06 Fakultät Luft- und Raumfahrttechnik und Geodäsie
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/7
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Item Open Access Maintenance strategies for large offshore wind farms(2012) Scheu, MattiWhich 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.Item Open Access Design of a kite launch and retrieval system for a pumping high altitude wind power generator(2012) Haug, StefanAirborne 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.Item Open Access Assessing hydrological changes in the Murray-Darling basin(2010) Schmid, StefanieIn this diploma thesis GRACE-derived time-variable gravity, TRMM precipitation, and in-situ hydrological observations (water level, river flow) between May 2003 and April 2009 (6 years) are used to assess hydrological changes in the Murray-Darling Basin (MDB). The investigations in terms of Multiple Linear Regression Analysis (MLRA) and Principal Component Analysis (PCA) show distinct differences between the northern and southern climatic regions of the MDB. While the north is dominated by precipitation occurring within the early part of each year, precipitation appears in the south half a year later. Furthermore, a severe drought is detected, which appeared in 2006/07. The analysis of seasonal variations reveals that gravity changes are preceded by precipitation by about one month in the north of the MDB, while the south shows the inverse behaviour. The phase shift of gravity changes between the north and the south suggests a mass transport taking about 5 months to cross the MDB from the north to the south, which is confirmed by the in-situ hydrological observations.Item Open Access GOCE sensitivity studies in terms of cross-over analysis(2009) Xue, YangThe GOCE (Gravity field and steady-state Ocean Circulation Explorer) satellite, launched on 17 March 2009, for the first time applies satellite gravity gradiometry (SGG) to recover the Earth's gravity field with cm accuracy at a resolution of 100km. To meet the envisaged accuracy, measurement validation at cross-over points (XOs) is necessary. Typically, validation is based on gravity gradients (GGs). However, the coefficient matrix of the gravitational tensor is dependent on orientation. In order to avoid matrix rotation, analysis based on orientation-independent invariants is possible. By applying various noise models, the goodness of XO-validation based on GGs and invariants will be studied in this thesis. First, by determining the maximum of scalar products from two tracks, the XOs can be predicted. Next, using local polynomial approximation, the geographical coordinates of XOs are calculated by solving a system of equations. Due to the orbit drift, the interpolation of height is performed separately along ascending and descending track before final comparison. Considering a sampling rate of 1Hz, GGs and invariants in all points of a one-week orbit are simulated for the further interpolation at the XOs. To determine the goodness of the selected interpolation algorithm, a closed loop test with noise-free data is investigated first. Since signal to noise ratios of GGs and invariants are all above 70dB, the same algorithm is applied in closed loop tests with noisy data. Since GOCE can only provide high accuracy for the main diagonal tensor components, various noise models, i.e. homogeneous and inhomogeneous white noise as well as homogeneous and inhomogeneous coloured noise, are added to the simulated values. The comparison of the goodness of GGs opposed to invariants is based on the signal to noise ratio (SNR). In this study, the second invariant demonstrates better SNR than GGs and the third invariant in the case of homogenous noise. However, due to the impact of inaccurate GGs, the SNR of invariants is poorer than the SNR of all GGs in the case of inhomogeneous noise.Item Open Access Optimization of the orbit parameters of future gravity missions using genetic algorithms(2011) Ellmer, MatthiasThe best global models for Earth's gravitational field are provided by recent satellite missions like CHAMP, GRACE and GOCE. The research in this thesis is dedicated to finding favorable parameters for possible future multi-pair satellite formations that follow in