Annual Probability Of Failure Research Articles

Time-Dependent Risk Assessment of Bridges Based on Cumulative-Time Failure Probability

AbstractThis paper presents an approach for the time-dependent risk assessment of bridges based on cumulative-time failure probability of bridge members. The cumulative-time failure probabilities of girders under traffic load and pier columns under scour are computed. Different scenarios of lane closure are identified and expressed in terms of failure events of bridge components. Two extreme correlation cases among the failure modes of bridge components are considered. After estimating the occurrence probabilities and consequences associated with these scenarios, the time-dependent risks are assessed. Considering the entire history of loadings and structural deteriorating processes by using the cumulative-time approach provides a more realistic basis for risk assessment of bridges than the approach based on annual failure probabilities.

Shear band propagation analysis of submarine slope stability

The paper summarises recent developments in the shear band propagation approach, which enables simplified submarine slope stability analysis to account for catastrophic and progressive failure. This approach covers a wide variety of potential failure mechanisms, such as slab failures, spreadings, ploughings and run-outs, and provides analytical energy balance criteria for predicting their occurrence. The simple form of the resulting criteria enabled their incorporation into deterministic and probabilistic slope stability analysis of offshore developments within the geographical information system. In contrast to conventional limiting equilibrium approaches used in such analysis, the shear band propagation approach is capable of explaining enormous dimensions of observed palaeo-landslides and predicting annual probabilities of failure that are orders of magnitude higher, as validated by reconstructed historical frequencies.

Time-variant bulk carrier reliability analysis in pure bending intact and damage conditions

Time-variant reliability analysis of a corroded bulk carrier in intact and damage conditions is performed by First-Order (FORM), Second-Order (SORM) Reliability Methods and Importance Sampling simulation. Annual failure probabilities are determined up to 25-year ship lifetime, accounting for time-variant corrosion wastage of structural members contributing to hull girder strength. Statistical properties of hull girder capacity are determined by Monte Carlo simulation, applying three correlation models among corrosion wastages of structural members contributing to hull girder strength, namely no correlation, full correlation and full correlation among wastages of structural members belonging to the same category of compartments. A modified incremental-iterative method is applied, to account for instantaneous neutral axis rotation, in case of asymmetrical damage conditions, as for collision and grounding events. Incidence of intact/damage condition, as well as correlation among corrosion wastages, on annual sagging/hogging time-variant failure probability is investigated and discussed. Time-variant sensitivity analyses for intact and damage conditions are also performed, to investigate the incidence of random variables' uncertainties on the attained failure probability. Finally, the bulk carrier section scheme, benchmarked in the last ISSC Report, is applied as test case.

Time-Variant Risk Assessment of Bridges with Partially and Fully Closed Lanes due to Traffic Loading and Scour

This article presents an approach for assessing time-variant risk associated with the closure of bridge lanes due to traffic loading and scour. Scenarios of lane closure are identified for both traffic loading and scour. The time-variant failure probabilities of girders under traffic loading are evaluated using a deterioration model that accounts for the decrease in resistance and a live-load model that considers the increase in maximum load effects. The annual failure probabilities of pier columns due to scour are estimated by comparing pier depth and scour depth caused by the annual peak flow. The probabilities of occurrence of the identified closure scenarios are computed on the basis of the relations between these scenarios and the failure events of girders and pier columns, and the correlations among failure modes are considered. The consequences caused by the lane-closure scenarios are evaluated by considering repair, running, and time-loss costs. Finally, the time-variant risks associated with the identified scenarios are assessed. The proposed approach is illustrated using a highway bridge as an example.

A Simplified Method for Quantitative Reliability and Integrity Analysis of Steel Catenary Risers

