In 2015, composite repair materials were used for the first time in an extensive pipeline crack repair testing program. Since that time, multiple test programs have continued to show the functionality of using a carbon fiber, epoxy composite repair system to significantly slow or deter crack growth in axially oriented cracks or crack-like features. To further build onto this successful testing, CSNRI, in conjunction with Williams Pipelines and ADV Integrity, has received pipe spools with SCC that were removed from service for testing purposes. These samples are repaired with the Atlas repair system, cycled, and then pressurized to burst while continuous strain gauge measurements are made on the SCC colonies.
This paper will first provide a brief overview on the theory of how composite materials can be used to provide a permanent repair for standard pipeline SCC defects. The test results will then be discussed with a focus on strain-based results compared between three samples: A baseline sample with no repair, a repair performed at no pressure, and a repair performed at a pressure equivalent to 50% SMYS. All test samples have SCC ranging from 40-60% through-wall. The test results clearly show a reduction in peak-to-peak strain when repaired. The sample installed with pressure additionally shows the impact of installing at pressure, namely that the peak-to-peak strain values are similar when compared with the repair installed at 0 pressure.
Stress Corrosion Cracking, or SCC, is part of a group of cracks commonly known as Environmental Cracking. Additional types of cracks found in this group include corrosion fatigue and hydrogen embrittlement. It is generally known that SCC requires three factors to be present to form and continue growing. The first is a susceptible material. In the world of pipelines, carbon steel is quite susceptible to corrosion when buried but is typically protected from this threat utilizing a combination of external coatings and cathodic protection. However, in the case that the external coating fails, is damaged, or is non-existent, the carbon steel pipe then becomes susceptible to corrosion. Second in the list of requirements is a corrosion inducing environment. Like the first case, if a coating fails while the pipe is buried in certain environments, the corrosion that occurs can be very aggressive leading to the opportunity of SCC creation. The third requirement for active SCC growth is high stress environments, specifically tensile stresses. This stress can occur from significantly high pressures in the line or residual stresses that occurred during manufacturing or installation of the line. Specifics of SCC creation or growth are not addressed in this paper but are a heavily researched phenomenon.
