Esta dissertação aborda o desenvolvimento de um método numérico para a análise de forças e momentos aerodinâmicos em turbinas eólicas fixas e flutuantes no domínio do tempo, utilizando a teoria da quantidade de movimento do elemento de pá (Blade Element Momentum Theory, BEMT) em C++. As pás são divididas em segmentos menores, onde a influência da turbina no fluxo é realizada através do cálculo de fatores de indução. Cada segmento é considerado como um aerofólio bidimensional, sendo possível estimar forças e momentos através de coeficientes para asas infinitas. A teoria da quantidade de movimento do elemento de pá, embora conceitualmente simples, é usualmente empregada com algumas correções em suas equações para se ajustar aos resultados experimentais. A inclusão de turbinas flutuantes é realizada através do movimento de corpo rígido da plataforma, que tem um impacto direto no cálculo aerodinâmico. Por não ser o objetivo deste trabalho, as equações de movimento são calculadas através de uma fonte externa e posteriormente colocadas como dado de entrada do código, simplificando assim a análise e excluindo uma fonte potencial de erro na verificação. O caso de estudo é a turbina do projeto Offshore Code Comparison Collaboration Continuation (OC4), a qual é analisada como uma turbina fixa e flutuante, utilizando uma plataforma semi-submersível. Os resultados das forças e momentos aerodinâmicos do software FAST do Laboratório Nacional de Energias Renováveis (National Renewable Energy Laboratory, NREL) são comparados ao código desenvolvido, mostrando excelente concordância para todos os casos analisados. / This dissertation addresses the development of a numerical method for the analysis of aerodynamic forces and moments of fixed and floating wind turbines in time domain, using the Blade Element Moment Theory (BEMT) written in C++. The blades are divided into smaller segments, where the influence of the turbine in the flow is performed through the calculation of induction factors. Each segment is considered as a two-dimensional airfoil, and it is possible to estimate forces and moments through coefficients for infinite wings. The Blade Element Moment Theory, though conceptually simple, is usually employed with some corrections in its equations to fit experimental results. The inclusion of floating turbines is performed through the rigid body motion of the platform, which has a direct impact on the aerodynamic calculation. Since it is not the objective of this work, the equations of movement are calculated through an external source and then placed as input data of the code, thus simplifying analysis and excluding a potential source of error in verification. The case of study is the turbine of the Offshore Code Comparison Collaboration Continuation (OC4) project, which is analyzed either as a fixed or a floating turbine, using a semi-submersible platform. The results of aerodynamic forces and moments from FAST software of the National Renewable Energy Laboratory (NREL) are compared to the developed code, showing excellent agreement for all cases analyzed.

This research focuses on studying the feasibility of placing large wind turbines on deep-ocean platforms. Water tank studies have been conducted using the facilities at Alden Research Laboratories (ARL) on 100:1 scale Tension Leg Platform (TLP) and Spar Buoy (SB) models. Froude scaling was used for modeling the offshore wind turbine designs. Primary components of the platform turbine, tower, and cable attachments were fabricated in ABS plastic using rapid prototyping. A wireless data acquisition system was installed to prevent umbilical data cables from affecting the behavior of the platform when exposed to wave loading. In Phase I testing, Froude-scaled TLP and Spar Buoy models at a 100:1 scale were placed in a water flume and exposed to periodic waves at amplitudes ranging from 0.5 cm - 7.5 cm and frequencies ranging from 0.25 Hz - 1.5 Hz. The testing was conducted on simple tower and turbine models that only accounted for turbine weight at the nacelle. In Phase II testing, emphasis was placed on further testing of the tension leg platform as a more viable design for floating offshore wind turbines. The tension leg platform scale model was improved by adding a disc to simulate drag force incident at the top of the tower, as well as a rotor and blades to simulate the gyroscopic force due to turbine blade rotation at the top of the tower. Periodic wave motions of known amplitude and frequency were imposed on the model to study pitch, heave, roll, surge, sway motions and mooring cable tensions (in Phase II only) using accelerometers, inclinometers, capacitance wave gage, and load cells. Signal analysis and filtering techniques were used to refine the obtained data, and a Fourier analysis was conducted to study the dominant frequencies. Finally, Response Amplitude Operators (RAO's) were plotted for each data set to standardize the results and study the overall trend with respect to changes in wave amplitude and frequency. For Phase I testing, it is shown that surge motion of the platform dominates other motions for both the tension leg platform and spar buoy, and varying tether pretension has little effect on response amplitude operator values. For phase II testing, it was found that the introduction of thrust and gyroscopic forces increases sway and pitch motions as well as upstream tether forces. Coupling effects of pitch motion with roll and sway due to the presence of gyroscopic forces were also seen. The present experimental results can be used to validate the hydrodynamic kernels of linear frequency-domain models, time-domain dynamics models, and computational simulations on floating wind turbines. Numerical analysis and simulations have been conducted in a separate study at WPI. These simulations are comparable to the experimental results.

