Achieving Maximum Crack Remediation Effect From Optimized Hydrotesting

Abstract

Hydrostatic testing is a key method for managing SCC in oil and gas pipelines. Benefits are achieved by eliminating defects above a critical size for the hydrotest pressure and hence achieving a post-test period without operating failure. Other benefits are related to temporary growth retardation after hydrotest because of crack tip blunting. Conversely, benefits of such a test could be offset by stable flaw growth in the previously dormant population and growth of cracks during hydrostatic loading. Although this type of growth behaviour has been previously analyzed, it was only assessed from tests in air, which neglects the effects of corrosive environments.</jats:p> <jats:p>Recent research has shown that crack growth can occur during hydrotests at much smaller crack dimensions than those originally analyzed. The adverse effect of hydrotesting is negligible if it initiates crack growth only on large-size cracks that are near the final stage of pipeline life. However, benefits of hydrotest would become uncertain if hydrostatic loading induces crack growth when crack dimensions are relatively small. This reduction in pipeline remaining lifetime by hydrostatic testing can be significant considering the fact that several hydrotests may be performed in the lifetime of a pipeline. Although hydrotests would usually re-condition the crack tip so that a lower crack growth rate can be expected for a limited time, it is questionable whether the reduced growth rate would compensate for the loss of life because of the crack growth during hydrotesting. Therefore, the overall benefits of hydrotesting must be evaluated from the viewpoint of life extension and pipeline safety.</jats:p> <jats:p>Effects of hydrostatic tests in this investigation were analyzed based on two competing morphological conditions at the crack tip before, during and after hydrostatic testing, that is, crack tip blunting and crack tip sharpening. Crack tip sharpening is related to the hydrogen effects and mechanisms of cyclic loading, while the crack tip blunting is attributed to low temperature creep of the pipeline steels. From the investigation, strategies aimed at achieving maximum crack remediation are proposed.