), ease of manufacture and deployment, and operational performance.

Offshore wind energy is a fast-growing sector and it is quickly increasing its share in the European electricity mix. However, the costs are still high and the economic feasibility of offshore wind turbines is limited to shallow waters. This limitation plays a relevant role in the development of the sector and limits the offshore wind exploitation area to a small percentage of the potential one. In this scenario, floating support structures may represent a solution. Nevertheless, these type of support structures are not yet economically feasible as their design is challenging. In fact, with respect to bottom fixed structures, they present more complex dynamics and more degree of freedom. In order to cut the cost of floating support structures, it is necessary to be able to predict the wave- and wind-induced loads and motions of the floater and understand the coupling between them. The aim of this project is to develop a three-dimensional numerical model of a floating cylinder in order to study the Fluid-Structure Interaction (FSI) of a spar-buoy support structure for offshore wind applications. The model is based on the coupled use of the CFD solver fluidity, to resolve the fluid dynamics, and an in-house python-developed code, to numerically solve the equations of motion of a rigid body in three dimensions. To achieve this goal, firstly, the python code is developed. Secondly, a Numerical Wave Tank (NWT) containing both air and water is generated and validated with both linear and nonlinear waves. Here, both an unstructured adaptive mesh and a simplex structured mesh are used to describe the domain. Finally, a free heave decay test of a floating cylinder is performed to investigate the accuracy of the model in computing the hydrodynamic coefficients of the floater. Here, the immersed-body method is used to model the presence of the body in the fluid domain. The numerical wave tank developed in this work resulted to be quite accurate and capable of correctly describing both the linear and nonlinear waves propagation. The final model is developed with the use of a simplex structured mesh. In fact, mesh adaptivity resulted to be very challenging and the cost of its implementation exceeded the benefits. Also, a finite element based scheme including a Sweby slope limiter is used to limit advective fluxes in the setup of the NWT. Thanks to this, a less computationally demanding mesh could be used. Finally, the FSI analysis showed that, with the developed setup, the CFD solver is able to accurately predict the natural period of the floater but it underestimates the hydrodynamic damping. The cause of this was attributed to the use of the slope limiter aforementioned. In fact, it smooths the velocity field by means of numerical diffusion and this affects the resulting damping.

In recent years, the interest in deep-water wind energy projects has drastically increased, driven by the considerable wind resource in deep-water locations. In such locations, floating wind turbines are more economically attractive than bottom-fixed wind turbines. Nevertheless, the floating wind industry faces numerous challenges. One major challenge is the high investment costs involved in the manufacturing of the substructure. Additionally, it is important to maintain the wind turbines’ power output within the required standards for grid connection. Therefore, the design and optimization of substructures for floating wind turbines has to account for both capital costs and power quality. To investigate the existing research on the optimization of floating wind turbines, a literature review was conducted. Evidently, there is limited research investigating both cost and power quality optimization. Furthermore, the majority of the optimization techniques presented are accompanied by a computationally expensive substructure analysis. Moreover, in the early stage of the substructure design, a variety of different alternative scenarios are tested, requiring an extensive number of optimizations. Thus, the computational resources required by the optimization become a major concern. One promising approach to address this issue is substituting a computationally expensive analysis with a surrogate model. This solution could significantly increase the computational efficiency of the optimization process. This thesis investigates the trade-off between substructure capital cost and power quality optimization. To achieve that, a multi-objective optimization workflow is developed for the case of a semi-submersible substructure. In parallel with this task, the optimization’s analysis blocks requiring most of the computational resources are identified and substituted with a surrogate model. Two different surrogate model implementation approaches have been examined. The first approach substitutes the optimization’s substructure analysis tool (NREL’s RAFT) entirely. This approach is called the ”End-to-End” approach. The second approach substitutes a specific analysis block of RAFT that computes the floater’s hydrodynamic properties. This analysis block requires most of RAFT’s computational resources. This approach is defined as the ”PyHAMS” approach. The thesis outcome shows that improving the quality of power output comes with the drawback of increasing the substructure’s cost. The Pareto front between these two competing criteria is dominated by designs of long pontoons and reduced chain thickness. This is the most economical approach towards reduced cost and improved power quality. These Pareto designs are mainly driven by the boundaries of the design variables. Additionally, the Pareto designs are affected by constraints on the maximum pitch, maximum surge, and the existence of vertical loads on the mooring line’s anchor. Regarding the surrogate model-based optimizations, there is a dramatic reduction in the computational resources required to conduct the optimization. Among the two surrogate model approaches, the ”PyHAMS” approach achieves a better accuracy at mimicking the original optimization workflow than the ”End-to-End”. Instead, the ”End-to-End” approach is the most computationally efficient one to conduct the optimization. Nevertheless, when factoring in the computational resources to produce the training dataset, the ”PyHAMS” approach is the most computationally efficient one. This is the outcome of utilizing a reduced training dataset compared to one utilized by the ”End-to- End” approach.

