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.

Wellhead Fatigue Analysis Method: Benefits of a Structural Reliability Analysis Approach

2012 ◽  
Author(s):  
Torfinn Hørte ◽  
Lorents Reinås ◽  
Jan Mathisen
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Offshore Structures ◽  
Probability Of Failure ◽  
Fatigue Analysis ◽  
Life Extension ◽  
Inspection Planning ◽  
Structural Reliability Analysis ◽  
Extreme Load ◽  
Input Parameters

Structural Reliability Analysis (SRA) methods have been applied to marine and offshore structures for decades. SRA has proven useful in life extension exercises and inspection planning of existing offshore structures. It is also a useful tool in code development, where the reliability level provided by the code is calibrated to a target failure probability obtained by SRA. This applies both to extreme load situations and also to a structural system under the influence of a time dependent degradation process such as fatigue. The current analysis methods suggested for service life estimation of subsea wells are deterministic, and these analyses are associated with high sensitivity to variations in input parameters. Thus sensitivity screening is often recommended for certain input parameters, and the worst case is then typically used as a basis for the analysis. The associated level of conservatism embedded in results from a deterministic analysis is not quantified, and it is therefore difficult to know and to justify if unnecessary conservatism can be removed from the calculations. By applying SRA to a wellhead fatigue analysis, the input parameters are accounted for with their associated uncertainty given by probability distributions. Analysis results can be generated by use of Monte-Carlo simulations or FORM/SORM (first/second order reliability methods), accounting for the full scatter of system relations and input variations. The level of conservatism can then be quantified and evaluated versus an acceptable probability of failure. This article presents results from a SRA of a fictitious but still realistic well model, including the main assumptions that were made, and discusses how SRA can be applied to a wellhead fatigue analysis. Global load analyses and local stress calculations were carried out prior to the SRA, and a response surface technique was used to interpolate on these results. This analysis has been limited to two hotspots located in each of the two main load bearing members of the wellhead system. The SRA provides a probability of failure estimate that may be used to give better decision support in the event of life extension of existing subsea wells. In addition, a relative uncertainty ranking of input variables provides insight into the problem and knowledge about where risk reducing efforts should be made to reduce the uncertainty. It should be noted that most attention has been given to the method development, and that more comprehensive analysis work and assessment of specific input is needed in a real case.

Probabilistic Fracture Mechanics Structural Reliability Analysis of Reeled Pipes

2006 ◽  
Author(s):  
Hugo A. Ernst ◽  
Ricardo Schifini ◽  
Richard E. Bravo ◽  
Diego N. Passarella ◽  
Federico Daguerre ◽  
...  
Keyword(s):  
Fracture Mechanics ◽  
Reliability Analysis ◽  
Structural Reliability ◽  
Crack Size ◽  
Probability Of Failure ◽  
Statistical Distribution ◽  
Assessment Procedure ◽  
Statistical Distributions ◽  
Probabilistic Fracture Mechanics ◽  
Structural Reliability Analysis

Structural integrity analyses are used to guarantee the reliability of critical engineering components under certain conditions of interest. In general, the involved parameters have statistical distributions. Choosing a single set of values for the parameters of interest does not show the real statistical distribution of the output parameters. In particular, offshore pipes installation by reeling is a matter of concern due to the severe conditions of the process. Since it is necessary to guarantee the integrity of the pipes, a probabilistic fracture mechanics reliability analysis seems to be the most adequate approach. In this work, a probabilistic fracture mechanics assessment approach to perform the structural reliability analysis of tubes subjected to a reeling process was developed. This procedure takes into account the statistical distributions of the material properties and pipe geometry, using a fracture mechanics approach and the Monte Carlo method. Two-parameter Weibull distributions were used to model the variability of the input parameters. The assessment procedure was implemented as a self-contained executable program. The program outputs are: the statistical distribution of critical crack size, amount of crack extension, final crack size and the cumulative probability of failure for a given crack size. A particular case of interest was studied; a seamless tube - OD 323.9 × wt 14.3 mm, was analyzed. Tolerable defect size limits (defect depth vs. defect length curves) for different probability of failure levels were obtained. A sensitivity analysis was performed; the effect of material fracture toughness and misalignment was studied.