GRACE's footsteps. A software framework was developed in the Python programming language that allows the study of such satellite formations using a range-acceleration approach to gravity recovery. The computational performance of this framework was improved significantly through several optimization steps, amongst others involving successful parallelization of multiple simulations and the exploitation of a GPU-accelerated version of the mathematical library BLAS. A large number of parameter studies were performed in an attempt to define criteria for the automatic quantification of the objective quality of specific multi-pair mission scenarios. This was achieved by analyzing and assessing several characteristics of the recovered potentials in both the spatial and the spectral domain. Using the custom software framework as well as the developed benchmarks for a recovered potential, a genetic algorithm capable of autonomously finding optimal combinations of multiple satellite formations was implemented. This genetic algorithm was then tested in a simple scenario, intended to find the optimal complimentary formation to a single polar satellite pair modeled after the GRACE twin-satellite mission. The performed genetic algorithm simulation suggests that a pendulum on a ~58° inclined orbit with a long along-track baseline and an opening angle of around 40° would make a good companion for GRACE. While finalizing this thesis a flaw in the custom software was discovered that affected all simulations performed in the context of the parameter studies and the genetic algorithm. This means that the results presented in this thesis should be considered with a critical mindset. The developed methodologies used to arrive at these results are however sound and could be used in future studies.Item Open Access Gravimetrische Untersuchungen und Einflussmodellierung "Stuttgart 21"(2014) Efinger, BenjaminDurch das geplante Großbauprojekt "Stuttgart 21" und den damit notwendigen Tunnelbauarbeiten entsteht nach deren Fertigstellung im Untergrund eine Massenveränderung. Diese Massenveränderung führt zu einer veränderten Schwerkraft, die mithilfe von gravimetrischen Untersuchungen beobachtet werden kann. Da sich die Tunnelbauarbeiten noch im Anfangsstadium ihrer Durchführung befinden, können diese Veränderungen bisher nur modelliert werden. Dazu stehen verschiedene Modelle zur Verfügung, die eine Massenveränderung im Untergrund mit unterschiedlichen geometrischen Formen beschreiben. Diese werden auf Grundlage einer Beispielgeometrie miteinander verglichen und bewertet, um anschließend mit den geeigneten Modellen und der geplanten Tunnelgeometrien den Einfluss der Tunnel zu modellieren. Um abschließend den modellierten Einfluss nach Tunnelfertigstellung mit den dann beobachteten Schwerewerten vergleichen zu können, wurde vor dem Beginn der Tunnelarbeiten mit gravimetrischen Beobachtungen begonnen. Dazu wurde ein geeignetes Profil gewählt, das die Tunnelröhren kreuzt, um so eine generelle Aussage über den Einfluss der Massenveränderung treffen zu können. Wichtig ist hierbei unter anderem die Frage, ob es mit dem vorliegenden Instrument, einem Relativgravimeter, möglich ist, aus den beobachteten Schwerewerten einen Einfluss zu beobachten. Zusätzlich werden die Einflüsse auf den Schweregradienten modelliert und analysiert. Da diese jedoch in einem nicht mehr messbaren Bereich liegen, werden dazu nur Messungen in einem Testumfeld durchgeführt und ausgewertet. Dabei ist interessant, ob sich das Messprinzip für zukünftige Untersuchungen eignet.Item Open Access Applying stochastic constraints on time-variable GRACE data(2009) Lorenz, ChristofSince its launch in the year 2002, the space satellite mission GRACE provides spherical harmonic coefficients, which can be used to observe the time-variable part of the Earth's gravity field. It was initially assumed that the derived gravitational quantities from these coefficients are of high accuracy and would thus deliver reliable large scale mass estimates. However, the provided coefficients of higher harmonic degree and order turned out to be seriously contaminated with noise, yielding an unrealistic signal of mass variations in form of massive north-south stripes. In this work, two methods