Dissertations / Theses on the topic 'Offshore Floating Structures'

The gradual depletion of shallow water hydrocarbon deposits has forced the offshore oil and gas industry to develop reserves in deeper waters. Dynamically installed anchors have been proposed as a cost-effective anchoring solution for floating offshore structures in deep water environments. The rocket or torpedo shaped anchor is released from a designated drop height above the seafloor and allowed to penetrate the seabed via the kinetic energy gained during free-fall and the anchor’s self weight. Dynamic anchors can be deployed in any water depth and the relatively simple fabrication and installation procedures provide a significant cost saving over conventional deepwater anchoring systems. Despite use in a number of offshore applications, information regarding the geotechnical performance of dynamically installed anchors is scarce. Consequently, this research has focused on establishing an extensive test database through the modelling of the dynamic anchor installation process in the geotechnical centrifuge. The tests were aimed at assessing the embedment depth and subsequent dynamic anchor holding capacity under various loading conditions. Analytical design tools, verified against the experimental database, were developed for the prediction of the embedment depth and holding capacity.

The present work highlights some aspects related to the analyses of Arctic offshore floating structures. This thesis consists of five papers, which can be divided into two main categories. One category deals with the dynamics of slender structures with an emphasis on the prediction and suppression of vortex induced vibrations (VIV), and the other category examines the process of interaction between sloping structures and sea ice with focus on developing a numerical model to simulate this process in real time. Slender structures, such as mooring lines and marine risers, are very important for the offshore petroleum industry, which is currently approaching deeper waters. Increasingly, attention has been focused on predicting the susceptibility of these structures to VIV. In this thesis, two asymptotic techniques namely, the local analysis and the WKB methods, were used to derive closed-form solutions for the natural frequencies and mode shapes of slender line-like structures. Both the top-tensioned nearly-vertical configuration and the catenary configuration were considered. The accuracy of the solutions derived was established through comparison with other analytic solution techniques and with results of numerical finite element solutions. The effects of the bending stiffness and the effects of approximating the tension variation as a linear function were discussed. Experimental data on the multi-modal in-line and cross-flow response behaviour of a towed catenary model were analysed to examine the usefulness of the solutions for predicting the response frequencies and envelopes due to VIV. Helical strakes are often used as a mitigating measure to suppress the VIV of slender structures. This thesis presented an innovative method to fit ropes helically to a riser in the installation phase. Such a procedure will help to overcome the handling problem associated with the use of conventional sharp-edged strakes. Experimental investigations were then performed to verify the efficiency of these ropes (round-sectioned helical strakes) in suppressing VIV. Systematic experimental investigations including twenty-eight configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The effects of varying pitch, the surface roughness and the ratio between the cross-flow and in-line natural frequencies on the efficiency of the proposed configuration of round-sectioned helical strakes were also investigated. The process of interaction between sea ice and offshore sloping structures (e.g., conical structures and ship-shaped structures) is quite complex. Modelling this process is very demanding and often computationally expensive, which typically hinders the chances for realtime simulations. This kind of simulation can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion was overcome in the present work. This thesis developed a numerical model to simulate the process of interaction between sea ice and sloping structures in real time. In this model, only level- and broken-ice features were studied. New analytical closed-form solutions were established and used to represent the ice breaking process. PhysX was used for the first time to solve the equations of rigid body motions with six degrees of freedom for all ice floes in the calculation domain. The results of the simulator were validated against experimental data from model-scale and full-scale tests. Accurate predictions of ice actions are also vital to optimise the design of the structures in the Arctic regions. A good understanding of the role of seawater in the process of interaction between the sloping structures and level ice will help to establish reliable models to estimate the ice forces. This work formulated both the static and dynamic bending problems for a floating wedge-shaped ice beam interacting with an offshore sloping structure. For the dynamic interaction, the effects of the water foundation on the bending failure of the ice were studied by comparing the results of an elastohydrodynamic approach with a model of a Winkler foundation. The thesis also investigated the breaking lengths of the ice wedges (i.e., the frequency of the ice loads) as a function of the ice thickness, the compression in the ice and the acceleration of the interaction.

Wake interactions on a floating platform for offshore wind energy applications were investigated.The study is performed in collaboration with Hexicon AB which has a patent family for innovative floating platforms, which are able to turn automatically. The Jensen model is used for wake effect calculations and the simulations were performed in MATLAB. The present study starts with wind speed and wind direction data analysis for the specific site that Hexicon AB plans to construct its first platform. Data analysis is followed by wake interaction studies for H4-24MW type Hexicon AB platform. Wake interaction simulations were performed for three different cases. Fixed turbine and platform, Nacelle yawing and fixed platform and Nacelle yawing and turned platform. Different cases were investigated in order to see wake interactions for different wind directions. Wind direction effect on wake interactions were performed between _90_ and 90_ with an increment of 10_. After having the simulation results for Nacelle yawing and turned platform case the results were compared with ANSYS - CFX simulations results. The results didn’t match exactly but they were very close, which is an indicator to the validity of the Jensen Model. After finding out the possible behavior of wake interactions for different wind directions, power calculations were performed for the same three cases. In order to perform the power calculations the wake interactions for different wind directions were taken into account. In case of platform turning it was assumed that power losses were caused both by wake interactions and in case of thrusters activation. The losses that would be caused by different thrust forces on the turbine blades were not included. The last study was performed to suggest different layouts. In the second case, Nacelle yawing and fixed platform, it was found out that nacelle yawing for most of the angles is not possible because it creates wake regions in front of the rotor area. It was decided to propose new turbine configurations on the platform which are tolerant to different nacelle yawing angles. The simulations were run without considering any constructions limitations, meaning that the availability of platform structure was not included. The study is ended by performing some probabilistic results for platform turning behavior.

