Methods for Determining the Remaining Strength Factor of Pipelines with Volumetric Surface Defects

摘要

Knowing the values of the Remaining Strength Factor - RSF constitutes a mostly useful tool for the application of the proactive maintenance principles and for programming the maintenance (repair) works for the liquid and gas transmission pipelines. This paper proposes and analyses the pertinent modalities for the definition and determination of RSF for the transmission pipelines whose steel pipes have volumetric surface defects, i.e. of the type local metal loss (locally thin areas, pittings, grooves or gauges-like flaws). The mechanical strength of the pipes, with and without flaws, of a pipeline (through which liquids or gases under pressure are flowing) is quantitatively expressed by: - the pressure value for which the pipeline reaches the serviceability limit state, named Maximum Allowable Operating Pressure - MAOP; - the pressure value for which the pipeline reaches an ultimate limit state (global yielding / plastic collapse, fracture following crack growth or failure / bursting of pipes). The equations presented in the paper for the determination of the pressure at which the pipelines (with and without flaws) reach the ultimate limit state have been analysed on the basis of the results of their application for the case of pipes (with and without flaws) that have been the object of some experimental programs, consisting of their internal pressure testing, up to bursting; the validation and qualification criterion for each equation has been constituted by the concordance between the bursting pressure values obtained by using the equation and those determined experimentally. The results available in literature regarding such experimental programs have been used, together with the results of an experimental program performed by the authors and presented thoroughly in the paper.