Pipeline Survey Scheduling Through Structural Reliability Analysis

2000 ◽  
Author(s):  
Richard J. Espiner ◽  
Alan M. Edwards ◽  
Andrew Francis
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Probability Of Failure ◽  
Acceptable Level ◽  
Time Dependent ◽  
Offshore Pipelines ◽  
Probability Of Occurrence ◽  
Offshore Pipeline ◽  
Safety And Reliability ◽  
Survey Strategy

In order to reduce the risks associated with the occurrence of free spans, offshore pipelines are subject to external survey on a periodic basis. The true impact of this strategy on safety and reliability is generally unknown. This paper describes a structural reliability-based assessment that was carried out to investigate the probability of failure of an offshore pipeline due to the presence of free spans and develop a survey strategy that ensures a quantified and acceptable level of reliability. The probabilities of failure of free spans of varying lengths are evaluated using a structural reliability based approach. Then, the probability of occurrence of free spans as a function of time is evaluated in order to quantify the overall time-dependent reliability of the pipeline. Finally, the influence of the survey frequency is investigated. The outcome of the study is a recommended strategy for future surveys of the pipeline.

Casing Collapse Design Using Structural Reliability Analysis for a Subsea Well on the Norwegian Continental Shelf

2018 ◽  
Author(s):  
David Buchmiller ◽  
Arve Bjørset ◽  
Torfinn Hørte ◽  
Sune Pettersen
Keyword(s):  
Finite Element ◽  
Reliability Analysis ◽  
Structural Reliability ◽  
Probability Of Failure ◽  
Critical Parameters ◽  
Safety Level ◽  
Structural Reliability Analysis ◽  
Specific Input ◽  
Well Design ◽  
Casing Collapse

Casing collapse capacity was identified by Statoil as a critical operational parameter on one of its fields in production. This facilitated the need to re-evaluate the overall well design, specifically the production casing’s collapse capacity. Studies were performed to analyze and objectively increase the documented casing collapse capacity, while maintaining the safety level. Initially, the casing collapse capacity was evaluated using API TR 5C3 / ISO 10400, with insufficient capacity being documented. In order to investigate further, physical material testing and collapse testing were performed. Detailed finite element analysis was used to evaluate the casing collapse capacity, given well specific input parameters. The four critical parameters of axial load, casing ovality, casing wear, and temperature-dependent material properties were identified and the importance of each parameter was mapped. Using the testing results and the finite element models as a basis, structural reliability analysis (SRA) was applied to calculate the probability of failure for casing collapse of the production casing as a function of the differential pressure. The SRA provided results for the spread of the field and for individual wells given specific input on the key parameters of casing ovality, wear and temperature. At the selected target reliability level, the SRA results showed a higher collapse capacity of the production casing relative to conservative calculations commonly used from API TR 5C3 / ISO 10400 for well design. Applying SRA to well design, specifically collapse evaluations, has proven useful in concluding on the probability of failure. The SRA has transformed improved knowledge from testing and measurements to reduced uncertainty and a corresponding reduction in the failure probability. The potential over-conservatism in the conventional deterministic analysis is then avoided, while maintaining the overall safety level. The SRA results were used to assist in the risk evaluation resulting in an allowance for continued production on existing wells.

scholarly journals ANÁLISE DE CONFIABILIDADE ESTRUTURAL DE PROBLEMAS BASEADOS NA MECÂNICA DOS SÓLIDOS

2016 ◽  
Vol 12 (2) ◽  
Author(s):  
Vinícius Favaretto Defiltro ◽  
Wellison José Santana Gomes
Keyword(s):  
Monte Carlo ◽  
Reliability Analysis ◽  
Structural Reliability ◽  
Solid Mechanics ◽  
Structural Model ◽  
Simulation Method ◽  
Probability Of Failure ◽  
Structural Safety ◽  
Structural Element ◽  
Structural Problems

