Optimal Control of Performance Risk for Large Potentially Dangerous Systems (LPDS)

2002 ◽  
Author(s):  
Sviatoslav A. Timashev
Keyword(s):  
Optimal Control ◽  
Risk Analysis ◽  
Conditional Probability ◽  
Structural Reliability ◽  
Process Analysis ◽  
Probability Of Failure ◽  
Simultaneous Optimization ◽  
Normal Operation ◽  
Residual Lifetime ◽  
Parameter Problem

The paper considers the safety problem for large potentially dangerous systems (LPDS). Disruption of their normal operations may lead to casualties, ecological and property damage. Solution to the above problem is sought in the framework of risk control of LPDS during their normal operation, based on the principle of preventive actions. Risk is described as the product of conditional probability of failure and the overall consequences of such failure. Methods of brining down risk analysis problems to reliability problems are presented. They are based on the following: assessments of “cost of life” (as economic equivalent of casualty); simultaneous optimization of the LPDS and its safety subsystem (expansion of the object of optimization). Such an approach allows unification and merging of structural reliability theory and probabilistic risk analysis. A quantitative method of damage size (the first component of risk) assessment is described, based on computer modeling of a full group of scenarios of a structural failure developing into a full blown LPDS catastrophe. As a result of modeling, the destruction zones and the character, size and probabilities of all kinds of damage (casualties, ecological damage, loss of property) are assessed. It is proposed, as the main method of securing LPDS integrity and safety, to equip each LPDS with suitable monitoring/inspection/maintenance systems, designed as an instrument for controlling the second component of risk (conditional probability of failure), on the basis of a three-level (warning-alarm-failure) control policy. In the outlined format maintenance/repair is considered as optimal control of random degradation and renewal functions, interaction of which forms a certain regeneration process. Analysis of this process allows defining the optimal triggering levels of deterioration parameters or risk that minimize total expenditures of LPDS performance while ensuring its safety. The problem formulated above naturally embodies all existing maintenance methods (based on admissible performance time, rate of failure and on actual and prognosed system condition). Further, the problem of optimal cessation of performance is solved. It allows convoluting a multi-parameter problem into a one-parameter problem and defining the ultimate permissible level of conditional probability of failure. The described methods of risk analysis and control were used in residual lifetime monitoring systems for oil pumping aggregates and for main oil pipe line segments repair prioritization.

Efficient Reliability Assessment With the Conditional Probability Method

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Rami Mansour ◽  
Mårten Olsson
Keyword(s):  
Conditional Probability ◽  
Structural Reliability ◽  
A Priori ◽  
Reliability Assessment ◽  
Probability Of Failure ◽  
Assessment Method ◽  
Probability Method ◽  
Probability Of Survival ◽  
Design Variables ◽  
Reliability Method

Reliability assessment is an important procedure in engineering design in which the probability of failure or equivalently the probability of survival is computed based on appropriate design criteria and model behavior. In this paper, a new approximate and efficient reliability assessment method is proposed: the conditional probability method (CPM). Focus is set on computational efficiency and the proposed method is applied to classical load-strength structural reliability problems. The core of the approach is in the computation of the probability of failure starting from the conditional probability of failure given the load. The number of function evaluations to compute the probability of failure is a priori known to be 3n + 2 in CPM, where n is the number of stochastic design variables excluding the strength. The necessary number of function evaluations for the reliability assessment, which may correspond to expensive computations, is therefore substantially lower in CPM than in the existing structural reliability methods such as the widely used first-order reliability method (FORM).

