06 Fakultät Luft- und Raumfahrttechnik und Geodäsie

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7

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Institute: search.filters.UBSInstitut.Institut für FlugzeugbauPublication Type: search.filters.UBSTyp.Abschlussarbeit (Diplom)

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    ItemOpen Access
    Maintenance strategies for large offshore wind farms
    (2012) Scheu, Matti
    Which equipment is needed and how shall tasks be scheduled in order to implement the economically most efficient operation and maintenance strategy for large offshore wind farms? This is the question motivating this research project. Considering production losses due to turbine downtime as well as local geographical and weather conditions, an efficient operation and maintenance (O&M) solution shall be achieved for two reference sites at the UK east coast. For this purpose, a Matlab-based tool has been developed, consisting of the following five main modules: Weather, Failures, Resources, Strategy and Cost. The "Weather" module is able to generate future sea states and wind speeds based on historical data. It uses a finite state Markov chain in discrete time to model significant wave heights. Wind speeds are then generated according to their conditional probability distribution at the corresponding wave height. In order to validate the weather module, several time series were generated and compared with existing data. For comparison, the mean values, standard errors, linear correlations and cumulative distribution functions for persistence of operational weather windows were chosen, both for synthetic and observed wind speed and wave height time series. Both reference sites in the UK North Sea were considered for validation. Failure rates are the basis for the "Failure" module. As an input, data gathered from onshore reliability investigations are used, which can be updated once more detailed data is available for offshore turbines. The outcomes of this module are turbine-failures occurring at a certain time. Within the “Resources” module, it is defined which equipment and personnel is available for O&M activities. The equipment is specified by its technical characteristics, e.g., the maximum transportable personnel and the operational wave height boundary. Another key parameter is the "Strategy". The main goal of this module is to take the decision whether to perform an operation or not. Within this thesis, one specific strategy has been used, but references are made to possible modifications in the according paragraphs. The measurement of economic performance is done in the "Cost" module. Here, production losses are quantified by combining the wind speed during a failure with the linearized power curve of the turbine and the local buyback price system. Therefore, the worth of additional or better maintenance equipment can be seen directly as an increase in availability and a decrease of production losses. Results show how sensitive availability and therefore production losses change with respect to changes in the maintenance fleet, reliability characteristics of components and distance to shore. Major improvements of availability were achieved by applying maintenance vessels with a higher operational wave height boundary. An increase of this constraint from one to 1.8 m significant wave height raised the availability by up to 30 percent, leading to a much better economic performance. The influence of the weather forecast accuracy on the number of maintenance vessel and crane deployments is also stated, showing a significant increase of deployments if the weather forecast is only accurate for short times. An improvement of component-reliability, modeled as a 50% decreased annual failure rate, could save up to 440 k€ of yearly production losses for each modeled wind turbine. Higher transit times, due to a greater distance to shore, strongly decrease the wind park availability.
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    ItemOpen Access
    Design of a kite launch and retrieval system for a pumping high altitude wind power generator
    (2012) Haug, Stefan
    Airborne Wind Energy (AWE) is currently one of the most challanging topics in wind energy. Due to the little amount of material it is possible to reach low energy costs and a low environmental impact. Kites are perfect wings to gain AWE due to its good flight quality and a high surface to mass ratio. However, a good and cheap launch and retrieval system for automated kite usage has not been designed, yet. This thesis aims to analyze the properties which influence the launch and retrieval of a Leading Edge Inflatable (LEI) Kite and to design a complete launch and retrieval system for the automated launch and retrieval of a 100m2 LEI Kite. This thesis starts from close to the scratch. It develops requirement for a launch and retrieval system to later check the quality of different possible systems. It finds out that systems like balloons or UAVs fit the best to the requirements, however, they are not reliable enough in strong wind. Hence an aluminum truss mast is build as prototype to investigate the behavior of the kites during an upside down launch. As the results are auspicious the launch is mathematically analyzed. To create a final design the current kite is scaled up to 100m2. By using the upscaled forces and dimensions it designs - a mast construction - a foundation for on- or offshore applications - a function to store, protect and control the kite during idle time Furthermore it gives a first impression on the dimensions of bearings and engines which are necessary for the system. By scaling up known data and using current price of steel and concrete a price for the system is estimated. It is proven by field tests that the upside down launch is possible with a wind speed of approximately 4m/s and that higher wind speed is profitable for the launch. Analysis showed that the final design is safe in case of extreme temperature, long durability and high wind up to the 50 years gust. Furthermore the CO2 emission per kWh of the final design can be decreased down to 11gCO2/kWh in good conditions.
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    Reduced model design of a floating wind turbine
    (2012) Sandner, Frank
    Floating platform concepts offer the prospect of harvesting offshore wind energy at deep water locations for countries with a limited number of suitable shal- low water locations for bottom-mounted offshore wind turbines. The floating spar-buoy concept has shown promising experimental and theoretical results. Al- though various codes for a detailed simulation exist the purpose of this work is to elaborate a reduced Floating Offshore Wind Turbine (FOWT) model that mainly reproduces the overall nonlinear low-frequency behaviour of the system with a significant saving in simulation time. One objective is to extend the model predictive control algorithm that has previously been developed for onshore wind turbines to the FOWT for motion control and load reductions. Another objective is a fast dynamic assessment of new concepts during design phase with respect to load cases defined by the International Electrotechnical Commission (IEC). The platform and wind turbine structure is modelled as a three-dimensional multibody system consisting of four rigid bodies with nine degrees of freedom. That is, unconstrained platform motion, tower bending in two directions and variable rotor speed. The coupled nonlinear system of equations of motion is calculated symbolically using the Newton-Euler formalism that takes Coriolis- and centrifugal forces into account. Complex disturbances on the system arising from aerodynamics and hydrodynamics are simplified along with the model as efficiently and accurately as possible. Wind loads are predicted by reducing the three-dimensional turbulent wind field to a scalar rotor-effective wind speed also considering restoring torques resulting from oblique inflow. Linear wave theory provides the wave kinematics and wave loads are calculated using the relative formulation of Morison’s Equation. An approach is presented to estimate wave loads on the floating structure based only on real-time wave height measurements. This allows also for an analytical calculation of wave loads in time-domain with- out iterative or recursive algorithms so that a significant saving in computational time is achieved. The presented disturbance reduction to simple and measurable inputs for wind and waves is a precondition for the implementation of an opti- mal control algorithm. The reduced nonlinear model is compared to the certified aero-hydro-servo-elastic FAST model in time and frequency domain. The results are promising as there is good agreement in static and dynamic response.