are investigated, which add stochastic constraints to time variable GRACE coefficients. It is assumed that these techniques are able to reduce the noise level in the monthly datasets by assimilating the GRACE coefficients with more reliable data. Both approaches need prior estimates of a signal and error covariances. Hence, the signal covariance of the time-variable gravity field is assumed to be of isotropic nature and is thus computed as a Kaula-type power law, which is fit into the part where the signal degree variances of the GRACE solutions linearly attenuate. The error covariance is estimated according to the energy balance approach which allows the simulation of a fully populated GRACE covariance matrix. Stochastic constraining in the spectral domain combines both signal and error covariance estimates in a Bayesian type regularization procedure, which constrains the monthly GRACE solutions with the modelled signal covariance. It is shown that Bayesian type regularization can be used to build a spectral filter kernel. Furthermore, the weight between both GRACE coefficients and the regularization term is estimated by a variance component estimation. Tests with a full, block diagonal and diagonal covariance matrix are performed, as it is widely believed that full covariance information can be sufficiently approximated by a block diagonal matrix. Furthermore, the Bayesian type regularization filter is tested with three different monthly GRACE solutions and compared with other widely used filtering techniques. The second approach constrains the time-variable GRACE coefficients with hydrological observations, which are provided as monthly precipitation and run-off values on basin scale. Both the GRACE and hydrological observation group are assimilated in one linear model, which is solved by sequential least squares estimation, yielding an agreement between mass estimates from GRACE and observed hydrology.Item Open Access GOCE data analysis: optimized brute force solutions of large-scale linear equation systems on parallel computers(2010) Roth, MatthiasThe satellite mission GOCE (Gravity field and steady-state Ocean Circulation Explorer) was set up to determine the figure of the Earth with unprecedented accuracy. The sampling frequency is 1 Hz which results in a massive amount of data over the one year period the satellite is intended to be functional. From this data we can setup an overdeterminded linear system of equations to estimate the geopotential coefficients which are required for modelling the Earth's gravity field with spherical harmonics in the desired high resolution. The linear system of equations is solved "brute-force" which means that the normal equation matrix has to be kept in memory as a whole. The normal equations matrix has a memory demand of up to 65 GByte, hence we need a computer providing a sufficient amount of memory and fast multiple processors for the computations to get them done in a reasonable time. In this study, a program was written to compute the geopotential coefficients from simulated GOCE data, as GOCE real data were not available yet. As a first step, the program was optimized for the computations to become more efficient. As a second step, the program was parallelized to speed-up the computations by two different techniques: For a first version, the parallelization was done via OpenMP which can be used on shared memory systems which usally have a small number of processors. For the second version, MPI was used which is suited for a distributed memory architecture, hence can incorporate much more processors in the computations. In summation, we gained a huge boost in efficiency of the program due to the optimization. Furthermore, huge speed-up was gained due to the parallelization. The more processors are incorporated in the computation, the more the overall efficiency drops because of increasing communication between the processors. Here we could show that for huge problems the MPI version is running more efficient than the OpenMP version.Item Open Access Nachweis einer vulkanischen Struktur auf der Schwäbischen Alb mittels gravimetrischer Messungen und unterstützender Höhenbestimmung(2010) Scheider, AnnetteIm Bereich der Schwäbischen Alb und des