Reliability Based Assessment of Deck Elevations for Offshore Jacket Platforms

2004 ◽  
Vol 126 (4) ◽  
pp. 331-336 ◽  
Author(s):  
Ernesto Heredia-Zavoni ◽  
Dante Campos ◽  
Gallegher Ramı´rez
Keyword(s):  
Reliability Index ◽  
Structural Reliability ◽  
Failure Modes ◽  
Probability Of Failure ◽  
Upper And Lower Bounds ◽  
Series System ◽  
Wave Loading ◽  
System Components ◽  
Storm Wave ◽  
Wave Heights

Structural reliability analyses of fixed marine platforms subjected to storm wave loading are performed to assess deck elevations. Platforms are modeled as a series system consisting of the deck and jacket bays. The structural reliability analyses are carried out assuming dominant failure modes for the system components. Upper and lower bounds of the probability of failure are computed. The variation of the reliability index per bay component as a function of wave height, with a focus on those wave heights that generate forces on the deck, is analyzed. A comparison is given for the deck probability of failure and the lower bound probability of failure of the jacket in order to assess how the deck or the jacket controls the probability of failure of the system. Results are also given for reliability analyses considering different deck elevations. Finally, an analysis of the total probabilities of failure, unconditioned on wave heights, is given.

Reliability in Microjoining Processes for Instrumented Nuclear Fuel Element Fabrication

2020 ◽  
Vol 1157 ◽  
pp. 31-37
Author(s):  
Călin Truţă ◽  
Adrian Amzoi ◽  
Dumitru Barbos
Keyword(s):  
Fuel Element ◽  
Nuclear Fuel ◽  
Structural Reliability ◽  
Added Value ◽  
Impedance Match ◽  
Normal Operation ◽  
Local Loss ◽  
Steel Coil ◽  
Nuclear Fuel Element ◽  
Prolonged Contact

The paper presents the assembling flux of thermocouple-instrumented nuclear fuel element for research reactor, from the point of view of the welding / brazing engineer, considering nuclear quality and safety requirements: fuel element structural reliability (no radioactive leaks through joints) and temperature signal reliability (thermocouple sheath integrity), this signal being an essential parameter for reactor normal operation and emergency shut-down. The paper is a real case study for an experimental instrumented element recently developed at INR-Pitesti describing technology choices as balance between fabrication complexity and risk of failure in joining processes, especially in later stages when added value increases. All joints (welded or brazed) fall into microjoining category, and it is shown how some special provisions may influence reliability. Focus is put on brazing thin-walled Inconel sheathed thermocouples, where erosion and local loss of ductility are known issues, leading to sheath rupture. Choosing as filler the less aggressive BNi-9 helped too little. A simple but efficient technique has been developed to address this matter adequate to narrow spaces inside a nuclear fuel element, where no room is available for solutions described in literature e.g. distal preplacing of filler. The solution prevents sheath from having prolonged contact with large volume of molten filler by using locally a miniature barrier (thin stainless-steel coil or sleeve) which only allows capillary wetting, being also a perfect real-time visual indicator of brazing progress and completion. As proved in the present paper, this method along with using filler formulation with lower Carbon content (without organic binder) enhances significantly, 8 times at least, resistance to bending fatigue. A particular vacuum brazing chamber design is employed: narrow quartz tube with external induction coil and top fitting letting outside the long thermocouples attached, reducing much the chamber volume and degassing. Careful impedance match is therefore required to overcome induction power loss due to the larger coil-to-workpiece gap. Additional joining problems are discussed e.g. inherent differential expansion of long parts during induction heating which afterwards may put tension upon braze during solidification and determine delayed cracking, this being avoided through wise order of operations. Another concern is the final precision weld between instrumentation segment having attached the hard-to-handle long thermocouples bunch and nuclear segment with the heavy Uranium pellets. The result of this research is successful assembling of first Romanian prototype with joints exhibiting He leak rate bellow 1E-09 std.cc/sec and overall reliability proved during reactor irradiation testing.

