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A navegação consulta e descarregamento dos títulos inseridos nas Bibliotecas Digitais UC Digitalis, UC Pombalina e UC Impactum, pressupõem a aceitação plena e sem reservas dos Termos e Condições de Uso destas Bibliotecas Digitais, disponíveis em https://digitalis.uc.pt/pt-pt/termos.Conforme exposto nos referidos Termos e Condições de Uso, o descarregamento de títulos de acesso restrito requer uma licença válida de autorização devendo o utilizador aceder ao(s) documento(s) a partir de um endereço de IP da instituição detentora da supramencionada licença.Ao utilizador é apenas permitido o descarregamento para uso pessoal, pelo que o emprego do(s) título(s) descarregado(s) para outro fim, designadamente comercial, carece de autorização do respetivo autor ou editor da obra.Na medida em que todas as obras da UC Digitalis se encontram protegidas pelo Código do Direito de Autor e Direitos Conexos e demais legislação aplicável, toda a cópia, parcial ou total, deste documento, nos casos em que é legalmente admitida, deverá conter ou fazer-se acompanhar por este aviso. Reliability Analysis on Crucial Subsystems of a Wind Turbine through FTA Approach Autor(es):Katsavounis, S.; Patsianis, N.; Konstantinidis, E.I.; Botsaris, P.N. Publicado por:Imprensa Abstract -The wind turbine reliability is a crucial factor for the successful operation of a wind power plant, affecting its availability and efficiency. Operation and maintenance costs affect the performance of the whole system and reinforce the necessity of redesign of specific sub-assemblies achieving lower energy production costs.At the first stage, field data make up Weibull sets in order to form the appropriate distribution-curve of the failure rate in each corresponding top event, are presented. These sets are limited to sub-systems having not only adequate data of the corresponding top events, producing more realistic results, but also having great risk priority, according to FMEA approach. These Weibull sets are linked with the corresponding top event of each subsystem and used to quantify the failure rates.The validation of previous studies made on wind turbine reliability FMEA analysis through the FTA method is investigated in this paper, as well as the results from previous studies made on reliability of wind turbines using the FMEA method. Though, the reliability and importance results as derived from a quantitative analysis, seem to be following the same trend like previous studies from different and various approaches. As a result, Electrical and Control systems as far as the Hydraulic System need to be re-designed with better performance and reliability since they are crucial for the operation of each WT separately as well as for the whole wind farm.

The scope of the present work is the techno-economic study with concern for the environmental issues and the investigation of the viability of an offshore wind farm in the Greek sea area, northeast of the island of Limnos. In the context of this study, the wind data, the suitable location for the installation of the wind turbines beyond nature protected areas, the type of the wind turbines, the losses due to wind turbines interaction, and the visual impact at the respective study areas are analyzed. Moreover, reliable costing models are used and applied, internationally recognized for techno-economic studies of offshore wind farms. Thereafter, the viability of the project is studied through investment benchmarks. This means that specific economic indices are estimated indicating whether the realization of such an investment is viable or not. In conclusion, based on the analysis performed in this study, it is noted that in the Greek archipelago and especially at the island of Limnos, there is a sufficient offshore wind potential at low depths and at relatively short distances from the shore. These circumstances are necessary conditions for the development of offshore wind farms in these areas. Moreover, economic estimations based on current data of grants, taxation, interest rates, etc., showed that the investment is sustainable, having a cost of energy production equal to 87.92 €/MW h.

The perpetual energy production of a wind farm could be accomplished (under proper weather conditions) if no failures occurred. But even the best possible design, manufacturing, and maintenance of a system cannot eliminate the failure possibility.In order to understand and minimize the system failures, the most crucial components of the wind turbines, which are prone to failures, should be identified. Moreover, it is essential to determine and classify the criticality of the system failures according to the impact of these failure events on wind turbine safety. The present study is processing the failure data from a wind farm and uses the Fault Tree Analysis as a baseline for applying the Design Structure Matrix technique to reveal the failure and risk interactions between wind turbine subsystems. Based on the analysis performed and by introducing new importance measures, the "readiness to fail" of a subsystem in conjunction with the "failure riskiness" can determine the "failure criticality." The value of the failure criticality can define the frame within which interventions could be done. The arising interventions could be applied either to the whole system or could be focused in specified pairs of wind turbine subsystems. In conclusion, the method analyzed in the present research can be effectively applied by the wind turbine manufacturers and the wind farm operators as an operation framework, which can lead to a limited (as possible) design-out maintenance cost, failures' minimization, and safety maximization for the whole wind turbine system. KEYWORDS design structure matrix, failures, fault tree analysis, safety impact, wind turbines | INTRODUCTIONWind is a clean and inexhaustible energy source and has become an important factor in the field of sustainable development. The global wind energy potential was estimated to be 72 TW, sufficient to supply the entire world's energy demand for all purposes. 1 Wind turbines (WTs) are installed either onshore (on land) or offshore (beyond the coast). Offshore wind farms seem to be advantageous because of the enormous energy potential associated with the large continuous areas and the stronger winds that imply greater power generation, even though they have higher investment, operation, and maintenance costs. The size of WTs is of great significance as a bigger WT produces more power in a more efficient way-resulting in greener produced electricity. 2The issue of WTs' failures is of particular interest. Failure is the loss of the ability of an item to perform a required function. 4 In WT, the unusually tough ambient conditions and the extremely high dynamic loading of the components often result to failures. These two factors are carefully considered in the design phase and materials' selection of a WT. Common causes of WTs failure can be components failure, high wind, waves,

Nowadays, the availability of wind turbines usually approaches the 98%. The objective is to increase the turbines availability, by improving the wind turbine reliability especially for offshore plants. The wind turbines reliability is a pivotal factor in the successfully function of a wind power plant. High reliability can be achieved by understanding and minimizing the failures of the system. Maintenance planning could make maintenance more effi cient and could lead to a reduction of failure events. In order to decide the most effective maintenance strategy, it should be identifi ed the most critical components of the wind turbines, which are prone to failures. The maintenance strategy should take into both the reliability improvement and the reduction of maintenance cost. Moreover, it is essential to determine and record the main cause that provokes the failure event. The data logging and statistical processing of failure information could improve the reliability of the system components. A wind turbine is a complex power generating system consisting of several structural, electrical and mechanical components. Its effi ciency depends largely on its reliability level. In the present study, the factors that affect the reliability of a wind turbine are presented, as well as the analytical way of calculating its reliability. The reliability analysis of the wind turbine is based on the systemic approach of the wind turbine subsystems, as its total reliability depends directly on the reliability of the subsystems. Moreover, it is based on a technical analysis of the reliability of complex systems. The model used is the Fault Tree Analysis (FTA), which is an identifi cation method for probable causes of system failure. The relation of reliability, maintainability and availability is also presented and explained by the example of a small wind farm.