Fundamental Understanding of Pipeline Material Degradation under Interactive Threats of Dents and Corrosion

Overview

Main Objective

Enrich the knowledge base for evaluating interactive threats of external mechanical dents and secondary features, through and integrated lab-scale experimental and numerical framework to characterize and better predict the remaining safe life and operating pressures, while projecting the needs for mitigation measures.

Public Abstract

The convolution of design, materials and operational practices have enabled a good safety records for oil and gas transmission pipelines. However, external mechanical interference results in local dents and failure of the corrosion protection systems. Operational codes employ fracture mechanics and plastic collapse based analysis to provide risk assessment for different classes of defects under operational conditions. As a result, pipeline operators sort and prioritize large number of these dents based on shape-factors or local strain-measures that are derived from the inline-inspection (ILI) measurements. While these risk assessment methods and protocols have been subjected to extensive experimental and full-scale validation, there still exists a large variation in the prognostics of various threats. The risk assessment is based on two parameter – geometric measure of defect and material strength. Consequently, the observed variability may possibly arise from the incomplete understanding of corrosion-induced reduction in material strength and toughness, or may arise from the evolution of the S-N fatigue life curve to a much degraded state, away from the design point. Furthermore, the threshold for SCC crack initiation is near the specified minimum yield strength (SMYS) of the pipe material, which is already exceeded under the external mechanical interference. Such synergistic interaction might be further exacerbated the fatigue life of the mechanically damaged line. Utilizing our understanding from previous CAAP projects on corrosion induced material strength reduction, we propose to develop an integrated experimental- computational framework to address the interacting threats of external mechanical damage, coupled with secondary feature as corrosion or cracking on the integrity management decision. The proposed methodologies utilize: (i) A high throughput lab-scale testing apparatus for simulating the combined threats of external damage induced plastic deformation combined with exposure to corrosion, (ii) A lab-scale experiments to assess the synergistic interaction between different threats and their effects on strength and toughness of corrosion degraded pipeline steel, (iii) Utilization of electrochemical impedance spectroscopy (EIS) as a path way for ILI-implementation to characterize the severity of the corrosion near an external mechanical damage, (iv) A detailed FEM analysis of the different geometric constraints on the variance of local stress and strain state (concentration), and (v) Systematic numerical sensitivity analysis in FEM to quantify metrics for material strength and toughness degradation with corrosion and under the interacting threats of external mechanical damage, and stochastic interpretation of the variation in reported full scale fatigue testing of dents (PRCI MD 4-2, 4-9). The PI is an expert in fracture and fatigue damage percolation mechanisms, the electrochemical processes of corrosion and multi-scale modeling of materials, and leverages the support and ongoing work with BP and Applied Research Associates Inc. on damage monitoring of pipelines. The proposed work leverages the provided sample set in a previous CAAP project by Kiefner and Associates. The proposed framework is a step towards enriching the knowledge base, and enhancing the ability to better predict the remaining safe life, characterize the interacting threats and support a statistically sound integrity management decisions.

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