Approximate Evaluation of Stresses in Dents of Cylindrical Shells

Abstract

A semi-empirical model is developed for analysis of stresses in a dent. Variation in maximum stress intensity in dents with different angular orientations relative to the shell's generatrix is demonstrated. Results of stress analysis are presented for a dent oriented along the generatrix of a pipe as a function of the dent's geometric parameters.Dents in pipelines and pressure vessels are seen as a rather widespread defect, which creates risk for continued operation of equipment as a function of the operating conditions of the structures and the mechanical properties of the material. If the load is predominately static and the material is sufficiently plastic, the existence of a dent does not exert a noticeable influence on the bearing capacity of the shell. In that case, the following basic restriction should be considered for a pipeline: the minimum diameter of the cross section should not be less than 85% of the nominal dimension.Dents with additional crack and score types of defects, and which also contain a defective welded seam, are actually dangerous. To analyze the risk associated with these defects during service, when corrosion, fatigue, or brittle (at low temperatures) failure is possible, it is necessary to evaluate concentrations of bending stresses in the wall within the zone of the dent.Considering the error generated in determining initial data, it is expedient here to utilize approximate semi-empirical models, which are based on analysis of experimental data and which make it possible to obtain simple analytical solutions. The method adopted on the basis of ASME standard B31G to evaluate the bearing capacity of pipes with corrosion damage is a typical example of this model [1].Let us examine an analytical expression, which can be used to analyze the notch-sensitivity index for technical diagnoses of pipelines and vessels.According to the exact analytical solution for flexible shells with an oval cross section [2], the maximum bending stresses

Application of the Cheng-Finnie method to the calculation of stress intensity factors in thin-walled pipes with long axial cracks with allowance for geometric nonlinearity

“…As concerns main pipelines, the assessment of man-caused dents, e.g., those produced by a power shovel, involves practical problems. Publications report many procedures for the calculation of dents [10,11]; for instance, admissible calculation of dents can be performed in accordance with the document API 579 [12], which was worked out for the evaluation of the state of petrochemical industry installations containing flaws of different nature.…”