O mexilhão-dourado é um organismo invasor que tem causado grandes prejuízos a sistemas de captação de água e usinas hidroelétricas (UHEs). Existem diversos métodos de controle da incrustação destes organismos em tubulações de usinas hidroelétricas, porém o impacto da aplicação desses métodos ainda não é amplamente conhecido. Este trabalho visa desenvolver uma metodologia para avaliar a degradação dos sistemas de resfriamento de UHEs submetidas a tratamentos químicos para o controle da incrustação por mexilhão-dourado. Neste trabalho é usado, como caso exemplo, o sistema de resfriamento do ar do núcleo do gerador e do óleo dos mancais de uma UHE com turbina Kaplan de potência aproximada de 150 MW. A análise proposta baseia-se na aplicação de ensaios acelerados de corrosão, a fim de determinar a taxa de corrosão nas condições normais de operação. Os ensaios executados neste estudo simulam condições operacionais mais severas do que as usualmente enfrentadas pelo sistema, visando reduzir o tempo de execução dos ensaios. Os ensaios acelerados de corrosão baseiam-se no aumento da solicitação térmica e da concentração das substâncias químicas injetadas no fluxo de água que são a adição de gás ozônio e de hipoclorito de sódio. Para realização dos ensaios foi construído um circuito experimental capaz de impor condições de temperatura e concentrações das substâncias químicas. Os resultados dos ensaios acelerados fornecem subsídios para aplicação dos conceitos de confiabilidade estrutural para determinação da probabilidade de falha dos equipamentos em estudo em função do tempo de aplicação do tratamento químico. Verificou-se entre os produtos químicos utilizados neste estudo, que o hipoclorito de sódio é 50% mais agressivo para a liga de cobre níquel e 700% mais agressivo para o aço inoxidável em relação a degradação observada pela ação do ozônio. / The golden mussel (Limnoperna fortunei) is an invader organism that has caused great damage to water catchment systems and hydroelectric power plants. There are various methods of fouling control of these organisms in hydroelectric power plants pipelines, but the impact of the application of these methods is not yet widely known. This work aims at developing a methodology to evaluate the degradation of hydroelectric power plants cooling systems pieces of equipment subjected to chemical treatments for control of golden mussel fouling. This work uses as a case example, the cooling system inside generator and oil bearing of Kaplan hydroelectric turbine with 150MW nominal power output. The analysis is based on the application of accelerated corrosion tests in order to determine the rate of corrosion under normal operation conditions given the rates of corrosion under various accelerated conditions. The tests run on this thesis simulate operating conditions more severe than those usually experienced by the system, to reduce the runtime of the tests. The proposed accelerated corrosion tests are based on increased thermal loading and concentration of chemical substances injected into water flow. The chemical treatments tested in this work are the addition of ozone and sodium hypochlorite. For carrying out the tests an experimental circuit capable of imposing different conditions of temperature and concentration levels was built. The results of accelerated tests provide subsidies for the implementation of structural reliability concepts for determining the failure probability of equipment under consideration. Among the chemical products used in the analysis, the sodium hypochlorite is 50% more aggressive for copper-nickel 90/10 alloy and 700% more aggressive for stainless steel in relation to degradation observed for ozone.

