Regular examinations of the pipe, both inside and out.
Figure 1: MFL Metal Loss Corrosion ILI Tool
Whether it’s the Eiffel Tower or underwater pipelines, steel infrastructure needs to be maintained and protected from corrosion. For the Eiffel Tower, that involves a new coating of paint every seven years to protect the iconic structure from the elements and keep it safe for millions of tourists. At Enbridge, we take a comprehensive approach to protecting and maintaining our pipelines to ensure the safety of the public and protection of the precious Straits of Mackinac waterway.
Our engineering analysis of the pipelines under the Straits of Mackinac tells us these pipes are in good condition, in almost ‘as new’ condition in fact. But how do we know that? One of the many ways we assess the fitness of our pipelines is through the analysis of data collected by specialized tools. These tools can determine if any of the tiniest of blemishes found on the steel is an indication of a developing problem. We’ll get into the data in a moment, but first, let’s talk about how we prevent corrosion.
We keep corrosion at bay—inside and out—through various means and measures. Externally, the parallel pipes travelling under the Straits of Mackinac are covered by a tough enamel coating that’s impermeable to water, providing a barrier protecting the steel from exposure to the elements. The potential for corrosion is further reduced by the use of a secondary protective measure called cathodic protection, where we apply a low-level electrical current to the pipeline to protect the steel from reacting to the environmental conditions.
We also minimize the potential for internal corrosion in a variety of ways, including:
Although the potential for corrosion is low because of all the measures we take to protect the pipelines, we routinely perform in-line inspections (ILI) with high-tech tools similar to technology used in the medical industry. They scan the pipelines inch by inch, and collect data that our expert engineers pore over and analyze to get a full picture of what’s happening.
Metal loss in the nearly one-inch-thick walls of Line 5’s steel pipe travelling under the Straits, is one of the conditions measured by the tools, even at minute levels. Our in-line inspection tools are capable of identifying small anomalies, called features, on both the inside and the outside surfaces of our pipes. Repeat inspections provide updated information and verify our engineering analysis.
The primary tool we use for metal loss and corrosion is the Magnetic Flux Leakage (MFL) tool. MFL technology uses powerful magnets to magnetize the steel in the pipeline. As the tool passes through the pipeline, precise measures of any changes to the magnetic field are measured. If any corrosion or thinning of the metal has occurred, the measure of the strength of the magnetic field is distorted, revealing an anomaly along with its size—both depth and length—and its location on the pipes.
The results of the MFL runs are reviewed by Enbridge pipeline engineers. Any anomalies are assessed for whether they are acceptable corrosion pits, or if they are large enough to require repair. If what’s found reaches a certain threshold, repairs are done. For the Straits, this would generally be through the installation of a steel reinforcing sleeve that would strengthen the pipe at the corrosion location.
The left and right plots below were taken directly from the summary sections of the 2013 MFL inspection reports for the East and West Straits pipelines, respectively (Line 5 branches into two parallel, 20-inch-diameter lines as it travels under the Straits of Mackinac). All of the metal loss features that were measured by this high-resolution tool are reported by the green triangles. The vertical axis represents the depth of the features, and the horizontal axis represents their length. The blue line represents the allowable corrosion depth and length for the pipeline to safely transport its product.
Figure 2 –2013 MFL Inspection Corrosion Results
As shown in the above figure, there were nine metal loss corrosion features identified by the MFL tool on the nearly five-mile East Straits pipeline, all of which were located on the on-shore portion of the pipeline. For these nine features, the average maximum depth was 10 percent of the wall thickness, and the average length was 1.7 inches. The deepest was 26 percent and 7.3 inches long, which makes it the longest feature as well—but still only half as deep as it would need to be to require repairs, as set out by regulatory standards. The allowable limit for safety, as set out by those regulatory standards, is a function of the metal loss size, pipeline properties, and operating parameters, and is represented by the blue curved line in these figures. The features that were found fall well below the line, which means well within safety thresholds.
Pipeline regulations require immediate action for areas where metal loss is greater than 80 percent of the wall thickness, or a predicted burst pressure is than less than the maximum operating pressure (MOP). Enbridge undertakes repairs when features are deeper than 50 percent of the pipe wall thickness, or based on operational safety factor calculations, which is designed to ensure the defect is repaired when its predicted failure pressure is well above the MOP.
All of these features found by the MFL tool were on the inside of the pipe, which means there was no external metal loss features on the East pipeline (remember, the outside of the pipe is covered with a tough enamel coating that provides a barrier between the steel and the elements, namely water).
For the West Straits pipeline, there were no metal loss features identified by the MFL tool on either the inside or outside of the line, which is why there are no green triangles on the right-hand-side Figure 2 plot.
Additionally, Enbridge conducts its own assessments of these features, above and beyond those conducted by the ILI vendor, to assess and evaluate the accuracy of the ILI tool; all nine were assessed by our engineers, and considered well within safety thresholds. If they were found to be above the acceptability limits, or the blue line in Figure 2, preventative maintenance action would be taken.
In addition to the metal loss anomalies shown in Figure 2, the ILI report also shows other features with reported length and depth that are characterized as mill anomalies. All pipe, even brand new, will contain some anomalies or imperfections, most of which will never grow or represent a threat. In the case of Line 5, which consists of specially manufactured seamless piping for extra strength and safety, some variations in wall thickness result from (and are expected from) the manufacturing process itself.
These wall thickness variations passed the original hydrotests and been safely accommodated by the pipelines since construction. They are picked up and reported by the corrosion tools, and while stable in nature, are nonetheless monitored and assessed from inspection to inspection, as shown in Figure 3 and described in Table 1.
Taken directly from the 2013 ILI Metal loss inspection reports, Figure 3 gives a graphical representation of the depth of features found along the East and West pipelines, including both metal loss and mill anomalies. The chart is categorized by total features, and then subsequently by increasing depth from front to back. The peak depth of mill anomalies on the East and West pipelines was 37 and 41 percent of the wall thickness, respectively. Table 1 below shows the distribution of features for both Straits pipelines, where there were 141 and 294 features identified by the MFL inspections of the East and West pipelines, respectively. As stated above, these features all exhibited a depth below 50 percent of wall thickness and well within safety thresholds.
Figure 3 – Summary of Number of Features by Depth of 2013 (East Leg) Metal Loss Inspections
|Features by Depth (percentage of wall thickness)||Number of Features|
|Predicted peak depth of > 50%||0||0|
|Predicted peak depth of > 40% and ≤ 50%||0||1|
|Predicted peak depth of > 20% and ≤ 40%||71||188|
|Predicted peak depth of ≤ 20%||70||105|
The low quantity of corrosion features identified by the MFL inspections are as expected, given the Line 5 crossing’s high-performance enamel coating, the current flow of the lake crossing environment, and low corrosivity of the NGL and light crude transported product.
The 2013 corrosion inspections represent the latest in a long-standing program of regular inspections that have been established as part of our integrity management program. Subsequent in-line inspections will continue as part of this program on a five-year interval to continually monitor these features for any changes or growth. Mitigation activities will be conducted as required by these assessments to ensure the ongoing safe operation of the line.
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