PSI - Issue 18

Kostina A. et al. / Procedia Structural Integrity 18 (2019) 293–300 Author name / Structural Integrity Procedia 00 (2019) 000–000

298

6

The results of the performed numerical simulation of artificial freezing and thawing are used for strength analysis of the steel freezing pipe. The mechanical characteristics of the pipe are listed in Table 3. The geometry of the pipe is described by a hollow cylinder. The pipe diameter is 1.46×10 -1 m, a thickness of the pipe wall is 1.1×10 -2 m. The analysis is performed in the axisymmetric configuration.

Table 3. Mechanical properties of the freezing pipe K, Pa G, Pa UTS  , Pa y  , Pa 1.67ꞏ10 11 7.7ꞏ10 10 9ꞏ10 9 7.5ꞏ10 9

It has been suggested that failure of the pipe could be induced by an abrupt rise of the mechanical pressure p at the interfaces of the soil stratums. The maximum value of the pressure leap is obtained when p increases from 1 60 p  MPa to 2 240 p  MPa. It is observed during soil freezing at the interface between stratums of sand and clay. In Fig. 4(a) distribution of a damage parameter in the freezing pipe wall is shown. An initial value of the parameter is equal to 0. If the failure criterion (16) is fulfilled, the parameter equals 1. From Fig. 4(a) it can be seen that for the considered values of pressure the freezing pipe does not damage. It has been established that a crack in the pipe occurs if p increases from 60 MPa to 640 MPa (Fig. 4(b)).

a

b

Fig. 4. Distribution of the damage parameter in the pipe wall subjected to external pressures 1 60 p  MPa, 2 240 p  p  MPa (b). If damage is absent, that the parameter equals 0 (blue color). If the failure criterion (16) is fulfilled, that the parameter equals 1 (red color). In an operational process the freezing pipe is corroded. As a result, a thickness of the pipe wall could reduce to 30%. A numerical simulation has been performed to determine of an influence of corrosion on integrity of the pipe. Two position of the thinned pipe wall have been considered. In the first case the thinned wall is simultaneously subjected to pressures 1 60 p  MPa and 2 240 p  MPa. In the second case the pressure peak occurs at the location where the wall thickness decreases. At that the larger pressure 2 p acts on the thinned wall of the pipe. In Fig. 5 distributions of the damaged parameter are presented. If the pressures 1 p and 2 p simultaneously act on the thinned wall, the pipe does not damage (Fig. 5(a)). In the second case a stress concentration induces a crack initiation (Fig. 5(b)). Therefore, the failure of the freezing pipe arises if the pipe is corroded at the interface of two layers with different material characteristics. A critical situation for integrity of the freezing pipe could originates if the pipe does not fully contact to soil. MPa (a) and 1 60 p  MPa, 2 640

a

b

Fig. 5. Distribution of the damage parameter in the pipe wall that has been thinned due to corrosion. (a) the thinned wall is simultaneously subjected to pressures 1 60 p  MPa, 2 240 p  MPa; (b) the pressure drop from 1 60 p  MPa to 2 240 p  MPa occurs at the location where the wall thickness decreases. Blue color corresponds to the value 0 of the damage parameter, red color corresponds to the value 1 of the damage parameter.