PSI - Issue 42

Halyna Krechkovska et al. / Procedia Structural Integrity 42 (2022) 1406–1413 Halyna Krechkovska / Structural Integrity Procedia 00 (2022) 000 – 000

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3

composite rods were also examined. 3. Results and discussion

Loss of performance of sucker rods often occurs as a result of broken columns due to fatigue. At the same time, most often various defects, which act as stress concentrators, become zones of initiation of fatigue destruction. Their influence on resistance to fatigue failure of sucker rods becomes especially dangerous during operation in technological environments, which include aggressive components (in particular, hydrogen sulfide). Therefore, it is important to evaluate the impact of operational and technological defects detected in rod samples by non-destructive testing methods (DUK-6B) on the resistance of sucker rods to fatigue failure. We analyzed the influence of technological (formed at the stage of production, transportation and installation of rods) and operational (formed during operation under real factors of influence during oil production) defects on the resistance to corrosion destruction of sucker rods with a diameter of 19 and 22 mm made of 20N2M steel (Fig. 1). It turned out that, in general, surface technological defects have a weak effect on the resistance of sucker rods to corrosion-fatigue destruction. Only a decrease in the limit of endurance and durability of sucker rods as their diameter increases was established. In particular, with the number of load cycles N = 10 -7 , the endurance limit for rods with a diameter of 19 mm was 95 MPa, and for rods with a diameter of 22 mm – 85 MPa. Operational defects in rod samples, such as fatigue cracks, corrosion damage, and pitting were formed in the rods even during their long-term operation in wells. And these defects more intensively reduce the fatigue life of rods under the influence of an aggressive environment. Fatigue durability analysis showed that rods with operational defects had significantly lower fatigue failure resistance compared to rods with technological defects.

175

150

2 3 4 1

125

100

s , MPa

75

50

10 7

10 5

10 6

N , cycles

Fig. 1. Corrosion fatigue life of 20N2M steel during tests in a 3% aqueous solution of NaCl, saturated with hydrogen sulfide, of sucker rods samples with a diameter of 19 mm (1, 2) and 22 mm (3, 4) with technological (1, 3) or detected by a non-destructive method control of operational (2, 4) defects. Macrofractures of rods destroyed in the well and samples from rods tested in laboratory conditions were analyzed, on which the zones of crack initiation, its gradual spread to the critical size and spontaneous destruction were identified. It is clear that it is necessary to reject sucker rods according to the length of the crack at the stage of its pre-critical growth. Therefore, it is important to justify the critical size of the crack (the depth of its penetration into the working section of the rod) depending on the critical depth of corrosion-fatigue cracks a c in the rods and the corresponding areas of the fracture zones S sfz (spontaneous failure zone) from the applied cyclic stresses σ are presented in Figure 2. These curves obtained from the analysis of relevant data for field operated rods with a diameter of 19, 22 and 25 mm (solid lines in Fig. 2), destroyed in operational conditions, and for samples from rods with a diameter of 19 and 22 mm, destroyed in laboratory conditions (dashed lines in Fig. 2). The obtained results of measurements of different zones on the fracture surfaces of rods of different diameters were described by curves, which made it possible to show that the critical size of corrosion fatigue cracks formed in production conditions under the action of low stresses of 60 MPa increased from 40 to 62% for changes in the diameter of the rod from 19 to 25 mm, respectively, and under the action of stresses twice as high (about 120 – 140 MPa), the range of a c / d 0 changes decreased to 24 – 36% (Table 1).