Browsing by Author "Trarieux, F."
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Item Open Access Assessment of the power available in a fixed offshore oscillating water column plant(Cranfield University, 2012-12) Holzhauer, Eva; Amaral Teixeira, Joao; Trarieux, F.The early effects of the global warming can be observed and people around the world are beginning to realize the seriousness of the situation. Reducing the CO2 emissions produced by fossil energy seems to be one of the main worldwide technological challenges at the time of writing. Hence, since the oil crisis in the 70s, a growing interest in renewable energies has been noticed. In Europe, the European Commission fixed a target: to produce 20% of the EU energy from renewable sources by 2020. Similar initiatives, in varying degrees, are being considered around the globe. Among all the renewable energy technologies currently on the market, the ocean energy industry is still at an early stage, despite investigations that have been carried out on both tidal and wave energy devices over the past 40 years. The subject of this thesis focuses on one of the wave energy devices: the Oscillating Water Column. The information found in the literature about this type of plants is mainly about onshore and floating offshore OWCs. Very little information about fixed offshore OWC is available. Besides, the availability of large numbers of fixed offshore structures installed in the world oceans suggests that many of these could possibly host an OWC plant. Hence, the present study investigated a fixed offshore OWC. The aim of this thesis is to assess the power available in a fixed offshore OWC plant. To illustrate the procedure of power assessment, the fictional scenario of a platform located in the Santa Maria sea region, off the coast of Californian, is introduced. This work intends to develop a methodology to study the feasibility of such installation and estimate the power extractable through various complementary approaches. From a theoretical approach based on the wave climate of Santa Maria to wave tank experiments with various geometries and shapes of chamber (cylinder and bent duct buoy in frontward and backward position), the viability of a fixed offshore OWC plant is demonstrated for the chosen location. Results highlight the performance of the Backward Bent Duct Buoy (BBDB) for the Santa Maria characteristic sea conditions. With the intention of completing the study with a Computational Fluid Dynamics (CFD) analysis, numerical investigations about the implementation of an alternative method to generate regular waves demonstrates better results of wave propagation than the common wave generation method based on Linear Wave Theory previously used at Cranfield University. In the conclusion, the work achievements and recommendations for future CFD investigations to reproduce the wave tank experiments are discussed.Item Open Access Drag study of the nacelles of a tidal stream device using CFD(Cranfield University, 2010-11) Martinez, Fabien; Trarieux, F.Nowadays, renewable energy is in full growth. In particular, offshore wind farms will be at the centre of UK energetic strategy in the coming years. However, other types of marine renewable are still at an early development stage. That is the case for tidal energy. Many projects have been undertaken but there is no candidate for competitive commercial applications yet. Deltastream is one of these numerous pioneering projects. It consists of a set of three marine current turbines mounted on a triangular base put down onto the seabed. The device is not moored and no harm is done to the environment. However, that makes the structure more sensitive to water flows. And it is important to ensure that it will remain at its location and not being carried along with the tidal streams. Using CFD, the present study aims to evaluate the drag on the nacelles of the structure and come up with solutions to reduce it as much as possible.Item Open Access Dynamics of marine vehicles with aerodynamic surfaces(Cranfield University, 2008-11) Collu, Maurizio; Patel, M. H.; Trarieux, F.An assessment of the relative speeds and payload capacities of airborne and waterborne vehicles highlights a gap which can be usefully filled by a new vehicle concept, utilizing both hydrodynamic and aerodynamic forces. A high speed marine vehicle equipped with aerodynamic surfaces (called an AAMV, 'Aerodynamically Alleviated Marine Vehicle') is one such concept. The development of this type of vehicle requires a mathematical framework to characterize its dynamics taking account of both aerodynamic and hydrodynamic forces. This thesis presents the development of unified and consistent equations of equilibrium and equations of motion to predict the dynamic performance of such AAMV configurations. An overview of the models of dynamics developed for Wing In Ground effect 'WIGe' vehicles and high speed marine vehicles (planing craft) is given first. Starting from these models, a generic AAMV configuration is proposed and a kinematics framework is developed. Then, taking into account the aerodynamic, hydrostatic and hydrodynamic forces acting on the AAMV, equations of equilibrium are derived and solved. This is followed by deriving and solving the full equations of motion, using a small perturbation assumption. A static stability criterion, specific for the AAMV configuration, has been developed. This mathematical framework and its results are implemented in MATLAB and validated against theoretical and experimental data. The resultant capability for analysing novel AAMV configurations is presented through two parametric analysis. The analysis demonstrate that these models offer a powerful AAMV design tool.Item Open Access Negative lift generation in a tidal device structure(Cranfield University, 2009-10) Thilleul, Olivia; Trarieux, F.Becoming aware of the impact of global warming, all countries of the European Union have agreed to produce 20% of their electricity from renewable energy by 2020. Hence, a new market emerged to develop more efficient technologies of sustainable power production. Cranfield University has been involved since 2006 in wave and tidal energy design projects through various government agencies funded partnerships with the emerging industry of marine renewable energy. This thesis is based on one of these projects called “DeltaStream” which was developed by Tidal Energy Ltd (TEL). The tidal stream turbine is one of the most efficient concepts in marine renewable energy, because of the high predictability of tidal streams compared to wave and wind energy. Many devices are currently being developed in the UK. Most of them are moored to the seabed using traditional methods used in the offshore oil industry, which means drilling the sea floor. The goal of this work is to avoid drilling the seabed by adding hydrofoils on the structure. Indeed, by setting a hydrofoil in the tidal stream, the goal is to prove that a downward lift force is created and is efficient enough to maintain the structure at its location, avoiding mooring costs and ecological issues relative to drilling the sea floor. Computational Fluid Dynamics (CFD) models have been performed, studying the sensitivity of mesh size compared to results quality, and testing then different hydrofoils to get the best downward lift force in local conditions. The main characteristics of the simulation are unsteady and one phase. The chosen geometry is a “quasi-2D” domain in order to minimize computational requirements. A validation case has been first performed on a well-known geometry of a circular cylinder, to compare analytical results and CFD data. Small variations between results validated the model to enable us to use the model on a less-known simulation such as a hydrofoil. Different parameters influencing the efficiency of the hydrofoil in terms of lift production and drag reduction have been tested. These results have been compared with tank tests undertaken by the OENA Group in June-July 2009 in IFREMER (France). And complementary studies have been performed to compare these results and CFD results.Item Open Access Simulation and Experimental Verification of the Flooding and Draining Process of the Tidal Energy Converter “Deltastream” during Deployment and Recovery(Cranfield University, 2014-09) Rocolle, Guillaume; Trarieux, F.Deltastream is an on-going project carried by Tidal Energy Limited since almost twenty years. It is a tidal energy converter with a triangular shape and one turbine on each tower. It has gone through many evolutions of design but a first prototype will be installed in the end of 2014 at Ramsey Sound. The deployment and recovery operations will be carried out with a single lift point through a heavy lift frame. Two issues have to be tackled during the operation: the rate of flooding of the ballasts and the tension on the lift crane cable. The most favourable sea state must be found in order to minimise the crane cable tension as well as the best inlets and outlets configuration for the ballasts system. In order to tackle those issues, preliminary analytical work was conducted on the demonstrator to assess the stability during the flooding process. A scaled model was designed and built in order to be tested in a wave-towing tank. The results from the tests highlight that the deployment and the recovery operations are safe for both the barge and Deltastream for the range of wave conditions tested in the tank. However, the sea state has an important impact on the proceeding of the operations, especially the period of the waves.