A confiabilidade de estruturas apresenta sólidos desenvolvimentos teóricos e crescentes aplicações práticas. Durante os últimos anos, avanços significativos foram obtidos em termos dos métodos de transformação (FORM, SORM), bem como em termos das técnicas de simulação de Monte Carlo. Métodos de transformação se mostraram eficientes para problemas de dimensões e não-linearidades moderadas. Já técnicas de simulação sempre permitiram a solução de problemas de grandes dimensões e fortemente não lineares, embora o custo computacional possa ser uma séria limitação. Com o avanço da capacidade de processamento dos computadores e com o desenvolvimento de técnicas de amostragem inteligente, a simulação de Monte Carlo passa a ser cada vez mais viável. Este trabalho tem por objetivo estudar e programar em computador técnicas de amostragem inteligente em simulação de Monte Carlo. O StRAnD é um programa de computador que já possui implementadas as técnicas de simulação de Monte Carlo Bruto e com Amostragem por Importância, ambas utilizando a Amostragem Simples na geração das variáveis básicas. Assim, são adicionadas, ao StRAnD, as técnicas de Amostragem Assintótica, Amostragem Melhorada e Simulação de Subconjuntos. Além disso, são programadas as técnicas de Amostragem por Hipercubo Latino e Amostragem por Variáveis Antitéticas. Nesta dissertação, são analisados seis problemas distintos, de forma que as vantagens e desvantagens de cada técnica sejam avaliadas, em termos da probabilidade de falha, do coeficiente de variação da probabilidade de falha, do erro relativo da probabilidade de falha e do tempo de processamento. / The structural reliability presents solid theoretical developments and increasing practical applications. During the past few years, significant advances were achieved in terms of transformation methods (FORM and SORM), as well as, in terms of Monte Carlo Simulation. Transformation methods are effective in problems with moderate dimensions and moderate nonlinearities. On the other hand, simulation techniques can be used to solve high-dimensional problems and highly nonlinear problems, although the computational cost could be a serious limitation. With the progress of computer processing capacity and with the development of intelligent sampling techniques, the Monte Carlo Simulation becomes increasingly feasible. This work aims to study and program intelligent sampling techniques in Monte Carlo simulation. The StRAnD (Structural Reliability Analysis and Design) software already has Crude Monte Carlo and Importance Sampling Monte Carlo, both using Simple Sampling as basic samples generator. Thus, the Asymptotic Sampling technique, the Enhanced Sampling technique and the Subset Simulation were added to the software. Moreover, the Latin Hypercube Sampling technique and the Antithetic Variates techniques were also added to the software. Six problems were evaluated in order to evaluate the advantages and disadvantages of each technique, in terms of probability of failure, coefficient of variation of the probability of failure, relative error and processing time.

Processos aleatórios de corrosão e fadiga reduzem lentamente a resistência de estruturas e componentes estruturais, provocando um aumento gradual nas probabilidades de falha. A gestão do risco de falha de componentes sujeitos a corrosão e/ou fadiga é feita através de políticas de inspeção, manutenção e substituição, atividades que implicam em custos, mas visam manter a confiabilidade em níveis aceitáveis, enquanto o componente permanecer em operação. Aparentemente, os objetivos economia e segurança competem entre si, no entanto, a redução de recursos para inspeção e manutenção pode levar a maiores e crescentes probabilidades de falha, implicando em maiores custos esperados de falha, ou seja, maior risco. A otimização de risco estrutural é uma formulação que permite equacionar este problema, através do chamado custo esperado total. Nesta Tese, a otimização de risco é utilizada no intuito de encontrar políticas ótimas de inspeção e manutenção, isto é, quantidades de recursos a serem alocadas nestas atividades que levem ao menor custo esperado total possível. Os processos de corrosão e fadiga são representados através de modelos em polinômios de caos, construídos de maneira inédita, com base em dados experimentais ou observados da literatura. Com base nestes modelos, os problemas de otimização de risco envolvendo processos de fadiga e corrosão são resolvidos para diferentes configurações de custos de falha e de inspeções. Verifica-se que as políticas ótimas de inspeção, manutenção e substituição podem ser bastante diferentes para configurações de custo distintas, e que a determinação destas políticas é bastante desafiadora, devido, dentre outros fatores, à grande quantidade de mínimos locais do problema de otimização em questão, causadas por descontinuidades e oscilações da função custo esperado total. / Random corrosion and fatigue processes reduce slowly but gradually the resistance of structures and mechanical components, leading to gradual increase in failure probabilities. Risk management for mechanical components subject to corrosion and fatigue is made by means of policies of inspection, maintenance and substitution. These activities imply costs, but are made to maintain the reliability at acceptable levels, while the component remains in operation. Apparently, economy and safety are competing objectives; however, reduction in inspection and maintenance spending may lead to larger failure probabilities, increasing expected costs of failure (risk). Risk optimization allows one to solve this problem, by means of the so-called total expected cost. In this Thesis, risk optimization is used in order to find the best inspection and maintenance policy, i.e., the proper amount of resources to allocate to such activities in order to obtain minimum total expected cost. Corrosion and fatigue are modeled by means of polynomial chaos expansions, using a novel approach developed herein and experimental or observed data obtained from the literature. These models are employed within two risk optimization problems, solved for different failure and inspection cost configurations. Results show that the optimal policies of inspection, maintenance and replacements can be very different, for different cost configurations, and that the solution of the associated risk optimization problems is a very challenging task, due to the large number of local minima, caused by discontinuities and fluctuations in the total expected costs.

