PSI - Issue 42

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

1409

4

10 12 14 16

10 12 14 16

3

6

2

1

7

0 2 4 6 8

0 2 4 6 8

S sfz , %

a c , mm

4

5

40 60 80 100 120 140

s , MPa

Fig. 2. The influence of cyclic stresses σ that acted in the working sections of sucker rods (1, 2, 3) and samples from rods (4, 5, 6, 7) with a diameter of 19 (1, 4, 6), 22 (2, 5, 7) and 25 (3) mm, to the critical depth of corrosion-fatigue cracks a c (1-5), which precedes the spontaneous destruction of rods or samples, and to the area of their final or spontaneous failure zone S sfz (6, 7), which were measured at fracture surfaces of samples from rods tested in laboratory conditions (in a 3% NaCl aqueous solution saturated with hydrogen sulfide - dashed lines) and directly on the sucker rods themselves, destroyed during operation (solid lines).

Table 1. The influence of cyclic stresses σ during opera tion of sucker rods in production conditions on the critical depth of corrosion-fatigue cracks a c / d 0 in rods of different diameter. d , mm a c / d 0 at σ , М P а 60 80 100 120 140 25 0.62 0.48 0.40 0.36 0.30 22 0.51 0.43 0.36 0.32 0.27 19 0.40 0.33 0.29 0.25 0.24

The analysis of the constructed curves which are based on the results of production and laboratory tests showed that during the operation of rods with a diameter of 19 mm in wells, the value of a c , determined by their fractures, depended on the applied str ess σ and was 25 – 42% higher than a c in lab, obtained by fractures of laboratory samples. And on rods with a diameter of 22 mm, the critical size of the crack under operational conditions was increased to 36 – 64% (depending on the level of σ), compared to th e measured values on the fractures of the rod samples tested on the laboratory bench (Table 2). Table 2. The ratio a c op / a c lab between the critical values of the lengths of fatigue cracks formed under different cyclic loads σ in rods (or samples) with a diameter of 19 and 22 mm, destroyed in operation a c op and laboratory a c lab conditions.

The ratio a c

op / a

c lab , determited for sucker rods, mm

σ , М P а

19

22

60

7.8 / 5,5 5.6 / 4.2 5.0 / 4.0

1.42 1.33 1.25

11.2 / 6,8 7.8 / 5.4 6.8 / 5.0

1.64 1.44 1.36

100 120

The detected difference between the critical values of the length of corrosion-fatigue cracks could be caused by the difference between the amplitude and equivalent stresses in sucker rods and samples from them, as well as the different loading frequency f in operation (0.1 – 0.2 Hz) and laboratory (15 Hz) conditions. Fatigue tests of samples from sucker rods with technological and operational defects in an aggressive environment with the addition of hydrogen sulfide made it possible to determine the critical dimensions of corrosion-fatigue defects of oil well origin and, on this basis, to effectively reject rods with dangerous damage. From the analysis of the curves of the crack growth rate during the test da/d N , determined on samples from rods with a diameter of 22 mm, depending on the number of load cycles N , it was concluded that their critical state, determined by the crack growth rate curve, was reached when the crack length along the perimeter of the rod and its depth reached 15...16 and 4.5...5.6 mm,