Probabilistic methods for wind turbine blades

The European Energy Research Alliance (EERA) has as a key purpose to elevate cooperation between national research institutes to a new level, from ad-hoc participation in joint projects to collectively planning and implementing joint strategic research programmes. The RES directive and the SET Plan enforce a high rate of deployment of wind energy, on- and offshore for the European Union’s member states leading to a high challenge for research in the two priority areas: Integration and Offshore. Wind energy was therefore at an early stage identified as an area for a joint research programme where the key players are the national wind energy research institutes but open to and encouraging universities to participate in the activities

A Critical Evaluation of Structural Analysis Tools used for the Design of Large Composite Wind Turbine Rotor Blades under Ultimate and Cycle Loading

Rotor blades for 10-20MW wind turbines may exceed 120m. To meet the demanding requirements of the blade design, structural analysis tools have been developed individually and combined with commercial available ones by blade designers. Due to the various available codes, understanding and estimating the uncertainty introduced in the design calculations by using these tools is needed to allow assessment of the effectiveness of any future design modification. For quantifying the introduced uncertainty a reference base was established within INNWIND.EU in which the several structural analysis concepts are evaluated. This paper shows the major findings of the comparative work performed by six organizations (universities and research institutes) participating in the benchmark exercise. The case concerns a 90m Glass/Epoxy blade of a horizontal axis 10MW wind turbine. The detailed blade geometry, the material properties of the constitutive layers and the aero-elastic loads formed the base by which global and local blade stiffness and strength are evaluated and compared. Static, modal, buckling and fatigue analysis of the blade were performed by each partner using their own tools; fully in-house developed or combined with commercially available ones, with its specific structural analysis approach (thin wall theory and finite element models using beam, shell or solid elements) and their preferable analysis type (linear or geometrical non-linear). Along with sectional mass and stiffness properties, the outcome is compared in terms of displacements, stresses, strains and failure indices at the ply level of the blade structure, eigen-frequencies and eigen-modes, critical buckling loads and Palmgren-Miner damage indices due to cycle loading. Results indicate that differences between estimations range from 0.5% to even 40%, depending on the property compared. Modelling details, e.g. load application on the numerical models and assumptions, e.g. type of analysis, lead to these differences. The paper covers these subjects, presenting the modelling uncertainty derived

Detailed state of the art review for the different on-line/in-line oil analysis techniques in context of wind turbine gearboxes

The main driver behind developing advanced condition monitoring (CM) systems for the wind energy industry is the delivery of improved asset management regarding the operation and maintenance of the gearbox and other wind turbine components and systems. Current gearbox CM systems mainly detect faults by identifying ferrous materials, water, and air within oil by changes in certain properties such as electrical fields. In order to detect oil degradation and identify particles, more advanced devices are required to allow a better maintenance regime to be established. Current technologies available specifically for this purpose include Fourier transform infrared (FTIR) spectroscopy and ferrography. There are also several technologies that have not yet been or have been recently applied to CM problems. After reviewing the current state of the art, it is recommended that a combination of sensors would be used that analyze different characteristics of the oil. The information individually would not be highly accurate but combined it is fully expected that greater accuracy can be obtained. The technologies that are suitable in terms of cost, size, accuracy, and development are online ferrography, selective fluorescence spectroscopy, scattering measurements, FTIR, photoacoustic spectroscopy, and solid state viscometers

Perturbation-based stochastic multi-scale computational homogenization method for the determination of the effective properties of composite materials with random properties

Quantifying uncertainty in the overall elastic properties of composite materials arising from randomness in the material properties and geometry of composites at microscopic level is crucial in the stochastic analysis of composites. In this paper, a stochastic multi-scale finite element method, which couples the multi-scale computational homogenization method with the second-order perturbation technique, is proposed to calculate the statistics of the overall elasticity properties of composite materials in terms of the mean value and standard deviation. The uncertainties associated with the material properties of the constituents are considered. Performance of the proposed method is evaluated by comparing mean values and coeffcients of variation for components of the effective elastic tensor against corresponding values calculated using Monte Carlo simulation for three numerical examples. Results demonstrate that the proposed method has suffcient accuracy to capture the variability in effective elastic properties of the composite induced by randomness in the constituent material properties

Long-term research challenges in wind energy – a research agenda by the European Academy of Wind Energy

The European Academy of Wind Energy (eawe), representing universities and institutes with a significant wind energy programme in 14 countries, has discussed the long-term research challenges in wind energy. In contrast to research agendas addressing short- to medium-term research activities, this eawe document takes a longer-term perspective, addressing the scientific knowledge base that is required to develop wind energy beyond the applications of today and tomorrow. In other words, this long-term research agenda is driven by problems and curiosity, addressing basic research and fundamental knowledge in 11 research areas, ranging from physics and design to environmental and societal aspects. Because of the very nature of this initiative, this document does not intend to be permanent or complete. It shows the vision of the experts of the eawe, but other views may be possible. We sincerely hope that it will spur an even more intensive discussion worldwide within the wind energy community

