L52307 Remaining Strength of Corroded Pipe Under Secondary Biaxial Loading

Abstract

Corrosion metal-loss is one of the major damage mechanisms to transmission pipelines worldwide. Several methods have been developed for assessment of corrosion defects, such as ASME B31G, RSTRENG and LPC. These methods were derived based on experimental tests and theoretical/numerical studies of the failure behavior of corroded pipelines subjected only to internal pressure loading. In the vast majority of cases, internal pressure loading will be the main loading mechanism on the pipeline. However, there may be instances when pipelines could also be subjected to significant loading from the environment. For onshore pipelines, these additional loads could be as a result of ground movement due to landslides, mining subsidence, or even seismic activity. In the case of offshore pipelines the formation of free spans may impose significant bending loads. For instance, seabed scour can lead to the development and growth of free spans of pipelines resting on the seabed, particularly if they are not trenched. Whilst, the guidance detailed in standard assessment methods will be sufficient in the majority of cases, it may be inappropriate or non-conservative to use it in cases when the pipeline may also be subjected to significant external loading. As a result, this work focus on : The remaining strength of corroded pipelines subject to internal pressure and external loading cannot be explicitly assessed using the ASME B31G, RSTRENG and LPC assessment methods. However, these assessment methods have been validated using pipe with real corrosion and simulated (machined) defects welded to dome ends to form a pressure vessel and subsequently failed under internal pressure loading. Consequently, existing methods include some inherent biaxial loading and the remaining strength of corroded pipelines can be assessed with a limited amount of external loading. Ground movement due to landslides can impose significant external loading to transmission pipelines. Stresses in pipelines due to landslides can be greater than the stresses due to internal pressure loading. Methods developed by the nuclear industry for assessing corroded pipework are given in ASME Code Case N-597-2 and based on ASME B31G when the axial extent of wall thinning is limited. For more extensive corrosion, the assessment methods are based on branch reinforcement and local membrane stress limits. Strictly the methods given in ASME Code Case N-597-2 are only applicable to the assessment of piping systems designed to the ASME Boiler and Pressure Vessel Code, Section III. Failure loci of pipelines with isolated corrosion defects and subjected to combined loads have been derived for common pipeline geometries and materials. The failure loci have been validated using tests performed on 457.2mm (18-inch) and 1219.2mm (48-inch) diameter pipe under combined bending/pressure loading. These failure loci can be used to assess the limit of acceptability of existing assessment methods such as ASME B31G and RSTRENG under combined loading conditions.