PSI - Issue 5

Jan Kec, Ivo Cerny / Structural Integrity Procedia 00 (2017) 000 – 000

6

Jan Kec et al. / Procedia Structural Integrity 5 (2017) 340–346

345

Strain gauges 19, 20 and 21 show positive increments of deformation values, it suggests a spreading internal surface of the dent and, therefore, a decrease of the dent depth in the dependence on internal pressure inside the pipeline. The increment has reached the saturation state at 10 MPa (strain gauge 19) and at 12 MPa (strain gauges 20 and 21). In case of strain gauges 19, 20 and 21, high deformation values up to 14 000 µm/m were observed. Negative deformation values were found on strain gauges 22 and 23 located at the edge of dent A. At the dent edge, a pressing down of the pipe wall occurs, which is supported by negative values of measured deformations. Above the pressure value 13 MPa, the curve gradient changes. It can be concluded that the dent equilibrium state occurs no longer above this value and the expansion of the pipe profile takes place. Reference strain gauge 24 showed a linear dependence of deformation on pressure up to 14 MPa. The pressure value 14 MPa corresponds with the yield point of the material at biaxial stress state. A rapid increase of deformation was observed above the yield point. The measured values of strain gauges 3 and 4 show that the deformation behaviour of dent B and dent A is very similar. The increment of deformations was found above 7 MPa and the saturation value was reached above the yield point (17 MPa). Strain gauge 5 situated at the edge of dent B showed negative values of deformation up to 14 MPa (yield point of the material). The effect of balancing the dent profile finished above 14 MPa and the expansion of deformations inside the whole cross-section of the pipe took place. Destrution of the body occurred at the pressure of 19 MPa outside the both dents. Fracture initiated at the fusion boundary of the weld patch. The fracture was of woody type with dimple morphology (see Fig. 11).

Fig. 11 (a) Woody fracture; (b) Dimple morphology of fracture

3. Conclusions

In the present work, the effect of dents on the integrity of DN 300 pipeline operating several decades of years is evaluated. The dents were found during the internal inspection of the operator of the pipeline. Two dents of different dimensions and depth were investigated. The most important results can be summarized as follows: - Analysis of the chemical composition showed that the material is 11 353 steel which is commonly used for the manufacture of seamless tubes - The results of tensile test at room temperature and impact bend test at 0 °C comply with the 11 353 steel specification - The base structure in both directions is ferritic-pearlitic with a uniform grain size and some areas with row-like arrangement - In dent A, linear dependences of deformation on pressure were found. It can be concluded that there is no change of the dent profile caused by operating pressure level. In case of dent B, on the contrary, linear dependences of deformation on pressure were not observed and residual values of deformations remained after unloading. The profile (depth) of dent B was changed by the effect of the operating pressure. - During the cyclic pressure test, the profile changes of dent A were observed. These changes occurred in the course of the first 2000 cycles. In dent B, no reaction on pressure cycles was found. - The following pressure test revealed in both dents the decrease of the dent depth in the dependence on internal pressure under the yield point. Thus, there is a tendency to return back to the original circular profile of the pipe. Above the yield point, the gradient of the stress-strain curve changes to positive values and restriction of deformation in all directions takes place up to the failure of the body - Destruction of the pipeline occurred outside both dents at 19 MPa. This value is at least three times higher than that of the operating pressure.