EFFECTS OF COMPRESSIVE RESIDUAL STRESS INTRODUCED BY HYDRROSTATIC TESTING

Hydrostatic testing is the primary operational measure for eliminating major axial defects in pipelines. Since hydrostatic tests can be performed at pressure levels of 125% to 140% of the maximum operating pressure, the critical defect size at hydrotest pressure is smaller than that associated with normal service conditions. Because of this difference, hydrostatic testing provides a safety margin against subsequent service failure. In order to evaluate quantitatively the effects of hydrotesting on SCC growth behaviour, two independent test programs were carried out, one using pre-cracked CT-type specimens [7] and the other using an X-52 full-scale pipe [20]. In both cases, SCC growth was started by applying cyclic loading and a high load excursion was applied to simulate a field hydrotest event. Following the excursion, the SCC growth rate was measured again for some time until reliable, consistent growth rate data could be obtained. Figure 8 [20] shows a comparison of the crack growth rates for fifteen cracks before and after a hydrotest performed on a full-scale pipe. The highest pressure reached during the hydrotest equaled 108% of the yield stress of the line pipe. All cracks showed detectable reduction in growth rate after the hydrotest. Before the first hydrotest, three cracks showed growth rates in the order of 2.0*10-3 mm/day or about 0.73 mm per year. The highest growth rates of all 15 cracks, of depths generally between 35 to 50% of the wall thickness of the pipe, was about 0.8*10-3 mm per day after the test. In fact, two cracks became practically dormant, and their growth rates were not measurable by the crack detection [DCPD] system. It has been argued that hydrotesting could significantly increase the crack tip radius, thus reducing the effective mechanical driving force for subsequent SCC growth. However, in the full-scale study, metallographic examination suggested this is not the case. Most of the nine cracks examined metallographically following the test program had a crack tip opening of a few microns, usually less than 5 microns. Therefore, the crack was essentially a sharp one for

Figure 8 Effects of Hydrostatic Testing on SCC Growth Rates [20]

practical purposes. Again, the effect of hydrogen or the corrosion environment on the behaviour of a crack during and after the overload remains unclear. In one recently reported study using A537 steel (yield strength 380 MPa) [21], the behaviours of a fatigue crack during and after a single overload in air, in a 3.5% NaCl solution at the free corrosion potential, and in the same solution but under cathodic polarization were all different. Whereas the instantaneous crack extension upon the overload was significantly greater when the steel was under cathodic polarization, the overall overload retardation zone was much smaller when the steel was tested in the salt solution than in air. The embrittling effect of hydrogen was surmised by the authors to be the reason for this observation.

In the case of linepipe steel in near-neutral pH environment, the retarding effects of hydrotesting on SCC growth may be a result of the creation of compressive residual stress in front of the crack tip. It is well-known that a compressive region is generated at a crack tip by overloading; the compressive stress can be as large as the yield stress [22].

CONCLUSION

The following conclusions can be drawn from the preceding discussions:

1. Depending on the surface geometry of the pipe, the net total stress available for the initiation and growth of stress corrosion cracks may be considerably greater than the nominal operating stress as the presence of residual stress and stress raisers contribute to the local stress.
2. In the laboratory tests carried out using cyclic loading with the maximum load below the yield stress of the steels, stress fluctuation is required for crack initiation and growth. The crack growth rates are found to increase with the time rate of J on a log-log plot.
3. When a linepipe steel is stressed close to its yield point in a susceptible environment, cracks may develop with very minor pressure fluctuation. In these cases, low-temperature creep can be a factor in generating the necessary plastic straining and the presence of hydrogen in the steel may facilitate this creep process.
4. Hydrostatic testing retards subsequent crack growth. It is probable that compressive residual stress plays a key role in the retardation. Hydrogen effects may also be involved.  

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