Quantitative reliability and integrity analysis of steel catenary risers (SCRs) can provide important information about their safety and toward their cost-effective and optimal design. SCRs are one of the commonly used riser systems in offshore production stations. The consequence of an SCR failure is significant; however, the overall safety of the riser is typically not quantified. Especially, because of the uncertainties associated with environmental conditions and structural capacities, quantitative reliability methods can take advantage of available data and developments in computing technology to provide a strong basis for their reliable engineering decision making. This paper presents a simplified approach for assessing the strength and fatigue reliability of SCRs, accounting for the uncertainties with their yield strength and fatigue capacities as well as the environmental conditions. Moreover, the integrity-based optimal design of riser strength limit state for a target annual probability of failure is discussed. The fatigue reliability of the SCR system is also assessed in component and system levels. The proposed method is then applied to a typical SCR attached to a semisubmersible vessel under Gulf of Mexico (GOM) conditions. Results of dynamic (time-domain) analyses under various environmental conditions are used to quantify the SCR safety and integrity and to optimize its design for a target annual probability of strength failure. By estimating the riser system probability of strength and fatigue failure in its lifetime, the strength and fatigue integrity indices, and the optimality factors of the riser sections for the strength limit state, suggestions are provided to improve the riser design. For example, it was found that considering the two main limit states of strength and fatigue failure of the SCR system, a strength failure at the taper stress joint (TSJ) is the likely mode of failure in this riser system, which has a probability of 0.0035 in its 25 year lifetime.

INFLUENCE OF ADDED VISCOUS DAMPERS ON THE PROBABILISTIC SEISMIC PERFORMANCE EVALUATION OF RC FRAMES

This paper deals with the parameters which influence the probability of reaching the near collapse limit state of RC frame structures equipped with nonlinear fluid viscous dampers. The study can be divided into two steps. The first aims to assess how the median and the dispersion of seismic demand can vary in the RC frame structures with and without dampers, considering a wide set of ground motions. The second step evaluates the expression in closed form, given by 2000 SAC/FEMA method, to assess the annual probability of failure of RC structures. This probability has been estimated considering a wide set of ground motions and different methods to approximate the hazard curve. The evaluations have been made on the basis of the results of a large number of nonlinear dynamic analyses; in particular, 180 nonlinear dynamic analyses have been made for the case studies with and without dampers. In conclusion, it has been noticed that the probabilistic assessment depends on the number of records considered and that the simplified formula provided by the 2000 SAC-FEMA method is strongly sensitive to the variation of the hazard curve and the dispersion.

Tornado Risk Analysis for Residential Wood-Frame Roof Damage across the United States

AbstractApproximately 1,200 tornadoes impact the United States every year with a percentage of these resulting in significant damage, injuries, and fatalities. Initially, a probabilistic tornado hazard analysis was performed in order to develop tornado hazard curves at select locations across the United States. This analysis resulted in the annual probability of experiencing a tornado of any strength at the specific locations, which varied as a function of location-specific occurrence rates. Five different residential wood-frame building archetypes were designed at each of the locations based on current residential building code and/or practice. Fragilities for the roof sheathing and truss to wall top-plate connections were developed for each archetype. Because fragilities are independent of location, they were then convolved with the tornado hazard curves to compute annual failure probabilities for select roof components. This represents the first time absolute risk of roof failure due to tornadoes has b...

Hazard analysis of seismic submarine slope instability

To assess the risk associated with a submarine landslide, one must estimate the probability of slope failure and its consequences. This paper proposes a procedure to estimate the probability of earthquake-induced submarine slope failure (hazard) based on probabilistic seismic hazard analyses, ground response analyses and advanced laboratory tests. The outcomes from these analyses are treated in a probabilistic framework, with analytical simulations using mathematical techniques such as the first-order reliability method, Monte Carlo simulation and Bayesian updating. Fragility curves of slope failure during the earthquake (co-seismic) and after the earthquake (post-seismic) were developed in this study, and were shown to provide a clear and well-organized procedure to estimate the annual failure probability of a submarine slope under earthquake loading.

Study of risk acceptance criteria for dams

This paper discusses the methods of establishing risk criteria for dams and reviews the application of dam risk criteria for individuals and societies in different countries or districts. Given the conditions in China and considering the public safety and acceptance of dam risk, historical dam break data and current design standards, individual and societal risk criteria for dams are proposed. The tolerable dam risk criteria for individuals should be set to 10 -5 -10 -7 per annum based on project scale, for example, approximately 1.0×10 -7 per annum, which corresponds to a reliability index of 4.2 based on a 100-year lifespan for a first-class or large project. The societal limit for risk tolerance for dams should be set to approximately 10 -3 -10 -5 per annum, corresponding to the fatality range from1 to 100 and be horizontally extended to 1000, and F - N curves are proposed. It was also found that the reliability indices of Chinese Standard (GB 50199-2013) and Eurocode1 (2002) are different, but they have the same level of safety measured by the annual probability of failure. The research results have significance for establishing dam risk criteria.

