Offshore Wind Turbine Support Structures Research Articles

Accurate and efficient modeling of complex offshore wind turbine support structures using augmented superelements

ABSTRACTTraditionally, wind turbine dynamics are analyzed using computationally efficient but geometrically coarse aeroelastic models. With ever larger offshore turbines being installed in deeper waters, the wind industry is gradually moving toward more complex foundation types such as jackets and tripods. Even the simplest models of such structures have many more degrees of freedom (DoFs) than the complete wind turbine model, leading to excessive computation times. To cope with this, we can employ reduced ‘superelement’ modeling of the support structure. However, since these structures are subjected to hydrodynamic loading at a large portion of their DoFs, traditional reduction methods fail to properly describe the response to this excitation. In this paper, we therefore propose to combine superelement modeling with the concept of modal truncation augmentation, which consists in extending the reduction basis by adding ‘residual vectors’. Furthermore, we use principal component analysis to find the predominant hydrodynamic loading on the support structure.A case study is performed on a reference wind turbine model on a jacket structure, revealing both the need for coupled dynamic analysis and the shortcomings of traditional superelement models for offshore support structures. Most importantly, this case study shows that the proposed augmented superelement approach allows to create very compact yet accurate models of the complex support structure, thereby enabling efficient integrated simulation of offshore wind turbines.Copyright © 2013 John Wiley & Sons, Ltd.

Probabilistic demand models and fragility estimates for offshore wind turbine support structures

Probabilistic models are developed to predict the deformation, shear and moment demands on offshore wind turbine support structures subject to operational and environmental loadings (i.e., day to day wind, wave and current). An existing deterministic model is corrected by adding a correction term to capture the inherent bias, and model error arising from an inaccurate model form or missing variables. A database of structural responses is used to calibrate the proposed models. The database is obtained from detailed three-dimensional finite element analyses of a set of typical wind turbine systems with different design parameters. The finite element analyses account for the nonlinear soil-structure interaction. The proposed probabilistic demand models provide accurate and unbiased estimates of the demands on the support structure properly accounting for the underlying uncertainties. The models are then used to estimate the fragility of the support structure of wind turbines which is defined as the conditional probability of not meeting specified capacity levels.

Comparison of Structural Properties between Monopile and Tripod Offshore Wind-Turbine Support Structures

Offshore wind power provides a new kind of green energy. This paper presents a comparison study on the structural properties of monopile and tripod wind-turbine support structures, which are used extensively in offshore wind farms. Both structures have the same upper tower, but different lower structures, one with a monopile and the other with a tripod. Static, fatigue, and modal analyses indicate that both the tripod and monopile structures are feasible in the field, but that the tripod structure is superior to the monopile structure. Static analysis reveals that the location of maximum stress in the monopile structure is different from that in the tripod structure, and that the tripod structure shows higher stiffness and greater stress-control capacity than the monopile structure. Fatigue analysis indicates that the tripod structure has a longer lifetime than the monopile structure. Modal analysis indicates that the two structures exhibit large differences in their natural frequencies. Unlike the monopile structure, the third and first modes both have a substantial influence on the dynamic response of the tripod structure.

Tragverhalten von Offshore Gründungskonstruktionen

AbstractDieser Artikel enthält eine Zusammenfassung von Ergebnissen des Forschungsverbundvorhabens “OGOWIN: Optimierung aufgelöster Gründungsstrukturen für Offshore‐Windenergieanlagen hinsichtlich Materialeinsatz, Montageablauf und neuer Fertigungsverfahren”. Die Ergebnisse umfassen (1) eine Untersuchung des dynamischen Verhaltens der verschiedenen Gründungsvarianten Monopile, Tripod und Jacket, (2) die Ermittlung einer Wöhlerlinie für den Grundwerkstoff eines Stahlgussknotens, (3) die Ermittlung der Traglast eines Stahlgussknotens in einem 1:1‐Versuch sowie (4) Berechnungen zur Form‐ und Materialoptimierung. Zur Überwachung der Gebrauchstauglichkeit während des Betriebes wurden relevante Parameter hinsichtlich ihres Einflusses auf die ersten Eigenfrequenzen der o. g. Gründungskonstruktionen rechnerisch untersucht. Untersuchungen der Ermüdungsfestigkeit und die erstmalige experimentelle Ermittlung der Traglast eines Gussknotens tragen zu einem optimierten Entwurf durch genaue Kenntnis des Materialverhaltens und der höchstbeanspruchten Bereiche bei. Darüber hinaus konnte gezeigt werden, dass eine geometrische Optimierung des Stahlgussknotens unter konstruktiven Randbedingungen zu einer Verringerung des Materialaufwandes bzw. zu einer Erhöhung der Traglast führen kann.Structural behaviour of offshore support structures – Computational and experimental analysisThis paper contains a summary of the joint research project “OGOWIN: Optimisation of support structures for offshore wind turbines regarding the material consumption, the construction and production sequence as well as new production methodologies”. The results comprise (1) a modal analyses of various types of offshore wind turbine support structures, (2) the fatigue strength testing of the cast steel material, (3) a large scale ultimate capacity test of a cast steel tube connection and (4) an optimisation of the tube connection aiming at material and stress reduction. The sensitivity of the relevant parameters on the first natural frequencies was quantified and the most influencing parameters were identified. The analyses of the fatigue strength and of the ultimate capacity of the cast steel tube connection support an optimised design by precise knowledge of the failure mechanisms and material behaviour. Moreover, it is shown that optimising the cast steel node can reduce the material use and stress concentration.

