Line 5 inspection results: Cracking
NDT Global In-Line Inspection Tool: Evo Series 1.0 UCc
Imagine taking a piece of steel, and bending it back and forth repeatedly. Certainly, it would take an significant amount of force, but even steel, with its strength and malleability, can grow fatigued enough to the point where a crack will form.
When this happens, it constitutes what’s scientifically known as stress-induced separation of metal, which is what the Pipeline and Hazardous Materials Safety Association (PHMSA) defines as cracking.
For the Line 5 Straits pipelines (Line 5 branches into two parallel 20-inch-diameter pipes as it travels under the Straits of Mackinac), the primary form of cracking that requires active management is ‘circumferential cracking’ at the locations where pipes are welded together – the girth welds.
At the Line 5 Straits crossing, to minimize any pipeline bending stress, Enbridge uses a sturdy screw-anchor system, affixing Line 5 to the Straits lakebed. The Line 5 Straits crossing is also the thickest segment of pipe along our North American network, at a minimum of 0.812 inches. Finally, we have rigorous inspection and monitoring programs in place to assess any conditions that might indicate a crack in the steel could potentially grow to result in a leak.
Our engineering analysis of the pipelines under the Straits of Mackinac tells us these pipes are in excellent condition, almost as new as when they were built and installed.
How do we know that?
Our specialized in-line inspection tools can determine if there are any crack anomalies that need repairs to prevent them developing into a problem. We’ll talk about the test results in a moment. First, it’s important to understand what risks we assess and manage.
We’re focused on preventing potential cracking fatigue at the girth welds. Our in-line inspection program helps monitor the pipeline integrity condition, but we also take proactive care to minimize the potential for the development of cracks through other measures, such as span management. Long spans can potentially cause high bending stresses, and cause pre-existing girth-weld imperfections to grow through cracking. Enbridge performs bi-annual span inspections of the Straits pipelines, and installs supports at spans that exceed our 75-foot length criteria. This maximum span criteria is very conservative, representing half of the safe span length as determined through a 3rd party stress analysis. Furthermore, the actual spans distances tend to be much shorter, with the current average span distance being less than 50 feet. This program of span management is designed to ensure the bending stresses at girth-welds are maintained to a very low level.
To assess flaws that weaken the strength of the pipe when it’s under loading, Enbridge follows British Standard (BS) 7910 in its fracture and fatigue assessment—the same standard used to assess the structural integrity of airplanes, whose steel wings bend and give under the pressure and stress of wind. Unlike an airplane’s wings, our pipelines are prevented from bending or moving underwater.
In 2014, Enbridge conducted two separate girth-weld cracking inspections—one using a “free swimming” circumferential crack inspection tool, and the other using a tethered circumferential crack inspection tool. The tethered tool stops at each girth weld location to examine the weld condition with high resolution ultrasonics—similar to that of a medical exam. The data from the tool is then transmitted to a technician on the water surface for a real time assessment of the weld.
What did our girth-weld inspections tell us?
Figure 1 (below) shows that all identified girth weld features were shallow, relative to the Line 5 Straits crossing’s nearly one-inch-thick pipe wall. All girth welds will contain some anomalies or imperfections, and most will never grow or represent a threat. The table references four categories of features by depth. While a number of circumferential crack-like anomalies in the steel were identified, these anomalies are well within the safety margin and assessed to be minor, with the deepest being in the range of 39-78/1000ths of an inch. Such assessments involve complex scientific calculations, and as we’ve already mentioned, applying the same engineering methods used in the evaluation of the structural integrity of airplanes.
Figure 1 – Results of East (left) and West (right) 2014 GW NDT UCC in-line inspections
All identified girth weld anomalies were assessed by Enbridge using industry accepted methods that consider the anomalies’ size, material, and expected stresses. The engineering analysis was performed immediately following in-line testing to assess the current fitness-for-service of each feature identified by the inspection. The minimum factor of safety of these anomalies was assessed to be 2.4 times the Maximum Operating Pressure. This confirms to us that the cracking threat on the pipeline has been demonstrated to be minor.
As an additional validation, field investigations were performed for the East and West segments on the onshore portion of the Straits pipelines. Figure 2 below, from the field inspection’s non-destructive evaluation report, shows the inspection results for ILI feature 341436. This feature was identified by the ILI tool as an external crack-like feature less than five percent deep. Field investigation found no cracking, discovering instead a small surface anomaly that was removed by grinding. This reflects the sensitivity of the ultrasonic inspection tool to recognize the smallest of features and identify them for further assessment.
Figure 2 – Field Results of East Segment in-line Inspection
Other engineering analyses are also performed to assure the continued fitness-for-service of the line. This includes pressure cycle monitoring on a frequent basis to ensure operating pressures remain at appropriate levels to minimize any pressure-related fatigue.
In conclusion, Enbridge’s comprehensive program is designed to address the prevention of defects followed by regular ongoing monitoring to assure the effective performance of the prevention elements. Mitigation is then conducted as required in order to maintain an appropriate margin of safety for continued operation. For the recent crack inspection program, all circumferential cracking features were inspected and shown to be minor in nature. Most, if not all, are benign weld anomalies that are not growing; nevertheless, all anomalies will be monitored repeatedly for growth through future inspections.
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