Uma grande variedade de modelos é utilizada para estimar a pressão de ruptura de dutos contendo defeitos de corrosão. O presente trabalho tem como objetivo estudar a precisão dos modelos mais comuns e avaliar a pressão de ruptura de dutos submetidos à corrosão. Os modelos avaliados são: ASME B31G, ASME B31G modificado, DNV RP F101 e PCORRC. O estudo é baseado em mais de 400 resultados de ensaios de ruptura em dutos corroídos, todos coletados da literatura. A base de dados contem defeitos de corrosão reais e artificiais. Uma análise estatística foi realizada para a variável erro de modelo. Uma análise de regressão não-linear foi realizada para investigar os efeitos da variável erro de modelo, das variáveis mais relevantes, como profundidade e comprimento do defeito, e tensão de ruptura do aço. Uma análise de confiabilidade foi realizada a partir das estatísticas obtidas da variável erro de modelo, sendo estimado o índice de confiabilidade e a probabilidade de falha do duto com defeitos de corrosão, através do método iterativo de primeira ordem, denominado FORM (First Order Reliability Method). Nesta análise avaliou-se a evolução da probabilidade de falha com o aumento da profundidade do defeito, bem como foram identificadas as variáveis aleatórias mais importantes na falha do duto. O estudo pode ajudar aos operadores a eleger qual modelo utilizar em análises de risco, proporcionando mais segurança às operações dutoviárias. / A variety of models exist to estimate burst pressures of pipelines containing corrosion defects. The objective of this work is to study the accuracy of some of the most popular empirical burst pressure models. The study addresses the models: ASME B31G, ASME B31G Modified, DNV RP-F101 and PCORRC. The investigation is based on over 400 burst test results, all collected from the literature, containing both real and artificial corrosion defects. A statistical analysis is performed for assessing the accuracy of semi-empirical models by using a model error variable. A non-linear regression analysis is performed to identify the influence, on model errors, of the most relevant variables, such as defect depth and length and steels rupture tension. A reliability analysis was carried out, using model error statistics developed herein, in order to evaluate reliability index and probability of failure of pipelines containing corrosion defects, through the iterative first order reliability method, or FORM - First Order Reliability Method. The evolution of failure probabilities, with increasing defect depth, was investigated. The most relevant random variables were identified. The study can help operators choose a proper empirical model to use in their risk analysis, leading to greater safety in pipeline operations.

Hurricanes have caused severe damage to the electric power system throughout the Gulf coast region of the U.S., and electric power is critical to post-hurricane disaster response as well as to long-term recovery for impacted areas. Managing hurricane risks and properly preparing for post-storm recovery efforts requires rigorous methods for estimating the number and location of power outages, customers without power, and damage to power distribution systems. This dissertation presents a statistical power outage prediction model, a statistical model for predicting the number of customers without power, statistical damage estimation models, and a physical damage estimation model for the gulf coast region of the U.S. The statistical models use negative binomial generalized additive regression models as well as negative binomial generalized linear regression models for estimating the number of power outages, customers without power, damaged poles and damaged transformers in each area of a utility company’s service area. The statistical models developed based on transformed data replace hurricane indicator variables, dummy variables, with physically measurable variables, enabling future predictions to be based on only well-understood characteristics of hurricanes. The physical damage estimation model provides reliable predictions of the number of damaged poles for future hurricanes by integrating fragility curves based on structural reliability analysis with observed data through a Bayesian approach. The models were developed using data about power outages during nine hurricanes in three states served by a large, investor-owned utility company in the Gulf Coast region.

