Reliability Assessment of Underground Pipelines Under Active Corrosion Defects

Abstract

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.