Threat Assessment Considerations for Vintage Pipes

Abstract

Abstract Vintage pipelines often pose unique potential threats compared to modern pipelines, especially as they relate to material properties and construction. Material toughness is a critical parameter governing the resistance of these early pipelines to fracture initiation and propagation. Methodologies for threat assessment and repair must consider actual or conservative material property estimates, which may vary substantially from pipeline to pipeline. This is an overview paper that discusses threat assessments and repair of vintage pipelines, especially those with low toughness. The threats that will be addressed in the paper are corrosion, cracking (including cracking in seam welds), mechanical damage, and pipe displacements under soil movement (geohazards). Primary emphasis is given to corrosion and cracking, with less coverage of threats due to mechanical damage and pipe displacement. Prior work on analysis methods for corrosion shows that empirical approaches, such as ASME B31G, are valid for lower toughness materials used in vintage pipelines, provided the corrosion features are blunt. Preferential corrosion of the seam weld merits special consideration, as does the potential impact of corrosion (and hydrogen) on the material properties of extremely hard seam welds. A variety of analysis methods exist for analyzing cracks in vintage pipe, including the ln-secant method, PRCI’s MAT-8, Raju-Newman, CorLAS™, and API 579, some of which are intended to address lower toughness materials. The main challenge in these assessments is often correlating known (or assumed) material properties, such as Charpy impact energies, to the stress intensity factors often used as input. Differences in fracture initiation toughness as a function of temperature and constraint are often misunderstood, creating confusion when selecting an appropriate toughness input for analysis. However, simply using conservative toughness values for vintage pipe when measured values are either sparse or unavailable can produce overly conservative results. A study was conducted on measured Charpy impact energies that quantifies the statistical scatter in the upper shelf and lower shelf values that can be used as more reasonable estimations of toughness in unknown vintage pipe. Mechanical damage poses additional considerations. Analysis methods need to consider the type of mechanical damage, such as plain dents, dents with gouges, or restrained dents, and the origin of the damage itself (mechanical interference, rocks, geohazards). Some of the conventional analysis methods for mechanical damage have significant limitations when applied to lower toughness materials. Finally, pipe displacement under soil movement or geohazards pose threats related to the tensile and compressive strain capacity of the girth welds. Frost-heave, settlement, landslides, and soil liquefaction all affect girth weld integrity in different ways. Several strain capacity models (e.g., PRCI and ExxonMobil) have been proposed. Their applicability to early generation low-toughness pipelines is often not well defined.