Development and urban-scale application of a simplified method for seismic fragility assessment of RC buildings

A simplified analytical method for the seismic fragility assessment of Reinforced Concrete buildings at large scale is presented. The proposed method is based on a simulated design procedure to define the structural model and on non-linear static analysis of a simplified structural model based on Shear-Type assumption to evaluate seismic capacity. Damage States are defined according to the observational-based Damage States provided by the European Macroseismic Scale (EMS-98). Presence of infills is considered, both taking into account their influence on the structural response and evaluating the damage to such non-structural elements. The method is applied to the Reinforced Concrete building stock data provided by the field survey carried out on a city in a high seismic area in Southern Italy, which are illustrated and compared with data from other sources. Uncertainties in seismic demand, material characteristics, and capacity models are taken into account through a Monte Carlo simulation technique. Fragility curves are obtained for each building, leading to the evaluation of annual failure probability at the assumed Damage States. The influence of key parameters in predicting seismic fragility, such as the number of storeys and the age of construction, is illustrated. The spatial distribution of annual failure probability at the different Damage States provides information on areas most prone to seismic risk within the city, depending both on building stock characteristics and on local amplification of seismic hazard due to soil conditions. A comparison with empirical-based fragility curves from literature is also illustrated.

Assessment of Gearbox Operational Loads and Reliability under High Mean Wind Speeds

This paper investigates the dynamic loads occurring in the drivetrain of wind turbines with a focus on offshore applications. Herein a model of the gearbox of the 5 MW wind turbine is presented. The model is developed in a multi-body framework using commercial software MSC ADAMS. Validation of the model was based on the experimental data provided by NREL for 750kW prototype gearbox. Failures of gearboxes caused by high dynamic loads have a significant influence on the cost of operation of wind farms. For these reasons in the study, the probability of failure of the gearbox working in an offshore wind turbine that operates in storm conditions with mean wind speeds less than 30 m/s is presented. In the study, normal shut-downs of a wind turbine in storm conditions were investigated. The analysis were conducted for two storm control strategies and different wind conditions from an extreme operating gust, normal turbulence model and extreme turbulence model. In the paper, loads in the planetary gear are quantified as well as the torsional moments in the main shaft. On the basis of simulation results the annual probability of failure of the gearbox in a wind turbine with soft storm controller is calculated, and compared with the one had the gearbox working in a wind turbine operating with hard storm controller. In the study, it was found that normal shut-downs do not have a significant influence on the ultimate loads in the gearbox, since they are related mostly to the gusts occurring during turbulence. Application of the storm controller with reduction of the wind turbine power allowed the decrease of the probability of failure for ultimate stresses.

Use of risk-based methods to evaluate spillway capacity: case histories

The term ‘risk-based’ in relation to dam safety has now been in use for many years with quantitative methods available for over a decade. This paper explores the meaning of the term ‘risk-based’ and provides examples of its use in relation to assessing the need for spillway upgrades, including its use to asses when risk has been reduced to ‘as low as reasonably practicable’ (Alarp); that is, when the costs of further upgrading are disproportionate to the additional reduction in risk achieved. The paper then discusses the issues arising, including reasons for the apparent adherence to deterministic standards and lack of take-up of the Alarp approach in the UK as well as the management of potential conflict between recommendations from Section 10 inspections under reservoir safety legislation and portfolio risk assessment. The paper does not cover the wider issues of other threats to dams, which may have higher annual probabilities of failure; however, similar principles can be applied to the management of these threats.

Safety approach for composite pressure vessels for road transport of hydrogen. Part 1: Acceptable probability of failure and hydrogen mass

Hydrogen is a promising alternative for current energy carriers. Compressed gas cylinders are the storage systems closest to the commercialization of hydrogen in vehicles. The safety factors in current standards are seen as restrictive for further growth and competitiveness of hydrogen infrastructure. A probabilistic approach can be employed in order to give a rational background to the safety factors. However, an acceptable probability of failure needs to be estimated before calculating the safety factors. The discussion of determining the acceptable probability must include the mass of hydrogen since this determines the consequences of an accident. It is concluded that an annual probability of failure of 10−7 would be appropriate for small pressure vessels containing a few kilograms of hydrogen. Larger pressure vessels of a few hundred kilograms or more should be designed for an annual probability 10−8.