RESUMO: A Mecânica dos Sólidos, a partir de hipóteses simplificadoras, fornece modelos de cálculo que podem ser aplicados a vários problemas estruturais e estabelece as bases e o entendimento para o desenvolvimento de teorias e a construção de modelos mais complexos. Entretanto, dentro deste contexto, é comum desprezar as incertezas inerentes às propriedades dos materiais envolvidos, às condições de contorno e à geometria do problema. Neste artigo, ferramentas da Teoria da Confiabilidade Estrutural são aplicadas a problemas estruturais baseados na Mecânica dos Sólidos no intuito de analisá-los considerando algumas das incertezas envolvidas. Para isso, o método de Simulação de Monte Carlo é empregado na análise de confiabilidade de duas vigas. No estudo da primeira estrutura, busca-se investigar a influência da correlação entre variáveis aleatórias na probabilidade de falha do elemento estrutural. Na segunda estrutura, analisa-se o efeito da utilização de materiais com diferentes comportamentos (frágeis ou dúcteis) e, consequentemente, diferentes critérios de ruptura, sobre a probabilidade de falha estimada. Verifica-se que as análises de confiabilidade estrutural podem fornecer muitas informações que estão fora do escopo das soluções determinísticas. Tais informações permitem uma avaliação mais precisa da segurança estrutural e podem também levar a um melhor entendimento do modelo estrutural em questão. ABSTRACT: The Solid Mechanics, from simplifying assumptions, provides calculation models that can be applied to various structural problems and establishes the foundation and the understanding for the development of theories and the construction of more complex models. However, within this context, it is common to despise the uncertainties inherent to the properties of the materials involved, the boundary conditions and the geometry of the problem. In this article, Structural Reliability Theory tools are applied to structural problems based on Solid Mechanics in order to analyze them considering some of the uncertainties involved. For this, the Monte Carlo simulation method is used in the reliability analysis of two beams. In the first structure, study seeks to investigate the influence of correlation between random variables on the probability of failure of the structural element. In the second one, the effect of using materials with different behavior (ductile or brittle) and, consequently, different rupture criteria on the estimated probability of failure is analyzed. Structural reliability analysis can provide information which usually is outside the scope of deterministic solutions. Such information enables a more accurate assessment of structural safety and may lead to a better understanding of the structural model in question.

Acceptable Target Probability of Failure for Subsea Pipeline UHB Design

2017 ◽  
Author(s):  
M. Liu ◽  
C. Cross
Keyword(s):  
Reliability Analysis ◽  
Structural Reliability ◽  
Current Practice ◽  
Probability Of Failure ◽  
Safety Level ◽  
Subsea Pipeline ◽  
Target Probability ◽  
Load Factors ◽  
Critical Issues ◽  
Acceptable Probability

For subsea pipeline UHB design, it is essential to establish appropriate UHB load factors by undertaking a structural reliability analysis based on a target acceptable probability of failure. The current practice is to setup the acceptable probability of failure according to the best industrial practice coded in DNV OS F101 and ISO16708 but regardless of trench performance, imperfection details and inspection results. This paper is intended to discuss the relationship between the UHB failure probability for the whole pipeline and the trench performance and imperfection frequency statistics. A detailed statistic and reliability analysis is undertaken to address some disparity in the current practice. It is shown that in order to achieve a specific reliability level required for the entire pipeline, it is paramount to calibrate the target probability of failure based on the survey data. The common practice for UHB is shown un-conservative giving rise to an insufficient safety level and non-compliant OOS design. An improved approach is outlined to address the critical issues allowing for a robust OOS assessment by means of case studies.

