The Experts below are selected from a list of 156 Experts worldwide ranked by ideXlab platform
Raphael T. Haftka - One of the best experts on this subject based on the ideXlab platform.
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reliability based design optimization using probabilistic sufficiency factor
Structural and Multidisciplinary Optimization, 2004Co-Authors: X Qu, Raphael T. HaftkaAbstract:A probabilistic sufficiency factor approach is proposed that combines Safety factor and probability of failure. The probabilistic sufficiency factor approach represents a factor of Safety relative to a Target probability of failure. It provides a measure of Safety that can be used more readily than the probability of failure or the Safety index by designers to estimate the required weight increase to reach a Target Safety Level. The probabilistic sufficiency factor can be calculated from the results of Monte Carlo simulation with little extra computation. The paper presents the use of probabilistic sufficiency factor with a design response surface approximation, which fits it as a function of design variables. It is shown that the design response surface approximation for the probabilistic sufficiency factor is more accurate than that for the probability of failure or for the Safety index. Unlike the probability of failure or the Safety index, the probabilistic sufficiency factor does not suffer from accuracy problems in regions of low probability of failure when calculated by Monte Carlo simulation. The use of the probabilistic sufficiency factor accelerates the convergence of reliability-based design optimization.
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Design Under Uncertainty Using Monte Carlo Simulation and Probabilistic Sufficiency Factor
Volume 2: 29th Design Automation Conference Parts A and B, 2003Co-Authors: Raphael T. HaftkaAbstract:Monte Carlo simulation is commonly employed to evaluate system probability of failure for problems with multiple failure modes in design under uncertainty. The probability calculated from Monte Carlo simulation has random errors due to limited sample size, which create numerical noise in the dependence of the probability on design variables. This in turn may lead the design to spurious optimum. A probabilistic sufficiency factor (PSF) approach is proposed that combines Safety factor and probability of failure. The PSF represents a factor of Safety relative to a Target probability of failure, and it can be calculated from the results of Monte Carlo simulation (MCS) with little extra computation. The paper presents the use of PSF with a design response surface (DRS), which fits it as function of design variables, filtering out the noise in the results of MCS. It is shown that the DRS for the PSF is more accurate than DRS for probability of failure or for Safety index. The PSF also provides more information than probability of failure or Safety index for the optimization procedure in regions of low probability of failure. Therefore, the convergence of reliability-based optimization is accelerated. The PSF gives a measure of Safety that can be used more readily than probability of failure or Safety index by designers to estimate the required weight increase to reach a Target Safety Level. To reduce the computational cost of reliability-based design optimization, a variable -fidelity technique and deterministic optimization were combined with probabilistic sufficiency factor approach. Example problems were studied here to demonstrate the methodology.
John Dalsgaard Sørensen - One of the best experts on this subject based on the ideXlab platform.
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Fatigue reliability and calibration of fatigue design factors of wave energy converters
International Journal of Marine Energy, 2015Co-Authors: Simon Ambühl, Francesco Ferri, Jens Peter Kofoed, John Dalsgaard SørensenAbstract:Abstract Target reliability Levels, which are chosen dependent on the consequences in case of structural collapse, are used in this paper to calibrate partial Safety factors for structural details of wave energy converters (WECs). The consequences in case of structural failure are similar for WECs and offshore wind turbines (no fatalities, low environmental pollution). Therefore, it can be assumed that the Target reliability Levels for WEC applications can be overtaken from offshore wind turbine studies. The partial Safety factors cannot be directly overtaken from offshore wind turbines because the load characteristics are different. WECs mainly focus on wave loads where for offshore wind turbine the wind loads are most dominating. Fatigue failure is an important failure mode for offshore structures. The scope of this paper is to present appropriate Fatigue Design Factors ( FDF ), which are also called Design Fatigue Factors ( DFF ), for steel substructures of WECs. A reliability-based approach is used and a probabilistic model including design and limit state equation is established. For modelling fatigue, the SN-curve approach as well as fracture mechanics are used. Furthermore, the influence of inspections is considered in order to extend and maintain a certain Target Safety Level. This paper uses the Wavestar prototype located at Hanstholm (DK) as case study in order to calibrate FDFs for welded and bolted details in steel structures of an offshore bottom-fixed WEC with hydraulic floaters.
