Effects of Hydrostatic Testing on the Growth of Stress...

International Pipeline Conference — Volume IASME 1998

Effects of Hydrostatic Testing on the Growth of Stress-Corrosion Cracks

W. Zheng, W.R. Tyson, R.W. Revie, G. Shen, J.E.M. Braid CANMET/Materials Technology Laboratory

568 Booth St.Ottawa, Ontario, Canada Kl A 0G1

Abstract

Two hydrostatic tests were carried out on an X-52 (Grade 359) pipe containing sixteen cracks of depths up to 55% wall thickness. Stress corrosion cracking (SCC) growth rates were measured in full-scale tests performed before and after the first hydrotest in order to demonstrate the effects of hydrostatic testing on subsequent crack growth rates. The effects on crack tip deformation were investigated by metallographic examination of crack cross-sections immediately following the second hydrotest.

The SCC tests were performed using a saw-tooth type load spectrum with the maximum stress set at 95% of the actual (as opposed to specified minimum) yield strength of the linepipe and R = 0.8. During the first hydrotest, the maximum applied stress was 108% of the yield stress; the total hoop strain in the pipe body reached about 0.2%, which is less than would have been reached in a uniaxial tensile test at this stress level because of the effect of the biaxial stress state in the pipe. The highest SCC growth rate measured before the first hydrotest was about 0.88 mm per year (2.4 * 10° mm/day), and the growth rate of the same crack after this hydrotest was about 0.37 mm per year (0.79 *10° mm/day). The other cracks all showed varying degrees of reduction in growth rate.

Post-mortem examination indicated that the hydrotests did not cause significant crack blunting. The beneficial effects of hydrotests are attributed primarily to the presence of compressive residual stresses in the heavily deformed region in front of the crack tip.

The majority of cracks showed some growth during the first hydrotest. SCC growth and growth during the hydrotests are associated with distinctly different microscopic features on the fracture surface.

© Minister of Natural Resources, Canada, 1998.

1. Introduction

Hydrostatic testing is the primary method for demonstrating the mechanical serviceability of a pipeline. For pipelines affected by SCC, repeated hydrostatic testing may offer an effective mitigation tool, as it eliminates major axial defects having dimensions greater than the critical size. Since hydrostatic tests are performed at pressure levels equivalent to 125% to 140% of the service pressure level, the critical defect size at hydrotest pressure is smaller than that associated with the operating pressure. Therefore, a safety margin is provided against service failure for the defects that survive a hydrotest, notably those in the axial direction [1].

However, the effects of repeated hydrostatic testing on the long-term growth behaviour of stress corrosion cracks are still not clearly understood. For cracks of sufficiently large size, the possibility of growth during a hydrotest exists, and repeated hydrotests may cause cumulative crack extension in addition to that occurring during service. On the other hand, the high stress level of a hydrotest may retard crack growth, as a result of crack blunting and compressive residual stresses in the metal ahead of the crack tip [1]. Systematic studies on these issues are lacking.

The primary objective of this project was to evaluate the effects of hydrotesting on the SCC growth behaviour, by comparing the growth rates of individual cracks under identical loading conditions before and after a hydrotest.

2. Experimental Details

The linepipe used for this work was a 24-in. diameter Grade 359 (X-52) pipe with a wall thickness of 6.4 mm. The pipe was buried in a clay-type soil. The actual yield and ultimate tensile strengths of the linepipe material were 421 and 538 MPa, respectively. The coating conditions, crack lengths,

IPC1998-2053

Downloaded From: http://proceedings.asmedigitalcollection.asme.org/pdfaccess.ashx?url=/data/conferences/asmep/89945/ on 05/19/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use

and depths of the sixteen cracks on the pipe at the beginning of the first hydrotest are given in Table 1.

Two hydrotests were carried out on the same pipe, the first on February 25, 1997 and the second on April 29, 1997. The conditions for the SCC test and for the hydrotests are given in the following sections.