Bed connected support structures such as monopiles are expected to be impractical for water depths greater than 30 m and so there is increasing interest in alternative structure concepts to enable cost-effective deployment of wind and tidal stream turbines. Floating, moored platforms supporting multiple rotors are being considered for this purpose. This thesis investigates the dynamic response of such floating structures, taking into account the coupling between loading due to both turbulent flow and waves and the dynamic response of the system. The performance and loading of a single rotor in steady and quasi-steady flows are quantified with a Blade Element Momentum Theory (BEMT) code. This model is validated for steady flow against published data for two 0.8 m diameter rotors (Bahaj, Batten, et al., 2007; Galloway et al., 2011) and a 0.27 m diameter rotor (Whelan and Stallard, 2011). Time-averaged coefficients of thrust and power measured by experiment in steady turbulent flow were in agreement with BEMT predictions over a range of angular speeds. The standard deviation of force on the rotor is comparable to that on a porous grid for comparable turbulence characteristics. Drag and added mass coefficients are determined for a porous disc forced to oscillate normal to the rotor plane in quiescent flow and in the streamwise axis in turbulent flow. Added mass is negligible for the Keulegan Carpenter number range considered ( less than 1). The drag coefficient in turbulent flow was found to decay exponentially with number, to 2±10% for values greater than 0.5. These coefficients were found to be in good agreement with those for a rotor in the same turbulent flow with disc drag coefficient within 12.5% for less than 0.65. An extreme-value analysis is applied to the measured time-varying thrust due to turbulent flow and turbulent flow with waves to obtain forces with 1%, 0.1% and 0.01% probability of exceedance during operating conditions. The 1% exceedance force in turbulent flow with turbulence intensity of 12% is around 40% greater than the mean thrust. The peak force in turbulent flow with opposing waves was predicted to within 6% by superposition of the extreme force due to turbulence only with a drag force based on the relative wave-induced velocity at hub-height estimated by linear wave theory and with drag coefficient of 2.0. Response of a floating structure in surge and pitch is studied due to both wave- forcing on the platform defined by the linear diffraction code WAMIT and due to loading of the operating turbine defined by a thrust coefficient and drag coefficient. Platform response can either increase or decrease the loading on the rotor and this was dependant on the hydrodynamic characteristics of the support platform. A reduction of the force on the rotor is attained when the phase difference between the wave force on the support and the surface elevation is close to ± and when the damping of the support is increased. For a typical support and for a wave condition with phase difference close to , the 1% rotor forces were reduced by 8% when compared to the force obtained with a rotor supported on a stiff tower.

Pour combler l'écart entre l'augmentation de la demande énergétique et l'épuisement de la production d'hydrocarbures onshore, l'exploitation des hydrocarbures offshore est de plus en plus envisagée, en particulier les gisements de gaz / pétrole dans les eaux plus profondes. En attendant, un grand nombre d'unités de traitement déployées pour la production d'hydrocarbures doivent respecter les contraintes environnementales conçues pour la protection maritime. Les systèmes tels que les réacteurs et les épurateurs à lit fixe embarqués deviennent inévitablement l'une des options les plus prometteuses pour atteindre ces deux objectifs. De nombreux efforts dans la littérature pour dévoiler l'hydrodynamique de l'écoulement multiphasé dans les lits garnis révèlent que des défis persistent soit dans leur conception / mise à l'échelle, soit dans leurs opérations. De plus, exposer ces réacteurs à des conditions marines difficiles telles que la convolution de la dynamique des navires et de l'hydrodynamique à l'intérieur des réacteurs à lit fixe conduit à des situations encore plus compliquées pour maintenir des performances de fonctionnement acceptables dans les conditions flottantes. Un grand nombre de preuves issues de la littérature a jusqu'à présent mis en évidence l'échec des colonnes garnies avec des garnissages aléatoires, des garnissages structurés ou des mousses à alvéoles ouvertes, pour empêcher la maldistribution des liquides dans les lits fixes destinés à fonctionner à bord de navires ou de platesformes flottantes. Les efforts de recherche doivent donc se poursuivre dans le but de trouver des composants internes robustes et capables de résilience contre la maldistribution des liquides dans les réacteurs / unités de séparation gaz-liquide. Ce projet de doctorat s’est proposé des recherches visant dans un premier temps de tester des internes disponibles commercialement pouvant préserver des performances similaires à celles des unités terrestres classiques. Au meilleur de notre connaissance, la sensibilité et la susceptibilité des réacteurs monolithes à une mauvaise distribution soumis à des conditions offshore n'ont pas encore été étudiées. Plutôt que de se concentrer uniquement sur une étude des lits monolithiques, le chapitre 1 opte pour une campagne expérimentale plus large comprenant un garnissage aléatoire et un garnissage en mousse à cellules ouvertes pour des comparaisons systématiques de la distribution des liquides en conditions flottantes. La distribution liquide des colonnes embarquées garnies de divers garnissages et pour une large plage de débit gaz / liquide est systématiquement comparée à l'aide d'un capteur à treillis métallique (WMS) et d'un émulateur hexapode à six degrés de liberté. La vraisemblance de conditions météorologiques extracôtières rudes pourrait menacer la sureté de l'exploitation des lits fixes, en particulier dans des situations extrêmes telles que des cyclones, des épisodes d'icebergs, etc. Pour assurer la sécurité du personnel et des installations, l’opération des colonnes garnies à bord doit être immédiatement interrompue pour éviter des problèmes de sécurité critiques sous de telles circonstances. Par conséquent, la base de connaissances sur la dynamique de drainage des liquides dans les lits flottants est iv essentielle pour assurer une vidange rapide du liquide. Néanmoins, l'étude de la dynamique du drainage liquide des lits fixes en conditions flottantes est à tout le moins rare. Par conséquent, le chapitre 2 se propose de comparer expérimentalement le drainage du liquide dans des colonnes garnies dans les conditions marines à celui observé dans une colonne statique verticale à l’instar des applications terrestres. En dehors de cela, l'influence des mouvements du navire (par exemple, cavalement, embardée, pilonnement, roulis, tangage, et lacet) à différentes amplitudes et périodes d'oscillation sur la dynamique de drainage des liquides est étudiée pour approfondir nos connaissances. Parallèlement à l'étude expérimentale, un modèle numérique Euler-Euler transitoire et en trois dimensions est utilisé en complément pour tenter de prédire la dynamique du drainage des liquides dans les lits flottants. D'autres facteurs susceptibles d'affecter la dynamique de drainage sont analysés par la simulation numérique. Ainsi, le chapitre 3 met en évidence l'influence globale des propriétés des liquides, de la structure du lit et des types de mouvement associé à la sollicitation marine. Par ailleurs, la campagne expérimentale en fournissant des données mesurables a permis de valider le modèle dans les conditions de roulis et de tangage testées au laboratoire.
To fill the gap between increasing energy demand and depletion of onshore hydrocarbon production, offshore hydrocarbon exploitation is increasingly contemplated especially the gas/oil fields in the deeper water. Meantime, large amount of deployed processing units for hydrocarbon productions must comply with the environmental codes designated for maritime protection. Systems such as embarked packed-bed reactors and scrubbers inevitably become one of the most promising options to achieve both purposes. Numerous efforts in literature to unveil the hydrodynamics of multiphase flow in packed beds reveal that challenges persist either in their design/scale-up or during the operations. Moreover, exposing these reactors to harsh marine conditions such as the convolution of ship dynamics and hydrodynamics inside packed-bed reactors leads to even more complex situations to maintain the proper operation performance of packed-bed reactors under floating conditions. A lot of evidence from literature has pointed out the failure of random and structured packings and open-cell foams, to prevent liquid maldistribution in packed beds destined to operate on-board sailing ships and floating platforms. Research efforts must therefore continue in the quest for robust internals capable of resilience against liquid maldistribution in gas-liquid reactors/separation units. The proposed Ph.D. research aims at firstly following a sound path to adapt commercially existing internals being capable of preserving performance similar to landbased packed beds. To the best of literature exploring, the sensitivity and susceptibility of monolith reactors to maldistribution subjected to offshore conditions have yet to be investigated. Rather than focusing on a study of monolith beds alone, Chapter 1 opts for a broader experimental campaign including a random packing and an open-cell foam packing for the sake of systematic comparisons of the liquid distribution under floating conditions. Liquid distribution of embarked columns packed with various internals under wide gas/liquid flow range is systematically compared with the assistance of wire mesh sensor (WMS) and six-degree-of-freedom emulator hexapod. Severe offshore weather conditions threaten the operation safety of floating packed beds especially encountering extreme situations such as cyclone, iceberg episodes and so forth. To ensure the safety of staff and facilities, the onboard packed columns must be immediately shutdown to avoid critical safety concerns under such circumstances. Therefore, knowledgebase of liquid draining dynamics in floating packed beds is the essence to ensure timely discharge of liquid. Nevertheless, the study regarding liquid drainage dynamics of packed beds under floating conditions is scarce to say the least. Then, Chapter 2 compares liquid draining of packed columns embarking on floating platforms with static land-based one experimentally. Other than that, the influence of ship motions (e.g., roll, roll & pitch, heave etc.) with different oscillation amplitudes and periods on liquid draining dynamics is investigated to deepen the insights. vi In parallel with the experimental study, a 3D transient Euler-Euler CFD model is employed as a supplementary analysis to further deepen the understanding of liquid drainage dynamics in floating packed beds. More factors possibly affecting the draining dynamics are exploited by numerical simulation. Consequently, Chapter 3 highlights the comprehensive influence of liquid properties, bed structure and moving types instead of focusing on impact of movements alone. Meanwhile, with sufficient body of experimental campaign, the validity and accuracy of model are strongly endorsed.