A simplified method for reliability- and integrity-based design of engineering systems and its application to offshore mooring systems

This paper presents a simplified method for the reliability- and the integrity-based optimal design of engineering systems and its application to offshore mooring systems. The design of structural systems is transitioning from the conventional methods, which are based on factors of safety, to more advanced methods, which require calculation of the failure probability of the designed system for each project. Using factors of safety to account for the uncertainties in the capacity (strength) or demands can lead to systems with different reliabilities. This is because the number and arrangement of components in each system and the correlation of their responses could be different, which could affect the system reliability. The generic factors of safety that are specified at the component level do not account for such differences. Still, using factors of safety, as a measure of system safety, is preferred by many engineers because of the simplicity in their application. The aim of this paper is to provide a simplified method for design of engineering systems that directly involves the system annual failure probability as a measure of system safety, concerning system strength limit state. In this method, using results of conventional deterministic analysis, the optimality factors for an integrity-based optimal design are used instead of generic safety factors to assure the system safety. The optimality factors, which estimate the necessary change in average component capacities, are computed especially for each component and a target system annual probability of system failure using regression models that estimate the effect of short and long term extreme events on structural response. Because in practice, it is convenient to use the return period as a measure to quantify the likelihood of extreme events, the regression model in this paper is a relationship between the component demands and the annual probability density function corresponding to every return period. This method accounts for the uncertainties in the environmental loads and structural capacities, and identifies the target mean capacity of each component for maximizing its integrity and meeting the reliability requirement. In addition, because various failure modes in a structural system can lead to different consequences (including damage costs), a method is introduced to compute optimality factors for designated failure modes. By calculating the probability of system failure, this method can be used for risk-based decision-making that considers the failure costs and consequences. The proposed method can also be used on existing structures to identify the riskiest components as part of inspection and improvement planning. The proposed method is discussed and illustrated considering offshore mooring systems. However, the method is general and applicable also to other engineering systems. In the case study of this paper, the method is first used to quantify the reliability of a mooring system, then this design is revised to meet the DNV recommended annual probability of failure and for maximizing system integrity as well as for a designated failure mode in which the anchor chains are the first components to fail in the system.

Improved reliability of wind turbine towers with tuned liquid column dampers (TLCDs)

Modern wind turbines are supported by slender, flexible and lightly damped tall towers, which exhibit high susceptibility to wind-induced vibrations. This paper develops a framework that combines efficient dynamic response models and probabilistic assessment tools to show how to structural response and improve structural reliability when equipping modern turbines with tuned liquid column dampers (TLCDs). This study improves a dynamic model of a wind turbine to accommodate single or multiple TLCDs to control excessive vibrations. The model is subjected to stochastically generated wind loads of varying speeds to develop wind-induced probabilistic demand models for towers of modern turbines under structural uncertainty. Results indicate that a baseline TLCD with 1% of the total wind turbine mass achieves up to 47% reduction in peak displacements. Furthermore, the study constructs fragility curves, which illustrate reductions in the vulnerability of towers to wind forces owing to the inclusion of the damper. Annual failure probabilities computed based on the likelihood of wind speed realizations in West Texas and West California, U.S. demonstrate reliability gains of up to 8% using the baseline TLCD. This paper also observes that the use of two TLCDs with a total mass ratio of 1.5% yields marginal benefits over the baseline TLCD gains. Rather, increasing the mass of the baseline TLCD to 1.5% of the turbine mass reduces peak responses by 53% and the annual failure probability by as much as 11%. These results suggest a viable alternative to help achieving long-term reliability and risk targets for utility scale wind turbines.

Economic Evaluation of Frequent Home Nocturnal Hemodialysis Based on a Randomized Controlled Trial

Provider and patient enthusiasm for frequent home nocturnal hemodialysis (FHNHD) has been renewed; however, the cost-effectiveness of this technique is unknown. We performed a cost-utility analysis of FHNHD compared with conventional hemodialysis (CvHD; 4 hours three times per week) from a health payer perspective over a lifetime horizon using patient information from the Alberta NHD randomized controlled trial. Costs, including training costs, were obtained using microcosting and administrative data (CAN$2012). We determined the incremental cost per quality-adjusted life year (QALY) gained. Robustness was assessed using scenario, sensitivity, and probabilistic sensitivity analyses. Compared with CvHD (61% in-center, 14% satellite, and 25% home dialysis), FHNHD led to incremental cost savings (-$6700) and an additional 0.38 QALYs. In sensitivity analyses, when the annual probability of technique failure with FHNHD increased from 7.6% (reference case) to ≥19%, FHNHD became unattractive (>$75,000/QALY). The cost/QALY gained became $13,000 if average training time for FHNHD increased from 3.7 to 6 weeks. In scenarios with alternate comparator modalities, FHNHD remained dominant compared with in-center CvHD; cost/QALYs gained were $18,500, $198,000, and $423,000 compared with satellite CvHD, home CvHD, and peritoneal dialysis, respectively. In summary, FHNHD is attractive compared with in-center CvHD in this cohort. However, the attractiveness of FHNHD varies by technique failure rate, training time, and dialysis modalities from which patients are drawn, and these variables should be considered when establishing FHNHD programs.

