An Integrated Nonlinear Wind-Waves Model for Offshore Wind Turbines

This thesis presents a numerical model capable of simulating offshore wind turbines exposed to extreme loading conditions. External condition-based extreme responses are reproduced by coupling a fully nonlinear wave kinematic solver with a hydro-aero-elastic simulator. First, a two-dimensional fully nonlinear wave simulator is developed. The transient nonlinear free surface problem is formulated assuming the potential theory and a high-order boundary element method is implemented to discretize Laplace's equation. For temporal evolution a second-order Taylor series expansion is used. The code, after validation with experimental data, is successfully adopted to simulate overturning plunging breakers which give rise to dangerous impact loads when they break against wind turbine substructures. Emphasis is then placed on the random nature of the waves. Indeed, through a domain decomposition technique a global simulation framework embedding the numerical wave simulator into a more general stochastic environment is developed. The proposed model is meant as a contribution to meet the more and more pressing demand for research in the offshore wind energy sector as it permits taking into account dangerous effects on the structural response so as to increase the global structural safety level

Lectures on Solid Mechanics

This volume presents the theoretical basics of solid mechanics collecting the lectures held by the Authors for the course of Mechanics of Solids to environmental engineering students at the University of Florence. Lectures on Solid Mechanics is organized in two parts. The first one introduces the theory of three-dimensional elasticity where, after a preparatory synthesis of the basic concepts of mathematics and geometry, the fundamental framework of strain and stress in elastic bodies are introduced. Then the classical law of linear elasticity is presented and finally the part concludes with the "Principle of Virtual Work and variational methods". Moreover, at the end of selected chapters the essential notions of the theory of shells are discussed. The second part concerns the traditional theory of beams focusing on the four fundamental cases: beam under axial forces, terminal couples, torsion, bending and shear. The Readers addressed by this volume are mainly the undergraduate students of Engineering Schools

Ein nichtlineares Modell zur kombinierten Berechnung von Wind- und Wellenlasten auf Offshore-Windenergieanlagen

This thesis presents a numerical model capable of simulating offshore wind turbines exposed to extreme loading conditions. External condition-based extreme responses are reproduced by coupling a fully nonlinear wave kinematic solver with a hydro-aero-elastic simulator. First, a two-dimensional fully nonlinear wave simulator is developed. The transient nonlinear free surface problem is formulated assuming the potential theory and a higher-order boundary element method (HOBEM) is implemented to discretize Laplace's equation. For temporal evolution a second-order Taylor series expansion is used. The code, after validation with experimental data, is successfully adopted to simulate overturning plunging breakers which give rise to dangerous impact loads when they break against wind turbine substructures. The impact force is quantified by means of an analytical model and the total hydrodynamic action is finally obtained by adding the impulsive term to the drag and inertial ones. In the second main core of the thesis, emphasis is placed on the random nature of the waves. Indeed, a global simulation framework embedding the numerical wave simulator into a more general stochastic environment is developed. Namely, first a linear irregular sea is generated by the spectral approach, then, only on critical space-time sub-domains, the fully nonlinear solver is invoked for a more refined simulation. The space-time sub-domains are defined as the wind turbine near field (space) times the time interval in which wave impacts are expected (time). Such a domain decomposition approach permits systematically accounting for dangerous effects on the structural response (which would be totally missed by adopting linear or weakly nonlinear wave theories alone) without penalizing the computational effort normally required. At the end of the work the attention is moved to the consequences that the proposed model would have in the quantification of the structural risk.In dieser Arbeit wird ein numerisches Modell zur Simulation von Offshore-Windenergieanlagen unter extremen Lasteinwirkungen entwickelt. Dazu wird ein vollständig kinematisch nichtlineares Wellenmodell mit einem hydroaeroelastischen Modell kombiniert. Zunächst wird das instationäre nichtlineare Problem der freien Wasseroberfläche unter Verwendung der zweidimensionalen Potentialtheorie beschrieben. Die sich ergebende Laplace-Gleichung wird mit einer Randelementmethode höherer Ordnung räumlich diskretisiert. Für die zeitliche Entwicklung wird eine Taylor Reihe zweiter Ordnung verwendet. Nach Abgleichung mit experimentellen Daten wird der entwickelte Algorithmus angewendet, um die für die Stoßbelastung von Windkraftanlagen ursächlichen überschlagenden brechenden Wellen zu simulieren. Die gesamte hydrodynamische Last wird schließlich durch ein analytisches Modell beschrieben, bei dem ein Term, der die Stoßwirkung der Wellen berücksichtigt, zu den Längs- und Trägheitskräften hinzugefügt wird. Im zweiten Teil der Arbeit wird das Wellenmodell in eine Simulationsumgebung eingebettet, welche die stochastischen Natur des Wellengangs erfasst. Hierbei wird zuerst ein linear beschriebener breitbandiger Seegang mithilfe des Spektralansatzes erzeugt. Beschränkt auf kritische Bereiche in der räumlichen und zeitlichen Simulation wird im Anschluss das vollständig nichtlineare hydrodynamische Modell für eine genauere Lösung herangezogen. Die kritischen Bereiche sind auf die nähere Umgebung der Windenergieanlage beim Eintreffen der brechenden Welle begrenzt. Diese Substrukturtechnik erlaubt es, für die Strukturantwort maßgebende Effekte systematisch zu erfassen, die bei einer Verwendung von linearen oder schwach nichtlinearen Wellentheorien komplett vernachlässigt werden, ohne dabei den herkömmlichen Rechenaufwand substantiell zu erhöhen. Zum Abschluss der Arbeit wird diskutiert, wie sich das vorgestellte Modell auf die Quantifizierung des Risikos der Struktur auswirkt