Le calcul des performances et des efforts appliqués sur une éolienne offshore est actuellement réalisé à l'aide d’outils basés sur des approches quasi-statiques. Ces approches sont intéressantes pour leur vitesse de calcul, elles sont cependant perfectibles suivant la méthode de mise en oeuvre et suivant les cas de chargement étudiés. Une approche alternative consiste à utiliser la modélisation CFD. Cette thèse s’intéresse à des méthodes d’une haute précision, ayant le potentiel de fournir des écoulements et efforts précis. La plateforme logicielle hautement parallélisée ICI-tech est utilisée dans cette thèse. Elle se base sur une résolution des équations de Navier-Stokes dans une approche multi-échelle, effectuée à l’aide d’éléments finis stabilisés. La représentation des phases dans le domaine de calcul est réalisée grâce à une méthode frontières immergées. Des implémentations ont été réalisées dans ICI-tech afin de pouvoir simuler des éoliennes flottantes. L’interaction fluide-structure et un bassin de houle numérique ont notamment été considérés. Un processus de vérification et validation s’est intéressé au comportement du solveur dans des conditions reproduisant celles impactant des éoliennes flottantes. Le niveau de précision atteint par les écoulements à haut Reynolds et la propagation de champs de houle s’est avéré être décevant. L’influence du maillage anisotrope sur les résultats obtenus a été quantifiée. Plusieurs pistes visant à améliorer la précision des simulations ont été introduites
The simulation of Floating Offshore Wind Turbines (FOWTs) is a tool to help this technology reach an industrial scale. Nowadays, low-precision numerical methods are used for the dimensioning of the structures, as they involve a reduced computational effort. This PhD thesis focused on the development of highly-accurate numerical methods, with a potential to provide a thin description of the flows and efforts around FOWTs. The simulations presented in this thesis have been realized on the highly-parallelized software platform ICI-tech. A resolution of the Navier- Stokes equations in a Variational MultiScale formulation is performed using Stabilized Finite Elements. The representation of the different phases in the computational domain is achieved using immersed boundary methods. Several numerical tools have been implemented in ICItech towards an application to the simulation of FOWTs. A fluid-structure interaction paradigm has been set up, and a numerical wave tank has been defined. Verification and validation studies have been realized to assess the solver results for environmental conditions representative of those observed for operating FOWT. The accuracy achieved for both the aerodynamics at high Reynolds numbers and the propagation of wave fields has been disappointing. The influence of the anisotropic meshing on the results presented has been quantified. Several options aiming at increasing the accuracy of the simulations have been discussed