Risk Analysis of Running a Deepwater Production Test From a Dynamically Positioned Vessel in the North-Atlantic

2003 ◽  
Author(s):  
Marc A. Maes ◽  
Jeff Sinclair ◽  
David B. Lewis
Keyword(s):  
Risk Analysis ◽  
North Atlantic ◽  
Normal Operation ◽  
Optimal Decision ◽  
Production Test ◽  
The North ◽  
Failure Cost ◽  
Optimal Decision Making ◽  
Key Steps ◽  
Fixed Structure

The present paper describes the key steps and issues involved in performing a quantitative risk analysis (QRA) for a dynamically positioned (DP) offshore vessel that is used to perform a short-term production test (PT) in North Atlantic deep waters. The basic approach is to focus on the “incremental” risk that would occur if the PT were run from a DP vessel as opposed to a fixed structure. The analysis is structured around two basic groups of risk: those specifically associated with DP vessel disconnection decisions and activities (all of which are seasonal) and those occurring during normal operation of the DP vessel. In the case of disconnection caused by hazards such as severe weather, ice, equipment or reference system malfunction, or human/operating error, a large variety of event sequences is assumed, each resulting in different consequences and risks. These are formulated for each analysis outcome in terms of loss of life, release of chemicals into the environment, damage and loss of assets and equipment, as well as overall failure cost. It is shown that the QRA provides a very useful basis for optimal decision making with respect to the feasibility, the planning, and the risk/benefit of deep-water production testing from a DP vessel.

Adjusting Environmental Contours for Specified Expected Number of Unwanted Events

2020 ◽  
Author(s):  
Erik Vanem
Keyword(s):  
Reliability Analysis ◽  
Return Period ◽  
Environmental Conditions ◽  
Structural Reliability ◽  
Probability Of Failure ◽  
Expected Number ◽  
Probability Of Exceedance ◽  
Underlying Distribution ◽  
Extreme Loads ◽  
Time Period

Abstract Environmental contours are applied in probabilistic structural reliability analysis to identify extreme environmental conditions that may give rise to extreme loads and responses. Typically, they are constructed to correspond to a certain return period and a probability of exceedance with regards to the environmental conditions that can again be related to the probability of failure of a structure. Thus, they describe events with a certain probability of being exceeded one or more times during a certain time period, which can be found from a certain percentile of the underlying distribution. In this paper, various ways of adjusting such environmental contours to account for the expected number of exceedances within a certain time period are discussed. Depending on how such criteria are defined, one may get more lenient or more stringent criteria compared to the classical return period.

Reduction Factors for Estimating the Probability of Failure of Mechanical Damage Due to External Interference

2008 ◽  
Author(s):  
Andrew Cosham ◽  
Jane Haswell ◽  
Neil Jackson
Keyword(s):  
Structural Reliability ◽  
Historical Data ◽  
Mechanical Damage ◽  
Probability Of Failure ◽  
Risk Assessments ◽  
External Interference ◽  
Failure Data ◽  
Major Accident ◽  
The Uk ◽  
Reduction Factors

Quantified risk assessments (QRAs) are widely used in the UK to assess the significance of the risk posed by major accident hazard pipelines on the population and infrastructure in the vicinity of the pipeline. A QRA requires the calculation of the frequency of failures and the consequences of failures. One of the main causes of failures in onshore pipelines is mechanical damage due to external interference, such as a dent, a gouge, or a dent and gouge. In the published literature, two methods have been used to calculate the probability of failure due to external interference: • historical failure data and • limit state functions combined with historical data (i.e. structural reliability-based methods). Structural reliability-based methods are mathematically complicated, compared to using historical failure data, but have several advantages, e.g. extrapolation beyond the limited historical data, and the identification of trends that may not be apparent in the historical data. In view of this complexity, proposed supplements to the UK pipeline design codes IGE/TD/1 (natural gas) and PD 8010 (all substances) — on the application of QRAs to proposed developments in the vicinity of major accident hazard pipelines — include simple ‘reduction factors’ for use in ‘screening’ risk assessments. These ‘reduction factors’ are based on a comprehensive parametric study using a structural reliability-based model to calculate the probability of failure due to mechanical damage, defined as: gouges, and dents and gouges. The two ‘reduction factors’ are expressed in terms of the design factor and wall thickness of the pipeline. It is shown that, through appropriate normalisation, the effects of diameter, grade and toughness are secondary. Reasonably accurate, but conservative, estimates of the probability of failure can be obtained using these ‘reduction factors’. The proposed methodology is considerably simpler than a structural reliability-based analysis. The development and verification of these ‘reduction factors’ is described in this paper.