comparison of hydrodynamic loading models for vertical cylinders in nonlinear waves

Abstract This paper introduces a comparison study between various hydrodynamic loading models in highly nonlinear waves and discusses its first phase - comparing Morison equation and Rainey corrections on a fixed cylinder in regular steep waves. In this study both of these two models showed similar results when compared against experimental data. Morison equation is found to capture the amplitude of the loading sufficiently well. However, neither model was able to capture higher-order loading components which are apparent in very steep waves and are associated with ringing. The main conclusion of this work is the identification of the need of a more appropriate loading model

Comparison of nonlinear wave-loading models on rigid cylinders in regular waves

© 2019 by the authors. Monopiles able to support very large offshore wind turbines are slender structures susceptible to nonlinear resonant phenomena. With the aim to better understand and model the wave-loading on these structures in very steep waves where ringing occurs and the numerical wave-loading models tend to lose validity, this study investigates the distinct influences of nonlinearities in the wave kinematics and in the hydrodynamic loading models. Six wave kinematics from linear to fully nonlinear are modelled in combination with four hydrodynamic loading models from three theories, assessing the effects of both types of nonlinearities and the wave conditions where each type has stronger influence. The main findings include that the nonlinearities in the wave kinematics have stronger influence in the intermediate water depth, while the choice of the hydrodynamic loading model has larger influence in deep water. Moreover, finite-depth FNV theory captures the loading in the widest range of wave and cylinder conditions. The areas of worst prediction by the numerical models were found to be the largest steepness and wave numbers for second harmonic, as well as the vicinity of the wave-breaking limit, especially for the third harmonic. The main cause is the non-monotonic growth of the experimental loading with increasing steepness due to flow separation, which leads to increasing numerical overpredictions since the numerical wave-loading models increase monotonically

coupling effects on the dynamic response of moored floating platforms for offshore wind energy plants

Abstract The increasing importance of offshore deep-water wind energy together with the complexity of the wind-wave-structure interaction problem makes the dynamic analysis of floating platforms a case of considerable interest. In this work, the dynamics of moored floating platforms for deep-water wind energy purposes is analysed in regular waves in order to discuss the effects on the motion due to the coupling of different degrees of freedom, usually associated with the operation of the mooring system and the hydrodynamic action, and the role of the main parameters affecting the motion. The platform is modelled as a rigid body and the associated differential dynamic problem is solved by using a suitable Lie group time integrator. The loads associated with mooring lines and waves are respectively assessed through a quasi-static model and a linear hydrodynamic model. The coupling of different degrees of freedom is usually related to loads with higher-frequency components and non-zero mean value that could bring the system into a mean dynamic configuration rather different from the static equilibrium configuration. Moreover, very interesting to limit the oscillations of the body is the effect of the location of the center of mass, the lower the center the lower the amplitude of pitch and roll response

Metastasis to parotid gland from non Head and Neck tumors

Most primary tumors spreading metastasis to the parotid gland are usually located in the head and neck region, nonetheless, rarely, parotid gland can also be the target of metastatic localization site of distant primary tumors. The purpose of this study was to describe a clinical series of metastasis to the parotid gland from distant primary tumors (non Head & Neck)

On long-term fatigue damage estimation for a floating offshore wind turbine using a surrogate model

This study is concerned with the estimation of long-term fatigue damage for a floating offshore wind turbine. With the ultimate goal of efficient evaluation of fatigue limit states for floating offshore wind turbine systems, a detailed computational framework is introduced and used to develop a surrogate model using Gaussian process regression. The surrogate model, at first, relies only on a small subset of representative sea states and, then, is supplemented by the evaluation of additional sea states that leads to efficient convergence and accurate prediction of fatigue damage. A 5-MW offshore wind turbine supported by a semi-submersible floating platform is selected to demonstrate the proposed framework. The fore-aft bending moment at the turbine tower base and the fairlead tension in the windward mooring line are used for evaluation. Metocean data provide information on joint statistics of the wind and wave along with their relative likelihoods for the installation site in the Mediterranean Sea, near the coast of Sicily. \textcolor{black}{A coupled frequency-domain model} provides needed power spectra for the desired response processes. The proposed approach offers an efficient and accurate alternative to the exhaustive evaluation of a larger number of sea states and, as such, avoids excessive response simulations