Reliability Analysis of Land Pipelines for Hydrocarbons Transportation in Mexico

Abstract

Pipelines are the most economical way to transport hydrocarbons. In Mexico, PEMEX manages more than 60,000 Km of oil and gas land and marine pipelines. Therefore, their structural integrity must be carefully assessed. Pipeline managers require reliable and realistic codes in order to back up their decisions about design, maintenance and operation. In particular, for safety prediction, the failure modes and uncertainties involved in each loading condition need to be incorporated in the analysis in order to specify the pipelines use thresholds that keep them over acceptable safety levels within their operating lifetimes [1, 2]. For these reasons, a structural reliability formulation appears to be the appropriate framework to perform the evaluation. In this paper, the land pipeline reliability is estimated for the internal pressure, bending and tension failure mode conditions. These loading conditions are applied individually and tension and bending in a combined fashion, and random variability on the internal pressure, steel mechanical properties as well as the degradation effect of internal corrosion due to the transported fluid is included. So far, seamless pipeline is considered as used in Mexico. A set of internal pressures and mechanical properties are randomly generated through Monte Carlo simulation and the pipeline response under each simulated condition is obtained by making use of commercial software. The response analysis resorts on the nonlinear finite element method and it involves the calculation of maximum stresses and stress concentration factors under no corroded and corroded conditions. The following limit states are assessed: 1) the margin between maximum stresses due to internal pressure, tension and bending and the material capacity and 2) the margin between stress concentration factor and fracture initiation toughness. The above described limit states are calculated for no corroded condition and, once the critical failure modes are identified, corrosion effect is included on them. The failure probability is estimated from the response statistics for the considered limit state. The Cornell reliability index and the respective safety factor are also estimated. These results may be further extended and used for risk assessments and code calibration for design, inspection and maintenance of pipelines in Mexico.