Les systèmes flottants de production, stockage et de déchargement (FPSO) ont été introduits dans les secteurs d'exploitation des hydrocarbures offshore en tant qu'outils facilement déplaçables pour l’exploitation de champs de pétrole et de gaz de petites ‘a moyenne tailles ou lorsque ceux-ci sont éloignés des côtes ou en eaux profondes. Ces systèmes sont de plus en plus envisagés pour les opérations de traitement et de raffinage des hydrocarbures à proximité des sites d'extraction des réservoirs sous-marins en utilisant des laveurs et des réacteurs à lit fixe embarqués. De nombreuses études dans la littérature pour découvrir l'hydrodynamique de l'écoulement polyphasiques dans des lits garnis ont révélé que la maîtrise de tels réacteurs continue d’être un défi quant à leur conception /mise à l'échelle ou à leur fonctionnement. De plus, lorsque de tels réacteurs sont soumis à des conditions fluctuantes propres au contexte marin, l'interaction des phases devient encore plus complexe, ce qui entraîne encore plus de défis dans leur conception. Les travaux de recherche proposés visent à fournir des informations cruciales sur les performances des réacteurs à lit fixes à deux phases dans le cadre d'applications industrielles flottantes. Pour atteindre cet objectif, un simulateur de mouvement de navire de type hexapode avec des mouvements à six degrés de liberté a été utilisé pour simuler les mouvements des FPSO tandis que des capteurs à maillage capacitif (WMS) et un tomographe à capacitance électrique (ECT) couplés avec le lit garni ont permis de suivre en ligne les caractéristiques dynamiques locales des écoulements diphasiques. L'effet des inclinaisons et des oscillations de la colonne sur le comportement hydrodynamique des lits garnis biphasiques a été étudié, puis les résultats ont été comparés à leurs analogues terrestres correspondants (colonne verticale immobile). De plus, des stratégies opérationnelles potentielles ont été proposées pour atténuer la maldistribution des fluides résultant des oscillations du lit ainsi que pour intensifier le processus de réactions dans les réacteurs à lit fixe. Parallèlement aux études expérimentales, un modèle Eulérien CFD transitoire 3D a été développé pour simuler le comportement hydrodynamique de lits garnis polyphasiques sous des inclinaisons et des oscillations de colonnes. Enfin, pour compléter le travail expérimental, une étude systématique a été réalisée pour étudier les performances de capture de CO2 à base d'amines d’un laveur à garnissage (en vrac et structuré) émulant une colonne à bord des ...
Floating production storage and offloading (FPSO) systems have been introduced to offshore hydrocarbon exploitation sectors as readily movable tools for development of small or remote oil and gas fields in deeper water. These systems are increasingly contemplated for onboard treatment and refining operations of hydrocarbons extracted from undersea reservoirs near extraction sites using embarked packed-bed scrubbers and reactors. Numerous efforts in the literature to uncover the hydrodynamics of multiphase flow in packed beds have disclosed that such reactors continue to challenge us either in their design/scale-up or their operation. Furthermore, when such reactors are subjected to marine conditions, the interaction of phases becomes even more complex, resulting in further challenges for design and scale-up. The proposed research aims at providing important insights into the performance of two-phase flow packed-bed reactors in the context of floating industrial applications. To achieve this aim, a hexapod ship motion simulator with six-degree-of-freedom motions was employed to emulate FPSO movements while capacitance wire mesh sensors (WMS) and electrical capacitance tomography (ECT) coupled with the packed bed scrutinized on-line and locally the two-phase flow dynamic features. The effect of column tilts and oscillations on the hydrodynamic behavior of multiphase packed beds was investigated and then the results were compared with their corresponding onshore analogs. Moreover, potential operational strategies were proposed to diminish fluid maldistribution resulting from bed oscillations as well as for process intensification of heterogeneous catalytic reactions in packed-bed reactors. In parallel with the experiment studies, a 3D transient Eulerian CFD model was developed to simulate the hydrodynamic behavior of multiphase packed beds under column tilts and oscillations. Ultimately, a systematic experimental study was performed to address the amine-based CO2 capture performance of packed-bed scrubbers on board offshore floating vessels/platforms. Apart from gaining a comprehensive knowledge on the influence of translational and rotational movements on multiphase flows in porous media, oil and gas sectors and ship industry would benefit from the results of this work for design and scale-up of industrial reactors and scrubbers.
Unité flottante de production, de stockage et de déchargement

The objective of this thesis is to identify, which hazards and failures in operation process will affect Reliability, Availability, Maintainability and Safety of floating offshore structures. The focus is on Dynamic Positioning (DP) system that has the responsibility of keeping the offshore structure in the upright position operation. DP system is one of the most critical subsystems on these types of structures in terms of safety of operation and failure risk costs. Reliability of the system in this thesis has been analyzed in qualitative and quantitativeb methods. In qualitative method to find the effective parameters on the reliability of the DPb system, Reliability Centered Maintenance (RCM ) and its application as a main tool have been used. To achieve the aim it has been tried to define the events and accidents which could be generated by the identified hazards then tried to determine the consequences of the realized accidents. In this step three categories are taken in to account including, safety, operation, and equipment. Next phase should be concentrated on considering and analyzing the relevant processes and the root causes which result in the identified hazard. After clarifying all probable root causes it has been tried to prioritize the root causes and specifying the necessary preventive actions. The aim of this step is either decreasing the occurrence of root causes or increasing the detectability of hazards. In the last part quantitative method has been used to measure the amounts of Reliability, Availability and Maintainability of the system, based on MTBF and MTTR of different components of the system and it has been tried to present the solutions to improve system reliability based on components RCM tables. Further, assuming DP system as human- machine system safety assessment has been included to indicate human factors in the reliability of the system beside probable failure of the components of the system.