Although significant strides have been made with regards to increasing the fuel economy of commercial passenger aircraft, the reduction of the environmental footprint remains of the utmost importance. The flow development over an aero-engine spinner will affect the velocity distribution over the fan of the engine and this affects the estimation of the losses generated in the fan and consequently the performance of the engine itself. Previous experimental studies have shown the existence of spiral vortices formed due to boundary layer instabilities over a rotating cone under axial and non-axial inflow. To study the effects of hub-corner separation in detail, it is first important to check the efficacy of existing commercial numerical simulation tools in the prediction of this boundary layer transition development of flow over a rotating nose-cone. The objective of the thesis was thus to simulate using URANS and LES, the formation of the counter-rotating vortices over a rotating cone. To this end. ANSYS DesignModeler and ICEM CFD were used to generate a meshed computational domain and ANSYS CFX was used as the solver. With axis-symmetric cones considered as an idealised geometry for the spinner of an aircraft engine, a 15° half-angle cone with a diameter of 47 mm was chosen. The tip of this cone was blunted by a factor of 1/100th of the cone-diameter. The domain diameter was set to 10 times the cone diameter to reduce the impact of the flow over the side walls. Two structured hexahedral meshes were generated. The BSL EARSM was chosen for the URANS simulation and the WALE Model for the LES run. To check if the simulations were able to capture the vortices, the footprint left behind by them were visualised using the non-dimensionalised instantaneous wall shear values.  The magnitude of the inlet flow velocity given was 2.46 m/s. A 2° incidence is given to the flow for the non-axial case. The effect of mesh refinement was studied for the axial case using both meshes and the finer mesh was used for the non-axial case. The axial case was studied using both the URANS and LES runs, while the non-axial case was studied using only the LES run. The variation of local Reynolds number (defined using boundary layer edge velocity) with local rotation ratio (defined using local radius, boundary layer edge velocity, and rotation velocity of the cone) was also studied and compared with available experimental data. The wall parallel velocity profiles over the length of the cone were also studied. Using the contours of the wall parallel velocity, the momentum mixing in the boundary layer due to the vortices was visualised and compared with the results from the experiments. To characterise the spatial development of the spiral vortices, a critical point was defined using the wall friction coefficient. This critical point was then compared to the one obtained through the experiments. The number of counter-rotating vortices formed over the length of the cone was also checked and the trend between the simulations and experiments were compared.

Offshore wind energy is a fast-growing sector in the energy industry, and the cost of electricity per kilowatt-hour from offshore wind is decreasing significantly. Until now, offshore wind turbines are mainly installed on bottom-mounted foundations, which are economically feasible in shallow waters. This factor has limited the development of offshore wind to those countries that can benefit from abundant wind resources located in shallow waters. To further extend the wind energy market, floating support structures for wind turbines are being studied and developed. This technology allows harvesting the wind energy resources located in deep waters, avoiding the costly construction and installation of large towers. A floating support introduces different challenges with respect to a bottom mounted one, not least the fact that the dynamics of the system are significantly different. The aim of this Master thesis is to generate a numerical model that can predict the dynamics of a geometrically simple, two-dimensional floater under the action of a train of regular waves. The wave-structure interaction problem is solved coupling the CFD solver fluidity with a rigid body code developed in Python that numerically solves the equations of motion for a rigid body and uses NURBS to define the geometry of the body. The immersed body method is used to represent the solid body into the fluid. The results obtained with this approach are then compared with potential flow results, experimental results and results obtained with other CFD solvers available in the literature. Similarities and differences are outlined for the different numerical experiments that have been simulated. In general, good agreement has been found with experimental results and other CFD solvers results. With respect to potential flow theory, good agreement has been found for low-frequency waves, while differences have been noticed for the high-frequency waves.

