PSI - Issue 40

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

294 12

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

( I dl C K dN    Ie dl C K dN   e

(23)

) , m 

( (24) where , , , – characteristics of the material and loading conditions determined experimentally; ∆ and ∆ – extents of stress and deformation factors, determined by calculation using expressions (18), (19), (20) for extents of nominal ∆ . For pipe steels = 3,2 − 4,0 and = 2 ÷ 2,2. Durability (service life) at the cracks development stage, estimated using their growth diagrams l based on number of cycles N until fracture by integrating the expressions upon achievement of critical values of с and с , respectively. Permitted durability [ ] is obtained by calculation using the same method, if instead of critical values of and с their permitted values [ ] and [ ] are used in expressions (18) and (19). If, according to the calculation, the permitted number of cycles [ ] is greater than 104, operation of the pipeline section being analyzed can be continued. 6. Making of final decisions based on diagnostics and verification calculation results Data on the design solutions, construction and operation for the pipelines being analyzed is used for elaboration by Lisin et al. (2018), Mahutov et al. (2021) and Neganov et al. (2019) of the diagram of their actual elevation and horizontal passage over L (km), pressure diagram P( τ S ) along the route with consideration of hydrostatic pressure, distribution of critical defective zones (ellipticities, corrosion, cracks), design safety margins n γ , n u, n eu, n k ,n ke and permissible durabilities [ N ] . They are marked with sections with insufficient margins, for which decisions are made on possibility or impossibility of further operation. If no deviations from design solutions are identified during the diagnostic process, it is possible to recommend extension of operation until the next diagnostics following the established norms by SNiP 2.05.06-85 (1986), ASME B31.4 – 2019 (2019), DIN 2470-1:1987-12 (2015) and Giurgiutiu (2014). If dangerous conditions are identified in excess of the design norms and rules based on individual factors (corrosion, development of defects and cracks), one of the three solutions can be recommended: - continue operation until the next diagnostic works with design period determined on the basis of permitted number of cycles [ ] and average (equivalent) frequency of loadings per year; - performance of repair and restoration works at dangerous sections (welding of cracks, installation of reinforcing devices and corrosion protection, replacement of damaged pipes); - total suspension of operation until elimination of dangerous sections or replacement of a pipeline section in case of wide-scale identical damages. In cases when technical diagnostics identifies complex numerous multi-factor combinations of damages (corrosion, cyclic, mechanical), and calculations become difficult to implement, it is recommended to perform bench or field tests of cut-out most damaged pipes with a system of operational defects and additional defects. These tests are performed to determine: - actual bearing capacity of pipes when loaded mainly by internal pressure; - actual durability of pipes when loaded by cyclic internal pressure in combination with bending and twisting, as well as additional impact of corrosive medium. Such complex and expensive tests are the most informative for evaluation of durability of proposed diagnostics and strength and service life monitoring methods. Experience with such tests applicable to oil pipeline transportation is accumulated in our country and abroad. An example of rig tests of over 1000 physical pipes in the initial condition and after extended operation (Fig. 8) has demonstrated applicability of practical used of the approaches described above. ) , e m