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

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Various local damages of LDP leading to drastic decrease of their strength and lifetime will be analyzed below. Potentially dangerous in terms of crimp formation are heated and fixed zones of the pipelines, arches, floating up zones. For example, on the northern gas pipelines of the Russian Federation one could observe arches that are 80 150 meters long, deflections whose length is up to 500 meters, floating up sections on the length of up to 10,000 meters. Crimp formation can occur in the process of LDP component fabrication and transportation or during LDP construction, operation, maintenance and repair. The size of crimps can be compared with the thickness of the pipeline wall and its diameter. Symmetrical and asymmetrical crimps located within the zones of welded joints as well as combination of crimps and welded joins are a specific threat since they change the geometrical form of the pipeline and mechanical properties of its material, and can trigger the pipeline fracture. Height h , external radius R ex , internal radius R in , the difference between them Δ R = R ex - R in , and the fold opening l are the main geometrical characteristics of crimps (Fig. 1) . In operating pipelines the crimp characteristic l / h is relatively constant and depends on the relation δ / R 0 (where R 0 is an internal radius of the pipeline, δ is the wall thickness) and the extent of crimp development Δ R / R 0 . The maximal (statistical) value of l / h when the fracture of the pipelines goes on is in the range of 2.5 - 4. This value does not depend much on the pipeline diameter D 0 = 2R 0 . A crimp on the straight or elastically bent part of the pipeline may occur in the constraint places or in the zones of heterogeneity of the physical and mechanical properties of the pipe material. A circular weld joint is also a zone of such heterogeneity. A reinforced weld joint has a higher rigidity and usually (if it is free from macro defects) proves to be stronger than the base metal.

Fig.1. Geometrical characteristics of crimps

Fig. 2. Deformation curve in true coordinates and its parameters

Crimp formation is possible in the bent segments as well. Geometrical nonlinearity of the pipe shape, nominal and local plastic deformations of the wall and residual stresses appear after a cold bending. The combination of those factors as well as local damages that can occur during manufacturing, construction or operation can result in the crimp formation in this part of the pipe. A symmetrical circular crimp is a comparatively rear phenomenon. Its formation is usually caused by temperature drop and axial compression that may result from pipeline constraint. In this case the main displacement of the pipeline occurs along its axis only. The widely occurred semicircular crimp is formed due to the combined action of longitudinal forces and bending moments with the possibility of the in-plane and out of plane displacement of the pipeline. The deformation then develops in the weakest zones and is localized as asymmetric crimps. The crimp is formed in the compressed zone of the pipe on the inner radius of the bent. Fracture of pipelines in the zones of crimps can be caused by both static and cyclic loads. The pipe wall in the crimp zone is subjected to significant local plastic deformations under the action of all force factors, which results in the change of physical and mechanical properties of the pipe metal that are commonly neglected in traditional