Albvorlandes existieren eine Vielzahl von vulkanischen Strukturen unterschiedlicher Größe, die man zusammen als Schwäbischer Vulkan bezeichnet. Sie entstanden bereits im Miozän und sind heute oft nicht mehr als solche zu erkennen. Besonders viele davon befinden sich im Gebiet von Kirchheim u.T. und Bad Urach. Das Maar am Hofbrunnen ist eine dieser Strukturen und liegt in der Gemeinde Münsingen, nahe des Ortes Trailfingen auf der Schwäbischen Alb. Diese Struktur ist bereits seit langer Zeit bekannt, dennoch ist sie in einigen neueren Karten nicht enthalten. In dieser Diplomarbeit soll ein Teilgebiet im Süden des Maars untersucht werden und es wird überprüft, ob eine vulkanische Struktur mittels gravimetrischer Messungen und unterstützender Höhenbestimmungen nachgewiesen werden kann. Zunächst werden die geologischen Grundlagen, die die Entstehung dieser vulkanischen Strukturen und speziell zu der des Maars am Hofbrunnen erklären, betrachtet. Da sowohl Höhen- als auch Schweremessungen durchgeführt werden sollen, werden anschließend die verschiedenen möglichen Messverfahren genauer betrachtet und die für diesen Zweck am besten geeigneten ausgewählt. Um die notwendigen Messungen durchführen zu können, muss im nächsten Schritt ein geeignetes Messkonzept aufgestellt werden, das dann bei der Durchführung der Messungen im Gelände umgesetzt werden kann. Bei der Auswertung wird zuerst aus den erhaltenen Höhendaten ein DGM generiert, dass die Ausgangslage für die Auswertung der Schwere-Messergebnisse liefert. Für die Herleitung der Schwereanomalien auf das Geoid müssen einige Reduktionen und Korrekturen angebracht werden, so dass im Ergebnis die vulkanische Struktur sichtbar wird. In einem letzten Schritt wird der Einfluss der vulkanische Struktur, die eine Störmasse im Untergrund darstellt, auf die Schwere modelliert. Dazu wird die Struktur aufgeteilt, vereinfacht und als geometrische Körper aufgefasst, die aus einem Gestein bestehen, dessen Dichte sich vom Umgebungsgestein unterscheiden. Die Ergebnisse dieser Modellierung werden abschließend mit den Messergebnissen verglichen.Item Open Access Orbitverdichtung mittels Kalman-Filterung am Beispiel der Satellitenmission GRACE(2011) Leinss, BenediktDas übergeordnete Thema der vorliegenden Arbeit ist die Verdichtung von Satellitenorbits. Grundsätzlich besteht hierbei die Aufgabe darin, für einen gegebenen Orbit eine korrespondierende Satellitenbahn mit einer höheren zeitlichen Auflösung zu bestimmen. Eine solche Orbitverdichtung ist etwa bei einigen Ansätzen zur Schwerefeldbestimmung aus Level-1B-Daten der Satellitenmission GRACE nötig, weil dabei die GRACE-Navigationslösungen mit den K-Band-Beobachtungen zu kombinieren sind und sich diese Datensätze gerade hinsichtlich ihrer zeitlichen Abtastung unterscheiden. Angesichts dieser Situation wird hier eine Orbitverdichtung mittels der erweiterten Kalman-Filterung (EKF) konzipiert und in einer Matlab*-Toolbox umgesetzt. Neben einer Funktion, mit der sich prinzipiell weitgehend beliebige Orbits verdichten lassen, wird außerdem ein Filter entworfen, das einzig für GRACE ausgelegt ist und es ermöglicht, auch die hochgenauen K-Band-Relativgeschwindigkeiten in die Verdichtung einzubeziehen. Um die Systemzustände und die zugehörigen Kovarianzen zu prädizieren, werden unterschiedliche Orbitintegrationen realisiert. Die meisten davon basieren auf der numerischen Integration von gewöhnlichen DGL-Systemen erster Ordnung in kartesischen, quasi-inertialen Koordinaten. Der hier komplexeste Integrator trägt den Störbeschleunigungen aufgrund der Anisotropie des Erdgravitationsfelds Rechnung, indem geeignete Gravitationsfeldmodelle genutzt werden. Sämtliche Orbitintegratoren und die Routinen zur EKF-Orbitverdichtung werden mithilfe simulierter Daten durch Tests verifiziert. Außerdem vergleicht man die verschiedenen Prädiktionsmethoden miteinander im Hinblick auf die erzielten Genauigkeiten und deren Berechnungseffizienz. Bei der gegen Ende durchgeführten Prozessierung realer GRACE-Daten zeigt sich, wie auch bei den vorherigen Tests, dass es dank der Kalman-Filterung gelungen ist, verdichtete GRACE-Orbits zu berechnen, welche erheblich besser zu den gemessenen K-Band-Relativgeschwindigkeiten passen als die gegebenen Navigationslösungen.