Reliability Assessment of Offshore Pipeline Due to Pitting Corrosion (original) (raw)
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Reliability of pipelines with corrosion defects
International Journal of Pressure Vessels and Piping, 2008
This paper aims at assessing the reliability of pipelines with corrosion defects subjected to internal pressure using the first-order reliability method (FORM). The limit-state function is defined based on the results of a series of small-scale experiments and threedimensional non-linear finite element analysis of the burst pressure of intact and corroded pipelines. A sensitivity analysis is performed for different levels of corrosion damage to identify the influence of the various parameters in the probability of burst collapse of corroded and intact pipes. The Monte Carlo simulation method is used to assess the uncertainty of the estimates of the burst pressure of corroded pipelines. The results of the reliability, sensitivity and uncertainty analysis are compared with results obtained from codes currently used in practice. r
Failure Probability Assessment for Pipelines under the Corrosion Effect
American Journal of Mechanical Engineering, 2014
In this work, a numerical method was developed, by a reliability mechanical coupling, in order to define the reliability index and probability of failure evolutions for pipelines under corrosion effect. The chosen model, takes into account uniform and localized corrosion. Thus, the hardness and tensile tests were worked out to characterize the mechanical properties of pipelines material. Once the model was defined, a simulation was carried out by the software Phemica. The importance factors were also estimated. A methodology has been presented for the reliability analysis of pipelines subjected to localized corrosion. The variables influencing the reliability are treated as random variables and represented by suitable statistical distributions. An approximate limit state function was developed. Advanced first-order second moment reliability theory was employed for the estimation of the probability of pipeline failure by Phimeca software logiciel. From a numerical investigation, it was found that both defect depth and fluid pressure have significant influences on pipeline reliability.
Pipelines Reliability Analysis Under Corrosion Effect and Residual Stress
2015
This work focuses on a development of a finite element model that simulates corrosion phenomenon and its influence on structure rupture (pipelines). Subsequently, the pitting effect on reliability and pipelines lifetime is studied. In this paper, the investigated structure material is characterized experimentally, in order to determine the maximum strength and stress resistance. Therefore, a numerical model was developed under ANSYS code to simulate different loads to obtain the stress concentration factor as the stresses could surpass the yielding limit in the corrosion impact, and to determine the Von Mises stress. A corrosion model and residual stresses used in the literature are coupled with the probabilistic model to find the limit state function. Thus, failure probability and structure reliability index under the effect of pitting corrosion phenomena are calculated and the obtained results are discussed and analyzed.
Corrosion, 2014
The reliability and risk of non-piggable, corroding oil and gas pipelines can be estimated from historical failure data and through reliability models based on the assumed or measured number of corrosion defects and defect size distribution. In this work, an extensive field survey carried out in an upstream gathering pipeline system in Southern Mexico is presented. It has helped determine realistic values for the number of corrosion defects per kilometer (defect density) and obtain a better description of the corrosion defect size distributions in this system.
Structural Safety, 2020
The remaining strength and the fatigue life of externally pitted corroded metallic pipeline are often assessed using an idealization to model the pit morphology, typically semi-ellipsoidal and cuboidal. Such idealized pit geometries are shown herein to underestimate Stress Concentration Factors (SCFs). By implication this leads to significant overestimates of fatigue life for pitting corroded pipes under sustained cyclic loading which could be the reason behind some of the recent unexpected pipeline failures. Further investigation clarified that this can occur only when there is little or no plastic deformation within the pit which is typical for pipelines constructed from brittle and quasi-brittle materials. Therefore, applying the conventional approach that uses simplistic geometries to model corrosion pits is not reliable for stress/fatigue analysis of pipelines with brittle or quasibrittle materials such as cast iron or high-grade steel. However, for pipelines made of ductile materials, there is a specific internal pressure, termed critical operating pressure herein, below which there is no plastic flow within the pit and simple idealization of pit morphology results in underestimated SCFs. A semi-empirical equation is developed herein to allow this critical operating pressure to be calculated, based on which a protocol is outlined for correct estimation of the SCFs of pitting corroded pipelines. The conclusions of this study are supported by numerical results validated against novel full-scale burst capacity tests with both simplistic and complex-shaped pits.