Application and extension of the TOPSIS method for the assessment of floating offshore wind turbine support structures

The offshore wind industry is shifting its focus towards deeper water sites, more suited to floating rather than bottom fixed support structures. Floating support structures currently used for oil and gas platforms provide a starting point for the floating wind industry; however, the selection of an optimum structure is not trivial with several unique factors that contribute to its complexity. In this work programme, a methodology to rapidly assess several concepts for wind turbine floating support structures is proposed. Using the multi-criteria decision-making method, TOPSIS, configurations are rated for a range of attributes. In addition, two complementary methodologies have been developed that extend the TOPSIS capability by including an estimated impact of the uncertainties associated with each of the selected criteria, on the final choice of an optimum support structure. This methodology represents a robust yet flexible design tool to complement the early conceptual design process that is able to rapidly investigate a broad design space and narrow down the number of potential options suitable for floating wind turbines.

Optimierung des Managements der Tragwerksintegrität für Offshore‐Windenergieanlagen

AbstractDieser Artikel enthält aktuelle Forschungsergebnisse im Bereich des überwachungsgestützten Managements der Tragwerksintegrität. Das Management der Tragwerksintegrität hat zum Ziel, die Funktionalität eines Tragwerks während des gesamten Lebenszyklus durch Einhaltung der Zielzuverlässigkeiten sicherzustellen. Ausgehend von Forschungsergebnissen über die Charakteristik von Überwachungsverfahren und ‐daten im Rahmen von Zuverlässigkeitsanalysen, wird ein Ansatz vorgestellt, wie die erwarteten Kosten für das Management der Tragwerksintegrität durch Überwachungsverfahren optimiert werden können. Dazu wird eine Kosten‐Nutzen‐Analyse für den Lebenszyklus einer Gründungstruktur von Offshore‐Windenergieanlagen durchgeführt. Im Rahmen der Bayes'schen Entscheidungstheorie werden durch eine Prä‐posteriori‐Entscheidungsanalyse die optimalen Entscheidungsparameter für den Entwurf eines Überwachungssystems bestimmt. Als Fallstudie wird ein Überwachungssystem für die Gründungsstruktur eines Multibrid‐M5000‐Prototyps konzipiert und die mögliche Reduktion der Risiken und erwarteten Betriebskosten quantifiziert.Optimisation of the structural integrity management for offshore wind turbinesThis paper contains recent research results in the field of monitoring supported structural integrity management. The structural integrity management aims at ensuring the functioning of a structure throughout the life cycle by compliance with the target reliabilities. Building upon research results concerning the characteristics of monitoring techniques and data within the framework of reliability analyses, an approach is introduced for the optimisation of the expected structural integrity management costs by monitoring information. For this aim, a life cycle cost benefit analysis for offshore wind turbine support structures is formulated. The optimal monitoring decision parameters are then assessed utilizing the Bayesian pre‐posterior decision theory. As a case study a monitoring system for a Multibrid M5000 offshore wind turbine prototype support structure is designed and possible reductions of the risks and expected costs are quantified.

Fatigue reliability analysis of the jacket support structure for offshore wind turbine considering the effect of corrosion and inspection

Due to the high level of fatigue loads as well as a large number of load cycles caused by wind and wave loads together, fatigue performance of welded connections is a design driving criterion for offshore wind turbine support structures. In this paper prediction of fatigue reliability of welded multi-planar tubular joints of the support structure of a fixed jacket offshore wind turbine designed for a northern North Sea site in a water depth of 70m is performed. Dynamic response of the jacket support structure due to wind and wave loads is calculated by using a decoupled procedure with good accuracy. Hot-spot stresses at failure-critical location of the relative reference brace of the selected tubular joint are derived by summation of the single stress components from axial, in-plane and out-of-plane action, the effects of planar and non-planar braces are also considered. A two-parameter Weibull function is used to fit the long-term statistical distribution of hot-spot stress ranges by combination of time domain simulation for representative environmental conditions (wind/sea states) in operational condition. The main uncertainties associated with the whole procedure of reliability analysis are identified and quantified. The load histories are normalized to ensure that fatigue design criteria based on the SN-Miner-Palmgren approach is satisfied. Hence the reliability estimates obtained refer to fatigue design of tubular joints that satisfy design criteria. The reliability analysis is based on fracture mechanics (FM) analysis of crack growth. The corrosion-induced increased crack growth rate is taken into account by considering the increased hot-spot stress range due to changes of nominal stress and stress concentration factors (SCFs) produced by the thickness thinning (wastage) effects of all braces and chord of the selected tubular joint with a general uniform corrosion model. The geometry function effect and the material degradation effect due to corrosion on the reliability analysis are also investigated. The effects of inspection and repair with and without consideration of corrosion are quantified based on the quality of inspection in terms of probability of crack detection curves. The sensitivity of the reliability index on important random variables is estimated.

Multi-criteria assessment of offshore wind turbine support structures

Wind power, especially offshore, is considered one of the most promising sources of ‘clean’ energy towards meeting the EU and UK targets for 2020 and 2050. Deployment of wind turbines in constantly increasing water depths has raised the issue of the appropriate selection of the most suitable support structures’ options. Based on experience and technology from the offshore oil and gas industry, several different configurations have been proposed for different operational conditions. This paper presents a methodology for the systematic assessment of the selection of the most preferable, among the different configurations, support structures for offshore wind turbines, taking into consideration several attributes through the widely used multi-criteria decision making method TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) for the benchmarking of those candidate options. An application comparing a monopile, a tripod and a jacket, for a reference 5.5 MW wind turbine and a reference depth of 40 m, considering multiple engineering, economical and environmental attributes, will illustrate the effectiveness of the proposed methodology.

Estimation of Section forces due to Impact Breaking Wave Force on an Offshore Wind Turbine Support Structure

Estimation of Section forces due to Impact Breaking Wave Force on an Offshore Wind Turbine Support Structure