SCC Tests. SCC growth was measured at the beginning of the test period in which the maximum applied pressure of the sawtooth-shaped load spectrum was 1210 psi, corresponding to about 95% of the actual yield stress of the pipe, with R = 0.80. The static hold time in a load cycle (S) was 5 min, and the sawtooth part of the load cycle (Dyn.) was 5 min. This load condition was applied for 28 days; subsequently, the static hold time was reduced to 1 min while the other load parameters remained unchanged, i.e., P = 1210 psi, R = 0.8, S = 1 min. and Dyn = 5 min.. This test condition was maintained for 42 days, when the first hydrotest was performed.

The first hydrotest During the first hydrotest the pressure was first dropped from 1210 psi to 0 psi, then increased, in steps, to 500, 1000, 1205, 1265, 1285, 1315, 1335, 1354 and finally 1360 psi; the pressure history is shown in Fig. 1. The hold time for each pressure level up to 1354 psi varied from 2 to 20 min. At the maximum pressure of 1360 psi, the total amount of strain in the pipe, as measured from a strain gauge oriented in the hoop direction of the pipe, reached about 0.2%. The pressure-strain relationship for the test pipe measured during the first hydrotest is shown in Fig. 2.

After reaching 1360 psi, the pressure was held constant for one hour, then lowered to 1210 psi, and SCC testing was resumed. The load spectrum used in the pre- hydrotest period (P = 1210 psi, R = 0.80, S = 1 min., Dyn = 5 min) was then reapplied, and the pipe was monitored for SCC growth for 57 days until April 25, 1997, when the SCC test was terminated. The liquid in the soil box was then drained, and the crack sites were flushed with ethanol. On April 29, 1997, the second hydrotest was performed.

The second hydrotest During the second hydrotest, the pressure was initially increased from 0 psi to the previous hydrotest pressure level. The pressure-time plot for the second hydrotest is also shown in Fig. 1. Since no significant crack growth was measured by DCPD, the pressure was subsequently increased to 1376 psi, 1392 psi and finally to 1395 psi. It was intended to hold the pressure at 1395 psi for one hour, but rapid growth and leak of one crack, 12BD, caused the second hydrotest to be terminated after about 15 min of pressure hold.

3. Test Results

Figure 3 shows a typical growth curve of the cracks on the pipe. It shows the depth of the semi-elliptical crack at its maximum penetration point (mid-length) before the first hydrotest, during the first hydrotest, after the first hydrotest and during the second hydrotest.

The growth rates of cracks before and after the first hydrotest were obtained by linear curve-fitting of plots of crack growth versus time such as that shown in Fig. 3. The slope of a fitted line in Fig. 3 is the growth rate in mm per day for the respective test period. Table 2 summarizes the growth rates of the fifteen cracks that were monitored. It should be noted that there is an experimental scatter of up to 25% in the crack growth data obtained using the DCPD system.

Figure 4 shows a comparison of the crack growth rates for the fifteen cracks before and after the fust hydrotest. Before the first hydrotest, three cracks showed growth rates in the order of 2.0* I O'5 mm/day or about 0.73 mm per year. All other cracks grew at a rate less than 1.0 *10° mm per day or about 0.37 mm per year. After the first hydrotest, all 15 monitored cracks showed growth rates below 1.0*10'3 mm per day. It is evident that the hydrotest had a retarding effect on the growth of all cracks, since the growth rates for all cracks were significantly lower after the hydrotest. In fact, two cracks (6BD and 9FD) became practically dormant and their growth rates were not measurable by the DCPD system.

It is interesting to note that the most remarkable reduction in growth was achieved in the three cracks that, before the first hydrotests, were growing at rates above 2.0* 10‘3 mm/day. Cracks 12BS and 12BD showed the least reduction in growth rate.

The effect of a hydrotest to reduce crack growth rates lies mostly in its role in creating a compressive residual stress ahead of the crack tip and in causing microscopic crack branching; these points are further discussed in Section 3.2.