La fonction principale des systèmes d'ancrage des éoliennes offshore flottantes est de limiter les mouvements du support. Les lignes d'ancrage qui les composent sont typiquement constituées de chaînes, de câbles aciers, de câbles synthétiques ou d'une combinaison de ces composants.Dans cette thèse, on se concentre sur les câbles en acier qui permettent de réduire le poids et d'augmenter la résistance en tension par rapport aux chaînes. Leur dimensionnement dépend des chargements en tension et flexion, liés aux mouvements du flotteur sous l'action de la mer et du vent.L'objectif de la thèse est le développement d'un nouveau modèle numérique pour prédire la durée de vie en fatigue des câbles d'ancrage d'une éolienne offshore flottante. Il doit notamment simuler les glissements relatifs entre les fils au cours d'une flexion du câble. Des résultats d'essais de tension-flexion de la littérature ont en effet montré que la première rupture est localisée près du plan neutre de flexion, où ces déplacements relatifs sont les plus grands. Cet effet majeur sur la durée de vie du câble n'est pas pris en compte par les lois de fatigue en tension-tension des normes de design offshore actuelles.Il faut aussi remarquer que l'utilisation d'un modèle détaillé de câble dans une démarche de dimensionnement à la fatigue représente un vrai défi. Le nombre élevé d'interactions de contact à modéliser, de l'ordre de plusieurs milliers par mètre de câble, et le grand nombre de cas de chargement rendent ce type de calculs très coûteux.Les chargements qui sont utilisés dans le modèle local de câble sont issus de calculs globaux réalisés à l'aide d'un logiciel multiphysique (Deeplines). Ce logiciel permet de simuler les conditions environnementales (vent, houle, courant) appliquées sur l'ensemble de la structure offshore.Nous montrons que le comportement non linéaire en flexion du câble, lié aux interactions de contact entre les fils, n'influence pas significativement les résultats du modèle global. Cette observation justifie une démarche de type descendante, les calculs globaux pouvant être réalisés en première étape. Les évolutions temporelles des tensions et courbures globales sont appliquées uniformément sur le fil central du modèle local du câble. La continuité du câble est représentée par des conditions de périodicité reliant les sections de bord à des points internes du modèle situés sur la même position circonférentielle. Les fils sont modélisés par des éléments poutres. On obtient les contraintes généralisées sur les fils, les forces de contact et les glissements relatifs. Des premières analyses ont montré que les déplacements relatifs entre les fils restent petits dans notre cadre d'application. Afin de réduire le coût calcul, nous avons développé un nouvel élément de contact entre poutres non parallèles, avec un appariement fixe de contact, dans l'hypothèse de petits glissements mais en grands déplacements et grandes rotations. Des tests numériques montrent l'amélioration obtenue, avec un résultat plus proche d'un modèle de référence qui considère un contact surfacique. De plus, le nouveau modèle réduit significativement le coût calcul et se montre plus robuste en convergence, ce qui s'avère crucial pour un calcul de fatigue. Les sorties du modèle local sont ensuite utilisées pour prédire un état de contrainte 3D, en exploitant des solutions analytiques de contact entre corps cylindriques. Finalement, un critère de fatigue multiaxial de la littérature est appliqué pour évaluer le risque en dommage
The main function of mooring systems of floating offshore wind turbines is to ensure station keeping. The mooring lines can be composed of chains, wire ropes, synthetic ropes, or even a combination of them. In this thesis we focus on wire ropes, whose advantage over chain is to sustain high tension at a lower weight. Their design must consider the successive tension and bending loading induced by the floater movement for various wind and waves conditions.The thesis purpose is to develop a new numerical model, dedicated to the prediction of fatigue damage in mooring wire ropes of a floating wind turbine. In particular it has to simulate the relative movements between the wires when the rope is bent. Results from free-bending fatigue tests in the literature show the importance of these effects, since the first rupture is localized near the neutral plane, where fretting is more important. This phenomenon affecting the fatigue life is not considered by fatigue criteria of current offshore standards, which are related to tension-tension loading.It is worth noting that the use of a detailed model of wire rope in a fatigue design procedure represents a real challenge. The high number of contact interactions to be modeled, which are several thousands per meter of rope, and the large amount of loading cases make this type of computations extremely time-consuming.The loading used in the developed local model of wire rope is obtained from global computations performed with a dedicated multiphysics software (Deeplines). This software allows to simulate the environmental conditions (wind, waves, current) applied on the whole structural system.Some preliminary computations showed that the nonlinear bending behavior of the wire rope, linked to the wire contact interactions, does not significantly affect the output of the global model. This observation justifies the use of a top-down scheme, with a prior computation of the global scale.The global scale tension and curvature are then uniformly imposed on the central wire of the local model. The continuity of the rope is represented by periodic conditions which link the end sections to points within the model, at the same circumferential locations. The wires are modeled by beam elements. The outputs at the local scale are the stress resultants on the wires, and the contact forces and relative displacements at contact locations.Small sliding between the wires has been observed from first numerical analysis, for a representative loading case. Therefore, in order to reduce the computational cost of the wire rope model, a new node-to-node contact element has been developed, dedicated to the modeling of contact between non-parallel beams with circular cross section. It assumes fixed contact pairing and finite rotations. Numerical benchmarks and experimental tests on wire ropes show the improvement with results closer to a reference surface-to-surface model, when compared to standard algorithm for the simulation of contact between beams. Moreover, the new model reduces significantly the CPU cost and is also more robust, which is crucial for fatigue life estimates.The outputs of the local scale model are then used to obtain the complete 3D stress state by means of analytical solutions of contact between solids with cylindrical shape. Finally, a multiaxial fatigue criterion is applied in order to assess the safety of the system

The dynamic motion of floating wind turbines is studied using numerical simulations. Floating wind turbines in the deep ocean avoid many of the concerns with land-based wind turbines while allowing access to strong stable winds. The full three-dimensional Navier-Stokes equations are solved on a regular structured grid, using a level set method for the free surface and an immersed boundary method for the turbine platform. The tethers, the tower, the nacelle and the rotor weight are included using reduced order dynamic models, resulting in an efficient numerical approach which can handle nearly all the nonlinear wave forces on the platform, while imposing no limitation on the platform motion. Wind is modeled as a constant thrust force and rotor gyroscopic effects are accounted for. Other aerodynamic loadings and aero-elastic effects are not considered. Several tests, including comparison with other numerical, experimental and grid study tests, have been done to validate and verify the numerical approach. Also for further validation, a 100:1 scale model Tension Leg Platform (TLP) floating wind turbine has been simulated and the results are compared with water flume experiments conducted by our research group. The model has been extended to full scale systems and the response of the tension leg and spar buoy floating wind turbines has been studied. The tension leg platform response to different amplitude waves is examined and for large waves a nonlinear trend is seen. The nonlinearity limits the motion and shows that the linear assumption will lead to over prediction of the TLP response. Studying the flow field behind the TLP for moderate amplitude waves shows vortices during the transient response of the platform but not at the steady state, probably due to the small Keulegan-Carpenter number. The effects of changing the platform shape are considered and finally the nonlinear response of the platform to a large amplitude wave leading to slacking of the tethers is simulated. For the spar buoy floating wind turbine, the response to regular periodic waves is studied first. Then, the model is extended to irregular waves to study the interaction of the buoy with more realistic sea state. The results are presented for a harsh condition, in which waves over 17 m are generated, and linear models might not be accurate enough. The results are studied in both time and frequency domain without relying on any experimental data or linear assumption. Finally a design study has been conducted on the spar buoy platform to study the effects of tethers position, tethers stiffness, and platform aspect ratio, on the response of the floating wind turbine. It is shown that higher aspect ratio platforms generally lead to lower mean pitch and surge responses, but it may also lead to nonlinear trend in standard deviation in pitch and heave, and that the tether attachment points design near the platform center of gravity generally leads to a more stable platform in comparison with attachment points near the tank top or bottom of the platform.

