PSI - Issue 20

Nikolay A. Makhutov et al. / Procedia Structural Integrity 20 (2019) 9–16 Nikolay A. Makhutov et al. / Structural Integrity Procedia 00 (2019) 000 – 000

16 8

0 4 (1 2 / ) (1 / ) R R D R R

p

.

(26)

c cr

n cr

0

For the considered pipeline with crimp p c cr =12.1 MPa. This value is significantly (1,47 times) lower than fracture pressure p c calculated using expression (18) for an undamaged pipeline, but it exceeds the allowable pressure [ p ] calculated through expression (7) by 1.57 times. The presented calculated and experimental data shows that crimp formation can significantly (up to 50%) reduce the static strength of pipelines. The strength tests of pipelines of diameters 720-1020mm damaged by crimps showed that first cracks appear at relatively low nominal stresses σ i = σ c cr <σ y . This coincides with the calculations on the basis of equation (26). The initial cracks appear and develop in a circular direction at the top of the crimp at the inner side of it (Fig. 3a). The fracture caused by crimp at stresses close to yield stress σ y , or higher, is also possible in the longitudinal direction along the weld joint (Fig. 3b) when the crimp height is relatively small, and it is located in the heat-affected zone of the weld. 3. Conclusions Crimps that can be inflicted during the construction, operation and repair of pipelines may significantly (up to 50%) reduce their static strength, mostly due to high stress concentration and plasticity exhaustion. The account of the influence of those damaging factors allows one to evaluate the reduction of the load carrying capacity of damaged pipelines. The threat of crimp formation should be considered even more severe if the reduction of plasticity and the increase of yield limit due to (i) aging, (ii) decrease of the operation temperature, (iii) residual tensile stresses acting in the inner part of the pipe at the crimp top during plastic crimp formation and (iv) residual tensile stresses caused by welding are also taken into account. Cyclicity of loading (especially for oil pipelines) is also an important factor contributing to the increase of risk of fracture of pipelines damaged by crimps. Unfortunately the reduction of load carrying and strain capacity of pipelines damaged by crimps is either ignored or only partially taken into account in the current regulatory and normative basis. This requires that broad efforts be implemented to fill the gap and develop solid design procedures. Acknowledgements This work is financially supported by the Russian Foundation for Basic Research (grant 16-58-48008 IND_omi). References Makhutov, N. at al. 2002. Safety of Russia. Legal, Social, Economic, Scientific and Engineering Aspects. Safety of Pipeline Transportation. Znanie Publishers, Moscow, pp. 752. (in Russian) Makhutov, N., Permyakov, V., 2005. Safe Lifetime of Pressure Vessels and Pipelines. Nauka Publishers, Novosibirsk, pp. 516. (in Russian) Makhutov, N., 2008. Strength and Safety. Basic and Applied Research. Nauka Publishers, Novosibirsk, pp. 528. (in Russian) Makhutov, N., 2017. Safety and Risks: Systemic Research and Developments. Nauka Publishers, Novosibirsk, pp. 724. (in Russian)