In domestic and international regulations the calculation of pipeline durability is performed based on force criterion under stress in form of allowed limit states and allowed limit stresses. This assumes that pipeline level of defect, at the point of its installation, construction and operation, remains within the norm of flaw detection control. However, continuous practice of pipelines operation (up to 40-50 years) demonstrates that these calculations do not exclude the appearance and development of flaw and cracks with sizes not only exceeding the norms, but also leading to loss of tightness and breakdown.
This article covers the analysis problems of the most dangerous states of main oil pipelines for transportation of oil and oil products in the event of longitudinal welded cracks, leading to loss of tightness and further breakdown. Normative methods of durability main calculations based on allowed loads, combined with methods of linear and nonlinear destruction mechanics were used as a basis for analysis. For modern pipelines, manufactured from enhanced plasticity pipe steel, reaching limit states within and outside of crack area happens upon occurrence of developed plastic regions, notably altering rated and local stress strain state. Taking this into account, the necessity and possibility of using a combination of force criterion of linear breakdown mechanics - critical coefficients of load intensity, critical coefficients of deformation intensity within the system of relative parameters was justified and proposed. This allows performing calculations for cases of high rated and local deformation, exceeding elasticity limits by tens and hundreds of times. Calculations show significant increase of limit crack size after transition from brittle to ductile fractures.
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