PSI - Issue 40

N.A. Makhutov et al. / Procedia Structural Integrity 40 (2022) 283–295 N.A. Makhutov at al. / Structural Integrity Procedia 00 (2022) 000 – 000

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4. Strength substantiation with verification calculations Data from technical diagnostics and condition monitoring of continuously operating oil pipelines is used for evaluation of strength and remaining service life by DIN 2470-1:1987-12 (2015), Mahutov et al. (2021) and Mahutov (2008). The following three basic design scenarios are used: - pipelines in all inspected sections correspond to the design solutions in strength according to expression (1) – macro-defects in excess of regulated limits are missing; wall thickness is preserved; operation of such pipelines continues; - inspected pipelines have unacceptable dangerous conditions – penetration defects with leaks, loss of stability with buckling, significant distortion of cross-section affecting the capacity – operation of such sections is suspended until restoration measures are taken; - pipelines with intermediate condition according to Fig. 3, b) – they have critical corrosion damages outside of the limits of design solutions (general or local) with wall thickness reduction, cracks (or technological and operational origin), development of extensive ellipticity of cross-section and buckling, plastic deformation caused by excessive loads with increase in the diameter d or local external impacts (dents, grooves). For the latter case verification calculations are performed to substantiate strength and durability for time following the order shown in Fig. 5. For calculations with wall thickness reduction ( ) identified during diagnostics the expression (1) is used with replacement of with ( ) and regulated properties and with ( ) and ( ) , determined using expression (4). The same calculation method can be used for determination of the exact design pressure ( ) for time ( ) 2 ( ) ( ) ( ) , ( ) S S u S S S u p d n n                      (5) It shall not be lower than adopted in the design according to (1). When diagnostics show more complex deviations from the design according to Fig. 3, a) and 3, b) (the calculations take into account reduction of wall thickness ( ) from uniform corrosion, development of ellipticity (with diameters ( ), ( )), ), development of non-uniform pitting corrosion in the base metal and in the weld area, distortion of longitudinal pipeline axis with radius ( ), temperature fluctuations occur ( ) ). For this purpose, using the thin-walled shells theory, all forces in the critical cross-section are determined using Fig. 3, b) – ( ), ( ), ( ), ( ), ( ), ( ). Main nominal stresses { 1 ( ), 2 ( ), 3 ( )} are determined from expression (2) using these forces and pressure p( τ S ) and the thin-walled shells theory and numerical methods (Finite Elements Method – FEM). The resulting stresses are used to determine reduced (equivalent) stresses ( ) on the basis of III theory of tangential stresses or IV energy theory.