Compressive Strain Limits of Large Diameter X80 UOE Linepipe

摘要

For arctic pipeline systems, designers are required to deal with anumber of unique environmental loading conditions not normallypresent in other regions of the world. For buried arctic onshorepipelines, the key structural design issue is the potential for highbending strains resulting from permafrost: frost heave and/or thawsettlement. Because of the lack of traditional design solutions to theseunique deformation-controlled loading conditions, reliability-basedlimit state design methods are becoming increasingly preferred forarctic applications. They allow the integration of analytical andexperimental assessments into the overall design philosophy, which hasbeen shown to improve design concept confidence and reduce overalluncertainty.For a strain-based design approach, high pipeline bending strains due todeformation-controlled ground movements are addressed as a functionof the defined allowable tensile and compressive strain limits.Excessive compressive strains may give rise to serviceability issuessuch as restricted pipeline pig passage or coating damage. In somecases, excessive compressive strains may compromise pipelineintegrity due to other loads and could lead to ultimate limit stateconditions associated with burst, fatigue or corrosion under damagedcoating.To further develop the understanding of pipeline compressive straincapacity, this paper summarizes a test program performed at theC-FER Technologies (1999) Inc. (C-FER) testing facility in Edmonton,Canada, which quantified the critical compressive strain of 36-inchdiameter, 19.8-mm wall thickness, grade X80 UOE linepipe. Testvariables included heat treatment effects (simulated coating plantheating) and the presence of a girth weld. Work involved performingthermal treatment studies, material coupon tests and three full-scalebend tests on X80 UOE linepipe specimens subject to high internalpressure.As expected, the analysis of the results has shown that the presence ofan offset in the girth weld reduced compressive strain capacity. It wasalso demonstrated that, although heat treatment increased the Y/T ratio,it did not have a significant impact on the slope of the stress-straincurve in the region where buckling occurred, thus resulting in no loss oflocal buckling capacity.Equations to predict critical buckling strain, which agree well with thefull-scale test results, were formulated based on published work. Theseequations consider the influence of varying stress-strain curve shape.