Assessment of Interacting Volumetric Surface Defects

Abstract

On the inside and/or outside surfaces of the steel pipes of the transmission lines intended for the transportation of hydrocarbons (petroleum, liquid petroleum products, natural gas, liquefied petroleum or natural gas) or other fluids (water, ammonia, etc.) clusters or colonies of volumetric surface defects (VSDs) can be generated. Each defect from such a colony can interact with the neighbouring defects, such that the stress concentration effects determined by a group/cluster of defects can be superior to the ones generated by the presence of individual defects. Therefore, the present chapter has the following goals: (i) definition of the factors that determine VSDs interaction: the characteristic dimensions of the pipeline (diameter and wall thickness); mechanical properties of the steel from which the pipeline containing the defects is manufactured (Young modulus, Poisson’s ratio, specified minimum yield strength, minimum tensile strength, minimum specified value of the percentage elongation, etc.); type, shape and characteristic dimensions of VSDs (length/axial extent, width/circumferential extent, maximum depth, etc.); distances between adjacent flaws/defects (in both axial and circumferential directions); complexity and intensity of the mechanical loads of the pipeline on which defects are placed (operating pressure and maximum intensities of other loads/forces and moments); (ii) comparative analysis of the criteria (from Norms presently in force: standards, codes, recommended practices, specifications, etc.) with which it can be assessed if adjacent defects from a VSD group or colony interact; (iii) description of the methods for the assessment of the shape and characteristic dimensions of the defect equivalent to a cluster of interacting VSDs; (iv) definition of the modalities for the evaluation of the effects of interacting VSDs groups upon the remaining mechanical strength of pipelines with such defects; (v) investigation of the influence of the interaction between two adjacent defects on the stress and strain states in the defects area by means of finite element analyses.