On propose dans ce travail une méthodologie pour le suivi en service des lignes d'ancrage des éoliennes flottantes. Tout d’abord, une expression empirique de la raideur dynamique d'un câble en nylon est obtenue à partir des données d'essais dans la littérature. Une procédure pratique de modélisation est proposée en tenant compte de la raideur axiale dynamique non-linéaire des câbles en nylon. La deuxième partie est consacrée à la prédiction de la durée de vie des lignes d’ancrages. Des méthodes avancées pour l’analyse de fatigue dans le domaine fréquentiel et la simulation des réponses non-linéaires sont donc également étudiées afin de réaliser une estimation rapide de la fatigue et de la résistance dans un cadre fiabiliste. La présente méthodologie vise à faciliter la prise de décisions concernant la maintenance ou le remplacement des lignes en fonction du niveau de fiabilité estimé à différents instants
In this work a methodology for service life monitoring of mooring lines of floating wind turbines is proposed. First, an empirical expression of dynamic stiffness of a nylon rope is obtained from the testing data in the literature. A practical modeling procedure is proposed which allows accounting for the non-linear dynamic axial stiffness of nylon mooring ropes. The second part is devoted to the prediction of fatigue life of mooring lines. Cutting-edge methods for fatigue analysis in frequency domain and for simulation of nonlinear mooring response are investigated in order to perform a quick fatigue estimate and strength check in a reliability framework. The present methodology aims to support making decisions regarding maintenance or replacement of lines based on the level of reliability estimated during the expected service life

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

Le développement de Geps Techno est basé sur un concept innovant de structure flottante destinée à produire de l'énergie électrique à partir de plusieurs sources d'énergies marines renouvelables dont la source houlomotrice. Le système houlomoteur développé par Geps Techno repose sur la mise en circulation d'eau et la création d'un tourbillon en son sein. En profitant du phénomène de carène liquide, le concept est également déclinable en un système de stabilisation de navire ou de toute autre plateforme flottante. L'objectif à court terme de la société est le développement de cette technologie permettant la stabilisation et la récupération de l'énergie des vagues et pour lequel il reste des verrous technologiques à lever afin d'arriver à la viabilité et la rentabilité du système. Pour cela, Geps Techno a lancé en octobre 2015 le projet IHES (Integrated Harvesting Energy System) qui consiste à construire un démonstrateur de son concept de plateforme houlomotrice. Le projet IHES est un des projets de la feuille de route du plan "Navires écologiques" de la Nouvelle France Industrielle. Il est soutenu par Bpifrance dans le cadre du programme d'Investissements d'Avenir - Projets Industriels d'Avenir. Afin de maîtriser les objectifs de stabilisation et de récupération d'énergie, Geps Techno étudie les volets technologiques nécessaires permettant de passer de l'énergie disponible au niveau des vagues jusqu'à celle disponible au niveau de la turbine du houlomoteur. Les travaux de thèse soutenus par Fourestier en mai 2017 portaient sur un premier volet "Définition et contrôle des écoulements internes au système houlomoteur". A l'aide d'une modélisation des fluides numériques, ces derniers ont abouti à des modèles opérationnels caractérisant les écoulements internes. La présente thèse Cifre s'inscrit dans la continuité des travaux de Fourestier et traite d'un second volet "Modélisation du système couplé plateforme / houlomoteur". L'ensemble de ces travaux devra aboutir à un code de calcul opérationnel et corrélé à des résultats expérimentaux permettant d'étudier l'écoulement interne et le comportement du flotteur soumis à la houle
Geps Techno's development is based on an innovative concept of a floating structure intended to produce electrical energy from several renewable marine energy sources, including wave power. The wave power system developed by Geps Techno is based on circulating water and creating a vortex within it. By taking advantage of the liquid hull phenomenon, the concept can also be used as a stabilization system for a ship or any other floating platform. The short-term objective of the company is the development of this technology allowing the stabilization and recovery of wave energy and for which there remain technological obstacles to be removed in order to achieve the viability and profitability of the system. To do this, in October 2015 Geps Techno launched the IHES (Integrated Harvesting Energy System) project, which consists of building a demonstrator of its wave power platform concept. The IHES project is one of the projects of the roadmap of the "Ecological ships" plan of New Industrial France. It is supported by Bpifrance within the framework of the Investments for the Future - Industrial Projects for the Future program. In order to master the objectives of stabilization and energy recovery, Geps Techno is studying the technological aspects necessary to switch from the energy available at wave level to that available at the wave turbine turbine. The Ph.D. thesis work supported by Fourestier in May 2017 focused on a first part "Definition and control of internal flows in the wave power system". Using CFD modeling, the latter resulted in operational models characterizing internal flows. This Cifre Ph.D. thesis follows on from Fourestier's work and deals with a second part "Modeling of the coupled platform / wave power system". All of this work should lead to an operational computer code correlated with experimental results making it possible to study the internal flow and the behavior of the float subjected to swell

Les outils de calcul actuels pour la modélisation numérique multi-physique (vent, vagues, courant, etc.) d’une éolienne flottante ont besoin de validations par des campagnes expérimentales. L’objectif de ce travail est le développement et la validation d’un dispositif expérimental dédié aux essais en bassin à houle d’une éolienne flottante, en se concentrant sur la représentation des efforts du vent. Pour cela, une approche hybride combinant modélisations physique et numérique est développée, appelée “software-in-the-loop” (SIL). Le développement des différentes briques composant un système SIL inclut (i) la sélection et le développement du modèle numérique (ii) le dimensionnement du système de reproduction des efforts (actionneurs) et (iii) la définition de l’environnement temps-réel pour l’intégration du modèle numérique, le contrôle des actionneurs, et l’acquisition des grandeurs mesurables. Pour caractériser et identifier les performances du système SIL, des méthodologies dédiées sont développées. Des bancs d’essais spécifiques sont construits, et des essais en bassin d’une éolienne flottante sont réalisés. Ces essais en bassins sont ensuite comparés à des simulations couplées aéro-hydro-servo structure pour investiguer les hypothèses du modèle d’efforts hydrodynamiques
Actual calculation tools for the multi physical numerical modeling (wind, waves, current, etc.) of a floating wind turbine need validation through experimental campaigns. The objective of this work is the development and validation of an experimental apparatus dedicated to floating wind turbines wave tank testing, focusing on the representation of wind turbine forces. A hybrid approach combining physical and numerical modeling is developed, called “software-in-the-loop” (SIL). The development of the different subsystems of an SIL system includes (i) the selection and development of the numerical model (ii) the design of the force reproduction system (actuators) and (iii) the definition of the real time environment for the integration of the numerical model, the control of actuators, and the data acquisition. To characterize and identify the performances of the SIL system, dedicated methodologies are developed. Specific test benches are built, and wave tank tests of a floating wind turbine are carried out. These wave tank tests are then compared to coupled aero-hydro-servo structure simulations to investigate the hypotheses of the hydrodynamic force model