To assess the losses associated with future earthquakes, seismic vulnerability functions are commonly used to correlate the damage or loss of a structure to the level of seismic intensity. A common procedure in seismic vulnerability assessment is to estimate the seismic fragility, which is defined as the conditional probability that a structure fails to meet the specific performance level for given level of seismic intensity. This dissertation proposes a methodology to estimate the fragility of corroded reinforced concrete (RC) bridges with two-column bents subject to seismic excitation. Seismic fragility functions are first developed for the RC bridges with two-column bents. All available information from science/engineering laws, numerical analysis, laboratory experiments, and field measurements has been used to construct the proper form of the fragility functions. The fragility functions are formulated, at the individual column, bent, and bridge levels, in terms of the spectral acceleration and the ratio between the peak ground velocity and the peak ground acceleration. The developed fragility functions properly account for the prevailing uncertainties in fragility estimation. The probabilistic capacity and demand models are then combined with the probabilistic models for chloride-induced corrosion and the time-dependent corrosion rate. The fragility estimates for corroded RC bridges incorporates the uncertainties in the parameters of capacity and demand models, and the inexactness (or model error) in modeling the material deterioration, structural capacity, and seismic demands. The proposed methodology is illustrated by developing the fragility functions for an example RC bridge with 11 two-column bents representing current construction in California. The developed fragility functions provide valuable information to allocate and spend available funds for the design, maintenance, and retrofitting of structures and networks. This study regarding the vulnerability of corroding RC bridges will be of direct value to those making decisions about the condition assessment, residual life, and the ability of lifeline structures to withstand future seismic demands.

Civil infrastructure facilities play a central role in the economic, social and political health of modern society and their safety, integrity and functionality must be maintained at manageable cost over their service lives through design and periodic maintenance. Hurricanes and tropical cyclones, tornadoes, earthquakes and floods are paramount among the potentially devastating and costly natural disasters impacting civil infrastructure. Even larger losses may occur in the future, given the population growth and economic development accompanying urbanization in potentially hazardous areas of the world. Moreover, in recent years, the effects that global climate change might have on both the frequency and severity of extreme events from natural hazards and their effect on civil infrastructure facilities have become a major concern for decision makers. Potential influences of climate change on civil infrastructure are even greater for certain facilities with service periods of 100 years or more, which are substantially longer than those previously considered in life-cycle engineering and may extend across multiple generations. Customary risk-informed decision frameworks may not be applicable to such long-term event horizons, because they tend to devalue the importance of current decisions for future generations, causing an ethical and moral dilemma for current decision-makers. Thus, intergenerational risk-informed decision frameworks that consider facility performance over service periods well in excess of 100 years and extend across multiple generations must be developed. This dissertation addresses risk-informed decision-making for civil infrastructure exposed to natural hazards, with a particular focus on the equitable transfer of risk across multiple generations. Risk-informed decision tools applied to extended service periods require careful modifications to current life-cycle engineering analysis methods to account for values and decision preferences of both current and future generations and to achieve decisions that will be sustainable in the long term. The methodology for supporting equitable and socio-economical sustainable decisions regarding long-term public safety incorporates two essential ingredients of such decisions: global climate change effect on stochastic models of extreme events from natural hazards and intergenerational discounting methods for equitable risk-sharing. Several specific civil infrastructure applications are investigated: a levee situated in a flood-prone city; an existing dam built in a strong earthquake-prone area; and a special moment resisting steel frame building designed to withstand hurricanes in Miami, FL. These investigations have led to the conclusion that risks can and should be shared across multiple generations; that the proposed intergenerational decision methods can achieve goals of intergenerational equity and sustainability in engineering decision-making that are reflective of the welfare and aspirations of both current and future generations; and that intergenerational equity can be achieved at reasonable cost.