Reliability Assessment of Marine Drilling Risers With Correlated Random Variables

2016 ◽  
Author(s):  
Piyali Sengupta ◽  
Ying Min Low ◽  
Xiaodong Zhang ◽  
Peter Francis Bernad Adaikalaraj ◽  
Chan Ghee Koh
Keyword(s):  
Life Cycle ◽  
Computational Model ◽  
Reliability Analysis ◽  
Deep Water ◽  
Environmental Conditions ◽  
Life Cycle Cost ◽  
Structural Reliability ◽  
Random Variables ◽  
Density Functions ◽  
Total Life

Marine drilling risers are integral parts of the deep water offshore oil and gas industry. They are required to be designed for safe operations during their service lives with appropriate degree of reliability. With limited experience present in ultra-deep water, drilling risers are subjected to a range of uncertainties arising from untested environmental conditions. However, the current industry practice is limited to deterministic design of drilling risers which cannot account for uncertainties present in real life scenario. Under uncertain environmental conditions, deterministic methods may lead to undesired consequences, i.e. over conservative or unsafe design and misguided estimates of operability and down time of ultra-deep water drilling risers affecting the total life cycle cost. Thus, structural reliability analysis is particularly useful for prediction of the probabilities of downtime and disconnection of drilling risers incorporating the environmental uncertainties. In addition, structural reliability analysis can be used to reduce the total life cycle cost of ultra-deep water drilling risers. In reliability analysis, many studies use uncorrelated random variables to represent uncertainties for simplification. Nevertheless, uncertainties in environmental conditions may be strongly correlated (for example wind and wave loads). If the correlation is not accounted for, it may lead to erroneous probability estimates. Thus, a joint environmental model is proposed in this paper using the conditional modeling approach where a joint density function is defined in terms of a marginal distribution and a series of conditional density functions. The joint density functions of environmental conditions are constructed in the current study using the recorded metocean data for Gulf of Mexico available from National Oceanic and Atmospheric Administration (NOAA) website. Then a computational model of connected ultra-deep water drilling riser system is constructed in ORCAFLEX to conduct time domain dynamic analysis. Thereafter, the correlated random variables in combination with the drilling riser computational model are utilized for conducting Monte Carlo Simulation (MCS) to evaluate the probabilities of downtime and disconnection. MCS is a widely accepted and robust approach and generally used as a benchmark to verify the accuracy of other reliability methods. But, in presence of large number of random variables representing environmental uncertainties, MCS is computationally demanding especially for the large number of simulations required to estimate small failure probabilities associated with extreme values. To this end, probability density functions of drilling riser responses are evaluated using Shifted Generalized Lognormal Distribution (SGLD) and Generalized Extreme-Value (GEV) Distribution both of which show similar accuracy (compared to MCS results) at a fraction of computing time (around 1/500 times).

STRUCTURAL RELIABILITY ANALYSIS BASED ON P-BOXES

2020 ◽  
Vol 92 (6) ◽  
pp. 51-58
Author(s):  
S.A. SOLOVYEV ◽  
Keyword(s):  
Probability Distribution ◽  
Reliability Analysis ◽  
Structural Reliability ◽  
Epistemic Uncertainty ◽  
Random Variable ◽  
Strength Criterion ◽  
Structural Elements ◽  
Structural Element ◽  
Limit States ◽  
Distribution Shape

The article describes a method for reliability (probability of non-failure) analysis of structural elements based on p-boxes. An algorithm for constructing two p-blocks is shown. First p-box is used in the absence of information about the probability distribution shape of a random variable. Second p-box is used for a certain probability distribution function but with inaccurate (interval) function parameters. The algorithm for reliability analysis is presented on a numerical example of the reliability analysis for a flexural wooden beam by wood strength criterion. The result of the reliability analysis is an interval of the non-failure probability boundaries. Recommendations are given for narrowing the reliability boundaries which can reduce epistemic uncertainty. On the basis of the proposed approach, particular methods for reliability analysis for any structural elements can be developed. Design equations are given for a comprehensive assessment of the structural element reliability as a system taking into account all the criteria of limit states.

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