Some crack growth was detected during the first hydrotest using the DCPD system. In general, the amount of increase in DCPD output during the hydrotest increased with the crack depth, although there was some variability in the data. However, it should be noted that the increase in the DCPD output on increasing the test pressure resulted from not only the extension of the crack but also the increase in the level of plasticity in the metal ahead of the crack tip. Consequently, there remains some uncertainty concerning the correlation between the DCPD changes and the physical crack growth under conditions of increasing pressure. Some calibration


work is planned to differentiate the relative contributions of crack length change and plastic deformation.

During the second hydrotest, no increase in DCPD output was detected until the pressure was increased to 1376 psi (see Fig. 1 for the load sequence applied). For Crack 12BD (Table 2), most of the 2.52 mm growth occurred about 13 min after the pressure had reached 1395 psi, i.e., during the last few minutes of high pressure testing.

3.2 Post-Mortem Examination

The main objectives of the post-mortem analysis and examination, using metallographic and fractographic techniques, were: (1) to study the cracking mode and the crack tip geometry; and (2) to examine the fractographic features produced by hydrotesting, in contrast to those associated with normal SCC growth.

3.2.1 Cracking Mode and Crack Tip Geometry

Figure 5 shows the cross-section metallography of Crack 3BS. It can be seen that, during the first hydrotest, ductile crack growth (marked by “1st HT” in Fig. 5) initiated on a plane about 45 degrees from the original plane, i.e. close to the plane of maximum plastic strain. However, consistent with macroscopic crack growth perpendicular to the applied stress, the crack took a somewhat zig-zag path. Some crack branching also occurred under the high load.

Between the two hydrotest events, a small, relatively flat segment of growth occurred as shown in Fig.5 (“SCC”). To confirm that this flat segment was caused by slow SCC growth between the hydrotests, SEM fractographs were taken of a site in the vicinity (in terms of its position along the crack length) of that shown in Fig. 5, and are shown in Figs. 6 through 8. Figure 6 is the overall tip region. Note that the brittle cleavage facets on the top were produced, in the postmortem stage, by fracture at liquid nitrogen temperature. The rough zone immediately below the cleaved region was a result of ductile crack growth during the second hydrotest, and it involved some micro-void formation, as shown in the magnified view in Fig. 7. One can also see, from Fig. 7, that there is a small relatively “brittle” zone below the second hydrotest marking. The quasi-cleavage characteristics of this “brittle” zone, clearly illustrated in Fig. 8, suggest that this is the SCC growth produced after the first hydrotest. The first hydrotest caused an extension of the crack that is somewhat out of plane with the SCC zone on the fracture surface, in agreement with the 45° cracking seen in Fig. 5.

It was observed, during metallographic examinations, that not all cracks showed the same amount of blunting as that in Fig. 5. Figure 9 shows the tip region of another crack, Crack

3BD. The tip is very sharp. Such cracks cannot be regarded as blunted cracks.

3.2.2 Fractographic Examinations

Fracture surfaces were examined using the scanning electron microscope (SEM) to study the mode of crack growth under SCC and hydrotest conditions. Since the fractography of various samples was very similar for the same loading condition, a typical fracture surface is used here.

Figure 10 shows a montage of SEM fractographs, taken at 324x, of Crack 3BD. Crack growth during the second hydrotest, the fust zone below the liquid-nitrogen cleavage fracture, took place mainly by micro-void formation, as indicated by the presence of dimples that are clearly visible at this magnification. As described in Section 2, all crack sites were flushed with ethanol prior to the second hydrotest, and therefore there would be minimal environmental contribution to growth during this hydrotest In contrast, the SCC growth prior to the second hydrotest and prior to the first hydrotest, indicated by Letters “A” and “C” in Fig. 10, is much flatter and more brittle. Close-ups of these SCC zones are shown in Figs. 11 and 12, respectively. The SCC zones before the second hydrotest (Fig. 11) and before the first hydrotest (Fig. 12) are similar in their quasi-cleavage appearance, but differ in the amount of corrosion product still remaining and in the degree of surface etching by the soil liquid, reflecting the two-month time lag between the two hydrotests.

