PSI - Issue 42

Chmelko et al. / Procedia Structural Integrity 42 (2022) 315–320 Chmelko, V./ Structural Integrity Procedia 00 (2019) 000–000

317

3

Hĺbka defektu z vonkajšej

Deš trukčný

tlak

Deštrukčný tlak DNV-RP

strany plášťa

ASMEB31G [MPa]

F101

[MPa]

50% 75%

12,5 12,4

14,9 14,8

The results show that the used criteria for the limit pressure of the pipe are practically unresponsive to the depth of wall loss for small length in the pipe centerline direction. More realistic results for internal defects in the pipe casing under partial modifications are given by the relations of Kastner [6] and Schulze [7]. However, no prescriptions take into account the influence of other factors such as pipe deviations from the ideal (designed) shape and additional bending in combination with the existence of a small corrosion defects. These aspects will be discussed in this paper.

Nomenclature DN

nominal internal diameter of pipe

D

pipe outside diameter

t l

wall thickness

the length of the corrosion defect in the direction of the pipe axis

p

internal pressure

Rm

conventional ultimate stress

Rm, true true ultimate stress Re yield stress M

strengthening modulus for bilinear model of material

2. Pipe shape inaccuracies The most common geometric inaccuracies observed on pipelines in operation are deviation from circularity (ovality) and non-constant wall thickness. When loaded with internal pressure in operation, there is a straightening of the shape into an ideal circle. This effect causes the distribution of maximum stresses to the locations with the largest bending stresses, as documented in Fig. 3. The pipe used has a deviation of 4% from the prescribed DN1200mm diameter in the horizontal direction (this is the largest allowable ovality of gas pipelines) and a wall thinned at the top edge by 1mm (the prescribed wall thickness is 13mm). In Fig.3 a, b there is always a pipe with a diameter deviation on the left and a thinned wall on the right.

Fig. 3. Pipe with shape imperfections loaded by internal pressure at the level of yield stress (a) displacements; (b) stresses.

In case corrosion defect occurs at the location of maximum deformations and stresses caused by deviations from the ideal shape, a synergy of these effects occurs. Fig. 4 numerically expresses the effect of shape imperfections on stresses