As one of the fastest growing renewable energy sources, wind energy is playing an increasingly important part in addressing the climate change and energy crisis issues the world is currently facing. The abundance of wind resource in offshore areas makes them a popular choice for turbine installation. In the past few years, several floating wind projects have emerged where wind turbines are installed far offshore in deepwater sites on moored platforms. Compared to land-based or offshore fixed-bottom wind turbines, an FOWT is a fully coupled system where the wind turbine with flexible blades and the floating platform with its mooring system interact with each other in wind and waves, which makes old design tools inadequate. This work aims to develop a fully coupled high-fidelity aero-hydro-mooring-elastic analysis tool, and to better understand the sophisticated fluid-structure interactions for FOWTs. The numerical tool developed in this work takes advantage of the open source CFD toolbox OpenFOAM to accurately solve wind turbine aerodynamics and floating platform hydrodynamics, and utilises the open source MBD code MBDyn for structural dynamics within a multibody framework while modelling flexible bodies based on a nonlinear beam theory. Coupling of these two solvers is achieved by establishing an interface library to exchange data with the help of the TCP/IP protocol. Additionally, to tackle the complex mesh movement in FOWT simulations, a mesh motion solver is developed in OpenFOAM by combining the sliding mesh technique and the dynamic mesh morphing method. A mooring system analysis module comprising a quasi-static method and a lumped-mass based dynamic approach is also implemented to simulate mooring lines in an FOWT system. A series of test cases is firstly studied to validate the various features of the tool, including basic fluid flow solving, modelling of wind turbine aerodynamics, hydrodynamic analysis of a floating structure with its mooring system, dynamic analysis of a riser or mooring line and coupled analysis of flow induced vibration of a flexible beam. The developed tool is then applied to analyse FSI problems of FOWTs under three different scenarios. Firstly, a coupled aero-hydro-mooring analysis is carried out for the OC4 semisubmersible FOWT under regular waves and uniform wind speed. Blade flexibility is ignored, and mooring lines are solved using the quasi-static method. Interactions between the moored platform and the wind turbine are investigated, focusing on of platform motion on the aerodynamic performance of the wind turbine and the impacts of wind turbine aerodynamics on the responses of the floating platform and its mooring system. Subsequently, an aeroelastic analysis is conducted for the NREL 5-MW offshore wind turbine with flexible blades under uniform wind speed. Effects of blade flexibility on wind turbine aerodynamics and structural responses are studied using the developed CFD-MBD tool. The floating platform supporting the turbine is not directly modelled for simplicity and the influence of platform motion responses on the turbine are analysed via imposing a prescribed surge motion to the turbine base. Fully coupled aero-hydro-mooring-elastic analysis is lastly carried out for the OC4 semi-submersible FOWT under a combined wind/wave condition to demonstrate the capabilities of the developed CFD-MBD tool. Responses of the floating system are investigated in terms of platform hydrodynamics, mooring system dynamics, wind turbine aerodynamics and blade structural dynamics. Interactions between the FOWT and fluid flow are analysed by visualising results obtained via the CFD approach.

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

Floating offshore wind power is a relatively new technology that enables wind turbines to float above the sea level, tied by anchors at the seabed. The purpose of this work is to develop an economic model for the technology in order to calculate the total cost of a planned wind farm. Cost data are retrieved from reports and academic journals available online. Based on these data, a model in Microsoft Excel is developed which calculates the Levelized cost of energy (LCOE) for floating wind power plants as a function of several input values. As an addition to this model, financing offshore projects are described using literature study and by doing interviews with three major companies, currently investing in offshore wind. As a result, the model allows the user to calculate Capital expenditures, Operating expenditures and LCOE for projects at any given size and at any given site. The current LCOE for a large floating offshore wind farm is indicated to be in the range of 138-147 £/MWh. The outline from interviews was that today there is no shortage of capital for funding wind projects. However, in order to attract capital, the governmental regulatory of that market has to be suitable since it has a crucial impact on price risks of a project.

Over the past decade, numerical modelling of nonlinear wave-structure interaction problems has been an important topic of ocean engineering research, with practical applications relating to load and response predictions for large floating structures subjected to steep waves. With the increasing need to incorporate more accurate analyses of wave effects into existing offshore design procedures, the emphasis of recent hydrodynamic research has been on the development of numerical solutions to a wide variety of nonlinear wave-structure interaction problems. In particular, the present theoretical work considers the second-order wave radiation and combined diffraction-radiation problems in both two and three dimensions for the case of large floating offshore structures of arbitrary shape. The mathematical formulation of the corresponding initial-boundary value problem can be derived on the basis of potential flow theory. A recent time-domain approach is extended to simulate second-order hydrodynamic effects involving large floating structures which cannot be obtained by the conventional linear hydrodynamic theory. The method involves the application of Taylor series expansions and the use of Stokes perturbation procedure to establish the corresponding first- and second-order boundary value problems with respect to a time-independent fluid domain. A time-stepping scheme together with a suitable iterative procedure are used to solve the coupled fluid-structure governing equations, and an integral equation method based on Green's theorem is used to obtain the wave field at each time step. In relation to conventional nonlinear methods, the present method provides a relatively algebraically straightforward and computationally effective numerical algorithm for treating the second-order free surface flow problems. The method is used to study the second-order wave radiation problem in two and three dimensions, in which surface-piercing structures of arbitrary shape undergo forced sinusoidal motions in otherwise still water. The method is illustrated by numerical results obtained from two semi-submerged circular and rectangular cylinder sections and a truncated surface-piercing circular cylinder. The second-order oscillatory force component due to second-order wave potential, for both the two-dimensional vertical plane case and the general three-dimensional case, contributes significantly in the evaluation of the total hydrodynamic forces. Although the second-order problems have been studied quite extensively in the context of wave diffraction and radiation separately, results for the nonlinear diffraction-radiation problem are rather scarce. This combined problem involves the equations of motion of the structure as well as nonlinear interactions between the incident, diffracted and radiated wave components. The present method is subsequently extended to simulate second-order wave interactions with large floating structures in two and three dimensions, and is illustrated by applying respectively to the cases of a semi-submerged circular cylinder and of a floating truncated circular cylinder. Numerical computations presented relate to the transient motion of a freely-floating cylinder with a specified initial vertical displacement, and the diffraction-radiation of Stokes second-order waves by a moored floating cylinder. In general, numerical results indicate significant second-order hydrodynamic effects in the forces and motions of large floating structures subjected to regular waves, as well as in the corresponding free surface profiles and wave amplitudes.

