Material Characterization of Pipeline Steels: Inspection Techniques Review and Potential Property Relationships

The oil and gas industry in North America operates an aging infrastructure of pipelines, 70% of which were installed prior to 1980 and almost half of which were installed during the 1950s and 1960s. There is growing interest in having knowledge of pipe properties so that a safe operating pressure can be determined, yet there are a significant number of cases where records are incomplete. Current in-line inspection (ILI) technologies focus on defect detection and characterization, such as corrosion, cracking, and the achieved probability of detection (POD). As a part of the process in assessing defect significance it is necessary to know the pipe properties, so as to determine potential failure limits. The mechanical properties (yield strength, tensile strength and fracture toughness) of steel pipe must be known or conservatively estimated in order to safely respond to the presence of detected defects in an appropriate manner and to set the operating pressure. Material property measurements such as hardness, chemical content, grain size, and microstructure can likely be used to estimate the mechanical properties of steel pipe without requiring cut-outs to be taken from pipes for destructive tests. There are in-ditch methods of inspection available or being developed that can potentially be used to determine many of the material characteristics and at least some mechanical properties. Furthermore, there is also potential ILI data to be used for obtaining some information. Advances in ILI technologies for this purpose are currently being explored by several interested parties. ILI companies are specifically focusing on relating magnetic measurements from eddy current and magnetic flux leakage measurements to mechanical properties. ILI also regularly uses ultrasound measurements for wall thickness determination. Potential application of advances in ultrasound measurements for grain size and other properties are being explored. However, nondestructive methods of inspection in common use today usually do not enable determination of either the material or mechanical properties, leaving the only alternative to be destructive testing. This is costly, time-consuming, and often not practical for pipe that is in-service. ILI and in-situ techniques are reviewed in this paper and provide an analysis of a sample set of data is presented. The paper explores the possibility of obtaining mechanical property data from data potentially measurable by ILI and in-situ measurements. Ideally, results would allow mechanical property measurements desired to assess pipelines so as to ensure that at a specific operating pressure there is the proper response to anomalies that might pose a significant threat. The use of a multivariate regression analysis showed better results than the traditional two-variable regression plots, and may be key to determining which properties are necessary to provide the best results for reliably estimating the mechanical properties of pipe. However, there is still much work to done in understand and account for the many sources of variability within the pipe material, and how that relates to the resultant relationships between the mechanical and material properties.