Floating offshore wind energy is being widely implemented in deep-water sites thanks to its numerous advantages, which among others include the access to stronger and undisturbed wind resources. In addition, considering the fast-paced growth in size and power generating capacity of offshore wind turbines, the designs of floating foundations need to be continuously improved. Therefore, proper testing (at model-scale) is necessary. To achieve this, several factors need to be taken into account. For instance, down-scaling for testing of floating structures brings challenges due to the so-called Reynolds effects, which degrade the aerodynamic response of the mounted turbine. Likewise, the rotation of the blades and the motions of the platform as a result of its interaction with waves, have an impact on the wind inflow acting upon the rotor. Thereby, modifying its instantaneous performance. This Master thesis, carried out in cooperation with MARIN (Maritime Research Institute Netherlands), focuses on developing a reliable methodology that renders in the design of a scaled-down thrust-matched 10MW wind turbine rotor blade, for its further implementation in the testing of a floating offshore support structure. For this purpose, the aforementioned effects are investigated, and a suitable set of design tools is selected and coupled, in order to realise an optimised blade geometry. For predicting the wind turbine behaviour, this implementation relies on the Blade Element Momentum Theory. Furthermore, the resulting blade's aerodynamic performance is validated using ReFRESCO (MARIN’s in-house CFD code), against the non-dimensional parameters of the DTU 10MW reference wind turbine.

Energy transition is imperative to effectively address the pressing issue of climate change resulting from global warming. In this transition, offshore wind power assumes a pivotal role as a crucial and indispensable source of clean and renewable energy. Offshore benefits become more pronounced as the offshore locations progress far offshore. In deep-water areas, where 80% of the worldwide offshore wind energy can be potentially harnessed, the utilization of floating foundations becomes essential instead of traditional bottom fixed ones. The present study seeks to investigate the disparities in power generation and energy production that arise from the replacement of bottom fixed wind turbines with floating counterparts. The former is represented by the monopile foundation, while the latter by the spar buoy. The power performance difference lies in the ability of floating structures to move, which can lead to suboptimal positioning of the rotor relative to the incoming wind inflow, mainly due to spar’s pitch and surge motions. The investigation is conducted using two distinct aerodynamic model of lower and higher fidelities, BEM and OLAF, respectively, to assess their effects on the outcomes. In the field of offshore wind turbine design, engineers rely on aero-hydro-servo-elastic software codes to simulate the dynamic behavior of floating offshore wind turbine systems in offshore environments. OpenFAST, an open-source software, has been extensively developed and validated for conducting such investigations. In this study, OpenFAST is employed to develop both floating and bottom fixed wind turbine models. Specifically, a coupled aero-hydro-servo-elastic model of a floating spar wind turbine is created, and the simulated motion of the spar is compared with measurements obtained from an actual floating turbine deployed in the Hywind Scotland floating offshore wind farm. Metocean data, spar measurement data, and spar system descriptions are provided by Equinor to facilitate this benchmarking process. Additionally, an equivalent monopile wind turbine model is developed for energy yield comparison purposes. The simulated spar motion results of the developed OpenFAST model exhibit reasonable realism when benchmarked with the measured data for all load cases and modal analysis, despite assumptions and uncertainties influencing the model's accuracy. Model discrepancies primarily stem from undisclosed wind turbine parameters and controller strategy as well as some modeling simplifications inherent in OpenFAST. Nevertheless, through statistical, time series and power spectral density comparisons against full-scale Hywind measurements encompassing various wind speeds (below-rated, rated, and above cut-out), the developed floating model is validated, thereby ensuring the reliability of its energy production outcomes. It is observed that the spar’s pitch and surge mean offset is mostly affected by the current speeds while varying wave conditions (height and period) influence their oscillation amplitudes. Power is slightly affected by the ocean conditions, primarily the wave effects, while it is more strongly influenced by the spar nacelle’s velocity and direction relative to the incoming wind, especially when subjected to lower wind speed fields. Finally, for both BEM and OLAF simulations, monopile bottom fixed structures produce higher amounts of energy annually compared to the floating spar counterparts. The implementation of alternative controllers specifically designed for FOWT, with optimization objectives focused on either maximizing power performance or ensuring structural integrity and longevity, results in estimated AEP reductions when utilizing a spar floater instead of a monopile foundation. For the BEM model, the estimated AEP decrease ranges from 3.46% to 8.62%, depending on the specific controller optimization objective. On the other hand, when employing the VPM-OLAF model, the estimated AEP reduction for a spar floater compared to a monopile substructure ranges from 4.50% to 9.58%, under similar conditions as previously mentioned. By employing OLAF instead of BEM, the computed annual energy yield for both Bottom Fixed and Floating offshore wind turbines increases by approximately 25%. In conclusion, it is recommended that future research prioritizes resolving identified discrepancies in the model setup, addressing phenomena not adequately captured by the current OpenFAST model, and conducting additional validation of the spar model using a wider range of measured parameters.