A probabilistic framework is developed to assess the structural reliability of offshore wind turbines. Probabilistic models are developed to predict the deformation, shear force and bending moment demands on the support structure of wind turbines. The proposed probabilistic models are developed starting from a commonly accepted deterministic model and by adding correction terms and model errors to capture respectively, the inherent bias and the uncertainty in developed models. A Bayesian approach is then used to assess the model parameters incorporating the information from virtual experiment data. The database of virtual experiments is generated using detailed three-dimensional finite element analyses of a suite of typical offshore wind turbines. The finite element analyses properly account for the nonlinear soil-structure interaction. Separate probabilistic demand models are developed for three operational/load conditions including: (1) operating under day-to-day wind and wave loading; (2) operating throughout earthquake in presence of day-to-day loads; and (3) parked under extreme wind speeds and earthquake ground motions. The proposed approach gives special attention to the treatment of both aleatory and epistemic uncertainties in predicting the demands on the support structure of wind turbines. The developed demand models are then used to assess the reliability of the support structure of wind turbines based on the proposed damage states for typical wind turbines and their corresponding performance levels. A multi-hazard fragility surface of a given wind turbine support structure as well as the seismic and wind hazards at a specific site location are incorporated into a probabilistic framework to estimate the annual probability of failure of the support structure. Finally, a framework is proposed to investigate the performance of offshore wind turbines operating under day-to-day loads based on their availability for power production. To this end, probabilistic models are proposed to predict the mean and standard deviation of drift response of the tower. The results are used in a random vibration based framework to assess the fragility as the probability of exceeding certain drift thresholds given specific levels of wind speed.

In the present work, an investigation of the fatigue life benefits emerging from fillet weld geometries optimization has been carried out. At first, an introduction to ageing mechanisms, corrosion and especially fatigue, acting on operating marine structures has been made. Residual stresses at weld toes, stress modes, and types, geometrical factors (weld angle, toe radius, leg length), welding techniques selected, post-welding treatment and plate‟s material are some of the principal factors affecting the fatigue life of a fillet weld joint. Especially, the accuracy of various approaches in fatigue life estimation of specific geometries under pre-set types and levels of stress is studied. It is evident so far that even the notch stress concept is the most accurate method based on S-N curves, the Fracture Mechanics approach can offer more accurate solutions of a crack development through the material. Towards this, a literature review on crack evolution aspects in welded and non-welded plates under bending and tension was performed; substantial parameters were determined and finally implemented in the LEFM model which was used for the simulation purposes of Chapter 6. As far as the crack aspect ratio evolution is concerned, an extensive reference is available in literature since many researchers have investigated its contribution to the determination of geometrical paths, commonly known as “Preferred Propagation Paths”. Their significance is related with our ability to determine accurate SIF solutions leading to precise fatigue life estimations. A typical fillet weld joint 2-D model has been developed in CAE Abaqus software and a Finite Element Analysis of subject T-profile has been carried out. Through this analysis, the fillet weld angle, the weld leg length, the weld toe curvature radio ρ and the carrying load plate thickness are examined for their impacts on the maximum surface stress. Finally, a number of stress mitigating measures are proposed and their effects are analyzed. Undoubtedly, the notch stress concept today is gradually gaining more and more acceptance among other fatigue analysis practices, hence the need for an estimation of the actual surface stresses along fillet weld toes, has become imperative. Towards this, different 2-D geometries are tested against stress concentration factors developed at weld toes, which are calculated on the basis of maximum in-plane principal stresses over nominal stresses in mode I pure bending and pure tension respectively. Moreover, validation with corresponding results from literature is provided. Finally, three different concepts for reducing the maximum surface stresses are presented. The first one proposes grinding of the weld toe area and formulation of an artificial U-notch or a part- circular profile. The second one applies to non-penetrating welds and assumes the existence of a root gap of a specific geometry which is related to the fatigue life and stress concentration factor of the fillet weld joint. Last but not least, the relatively recent concept of the variable radius notch is discussed, even though it is applicable mostly to notched bodies, not weld joints. Afterwards, a Linear Elastic Fracture Mechanics analysis of reference 2D fillet weld model is demonstrated. A number of geometrical parameters considered at previous stage for their impact on surface Stress Concentration levels at the weld toe region, have been correlated to fatigue life benefits in terms of increased number of stress cycles till failure. An extensive analysis of 9 different T-butt weld joint geometries has been provided in order to investigate how positively a possible SCF reduction can affect the fatigue life of a weld joint. Essential geometric variations (weld angle, length, toe radius, root slot) were considered in the 2D model. All calculated benefits both in pure bending and pure tension cases have been reported accordingly. Based on a linear interpolation of the points scatter (SCF, N-cycles) both in banding and tension, it was observed that a surface stress mitigation of 1% could lead to 1,33 up to 2,5% fatigue life benefit in the range of SCF=2 – 2,5. It is evident so far that the geometrical optimization of a weld joint in respect of notch stress mitigation can be a powerful tool both in shipbuilding and maintenance practice in the future. However, technically wise their application may incur high initial costs of improved tools of welding and post welding treatment and robots even though it would consist a cost effective solution in a medium/long term basis. Finally, the above process is followed by a reliability analysis of the most critical geometrical parameters affecting the fatigue life of a fillet weld joint. Reliability assessment results concerning medium, high and low cycle fatigue are provided and a comparative analysis of each factor‟s impact on fatigue life has been carried out.