碩士
逢甲大學
土木工程學系
106
Wind power generation is one of the well-known clean energy source. It is one of the focuses of many countries in the world, and there are many factors that must be considered in the design and assembly of wind turbines, such as aerodynamics, oceanography and the motion of the floating structures, etc. The purpose of current study is to use the connection of floating modules with the offshore wind turbines to maintain the stability of overall floating structures on the sea surface. Review of the previous literatures, a new method by a combination of “Nonlinear network dynamics of flexibility connected multi-modules very large floating structures” and “Spar-type offshore wind turbines” has been develop to perform the current study.

As has been the case for onshore wind systems, the environmental effects of offshore wind farms are expected to play an important part of the development of future large-scale wind energy systems. This paper presents a detailed review of the status of, and recent developments in, research on the environmental impacts of fixed and floating offshore wind turbine systems. The primary information that has been reviewed has come from European sources where there are a significant number of offshore installations, but some work on this subject has been carried out recently in the United States. Information, from an extensive review, is presented on the environmental impacts of fixed and floating offshore wind turbines on benthic organisms, fish, marine mammals, avian species and bats. The environmental impacts of fixed and floating systems are anticipated to vary due to multiple parameters that need to be taken into account when identifying environmental impacts. Additionally, there are variations in the impact throughout the lifecycle of the offshore wind turbines. The primary focus for this paper is on the environmental impacts through the scope of barrier and habitat impacts in addition to the anticipated avian and bat fatalities. A noise propagation model is used to determine the extent of effects due to the installation of fixed and floating support structures using piling installation methods. Finally, a summary of progress in all the major environmental impact areas is given along with recommendations for future research.

Floating offshore wind turbines have the potential to become a significant source of affordable renewable energy. However, their strong interactions with both wind- and wave-induced forces raise a number of technical challenges in both modelling and design. This thesis takes aim at some of those challenges. One of the most uncertain modelling areas is the mooring line dynamics, for which quasi-static models that neglect hydrodynamic forces and mooring line inertia are commonly used. The consequences of using these quasi-static mooring line models as opposed to physically-realistic dynamic mooring line models was studied through a suite of comparison tests performed on three floating turbine designs using test cases incorporating both steady and stochastic wind and wave conditions. To perform this comparison, a dynamic finite-element mooring line model was coupled to the floating wind turbine simulator FAST. The results of the comparison study indicate the need for higher-fidelity dynamic mooring models for all but the most stable support structure configurations. %It was also observed that small inaccuracies in the platform motion time series introduced by a quasi-static mooring model can cause much larger inaccuracies in the time series of the rotor blade dynamics. Industry consensus on an optimal floating wind turbine configuration is inhibited by the complex support structure design problem; it is difficult to parameterize the full range of design options and intuitive tools for navigating the design space are lacking. The notion of an alternative, ``hydrodynamics-based'' optimization approach, which would abstract details of the platform geometry and deal instead with hydrodynamic performance coefficients, was proposed as a way to obtain a more extensive and intuitive exploration of the design space. A basis function approach, which represents the design space by linearly combining the hydrodynamic performance coefficients of a diverse set of basis platform geometries, was developed as the most straightforward means to that end. Candidate designs were evaluated in the frequency domain using linearized coefficients for the wind turbine, platform, and mooring system dynamics, with the platform hydrodynamic coefficients calculated according to linear hydrodynamic theory. Results obtained for two mooring systems demonstrate that the approach captures the basic nature of the design space, but further investigation revealed limitations on the physical interpretability of linearly-combined basis platform coefficients.. A different approach was then taken for exploring the design space: a genetic algorithm-based optimization framework. Using a nine-variable support structure parameterization, this framework is able to span a greater extent of the design space than previous approaches in the literature. With a frequency-domain dynamics model that includes linearized viscous drag forces on the structure and linearized mooring forces, it provides a good treatment of the important physical considerations while still being computationally efficient. The genetic algorithm optimization approach provides a unique ability to visualize the design space. Application of the framework to a hypothetical scenario demonstrates the framework's effectiveness and identifies multiple local optima in the design space -- some of conventional configurations and others more unusual. By optimizing to minimize both support structure cost and root-mean-square nacelle acceleration, and plotting the design exploration in terms of these quantities, a Pareto front can be seen. Clear trends are visible in the designs as one moves along the front: designs with three outer cylinders are best below a cost of \$6M, designs with six outer cylinders are best above a cost of \$6M, and heave plate size increases with support structure cost. The complexity and unconventional configuration of the Pareto optimal designs may indicate a need for improvement in the framework's cost model.
Graduate
0548
mtjhall@uvic.ca

Recent events regarding energy policies throughout the globe and advances in technology are making offshore wind farms become a reality. Most offshore wind farms are still, however, built close to land masses, and need to be rigidly attached to the seabed in one way or another. In many countries, both public and private entities are developing new concepts of floating platforms to overcome the thirty to thirty-five-metre depth limit. Some of these new platforms use and adapt previous Oil and Gas platform concepts, while others are built up from scratch. This Master Thesis covers a hydrodynamic and structural analysis of a new concrete floating platform concept developed for medium to deep waters. This work is based on data from experimental model-scale tests performed in a wave tank and from numerical models using linear potential theory, limited here only to regular wave trains. The study focused on the behavior of the heave plates attached to the platform: test data was analyzed in order to find indicators of the largest dynamic pressures on the plates when only motion data was available, and the structural behavior of the plates was studied under different static pressure distributions using a commercial Finite Element Method software. The results from these analyses show that the normal accelerations of the plates -assumed rigid- strongly correlate with the dynamic pressures measured; and that the general structural behavior of the plate, in terms of deformations and bending moments, is well captured when the hydrodynamic load distribution is simplified into a uniformly distributed load of the same magnitude. The results obtained will help reduce the computational effort currently needed in the design of these floating structures, especially at some stages, when numerous scenarios, load cases and combinations need to be studied.

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.