The advancement of floating wind turbine technology in recent years necessitates accurate predictions of their behaviour using various simulation tools. Utilizing multi-fidelity tools like OpenFAST allows for comprehensive calculations of the entire floating wind turbine structure. However, the precision of these calculations is challenged by the uncertainties associated with different system parameters. The main goal of this research is to conduct a comprehensive exploration of various parameters, with a particular emphasis on hydrodynamic factors, and their impact on the structural and aerodynamic performance of semi-submersible floating wind turbines. To achieve this, an extensive sensitivity analysis approach is employed, utilizing the multi-fidelity model OpenFAST. The simulations are conducted on a semi-submersible floating wind turbine, specifically the one used in the OC4 experiment, featuring the 5MW NREL wind turbine. The Elementary Effects (EE) method is employed in this study to assess the significance of various factors. These factors include the hydrodynamic drag coefficients, wave height, peak wave period, wind speed, wind direction, current speed, current direction, and turbulence intensity. Their influence on the Damage Equivalent Load (DEL) of different loads acting on the floating offshore wind turbine (FOWT) is thoroughly examined. The analysis conducted in this study has unveiled several key findings. The most influential parameters identified are the current speed, primarily impacting mooring line tension, and the significant wave height, which exhibits a balanced effect across various outputs, including hydrodynamic loads, tower loads, and rotor dynamics. Additionally, turbulence intensity emerges as a significant factor, particularly concerning rotor thrust and torque. When employing the Morison equation, the order of parameter significance remains consistent with the hybrid theory (Potential + Drag). However, it is noteworthy that the added mass coefficients carry greater significance compared to the drag coefficient. Furthermore, simulations conducted using probability distributions for different locations confirm that wave height consistently ranks as the most significant parameter. However, the overall significance may vary and decrease in more severe environmental conditions. The comparison with the OC3 spar platform revealed a similar pattern when examining the influence of current speed and significant wave height. Nonetheless, the simple hydrodynamic drag coefficient used for the whole platform also displayed significance, emphasizing the sensitivity of the spar platform loads to this modelling parameter. This significance is much higher than the corresponding one of the semisubmersible platform. The research underscores it is imperative to mitigate the uncertainty associated with various parameters. This can be achieved through experimental verification and establishing correlations between modelling parameters and the specific conditions being simulated. Additionally, given that the most influential parameters are related to external conditions, it becomes crucial to simulate conditions that closely resemble the anticipated deployment location for the floating offshore wind turbine (FOWT). By doing so, the accuracy and reliability of the simulations will be enhanced to better inform FOWT design and deployment strategies.

The field of Airborne Wind Energy is concerned with harvesting high-altitude wind power; the corresponding group in Delft, in particular, is investigating the suitability of a kite for this purpose. Due to the high complexity that such a system ensues, it is of interest to have a fundamental understanding of the critical aero-elastic modes of the structure in flight. The aim of this project was to further extend on the existing research efforts in this area by coupling a high fidelity Reynolds-Averaged Navier-Stokes solver with an improved version of the existing structural solver framework and validate the model on simplified test cases using a partitioned approach. The developed methodology uses the open-source Computational Fluid Dynamics solver foam-extend with an adapter to the coupling library preCICE. For the structure model, an in-house Python code based on a nonlinear shell element formulation is utilised. A literature survey revealed that while there exists an abundance of publications on strongly coupled Fluid-Structure Interaction problems, the majority of them are targeted at applications set in completely different flow regimes. Thus, the most difficult part of the project was to find appropriate validation data for membrane wings at high Reynolds numbers Finally, the capabilities of the method have been successfully showcased on a classic FSI benchmark case, and a partial validation on benchmark cases at more realistic Reynolds numbers has been carried out as well. Hence going forward a full quantification of the accuracy of the method for its target application range is recommended.