La simulation numérique des éoliennes flottantes est essentielle pour le développement des Energies Marines Renouvelables. Les outils de simulation classiquement utilisés supposent un écoulement stationnaire sur les rotors. Ces théories sont généralement assez précises pour calculer les forces aérodynamiques et dimensionner les éoliennes fixes (à terre ou en mer) mais les mouvements de la plateforme d’une éolienne flottante peuvent induire des effets instationnaires conséquents. Ceux-ci peuvent par exemple impacter la force de poussée sur le rotor. Cette thèse de doctorat cherche à comprendre et à quantifier les effets de l’aérodynamique instationnaire sur la tenue à la mer des éoliennes flottantes, dans différentes conditions de fonctionnement. L’étude montre que les forces aérodynamiques instationnaires impactent les mouvements de la plateforme lorsque le rotor est fortement chargé. Les modèles quasi-stationnaires arrivent néanmoins à capturer la dynamique des éoliennes flottantes avec une précision suffisante pour des phases de design amont. Les éoliennes flottantes à axe vertical sont elles aussi étudiées pour des projets offshore puisqu’elles pourraient nécessiter des coûts d’infrastructure réduits. Après avoir étudié l’influence de l’aérodynamique instationnaire sur la tenue à la mer de ces éoliennes, une comparaison est menée entre éoliennes flottantes à axe horizontal et à axe vertical. Cette dernière subit une importante poussée aérodynamique par vents forts, induisant de très grands déplacements et chargements. / Accurate numerical simulation of thesea keeping of Floating Wind turbines (FWTs) is essential for the development of Marine Renewable Energy. State-of-the-art simulation tools assume a steady flow on the rotor. The accuracy of such models has been proven for bottom-fixed turbines, but has not been demonstrated yet for FWTs with substantial platform motions. This PhD thesis focuses on the impact of unsteady aerodynamics on the seakeeping of FWTs. This study is done by comparing quasi-steady to fully unsteady models with a coupled hydro-aerodynamic simulation tool. It shows that unsteady load shave a substantial effect on the platform motion when the rotor is highly loaded. The choice of a numerical model for example induces differences in tower base bending moments. The study also shows that state of the art quasi-steady aerodynamic models can show rather good accuracy when studying the global motion of the FWTs. Vertical Axis Wind Turbines (VAWTs) could lower infrastructure costs and are hence studied today for offshore wind projects. Unsteady aerodynamics for floating VAWT sand its effects on the sea keeping modelling have been studied during the PhD thesis,leading to similar conclusions than for traditional floating Horizontal Axis Wind Turbines (HAWTs). Those turbines have been compared to HAWTs. The study concludes that, without blade pitch control strategy, VAWTs suffer from very high wind thrust at over-rated wind speeds, leading to excessive displacements and loads. More developments are hence needed to improve the performance of such floating systems.