Reliability estimation in the design of composite material structures

The dissertation intents to develop a suitable methodology and the relevant numerical tools for the estimation of the reliability of composite material structures, when not only the variability of the loading and the orthotropic material strength properties is taken into account, but also, the variability of the material elastic properties and the thermal expansion coefficients. Simultaneously aim of the work is to develop a method that is easy to use and relatively accurate so that it is practical for use during the design of such structures, that require a multitude of repetitive trails up to the final solutions. In the work the variability of the mechanical properties of composite materials is investigated especially those used in for the manufacturing of wind turbine blades, their statistical modeling as well as methods for the estimation of the structural reliability. Focusing the interest in the structural design of wind turbine blades an adequate numerical tool has been developed for the estimation of the wind turbine blade reliability on the material layer level taking into consideration the variability of the elastic and thermal properties in addition to the strength properties while on the loading side, the variability of the normal stresses as well as the shear stresses starting from the variability of the external loading of the blade is considered. The suggested methods for the estimation of the failure probability of the wind turbine blade are proved applicable also for other composite material structures sharing the characteristic of the applied loads being undertaken mainly by the composite material.Η διατριβή αποσκοπεί στην ανάπτυξη κατάλληλης μεθοδολογίας και των αντίστοιχων υπολογιστικών εργαλείων με σκοπό τον προσδιορισμό της αξιοπιστίας κατασκευών από σύνθετα υλικά όταν λαμβάνεται υπόψη όχι μόνο η στοχαστικότητα της φόρτισης και των ιδιοτήτων αντοχής του ορθοτρόπου υλικού αλλά και η μεταβλητότητα των ελαστικών του ιδιοτήτων και των συντελεστών θερμικής διαστολής. Ταυτόχρονα στόχος της εργασίας είναι η μέθοδος που θα αναπτυχθεί να είναι ευκολόχρηστη και αρκετά ακριβής ώστε να καταστεί πρακτική για χρήση σε σχεδιασμό τέτοιων κατασκευών οπού μέχρι το τελικό αποτέλεσμα απαιτούνται πολλές επαναληπτικές δοκιμές. Στην εργασία διερευνάται η μεταβλητότητα των μηχανικών ιδιοτήτων σύνθετων υλικών, κυρίως αυτών που χρησιμοποιούνται στην κατασκευή πτερυγίων Ανεμογεννητριών η στατιστική μοντελοποίηση της καθώς και μέθοδοι για τον προσδιορισμό της αξιοπιστίας της κατασκευής. Επικεντρώνοντας το ενδιαφέρον στον δομικό σχεδιασμό πτερυγίων ανεμογεννητριών αναπτύχθηκε κατάλληλο υπολογιστικό εργαλείο για την εκτίμηση της αξιοπιστίας του πτερυγίου υπό στατική φόρτιση, στο επίπεδο της στρώσης του υλικού λαμβάνοντας υπόψη την στοχαστικότητα των ελαστικών-θερμικών ιδιοτήτων και των ιδιοτήτων αντοχής του υλικού ενώ από την πλευρά της φόρτισης περιλαμβάνεται η μεταβλητότητα τόσο των ορθών τάσεων όσο και των διατμητικών τάσεων που αναπτύσσονται στο επίπεδο της στρώσης ξεκινώντας από την στοχαστικότητα της εξωτερικής φόρτισης του πτερυγίου. Οι προτεινόμενες μέθοδοι εκτίμησης της πιθανότητας αστοχίας του πτερυγίου ανεμογεννήτριας αποδεικνύονται εφαρμόσιμες και σε άλλες κατασκευές από σύνθετα υλικά, με κύριο γνώρισμα τους την παραλαβή της φόρτισης σε μεγάλο βαθμό από το σύνθετο υλικό

Measurement uncertainty of fatigue properties and its effect on the wind turbine blade reliability level

Wind turbine rotor blades are large composite structures performing most of their design life under random cycle loading patterns. Material properties of the building ply exhibit also inherent variability. In order to assure a safe and cost effective design, uncertainty related to the basic variables (material properties, loads, etc.) should be quantified and taken into account in design calculations. Herein, for the first time, measurement uncertainty for the fatigue properties of the composite material is quantified based on detailed experimental data and following principles of metrology. The effect of considering measurement uncertainty directly on the blade failure probability is investigated by enhancing probabilistic analysis tools developed for use on wind turbine blade design. The application is performed on the INNWIND.EU reference 10MW rotor blade of 90m length. Sensitivity analysis with respect to the final reliability estimation is further performed on the fatigue analysis details, such as the selection of constant life diagram and the discretization strategy followed in the rainflow counting method