The Effect of Residual Longitudinal Compressive Stress in Onshore Buried Steel Pipelines
Proceedings Publication Date
Vaughan Pryce-Jenkins
Vaughan Pryce-jenkins, Will Heaps, Elie Dib
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Onshore buried steel pipelines often use cold bend forming to reduce installation costs, as opposed to high temperature induction bending. A conservative approach to modelling these residual stresses has been to calculate an upper bound stress value for a specific geometry and to reduce the design code allowable stress value by this amount. Typically, this can either remove cold bending as a design option and significantly increase construction costs or require costly mitigation to reduce the force in the system. However, a third common option is to ignore residual stress in a pipeline cold bend without an associated explanation. Onshore buried steel pipelines are often in net longitudinal compression due to the frictional force from the surrounding backfill material. This results in localised regions of peak longitudinal stress which consist of both residual stress and stress from operational loading. Design code allowable stress limits tend to consider both allowable longitudinal stress and combined stress values such as von Mises yield criterion. This paper investigates whether residual stress contributes to either failure in net compression or reduced hoop stress capacity causing loss of pressure containment.

Code based allowable stress values are for the purposes of pressure containment therefore, using detailed numerical analysis to capture critical failure modes, an assessment can be made of whether loss of pressure containment due to residual stress is likely. Two identical models, with and without longitudinal residual stress are compared, observing both the mode of failure and the point at which it occurs, with possible recommendations being made for not including residual stress within the code based allowable stress limits. This study provides a methodology for justification in certain pipeline systems for the use of cold formed bends without costly mitigation measures, significantly reducing construction costs for onshore buried steel pipelines. 

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