The Role of Coatings in the Development of Corrosion and Stress Corrosion Cracking on Gas Transmission Pipelines

In order to protect pipelines against corrosion and stress corrosion cracking NOVA Gas Transmission (NGT) has employed a dual system involving the application of an insulating coating and effective cathodic protection to all pipelines. Since the installation of pipelines in the 1950’s the coatings of choice have changed with time. Construction in the 1950’s and 1960’s employed asphalt products which were hot applied over the ditch. Subsequently in the 1960’s and 1970’s polyethylene tape coatings were used extensively. Today the coatings of choice on the NGT system are fusion bond epoxies and extruded polyethylene systems."/jats:p" "jats:p"This variety in the types of coating employed to protect pipelines against corrosion has necessitated a requirement to understand the long term performance of coating products. Studies at NOVA Research & Technology Corporation have been aimed at developing an understanding of the modes of failure of coating systems. The mode of failure of a specific coating system can impact significantly on the nature and severity of the resultant corrosion scenarios. Different coating systems under identical field conditions can fail in completely different ways. Examples from field coating failures will be described which show the significance of the mode of failure with respect to corrosion and stress corrosion cracking."/jats:p" "jats:p"Laboratory studies using polyethylene tape, asphalt enamels and fusion bond epoxy coatings will be described which clearly demonstrate that coating failure modes are a key with respect to the type of corrosion scenario that is ultimately developed at the steel surface."/jats:p" "jats:p"Such information is important in the development of site specific risk assessment models for the management of pipeline integrity

Achieving Maximum Crack Remediation Effect From Optimized Hydrotesting

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