Special attention was paid to the morphology of the crack growth that occurred during the first hydrotest, for comparison with that of the second hydrotest. While the ductile dimples are readily visible on the fracture surface of the second hydrotest, they are not as obvious on the surface of the first hydrotest. Figure 13 was taken from the area indicated by the Letter “B” in Fig. 10, and it appears to show that there was a significant amount of shear, on the 45 degree planes. The micro-voids are indeed visible at 400x but smaller in size in comparison to those formed during the second hydrotest. The fact that the crack took a zig-zag path during the first hydrotest suggests that the growth process was essentially ductile in nature, although the hydrogen produced by the corrosion reaction of iron with water may have played a role in the ductile growth process. Hydrogen has been found to enhance ductile crack growth as well as to promote cleavage [2,3].

4. Discussion And Conclusions

The experimental results obtained from this project suggest that high-pressure hydrostatic testing is beneficial for reducing the growth rate of stress corrosion cracks in a pipeline, although the extent of reduction in growth rates may vary from one crack to another, reflecting the variations in the


local environmental and metallurgical conditions. The generic applicability of this conclusion is supported by the fact that none of the fifteen monitored cracks showed accelerated growth after the first hydrotest.

It has traditionally been thought that hydrotesting could significantly increase the crack tip radius, thus reducing the effective mechanical driving force for subsequent SCC growth. However, the results of metallographic examination suggest this is not the case in this study. In most of the 9 cracks examined metallographically, the crack tip opening (following the second hydrotest) was usually less than 5 pm. Therefore, the crack was essentially a sharp crack for practical purposes. The retarding effects of hydrotesting on SCC growth are more likely a result of the creation of compressive residual stress in front of crack tip. It is well known that such compressive regions are generated at crack tips by overloading; the compressive stress can be as large as the yield stress [4].

5. Acknowledgments

The authors acknowledge the contribution of all CANMET/MTL personnel involved in the project. This work is supported by the Federal Interdepartmental Program of Energy R&D (PERD) and by a group of industrial participants consisting of the following members:

AEC Pipelines, a Division of Alberta Energy Co. Ltd. Alberta Natural Gas Company Ltd.British Gas Holdings (Canada) Limited Foothills Pipe Lines Ltd.Interprovincial Pipe Line Inc.Mobil Oil Canada NOVA Corporation of Alberta Pembina Corporation TransCanada PipeLines TransGas LimitedTrans Mountain Pipe Line Company Ltd.Westcoast Energy Inc.

6. References

1. National Energy Board, “National Energy Board Report of the Inquiry on Stress Corrosion Cracking on Canadian Oil and Gas Pipelines”, Report # MHW-2-95, Nov. 1996.2. J. Maier, W. Popp and H. Kaesche, “Hydrogen Effects on Cyclic Deformation Behaviour of a Low Alloy Steel”, p 343 in H ydrogen E ffects in M ateria ls, Ed. A.W. Thompson and N.R. Moody, The Minerals, Metals and Materials Society, 1996.3. J.P. Hirth, “The Role of Hydrogen in Enhancing Instability and Degrading Fracture Toughness in Steels”, p

507 in H ydrogen E ffects in M ateria ls, Ed. A.W. Thompson and N.R. Moody, The Minerals, Metals and Materials Society, 1996.4. T.L. Anderson, “Fracture Mechanics: Fundamentals and Applications”, 2nd Edition, CRC Press, 1995.