Reliability Assessment of Underground Pipelines Under Active Corrosion Defects
Reliability assessment of underground pipeline steel structures for hydrocarbon transmission and distribution systems under active corrosion defects has been investigated. The aim of this work is to obtain a decision-making tool for risk-based inspection and maintenance program. The basic idea consists in statistical analysis of corrosion defect measurements in a buried pipeline as to sort out the corresponding depths and lengths after several years of service. As a result steady corrosion rate model is proposed to estimate the growth in the dimensions of corrosion defects. Then, a degradation model based on ASME modified B31G standard is used as a mechanical model in order to assess the failure probability or the reliability index through defect dimensions and associated uncertainties. Meanwhile, the main assumption of the probabilistic analyses for corroded pipelines is the randomness of load and resistance parameters determining the limit state functions. The statistical distribution of the uncertainties in the parameters involved in the limit state functions is modeled using normal and lognormal distributions. The reliability software PHIMECA is used to compute the reliability index β and the probability of failure Pf, when increasing dimensions of the defect within the respective measured values of depth and length during inspection and maintenance operations. The reliability calculation allows carrying out the sensitivity of the variables according to their importance, their mean value and their coefficient of variation. The probability of failure and reliability index are obtained assuming in one hand constant depth of defect with increasing length and in the other hand a relationship between the depth to length corrosion defect ratio is given as a function of reliability index β. The latter can be used as a parameter for risk-based inspection and maintenance operations.
Study of the reliability of corroded pipeline by the ASME B31G method
Modelling, Measurement and Control B
This work concerns the study of the reliability of 5LX60 steel pipes for the transport of gas under the effect of corrosion failures, particularly corrosion peaks. The objective is to determine the life time of the pipes by estimating the probability of failure based on a parametric approach where the variables are considered. Initially, an ASME B31G-based calculation model is used to calculate the operating safety pressures in the corroded pipes. This method is usually deterministic. The application of a probabilistic method implies the knowledge of the variables and the dimensions, and which give us the reliability of the models used to calculate the pressures.
Reliability and failure pressure prediction of various grades of pipeline steel
The reliability assessment of various grades of pipeline steel with corrosion defects was conducted through a finite element analysis (FEA) model. The failure pressure of pipelines was also evaluated by three industry models for comparison. Results demonstrate that the failure pressure of pipelines is reduced by the increasing corrosion depth and the decrease of the steel grade. Predictions by ASME B31G and the modified B31G models tend to be higher than FEA results for low grade steels, while the DNV model shows the close result to FEA. The predictive reliability by ASME B31G and the modified B31G decreases with the increases in corrosion depth and the steel grade. The geometry of corrosion defects affects remarkably the local stress and strain distributions, and plays a critical role in the failure pressure prediction. The applied strain in the longitudinal direction simulating the soil strain, regardless of tensile or compressive, would reduce the failure pressure of pipelines.
Reliability based integrity assessment of steel pipelines under corrosion
Corrosion Engineering Science and Technology
Currently, the assessment of mechanical integrity of pipelines damaged by corrosion is performed through deterministic procedures and, in special cases, by using random variables. Rigorously, the calculations require the use of probabilistic methods because they provide the proper framework to include explicitly the uncertainties involved. Some recent treatments consider the corrosion events to be independent, which may lead to errors in estimating the failure probability of the pipeline. In this paper, pipeline integrity is assessed in terms of failure probability and recent models are utilised that estimate the resistance to failure using improved representations of the corrosion defect. Also, a simplified way to consider the space correlation between different segments of a corroding steel pipeline is introduced and the procedure is applied to the specific case where the defect geometry is modelled to be of parabolic shape. The geometrical and mechanical pipeline properties are considered to be random variables; their mean values are taken from experimental results and the interdependence between them is characterised using the model's correlation matrix.