Assessing pipeline integrity using fracture mechanics and currently available inspection tools

摘要

Ppeline systems are designed to comply with the regulatory requirements of each country and their applicable engineering standards. In the USA, Title 49 of the Code of Federal Regulations (CFR) establishes the mandatory minimum federal safety standards of pipelines for the transportation of natural gas (Part 192) and hazardous liquids (Part 195). These mandatory regulatory requirements typically cite consensus standards promulgated by the American Society of Mechanical Engineers (ASME), the American Pipeline Institute (API) and ASTM International. Specific performance criteria for pipeline systems suitable for the transportation of gas and hazardous liquids are established in ASME B31.8 [Gas transmission and distribution piping systems] and ASME B31.4 [Pipeline transportation systems for liquid hydrocarbons and other liquids] and frequently quoted in the construction specification of pipeline systems throughout the world.rnCode compliance is established if the designer demonstrates that all specific code requirements and all reasonably foreseeable load conditions are addressed by the design. The load condition seen by all pipelines is the load resulting from internal pressure. Because the hoop stress resulting from the internal pressure in the pipeline is at least twice the axial stress, typically longitudinal cracks and welds are the most susceptible and a substantial volume of literature addresses longitudinal cracking in pipes. Several pipeline systems, however, are subjected not only to internal pressure but also to significant external loads, which need to be evaluated using the code's so-called occasional load condition. For example, pipeline systems buried in regions of active landslides, expansive soils, steep topography, and poor foundation conditions can be subjected to substantial external forces, which produce axial loads in the pipe. These loads can well exceed the axial pressure load and present a much greater risk for joints like circumferential welds. Guidance on how to implement some of these geotechnical considerations and how to estimate these external loads are described in more detail in Refs 1, 2, and 3.rnAs our analysis will show, circumferential growth of cracks has the potential of causing severe consequences, typically leading to the rupture of the pipeline with the potential of a full-bore pipe failure. This observation is also reflected in the spill incident data of the 6th EGIG report [4], where the leading cause of spill incidents is external interference at 49.7%, followed by construction defects/material failure at 16.7%, corrosion at 15.1 %, and ground movement at 7.1 %, with ground movement having the largest proportion of ruptures and landslides, causing more than half of the ground-movement-related spill incidents. Incident-spill data have been further segregated to only include pipelines in mountain areas [3]. The authors report an incident rate of 0.32 to 0.8 spill incidents per 1000 km years for mountainous areas in Europe and the USA and, depending on the sophistication of the geotechnical engineering, a rate of 0.33 to 2.8 spills per 1000 km year in the Andean Mountains. This rate is slightly larger than the most recent spill incident rates for pipelines at large, which are typically 0.2 spill incidents per 1000 km year [4]. However, this incident data [3] does not include the spill incident data from the most recently constructed pipeline system crossing the AndeanrnMountains, the Camisea transportation system, which is buried in a region where landslides and other geological hazards are common.rnIn this paper an elastic plastic fracture mechanics analysis of a pipeline is presented that ruptured due to external soil loading, to evaluate possible loading conditions and correlate the observed crack propagation with possible external loading conditions. Next a fracture mechanics based performance criterion is derived for the most commonly used in-line inspection (ILI) methods, to detect these circumferential cracks; i.e. the magnetic flux leakage (MFL) tool.