Environmental, aesthetic and political pressures continue to push for siting off-shore wind turbines beyond sight of land, where waters tend to be deeper, and use of floating structures is likely to be considered. Savings could potentially be realized by reducing hull size, which would allow more compliance with the wind thrust force in the pitch direction. On the other hand, these structures with large-amplitude motions will make dynamic analysis both more challenging and more critical. Prior to the present work, there were no existing dynamic simulation tools specifically intended for compliant wind turbine design. Development and application of a new computational method underlying a new time-domain simulation tool is presented in this dissertation. The compliant floating wind turbine system is considered as a multibody system including tower, nacelle, rotor and other moving parts. Euler's equations of motion are first applied to the compliant design to investigate the large-amplitude motions. Then, a new formulation of multibody dynamics is developed through application of the conservation of both linear momentum and angular momentum to the entire system directly. A base body is prescribed within the compliant wind turbine system, and the equations of motion (EOMs) of the system are projected into the coordinate system associated with this body. Only six basic EOMs of the system are required to capture 6 unknown degrees of freedom (DOFs) of the base body when mechanical DOFs between contiguous bodies are prescribed. The 6 x 6 mass matrix is actually composed of two decoupled 3 x 3 mass matrices for translation and rotation, respectively. Each element within the matrix includes the inertial effects of all bodies. This condensation decreases the coupling between elements in the mass matrix, and so minimizes the computational demand. The simulation results are verified by critical comparison with those of the popular wind turbine dynamics software FAST. The new formulation is generalized to form the momentum cloud method (M- CM), which is particularly well suited to the serial mechanical N-body systems connected by revolute joints with prescribed relative rotation. The MCM is then expanded to multibody systems with more complicated joints and connection types.

Le travail présenté vise à mettre en place une méthodologie pour le suivi en service de la fatigue mécanique pour l’ombilical dynamique d’un système EMR flottant. L’approche envisagée consiste à simuler à l’aide d’outils numériques la réponse de l’ombilical aux cas de chargement observés sur site. Le post-traitement des résultats de ces simulations devant permettre d’accéder à différentes quantités d’intérêt en tout point du câble. Pour quantifier et réduire l’incertitude sur la réponse calculée de l’ombilical ce dernier doit être instrumenté. Un certain nombre de paramètres du modèle numérique feront alors l’objet d’une calibration régulière pour suivre l’évolution des caractéristiques de l’ombilical susceptibles d’évoluer. Dans ce contexte ce manuscrit présente et compare différentes méthodes pour analyser la sensibilité de la réponse de l’ombilical aux paramètres susceptibles d’être suivis. L’objectif est notamment d’orienter le choix des mesures à mettre en oeuvre. L’analyse en composantes principales permet pour cela d’identifier les principaux modes de variation de la réponse de l’ombilical en réponse aux variations des paramètres étudiés. Différentes approches sont également envisagées pour la calibration des paramètres suivis,avec en particulier le souci de quantifier l’incertitude restante sur le dommage. Les méthodes envisagées sont coûteuses en nombre d’évaluations du modèle numérique et ce dernier est relativement long à évaluer. L’emploi de méta-modèles en substitution des simulations numériques apparait donc nécessaire, et là encore différentes options sont considérées. La méthodologie proposée est appliquée à une configuration simplifiée d’ombilical dans des conditions inspirées du projet FLOATGEN. / The present work introduces a methodology to monitor fatigue damage of the dynamic power cable of a floating wind turbine. The suggested approach consists in using numerical simulations to compute the power cable response at the sea states observed on site. The quantities of interest are then obtained in any location along the cable length through the post-treatment of the simulations results. The cable has to be instrumented to quantify and to reduce the uncertainties on the calculated response of the power cable. Indeed some parameters of the numerical model should be calibrated on a regular basis in order to monitor the evolution of the cable properties that might change over time. In this context, this manuscript describes and compares various approaches to analyze the sensitivity of the power cable response to the variations of the parameters to be monitored. The purpose is to provide guidance in the choice of the instrumentation for the cable. Principal components analysis allows identifying the main modes of power cable response variations when the studied parameters are varied. Various methods are also assessed for the calibration of the monitored cable parameters. Special care is given to the quantification of the remaining uncertainty on the fatigue damage. The considered approaches are expensive to apply as they require a large number of model evaluations and as the numerical simulations durations are quite long. Surrogate models are thus employed to replace the numerical model and again different options are considered. The proposed methodology is applied to a simplified configuration which is inspired by the FLOATGEN project.

