Within the last decade, a number of gas pipeline companies throughout the world have identified the presence of Stress-Corrosion Cracking (SCC) on their pipelines. While the cause of the SCC resulted from a multitude of factors, there appeared to be a commonality relating to pipeline coatings. Although SCC has occurred under polyethylene/PVC tapes and enamel coatings, not a single incident has been recorded where SCC has occurred on a fusion-bonded-epoxy coated line. The three major factors that were thought to account for the difference in the SCC performance of the various coatings were; (1) resistance of a coating to disbonding, (2) the ability to pass current through the coating and thereby protect the disbonded regions and (3) the surface preparation of the line pipe prior to coating application. These factors were addressed in this three-year program for PRCI. This report examines the relationship between coatings and surface preparation and the likelihood of subsequent stress corrosion cracking. Standard Cathodic disbondment tests, measurements of potential gradients beneath disbonded coating, electrochemical measurements of current passages through coatings and actual SCC tests are described and results reported.

External, corrosion-caused loss of wall thickness is the most common cause of repair for gas transmission pipelines. To prevent an area of corrosion damage from causing a pipeline to rupture, the area containing the corrosion damage must be reinforced. Since corrosion is a time dependent process, as pipelines become older, more repairs are required. Repair methods that can be applied from the inside of a gas transmission pipeline (i.e., trenchless methods) are an attractive alternative to conventional repair methods since pipeline excavation is precluded. This is particularly true for pipelines in environmentally sensitive and highly populated areas. Hydrostatic pressure testing was conducted on pipe sections with simulated corrosion damage repaired with glass fiber-reinforced composite liners, carbon fiber-reinforced composite liners, weld deposition, an adhesively bonded steel patch, and adhesively bonded/helically wound steel strip. To benchmark pipeline material performance, additional pipe sections were evaluated in the virgin and in the corrosion damaged/un-repaired conditions. Three repair technologies exhibited burst pressures that were greater than the burst pressures of the un-repaired pipe sections: adhesively bonded/helically wound steel strip repair exhibited the highest performance with burst pressures ranging from 0.4% to 144% higher; carbon fiber-reinforced liner repair had burst pressures ranging from 4% to 17% higher; and glass fiber-reinforced liner repair had burst pressures ranging from 1% to 7% higher. Two repair technologies exhibited burst pressures that were lower than the burst pressures of the un-repaired pipe sections: adhesively bonded steel patch repair was 1% lower and weld deposition repair was10% lower.

Pipeline repair by direct deposition of weld metal, or weld deposition repair, is an attractive alternative to the installation of full-encirclement sleeves or composite reinforcement, particularly for repair of wall loss defects in bend sections and fittings. Previous work concluded that the static strength of straight sections of pipe, field bends, elbows, and tees can be fully restored when the after-repair minimum wall thickness is equal to or greater than the nominal wall thickness, provided that the weld metal strength is at least equal to the strength of the parent material and that the repair overlaps the perimeter of the wall loss by at least one nominal wall thickness. This criteria was validated by full-scale tests that included a limited number of wall loss locations and end-restraint conditions. To allow the confident use of this repair method, additional validation experiments were required for different wall loss locations and end-restraint conditions. The objective of this project was to further validate the criteria developed in previous work.

The objective of this research project was to identify the environmental, metallurgical, and mechanical conditions that lead to dormancy and re-initiation of previously dormant stress corrosion cracks. These parameters would assist pipeline operators in mitigating near-neutral pH SCC on their systems, and allocating resources for pipeline maintenance. Unload-reload transients were found to increase the crack growth rate in the majority of the experiments and to re-initiate dormant stress corrosion cracks in several cases. On the other hand, there was no consistent effect of the presence or magnitude of overloads on crack growth behavior. The simulated hydrostatic tests had relatively little effect on the crack velocities for near dormant conditions, but consistently inhibited subsequent crack growth for actively growing cracks. The results of analyses of the data and modeling suggest that the R ratio (ratio of minimum to maximum pressure) and frequency of pressure fluctuations on an operating pipeline can be used as a tool to rank segments of pipelines base on the mechanical driving force for propagation of near neutral pH SCC. The concept of a critical crack tip strain rate, which can be related to the R ratio and frequency, also can be used to reasonably predict whether a given set of loading conditions will lead to dormancy.