Simplified Strength Reliability and Integrity Analysis of TTRs

2014 ◽  
Author(s):  
Mir Emad Mousavi ◽  
Sanjeev Upadhye ◽  
Kevin Haverty
Keyword(s):  
Extreme Events ◽  
Structural Reliability ◽  
Probability Distributions ◽  
Probabilistic Approach ◽  
Reliability Assessment ◽  
Cost Effective ◽  
Probability Of Failure ◽  
Annual Probability ◽  
Reliability Methods ◽  
Integrity Analysis

The design of riser systems can be improved if structural reliability methods are used to assess their safety and integrity and confirm that such design meets a target annual probability of failure. TTRs are typically multi–bore assemblies involving several sub-assemblies. The failure of any of the components of a TTR under extreme or service environmental conditions can lead to an immediate failure of the entire assembly and impose a direct risk of damaging the wellheads, conductors, casing and tubing hangers, or other subsea equipment, because they are installed directly on top of the wellhead. However, the actual strength safety of the TTR cannot be examined unless after it is installed and examined under extreme events. Because of the numerous uncertainties associated with the design of TTRs, a probabilistic approach based on structural reliability methods can account for many of those uncertainties and serve as a basis for their reliable and cost-effective design. In turn, a comprehensive reliability assessment of a TTR requires extensive analysis that is considerably more complex and time consuming compared to a conventional deterministic-based analysis. This paper presents a probabilistic-based simplified methodology for the strength reliability assessment of TTR systems. In this method, marginal values on some uncertain model inputs are considered similar to the conventional analysis methods but, some key random variables related to environmental demands and component capacities are considered with their associated probability distributions. As a result, this method can be used to estimate the minimum level of safety of the TTR under extreme events. Additionally, results of the proposed method are discussed for integrity analysis and integrity-based optimal design of the TTR system, which compare the safety of the TTR components and estimate the component Optimality Factors for improving the design integrity and meeting a target minimum annual probability of failure.

Optimum linear selection indexes for multiple generation objectives with non-linear profit functions

1995 ◽  
Vol 61 (1) ◽  
pp. 165-175 ◽  
Author(s):  
J. C. M. Dekkers ◽  
P. V. Birke ◽  
J. P. Gibson
Keyword(s):  
Optimal Control ◽  
Net Present Value ◽  
Selection Index ◽  
Index Theory ◽  
Planning Horizon ◽  
Simultaneous Optimization ◽  
Linear Selection ◽  
Non Linear ◽  
Profit Functions ◽  
Selection Indexes

AbstractA method to obtain linear selection indexes that maximize objectives that involve average profit in one or more generations within a planning horizon based on non-linear profit functions, was derived through application of optimal control theory. The method involves simultaneous optimization of indexes for each generation in the planning horizon. Optimum linear indexes were found to be conform indexes derived from selection index theory, with economic values equal to a weighted sum of partial derivatives of the profit function at the trait means which result in each generation of the planning horizon. Numerical procedures to derive optimum indexes are presented. Methods and properties of alternative strategies for selection witli non-linear profit functions are illustrated for selection on egg weight and rate of lay in poultry. In the example, the additional benefit of selection indexes that maximize cumulative net present value of profit over a planning horizon of10 years was small relative to use of traditional selection procedures. Optimal indexes were also derived with a derivative-free non-linear programming optimizer, with identical results. The latter method also allows incorporation of additional constraints.Possible extensions of the optimal control methodology to address other problems related to optimization of selection over multiple generations are discussed.

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