Offshore wind turbines have the potential to be an important part of the United States' energy production profile in the coming years. In order to accomplish this wind integration, offshore wind turbines need to be made more reliable and cost efficient to be competitive with other sources of energy. To capitalize on high speed and high quality winds over deep water, floating platforms for offshore wind turbines have been developed, but they suffer from greatly increased loading. One method to reduce loads in offshore wind turbines is the application of structural control techniques usually used in skyscrapers and bridges. Tuned mass dampers are one structural control system that have been used to reduce loads in simulations of offshore wind turbines. This thesis adds to the state of the art of offshore wind energy by developing a set of optimum passive tuned mass dampers for four offshore wind turbine platforms and by quantifying the effects of actuator dynamics on an active tuned mass damper design. The set of optimum tuned mass dampers are developed by creating a limited degree-of-freedom model for each of the four offshore wind platforms. These models are then integrated into an optimization function utilizing a genetic algorithm to find a globally optimum design for the tuned mass damper. The tuned mass damper parameters determined by the optimization are integrated into a series of wind turbine design code simulations using FAST. From these simulations, tower fatigue damage reductions of between 5 and 20% are achieved for the various TMD configurations. A previous study developed a set of active tuned mass damper controllers for an offshore wind turbine mounted on a barge. The design of the controller used an ideal actuator in which the commanded force equaled the applied force with no time lag. This thesis develops an actuator model and conducts a frequency analysis on a limited degree-of-freedom model of the barge including this actuator model. Simulations of the barge with the active controller and the actuator model are conducted with FAST, and the results are compared with the ideal actuator case. The realistic actuator model causes the active mass damper power requirements to increase drastically, by as much as 1000%, which confirms the importance of considering an actuator model in controller design.

The offshore wind energy industry is a rapidly growing industry as solutions are becoming cost-competitive and there is an increasing need to limit greenhouse gas emissions. New floating offshore wind turbine designs now enable the access to previously inaccessible offshore wind resources. In this research, a comprehensive analysis is made of the different factors influencing the macro environment for a potential floating offshore wind energy market in California. The analysis assesses the relevant political, economic,social, technological, environmental, and legal aspects in California. The outcome of this research shows the opportunities and challenges for a floating wind turbine market in California. It is found that there are many opportunities present due to California's political and economic climate. There is considerable support for offshore wind projects on the state level, demonstrated by the active engagement of the governor and the creation of the California Task Force. The large economy and high electricity prices are promising for future projects. Furthermore, wind resources are vast and the technical infrastructure is present, especially Southern California is well suited. There are technological threats present, but these are common for all renewable energy sources and seem unavoidable with the Renewable Portfolio Standards California has set. The main threats are posed by the complex regulatory environment and the financial uncertainty as a result of the lackof federal support. The Jones Act, for example, can be troublesome as it will likely increase costs and delay projects. Furthermore, the social environment and local willingness for such projects was shown to be very important for their success. The state of California has already been working pro-actively on involving the local members of thepublic in potential upcoming offshore wind energy projects. The research concludes that California offers many opportunities with surmountable threats.

The development of new types of offshore wind turbine on floating platforms requires the development of new approaches to modelling the combined platform-turbine system. In this thesis a linearised frequency-domain approach is developed which gives fast but approximate results: linearised models of the structural dynamics, hydrodynamics, aerodynamics and control system dynamics are brought together to find the overall response of the floating wind turbine to harmonic wind and wave loading. Initially, a nonlinear flexible multibody dynamics code is developed and verified, which is then used to provide reference nonlinear simulation results. The structural dynamics of a wind turbine on a moving platform are shown to be nonlinear, but for realistic conditions the effects are small. An approximate analysis of the second-order response of floating cylinders to hydrodynamic loads suggests slow drift motion may be relatively small for floating wind turbines, compared to other floating offshore structures. The aerodynamic loads are linearised using both harmonic and tangent linearisation approaches; the harmonic linearisation gives improved results when stall occurs. The wake dynamics can also be included. The control system behaviour is linearised using the same method, which works well when the wind speed is far from the rated wind speed; close to the rated wind speed the nonlinearity is stronger, but further improvement should be possible. These sub-models are combined to give a simple but complete model of a floating wind turbine, with flexible blades and a flexible tower, but neglecting the control system behaviour, wake dynamics and nonlinear hydrodynamic loads. For the OC3-Hywind turbine, the accuracy of the results is assessed by comparison to nonlinear time-domain simulations using the commercial code Bladed. Peak-peak errors of less than 5 % are achievable for many harmonic wind and wave inputs, but certain conditions lead to larger errors. The effect of including linearised control system behaviour is demonstrated for a subset of conditions. Overall, the results are promising but more work is needed for practical application.