IIW White Paper

technology based on the existing pipeline materials, welding technology and standards on transmission of hydrocarbons.

9.4.4 Testing of pipes Although a very small number of pipeline failures is caused each year by stress-corrosion cracking (SCC), it remains a concern to pipeline operators and regulators. The primary methods for managing the problem include periodic hydrostatic testing, in-line inspection (ILI), or SCC direct assessment (DA). The choice of methods depends on many factors unique to each pipeline. No single method is appropriate for every case. Since all three methods are very costly, especially compared to the relatively low risk of failures, it would be of considerable benefit to the industry if more-cost-effective ways to manage SCC were available. Hydrostatic Testing: Hydrostatic testing is the most common method for locating SCC. It is well accepted by the industry and by regulatory agencies, and it is certain to find every crack that is larger than a critical size, which depends upon the test pressure. There are a number of limitations to hydrostatic testing, however. It requires that flow of the product through the pipeline be interrupted, which may not be practical if the line is not looped. In areas with large elevation differences, it might not be possible to achieve sufficient pressure at the high points without excessive segmentation of the line. Furthermore, water might not be available, or, if it is, the cost of disposing of it, due to environmental concerns, might be prohibitive. In situations such as those, ILI or SCC DA may be preferable. In-Line Inspection: ILI has certain advantages over hydrostatic testing in that it can find cracks that are smaller than those that would fail at the hydrostatic-test pressure, thus potentially providing greater margins of safety, and, in many cases, it can be conducted in the fluid that the pipeline is meant to carry, thus not requiring interruption of service. Ultrasonic techniques have been most successful at locating and sizing stress-corrosion cracks in liquid pipelines where the liquid petroleum product serves as the couplant for energy flow between the transducers and the pipe. Achieving satisfactory coupling in gas pipelines has been a serious problem. A recently developed technology involving electromagnetic acoustic transducers shows promise for overcoming this problem, but it has not yet been validated through field trials. Another serious limitation of current ultrasonic ILI tools is their inability to find cracks in dents. This is because the geometry of the dent throws off the angles of transmitted and reflected waves. SCC Direct Assessment: SCC DA is an important method of integrity assurance, especially for portions of the pipeline where it is impractical or even impossible to do ILI or hydrostatic testing. The SCC DA process was formalised with the publication of an NACE Recommended Practice. Essentially, SCC DA involves making intelligent choices about where to excavate the pipeline and then directly examining the pipe to determine the severity of cracking as a basis for deciding what actions should be taken and how soon. The foundation to SCC DA is proper site selection, which remains a major challenge. Internal SCC in Ethanol Pipelines: With the increasing use of ethanol as an additive to gasoline, consideration is being given to transport of denatured ethanol by pipeline. Currently, denatured fuel grade ethanol is transported primarily by railroad tanker cars and tanker trucks. The discovery of SCC in user terminals, storage tanks, and loading/unloading racks in contact with denatured fuel-grade ethanol has raised concern about potential effects in pipelines. Research is needed to develop a better understanding of the causes of SCC in ethanol and, based upon that understanding, to suggest and evaluate possible ways to prevent it. Crack Arrest: When a crack in a welded structure propagates under the action of a load, it can extend in principally two different manners, stably by ductile tearing, or unstably by either brittle fracture or disruptive ductile fracture. Crack growing stably will arrest unless the stress intensity applied to the crack tip continues to increase at a greater rate than the crack resistance curve. Analysis and prediction of an arrest event of a dynamically running unstable crack, however, is a major challenge for higher grade pipeline steels (X 80 and beyond) and welds.

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