What if your burst pressure calculation results don’t make sense?
- It’s essential to find the right balance in taking action on anomalies
- Limitations of existing fracture toughness testing technologies
- Modern techniques are available to measure fracture toughness directly
It was supposed to be an uneventful second-round in-line inspection tool run through a liquids pipeline about 100 miles long. But it stopped being ordinary when analysis of the tool run data found 1,800 crack-like features that met burst pressure remediation criteria and were potentially hazardous to the integrity of the line.
A quick back-of-the-envelope calculation found that if one crew dug an anomaly each week and resolved it, the remediations would take 35 years to complete. Not a viable course of action, even with multiple crews.
Our other choices included lowering the operating pressure of the line, but of course this would reduce the amount of product that it could move – as well as hampering the ability of the liquid to sweep corrosion-causing water out of the low points. Another option was to develop better inputs to the fracture analysis.
We did both – lowering the operating pressure temporarily for safety while also investigating more accurate inputs. The better inputs would, we hoped, give us more realistic predicted burst pressures so we could replace the overly conservative values with more accurate ones.
Part of our research involved taking scraps from samples that had been cut out of the pipeline earlier and sending them to a lab to get a direct measure of fracture toughness. The toughness tests gave us accurate values specific to the line to plug into the calculations. In this way, we were able to bring the original 1,800 features that needed remediation down to about 1,400. This was certainly progress, but still wasn’t a practical number of features to dig.
So, we performed a pressure test (a brief spike test followed by an eight-hour test) of the entire line – filling it with water and bringing it to a pressure well above anything it would be subjected to while moving product. The line held, demonstrating that the anomalies the in-line inspection tool found would be safe for at least five years, until the next inspection could be used to check on them again. The test also established a new maximum operating pressure for the line, and it was put back in service with a capacity sufficient to serve the operator’s shippers, and with flow rates high enough to prevent internal corrosion.
Resolving this situation and others like it requires us to maintain a tension between competing priorities. Many times, the data coming back from an in-line inspection is inconclusive on its own, and the way forward needs to be considered carefully. Digging too many anomalies (i.e. digging anomalies that can be proven by more economical means to be non-injurious) misallocates resources that could be better used to address real threats to the line’s integrity. Digging too few anomalies (i.e. not digging some anomalies that do present an urgent threat) results in a greater chance that some of them will fail and cause a leak or a rupture.
Why it’s essential to find the right balance in taking action on anomalies
Finding that balance between too much and too little action is more important now than ever. This is partly because it’s so difficult to obtain permitting for new lines, or even for paralleling or expanding the capacity of existing lines. There’s also more concern about pipeline safety from the media, public, landowners, regulators, and investors.
As a result, there’s a pressing need to keep existing pipelines as available as possible – ready and able to move product at full capacity almost constantly. Integrity management of crack-related threats is particularly important for older (pre-1980s) lines that were constructed of line pipe with known seam issues (e.g. low-frequency electric resistance welded pipe), and which may now be showing signs of weakness or fatigue along those seams.
In addition, federal regulations were updated in 2021, and the code for natural gas lines now addresses toughness directly (49 CFR 192.712).
All these factors combine to highlight the need for good tools and good data on pipe properties to manage pipeline integrity. Despite the importance of these factors, it can be hard to “sell” the need to spend more on digs and repairs or materials testing, since these efforts are typically preventative. It can be difficult to communicate the significance of a threat that hasn’t yet resulted in a leak.
Limitations of existing fracture toughness testing technologies
So, what’s new in the figurative toolbox for pipeline operators wanting to manage cracks more effectively? On one hand, in-line inspection has become more sensitive, able to find more features. But there hasn’t been as much improvement in the ability to determine the risk that these features pose to the line. This means that it’s hard to set priorities in terms of digs.
Crack growth and fracture analysis models are the industry’s go-to tool for understanding the risk of cracks or crack-like features. A key input to these models is the fracture toughness of the pipe material. Fracture toughness is what matters for preventing or limiting crack propagation, but measuring or calculating the toughness of pipe is still difficult.
The industry’s primary fracture toughness metric, Charpy V-notch (CVN) energy, is not even a direct measure of fracture toughness. The two are separate material properties; they are correlated, but there is no equation to convert directly from one to the other. CVN energy is convenient because it’s easy to measure, and the test methodology is well over a century old, but using it as a measure of fracture toughness is like using standardized testing to measure intelligence: it doesn’t tell the whole story. Correlations between the two err on the side of conservatism, and correlations that yield good results at one temperature or toughness range may be inappropriately conservative at another, yielding red flags where there should be none.
Fortunately, modern techniques are available to measure fracture toughness directly. Unfortunately, those technics are not available in-the-ditch. Like CVN testing, fracture toughness test methodologies require material specimens cut out of the pipeline. Unlike CVN testing, fracture toughness testing can require a great deal of time. The material specimen must be loaded and unloaded hundreds, sometimes thousands of times with crack tip growth measured between each cycle.
Recommendations and next steps
If you are managing the integrity of lines that are vulnerable to cracking, our recommendations include:
Save scraps! Make sure you document where samples are cut from the pipeline during maintenance and store them in a place where they can be preserved securely. Include pertinent data like what line segment and location they were cut from, flow direction, seam type, vintage, manufacturer, MTRs and other available characteristics of the pipe. This is important because the new regulations require that, if the operator doesn’t have information on the toughness of the pipe, prescribed conservative values must be used. Operators need accurate toughness data on their pipe so they can avoid using the default values, which may be overly conservative.
Use higher resolution ILI tools combined with an analysis process that can discriminate between injurious and non-injurious features, like blunt crack-like features versus true sharp-tipped cracks.
Stay current on developments: The Pipeline Research Council International (PRCI), an industry body, is creating a database of different fracture toughness values for pipe produced by various mills at various times, with expected completion 2023. This will allow pipeline operators to use those values rather than having to obtain samples from an active line for lab testing or using default values.
Get the right technical expertise: Third-party engineering firms such as HT Engineering make it their business to bring the best available technology and practices to your pipeline system. They can help you walk the balance between digging features that don’t actually pose a threat to your line and missing out on the actions necessary to prevent potential problems.
For an exploratory conversation, reach out to one of the experts at HT Engineering.