Table 1 - Crack Sizes and Coating Conditions

IDAxial

position (in)Length(mm)

Coating

condition

Crack depth (DCPD, mm)

Crack depth (actual, mm)

12FS 17 36 Coated 2.55 2.28

12FD 21 46 Coated 3.45 3.70

12BS 27 36 Free surface 2.33 2.26

12BD 31 46 Free surface 3.51 3.50

3FS 17 36 Coating 2.80 2.70

3FD 21 46 Coating 2.51 **

3BS 27 36 Free surface 2.02 2.38

3BD 31 46 Free surface 3.43 3.35

6FD 40 46 Coating 2.90 **

6FS 44 36 Coating 2.00 2.10

6BD 50 46 Free surface 2.90 3.18

6BS 54 36 Free surface 2.00 2.14

9FDWeld 40 46 Free surface 3.06 **

9FSWeld 44 36 Free surface 1.99 **

9BD Weld 50 46 Coating 3.10 3.24

9BSWeld 54 36 Coating 2.30 1.89

♦♦these crack depths are estimated from the DCPD output, as the cracks have not been cut from the pipe material (for future reference use, as per discussion with the sponsoring members o f this project).


IDLength(mm)

Crack Depth Before 1st HT

(mm)

GrowthRate Before 1st HT

(*10'J mm/day)

Growth Rate after 1st HT (*10°

mm/day)

Crack Depth before 2nd HT

(mm)

12FS 36 2.28 2.4 0.79 2.46

12FD 46 3.70 0.90 0.36 4.05

12BS 36 2.26 0.31 0.24 2.41

12BD 46 3.50 0.26 0.19 3.88

3FS 36 2.70 0.53 0.34 2.79

3FD 46 2.51** 0.58 0.19 2.59

3BS 36 2.38 0.75 0.51 2.55

3BD 46 3.35 1.10 0.65 3.65

6FD 46 2.90** 1.97 0.35 3.17

6FS 36 2.10 0.30 0.10 2.23

6BD 46 3.18 0.67 NM 3.34

6BS 36 2.14 0.28 0.17 2.22

9FDW eld 46 3.06** 0.47 NM 3.24

9FSWeld 36 1.99** 0.07 0.04 2.06

9BD Weld 46 3.24 1.98 0.43 3.44

9BSWeld 36 1.89 NM NM NM

NM:Not measurable. ** Based on DCPD M easurement only.


Pres

sure

(psi

)

Figure 1. Test pressure applied to the pipe during the first and the second hydrotests.

Strain

Figure 2. The pressure-strain curve measured during the first hydrotest.


Crac

k G

row

th R

ates

(*10

‘J m

m/d

ay)

vvw* i------¡ i i ;i IL __l___ ----- ------j------------1 - - | ; i__________ 1__________!__________1__________ i____i____1___ i____i___

10 20 30 40 50 60 70 80 90 100 110 120 130 140

Time (days)♦Dec. 18. 1996

Figure 3. Growth behaviour of Crack 12FD during SCC tests and the hydrotests.

<r> <P <& <£> <r> <8> oP <& <r> oP ¿r <»-

Crack ID¿0 Jsr o»

<9 # <§F

*

4

Figure 4. Comparison of SCC growth rates before and after the first hydrotest


Figure 5. Cross-section of Crack 3BS showing the crack tip region.

Figure 6. SEM ffactograph showing the overall crack front region of 3BS.


Figure 7. Close-up view o f the fracture surface produced by the two hydrotests and the SCC growth between them.


Figure 8.

Figure 9.

The area indicated by the arrow in Figure 7 showing the quasi-cleavage features o f the SCC zone.


470

Fig. 10. Montage o f SEM fractographs showing the microscopic features o f SCC and ductile growth


;,^;y'S‘Spof:.Magn :0er\WD'; Exp;:'' i — ~ I 20 um-SO-.;- ^80-Pipeg3 ;

Figure 11 .Close-up view of the interface between regions of SCC and ductile growth caused by SCC and the second hydrotest, respectively.

Figure 12. Close-up view of a SCC region before the first hydrotest.


Figure 13. Close-up view o f the surface morphology produced by the first hydrotest.


Effects of Hydrostatic Testing on the Growth of Stress...

Documents

Transcript of Effects of Hydrostatic Testing on the Growth of Stress...