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Reliability-Based Operation of Offshore Wind Turbines
2013Co-Authors: Sergio Marquez-dominguez, John Dalsgaard Sørensen, José G. Rangel-ramírezAbstract:The offshore wind industry is growing significantly these and coming years at offshore places far away from the coasts, where wind and wave loads as well as fatigue, corrosion and wear of the substructures are main sources of uncertainty. These uncertainties are important for the design, installation, operation and maintenance of the offshore wind infrastructure. Due to the location, it is important to reach the optimal plans for operation of offshore wind turbines in order to reduce the costs, coming from inspection and maintenance activities. Offshore wind turbines (OWTs) are operated with specific power production specifications which are inherently linked with the reliability Levels for the fatigue loads due to wind impact and depending of the component to be analyzed. When OWTs are operated over its design capacity influenced by changes of generator, gearbox, blades or simply by the control configuration, they will be imposed to higher wind velocities and obviously larger fatigue loads. Besides operating over the design power capacity, the location at wind farms can play a detrimental role for the fatigue performance of OWTs. Wind turbines with a wind farm will face up (larger) turbulence coming from wakes generated by surrounding OWTs. Therefore, the structural components of offshore wind turbines have to be able to withstand large fatigue loads during the design life. The operational conditions are characterized by the wind speeds at the site, the wind turbine capacity, the operational modes, and the wind farm layout. Changes in the control configuration of the operational states imply an influence on the fatigue loads of the structural components. Varying the production periods, operational wind speed and desired energy production may therefore have a significant effect on the reliability of the wind turbine components. This paper addresses the influence of the operational configuration on the structural reliability of offshore wind turbines by assessing its life cycle during operational periods with different operational configurations. This implies different influences on the load stress ranges at different wind speeds. The influence of the operational control on the load may be important when offshore wind turbines are intended to be fully exploited in their fatigue life. In the same manner, the wind farm location can vary the operational conditions and add fatigue load. The in-wind farm location and associated wake effects is taking into account by a code-based reliability approach. The stochastic model will be explained in detail and it will be shown how the influence of the operational conditions influence the fatigue reliability by setting up different load-stress ranges, wind intensities and wind turbulence for the case of single/alone and in-wind farm locations. Therefore, a reliability-based approach is used and a probabilistic model has been developed where strength and load uncertainties are described by stochastic variables. SN-curve / Miner’s rule and fracture mechanics approaches are considered to model the fatigue life. Design and limit state equations are established for the accumulated fatigue damage for single (no wake effects) and in-wind farm condition (with wake effects). The acceptable reliability Level for optimal fatigue design of OWTs is discussed and results are presented. Further, the influence of inspections is considered in order to extend and maintain a given Target Safety Level. The probabilistic basis for the analysis of fatigue reliability is using the fatigue model, proposed by Sorensen et al. (2008). Welded steel joints in the support structure are considered in this paper. An application example is described for the impact of the operation configuration (control configuration due to over-rate power production) in the life cycle reliability for single (no wake effects) and in-wind farm condition (with wake effects).
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Fatigue Reliability and Calibration of Fatigue Design Factors for Offshore Wind Turbines
Energies, 2012Co-Authors: Sergio Marquez-dominguez, John Dalsgaard SørensenAbstract:Consequences of failure of offshore wind turbines (OWTs) is in general lower than consequences of failure of, e.g., oil & gas platforms. It is reasonable that lower fatigue design factors can be applied for fatigue design of OWTs when compared to other fixed offshore structures. Calibration of appropriate partial Safety factors/Fatigue Design Factors ( FDF ) for steel substructures for OWTs is the scope of this paper. A reliability-based approach is used and a probabilistic model has been developed, where design and limit state equations are established for fatigue failure. The strength and load uncertainties are described by stochastic variables. SN and fracture mechanics approaches are considered for to model the fatigue life. Further, both linear and bi-linear SN-curves are formulated and various approximations are investigated. The acceptable reliability Level for fatigue failure of OWTs is discussed and results are presented for calibrated optimal fatigue design factors. Further, the influence of inspections is considered in order to extend and maintain a given Target Safety Level.
X Qu - One of the best experts on this subject based on the ideXlab platform.
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reliability based design optimization using probabilistic sufficiency factor
Structural and Multidisciplinary Optimization, 2004Co-Authors: X Qu, Raphael T. HaftkaAbstract:A probabilistic sufficiency factor approach is proposed that combines Safety factor and probability of failure. The probabilistic sufficiency factor approach represents a factor of Safety relative to a Target probability of failure. It provides a measure of Safety that can be used more readily than the probability of failure or the Safety index by designers to estimate the required weight increase to reach a Target Safety Level. The probabilistic sufficiency factor can be calculated from the results of Monte Carlo simulation with little extra computation. The paper presents the use of probabilistic sufficiency factor with a design response surface approximation, which fits it as a function of design variables. It is shown that the design response surface approximation for the probabilistic sufficiency factor is more accurate than that for the probability of failure or for the Safety index. Unlike the probability of failure or the Safety index, the probabilistic sufficiency factor does not suffer from accuracy problems in regions of low probability of failure when calculated by Monte Carlo simulation. The use of the probabilistic sufficiency factor accelerates the convergence of reliability-based design optimization.
