PSI - Issue 42

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

1410

5

respectively (Fig. 3). These crack lengths were recorded at stresses of 100 MPa, which are commensurate with the operating loads of the rods in field. For rods of other standard sizes, the critical depth of the crack, which precedes the almost spontaneous destruction of rod samples, was 3...4 mm (for rods with a diameter of 16 mm), 4...5 mm (19 mm) and 5...6 mm ( for 25 mm). Thus, the conducted research made it possible to substantiate the critical sizes of cracks in rods of various standard sizes, the detection of which is available using the applied control tools during the reassembly of sucker rod columns. And although the defects of a smaller depth were not revealed by the applied method of controlling damage to the rods, the margin of durability caused by the development of damage smaller than 1...2 mm to a critical value made it possible to guarantee the resource of safe operation of the sucker rod columns until the next stage of their rearrangement.

-6

10

2 3

1

10 -7

da / dN , m/cycle 10 -8

10 -9

2 4 6 8 10 12 14

N, 10 -7 cycles

Fig. 3. Dependences of corrosion-fatigue cracks growth rates da / dN on the number of load cycles N in steel 20N2M of sucker rods with a diameter of 22 mm (without (1) and after (2, 3) treatment of their surfaces by rotating metal brushes), tested at stresses of σ = 100 (1, 2) and 150 (3) MPa in 3% NaCl water solution with additional saturation (1, 3) or without saturation (2) with hydrogen sulfide. It is also important to establish the residual life t rez of rods of different diameters in the presence of operational defects in them. It turned out that with comparable defect sizes, the life of rods with a diameter of 19 mm decreases more than rods with a diameter of 22 mm. In particular, the resource of pre-serviced sucker rods with a diameter of 22 mm reaches 60...70% of the resource of new ones without such damage. Whereas on rods with a diameter of 19 mm, only 25...40% of the resource of undamaged rods can be achieved. The reason for this difference is the smaller residual cross-section of rods of smaller diameter due to defects that arose during their operation in the well. Residual life ( t rez ) of sucker rods with fatigue damage detected by non-destructive testing was predicted according to the expression:

ൌ ∙ − Ͳ − Ͳ ൅ ͳ − ͳ

(1)

where t – is the duration of tests at the time of assessment of the residual resource; V – is the rate of slow (pre critical) crack growth with initial length a 0 < 2 mm; V x – is the crack growth rate at the moment of predicting t rez ; a c – is the critical length of the crack, at which the growth rate increases sharply ( a c = 3...6 mm, Fig. 2); a – is the depth of the crack after its propagation during time t . It was experimentally established that the maximum increase in the crack growth rate during the period of its pre critical propagation is achieved at the ratio V x / V = 1.5. For example, using the proposed expression, the residual life of a sucker rod with a diameter of 22 mm was estimated, in which the crack propagated to a length a = 3 mm during t = 12 months.

ൌ ͳʹ ∙ ͲǤ͸͹ ∙ ͷ − ͵ ʹ − ʹ ൅ ͳ − ͳ ൌ ͺǤͺ ‘–Š•

(2)

Thus, after 12 months of operation in a well of a sucker rod with a diameter of 22 mm with an initial technological defect with a depth of a 0 = 2 mm, the length of the defect in the rod increased to 3 mm. Then,