Second-order Logarithmic formulation for hazard curves and closed-form approximation to annual failure probability

Closed-form solutions to compute annual failure probabilities are helpful in the implementation of performance-based engineering. Closed-form solutions to compute the annual failure probability have been proposed in the context of seismic hazard using a linear approximation in the logarithmic scale for the hazard curves. However, a linear approximation is found to significantly deviate from the actual seismic hazard curve and hence can result in significant errors in the computation of the annual failure probability. This paper develops a closed-form solution to compute the annual failure probability of systems using a novel Second-order Logarithmic Formulation (SOLF) to model the hazard curves. Thereafter, we illustrate the proposed closed-form solution by computing the annual failure probabilities for reinforced concrete (RC) bridges subject to the seismic hazard of San Francisco, CA and Memphis, TN. We demonstrate that SOLF yields accurate estimates of the annual failure probabilities whereas the existing linear logarithmic formulation can significantly over or under estimates such probabilities. The proposed formulation is general and is expected to yield accurate estimates for hazards other than the seismic hazard (e.g., hurricanes and floods).

Delta Subsidence Reversal, Levee Failure, and Aquatic Habitat—A Cautionary Tale

Various schemes are often suggested to reverse the subsidence of lands below sea level in California's Sacramento—San Joaquin Delta, an area protected by levees (dikes) that have significant probabilities of failure. Elementary modeling is used to estimate the probability distribution of land elevations at time of failure for 36 of these subsided islands, assuming a reasonable potential subsidence rever- sal rate. Given estimated annual probabilities of levee failure, elevation gains at this rate are not expected to exceed 1 to 2 m before flooding, which would be insufficient to restore most subsided islands to mean sea level (msl). However, under some circumstances 1- to 2-m gains are significant. A framework is introduced for evaluating islands as promising candidates for subsidence reversal based on elevation goals other than msl, as dem- onstrated though a hypothetical aquatic habitat example. Here, we recommend relevant subsidence reversal strategies by comparing an elevation goal with each island's anticipated flooded depth, and we prioritize islands for investment based on trade- offs between anticipated outcome and lost agricul- tural revenues. This approach might help integrate subsidence-reversal activities into long-term Delta planning under a range of flooding, land use, and habitat management scenarios.

Practical derivation of operational dike failure probabilities

Probability forecasts for natural loads, such as river discharge or wind speed, allow deriving predictive dike failure probabilities within a future time interval, the so-called operational dike failure probabilities. The operational dike failure probabilities can be used to assess the safety of dikes and can support decision making in the face of flooding.In this article, a practical framework for deriving the operational dike failure probabilities is introduced. The framework is based on a presently applied technique that is used to estimate the annual dike failure probabilities in the Netherlands. The framework is applied to a dike located in the delta of the river Vecht, providing probability forecasts for water levels and waves at the dike and the operational dike failure probabilities. Several aspects of the framework are also analysed, giving insight into the properties and accuracy of the method.

Seismic reliability analysis of reinforced concrete framed buildings deteriorated by chloride ingress

The main purpose of this study is to propose an evaluation method that can be used for analysing the time-dependent seismic reliability of reinforced concrete (RC) buildings located in a corrosive environment with high seismic hazard. In this study, several models have been developed to investigate the deterioration induced by chloride ingress in order to estimate the initiation stage and rate of corrosion and to analyse the structural capacity of columns and beams with corroded reinforcing bars; then, the seismic evaluation for RC framed buildings was used for calculating the story shear capacity of each floor in a building. In addition, the hazard curve of the story shear demand for each floor obtained from the seismic hazard analysis was adopted; consequently, a Monte Carlo simulation was carried out for estimating the annual failure probability and seismic reliability index of the concerned building; in other words, a time-dependent seismic reliability function could be built.