A Discussion on Evaluating Pipeline Dents



Why are dents a problem?

First of all you have an immediate concern that you have to address.  That concern may be that if the dent is severe enough or sharp enough it can induce cracking. If it is third-party damage, the action of creating third-party damage, gouging can also initiate cracking. Both of those immediate concerns can also be related to coatings disbondment.  When you have a coating disbondment, that can lead to corrosion which also adds to the integrity concern of the dent. In the long-term, any pipeline is going to see some amount of cycling, liquid lines more severe than gas lines, but that has to be addressed as well. As a long-term, the action of pressure cycling on a dent is going to lead to the initiation of fatigue cracks and ultimately failure if it's not carefully mitigated against.

How have dents been historically evaluated?

Historically the industry has used one of the three methods. The two that are outlined in the code is either depth or strain. There is a more advanced technique which is gained a lot of popularity and its computational abilities have progressed recently is finite element analysis.  So as ILI tools have progressed and its computational power has progressed, you've really seen finite element analysis capabilities take off.  That type of high quality data combined with rapid computational power really affords you the ability to do finite element analysis on the dents on a scale that was impossible ten or even fifteen years ago.

How can we use in-line inspection data to assess dents?

ILI has really been critical throughout the history of dent assessments. Whether you're doing dent depth or you're doing dent strain, you're going to need to capture that data somehow. Now that ILI tools have progressed to the point where you can really get good resolution from the entire circumference of the pipe and through the profile of the dent, you can get a good picture of what the dent looks like in 3-D. Once you have that type of data it opens up the realm to do finite element assessments that would've been possible in ten or fifteen years ago and those finite element assessments will allow you to estimate the burst pressure of a dent, they may allow you to estimate the fatigue life of a dent, and those are the two things you are primarily going to be concerned with.

How is analysis and testing used to evaluate dents?

SES uses analysis as a means for estimating the stress concentration factor in a dent.  On the testing side, SES has been very successful in replicating dents that we found though ILI in the field by investigating the effects of constraint, looking at how stress concentration factors vary in the shape of a dent, understanding how a dent behaves under cycling, demonstrating shakedowns to elastic action, and ultimately demonstrating in a capacity in the lab what the fatigue life of the dent is.  So analysis is great we do actually perform destructive testing in the lab under cyclic or burst that's a very hands-on tangible evidence of how that dent performs. It gives you a level of confidence that you can't provide with a finite element assessment alone.

What does the future hold for evaluating dents?

Dent assessments historically have been a thing that you did only under certain conditions and that usually is whenever you're dealing with a dent in a very high profile, high risk, and high consequence areas.  I see finite element assessments becoming routine and coming to the point where you no longer say “I only want to do finite element assessments here or there,” you say “I've got 150 or 500 or even 1,000 dents and I want to do a finite element assessment on all of them because I can, and that's the best means of assessment that I can do.”