As demandas do mercado da construção civil têm exigido vãos cada vez maiores e ao mesmo tempo alturas cada vez menores das vigas. Isto tem levado muitos projetistas à adoção da solução do pavimento em laje lisa em concreto armado ou protendido. No entanto, a ausência das vigas torna possível a ruptura das lajes por puncionamento junto aos pilares. A norma NBR-6118:2014 prescreve as disposições para o projeto de lajes sob punção. O trabalho proposto teve como objetivo principal a análise da confiabilidade da ligação laje-pilar interno sob o efeito da punção em lajes que se apoiam diretamente sobre pilares de acordo com a NBR-6118:2014. Primeiramente, com o intuito de se entender melhor o fenômeno da punção, realizou-se uma breve revisão bibliográfica, identificando os principais parâmetros que influenciam na resistência da ligação, assim como os principais métodos de análise e trabalhos realizados na área. Em seguida, estudou-se o software de análise em elementos finitos ANSYS (Analysis Systems Incorporated), especificamente a ferramenta UPF (User Programmable Features), que foi utilizada para adoção de um modelo constitutivo para o concreto. Utilizando o software, foram modeladas lajes estudadas por outros autores, visando a validação do modelo numérico. Para o estudo da confiabilidade foi dimensionado um conjunto de lajes lisas seguindo as prescrições da NBR 6118:2014. A análise da confiabilidade foi realizada utilizando a ferramenta PDS (Probabilistic Design System), empregando o método de simulação numérica de Monte Carlo com amostragem por Latin Hypercube. Por fim, determinou-se o índice de confiabilidade em cada projeto e realizaram-se análises paramétricas com as variáveis adotadas no trabalho. Os resultados obtidos mostraram que as lajes lisas sem armadura de cisalhamento projetadas segundo a NBR 6118:2014 obtiveram, em sua maioria, índices de confiabilidade adequados. Porém, para as lajes lisas com armadura de cisalhamento, o índice de confiabilidade foi, em grande parte, menor que o índice de confiabilidade alvo adotado. / The demands of the construction market have required increasingly large spans while diminishing of the beam heights. This has led many designers to adopt the pavement solucion of reinforced or prestressed concrete flat slab. However, the absence of the beams makes it possible to slabs failure by punching shear. The Standard NBR-6118: 2014 prescribes the requirements for the design of slabs under punching. The aim of this work was to analyze the reliability of the internal slab-column intersection under punching in slabs supported directly on columns according to NBR-6118: 2014. Firstly, in order to better understand the punching phenomenon, a brief bibliographic review was carried out, identifying the main parameters that influence the connection strength, as well as the main methods of analysis and published in the area. Then, the finite element analysis software ANSYS (Analysis Systems Incorporated), specifically the UPF (User Programmable Features) tool, was used to adopt a concrete constitutive model. Using the software, slabs studied by other authors were modeled, aiming at the validation of the numerical model. For the reliability study, a set of flat slabs was designed following the requirements of NBR 6118: 2014. The reliability analysis was performed using the PDS (Probabilistic Design System) tool, using the Monte Carlo numerical simulation method with Latin Hypercube sampling. Finally, the reliability index was determined in each project and parametric analyzes were performed with the variables adopted in the study. The results from this study show that the flat slabs without shear reinforcement designed according to NBR Standarts obtained appropriate reliability index. However, for the flat slabs with shear reinforcement, the reliability index, in most cases, did not achieve the target reliability index.