Masahiko Fujikubo - One of the best experts on this subject based on the ideXlab platform.
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Investigations into Target Safety Level of Ships' Hull Girder Strength Based on Risk Assessment
Journal of the Japan Society of Naval Architects and Ocean Engineers, 2011Co-Authors: Kazuhiro Iijima, Masahiko FujikuboAbstract:In this study, the Target reliability Level of ships' ultimate hull girder strength is investigated by employing risk assessment. The risk can be defined as "Risk = Failure probability x Consequence." In the present case, the failure probability corresponds to the probability of hull girder collapse in bending while the consequence to a monetary loss accompanied by the failure. First, a risk model for the hull girder collapse and the limit state function are set up. Uncertainty models for the various parameters in the limit state function for the hull girder collapse are investigated. Among them are the yielding stress, plate thickness, corrosion, wave randomness, nonlinearity of wave loads, and so on. A series of reliability analysis is performed to obtain the relation between the Safety Level and deck plate thickness as a risk control option. Then, the Target reliability Level is investigated through Cost Benefit Analysis (CBA). It is shown that the obtained Target reliability Level is close to the reliability Level of the existing ships whose design complies with the current Common Structural Rules. Finally, a sensitivity study is conducted to investigate into the impact of the risk model parameters to the Target Safety Levels. It is discussed how different views may affect the Target Safety Levels.
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Structural Safety assessment of a pontoon-type VLFS considering damage to the breakwater
Journal of Marine Science and Technology, 2003Co-Authors: Masahiko Fujikubo, Taoyun Xiao, Kazuhiro YamamuraAbstract:A structural Safety assessment of a pontoon-type very large floating structure (VLFS) surrounded by a gravity-type breakwater was carried out for extreme wave conditions by considering the damage to the breakwater. Bending and shear collapses are considered to be a failure mode of the floating structure, while overturning damages the breakwater. The probability of the breakwater overturning, and the transmitted wave height before and after damage to the breakwater, are evaluated using design formulae for port and harbor facilities in Japan. The ultimate bending and shear strengths of the floating structure are calculated by the idealized structural unit method (ISUM) and FEM, respectively. The calculated failure probability for the floating structure is compared with the specified Target Safety Level. It was found that the floating structure under consideration is most likely to fail by bending in transverse waves, and that the corresponding failure probability satisfies the Target Level.
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Structural Safety Assessment of Pontoon-Type VLFS Considering Damage of Breakwater
21st International Conference on Offshore Mechanics and Arctic Engineering Volume 2, 2002Co-Authors: Masahiko Fujikubo, Taoyun Xiao, Kazuhiro YamamuraAbstract:A structural Safety assessment of a pontoon-type very large floating structure (VLFS) surrounded by a gravity-type breakwater was carried out for extreme wave conditions by considering the damage to the breakwater. Bending and shear collapses are considered to be a failure mode of the floating structure, while overturning damages the breakwater. The probability of the breakwater overturning, and the transmitted wave height before and after damage to the breakwater, are evaluated using design formulae for port and harbor facilities in Japan. The ultimate bending and shear strengths of the floating structure are calculated by the idealized structural unit method (ISUM) and FEM, respectively. The calculated failure probability for the floating structure is compared with the specified Target Safety Level. It was found that the floating structure under consideration is most likely to fail by bending in transverse waves, and that the corresponding failure probability satisfies the Target Level.
Eun Jeong Cha - One of the best experts on this subject based on the ideXlab platform.
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A framework for optimization of Target reliability index for a building class based on aggregated cost
Structural Safety, 2019Co-Authors: Vamshi Krishna Gudipati, Eun Jeong ChaAbstract:Abstract To effectively manage the seismic risk of the built-environment, the Target Safety Levels that design standards are based on should be properly managed by considering the total risk to the built-environment and to the community. This paper introduces a framework for optimizing the Target Safety Levels based on the total life-cycle cost of a portfolio of buildings. Building inventory is categorized to obtain a clear relationship between Target Safety Level and its realizations to design, which is reflected in the proposed framework. To address the issue of unrealistic computation cost in implementing optimization process in Target Safety Level determination, the framework utilizes a neural network for structural response estimation. A procedure to develop a neural network for dynamic response estimation of a group of buildings is also introduced in the paper. The proposed framework is illustrated to obtain the optimal Target reliability index for mid-rise office buildings.
