Numerical prediction of the current and limiting states of pipelines with detected flaws of corrosion wall thinning

Abstract

The main assumptions have been made for the physical and mathematical models of combined development of the stressstrain state and voids of ductile fracture in welded pipeline elements and pressure vessels with the detected 3D metal discontinuities (local corrosion losses of metal, macrovoids), including the ones located in the area of site welds for determination of the characteristic features of defective structures’ limiting state. Analysis of the stress-strain state of the welded pipeline section has been carried out based on finite element solution of the problem of non-stationary thermoplasticity by tracing elastic-plastic deformations from the beginning of welding to the complete cooling of the structure and subsequent loading to the limiting state. The continuum dilatation model of fracture is based on a stepwise prediction of micro- and macro-damage of material both in welding and under loading with internal pressure and external force bending moment to the limiting state on the basis of the Gurson–Tvergaard–Needleman theory. The criteria for macroscopic fracture of structure on a brittle-ductile mechanism have been proposed. Methods for probabilistic estimation of the stressed state of the pipeline structure from the point of view of fracture susceptibility, which are based on the integration of the calculated field of principal stresses within the framework of the Weibull statistics, have been developed. Functional dependences of Weibull coefficients on the properties of the metal, namely yield stress and degree of strain hardening, were obtained for correct quantitative assessment of the state of critical structures based on a complex analysis of the limiting state of steel pipelines under internal pressure. Specific features of the limiting state under loading with internal pressure and bending moment have been investigated based on an example of typical cases of operation damage of main pipeline elements such as local metal losses in the area of the site weld. It was shown that nature of interaction of residual post-weld stress state of metal with operation stresses from the geometry concentrator significantly affected the value of limiting pressure in the pipe. Besides, effect of the additional bending moment on load-bearing capacity of the pipeline section with isolated flaw of local corrosion wall thinning has been determined. The change of failure probability of the structure as a result of internal pressure and bending moment loading at various geometry dimensions of thinning flaw has been investigated based on the results of integral analysis of the pipeline flawed section state.