This week’s Pipeliners Podcast episode features first-time guest Dr. Jim Oswell of Naviq Consulting discussing the importance of soil mechanics to support pipeline stress analysis.
In this episode, you will learn about the benefits and value of understanding soil mechanics from the person who wrote the book on the topic. Dr. Oswell will also discuss the extent to which geotechnical issues are impacting the pipeline industry and how properly understanding soil mechanics can create a better pipeline project. Dr. Oswell and host Russel Treat also discuss how geotechnical input has helped the pipeline industry grow over time.
Soil Mechanics for Pipeline Stress Analysis: Show Notes, Links, and Insider Terms
- Dr. Jim Oswell is the prinicipal engineer at Naviq Consulting with 40+ years of industry experience. Connect with Jim on LinkedIn.
- ΔT (Delta T) is defined as the difference in temperatures between two measuring points. The temperature differs either in time and/or position.
- ALA (American Lifelines Alliance) is a cooperative agreement between the American Society of Civil Engineers (ASCE) and FEMA to prepare a guide for the design of buried steel pipe. The guide includes fundamental design equations and calculations.
- ALA (2005) design guidelines are considered the standard practice for soil mechanics for pipelines.
- Learn more by reading through Dr. Oswell’s technical workshop on Soil Mechanics for Pipeline Stress Analysis.
- The coefficient of Variation (COV or CV) is defined as the ratio of the standard deviation to the mean. It shows the variability, as defined by the standard deviation, relative to the mean.
- Yield Strength is defined as the stress at which a predetermined amount of permanent deformation occurs.
- SMYS (Specified Minimum Yield Strength) is a measurement of a pipe’s strength, as determined by the manufacturing specifications of the pipe.
- Undrained Shear Strength (US) is defined as maximum shear stress at yielding or at a specified maximum strain in an undrained condition.
- Shear Stress is the force tending to cause deformation of a material by slippage along a plane or planes parallel to the imposed stress.
- Bridge Abutments is a substructure that supports one terminus of the superstructure of a bridge and, at the same time, laterally supports the embankment which serves as an approach to the bridge.
- Kilopascal (kPa) is defined as one thousand times the unit of pressure and stress in a meter-kilogram-second system.
- Mohr-Coulomb Theory is a mathematical model describing the response of a material such as rubble piles or concrete to shear stress as well as normal stress.
- Exceeding the shear strength results in failure of the ground which can be described by the Mohr-Coulomb Failure Envelope.
- Horizontal Directional Drilling (HDD) is a steerable trenchless method of installing underground pipelines in a shallow arc along a prescribed bore path by using a surface-launched drilling rig, with minimal impact on the surrounding area.
Soil Mechanics for Pipeline Stress Analysis: Full Episode Transcript
Russel Treat: Welcome to the Pipeliners Podcast, episode 211, sponsored by EnerSys Corporation, providers of POEMS, the Pipeline Operations Excellence Management System, compliance and operations software for the pipeline control center to address Control Room Management, SCADA, and audit readiness. Find out more about POEMS at EnerSysCorp.com.
Announcer: The Pipeliners Podcast, where professionals, Bubba geeks, and industry insiders share their knowledge and experience about technology, projects, and pipeline operations. And now, your host, Russel Treat.
Russel: Thanks for listening to the Pipeliners Podcast. I appreciate you taking the time, and to show the appreciation, we give away a customized Yeti tumbler to one listener every episode. This week, our winner is Ashley Jesko with Energy WorldNet. Congratulations, Ashley, your Yeti is on its way. To learn how you can win this signature prize, stick around for the end of the episode.
This week, Jim Oswell joins us to talk about his book, “Soil Mechanics for Pipeline Stress Analysis.” Jim, welcome to the Pipeliners Podcast.
James Oswell: Thank you very much, Russel.
Russel: So glad to have you on. I think before we get started, I’ll just ask a general question. Maybe you could tell the listeners about yourself, your background, and how you got into pipelining.
James: Sure. I’m a geotechnical engineer. I live in Calgary and have been here in Calgary more or less for over 40 years. When I first started, the first pipeline project I started on was I started with a new company, a consulting company in Calgary, after completing my Ph.D. in geotech engineering.
The first guy to walk into my office asked me to help him with some spreadsheet work on a pipeline. Literally, that’s more or less how it happened. If the first guy who had walked into my office was a tailings guy, I would have become a tailings engineer, probably. Just as serendipity, it turned out that the first senior engineer to come into the office was this guy who needed some help on a pipeline project with some geotech work.
Russel: I took some courses in university from a Ph.D. geotechnical engineer, a gentleman by the name of Dr. Robert Litten, and I tell you what, I was a structural engineer. I wasn’t a geotechnical, but geotechnical’s part of what you had to take to get out in that program. He ran me through the paces. I learned more about geotechnical engineering than I ever wanted to know, and then I ended up using it over and over again in my career.
James: Yeah, and it’s been interesting for me, because I work in two areas. One, pipelines, and the other area that I have lots of experience in is permafrost. My real niche is pipelines in permafrost. What I’ve found is that it’s actually been quite helpful for me to understand things like delta T, temperature effects, and soil temperatures on backfill. A lot of that information that I now use came from my permafrost engineering background or experience. That has been a really good thing for me.
Russel: Let me ask this question of you, Jim. You wrote a book called Soil Mechanics for Pipeline Stress Analysis. The first question is why did you write the book?
James: Well, a number of years ago about eight years ago I was working on a pipeline project supporting the designers. My role was more or less to assist the prime designers with geotechnical issues. There was a geotechnical consultant on the project who was providing geohazard engineering and things like that. Also, some input into the soil properties.
The stress analysis folks did the stress analysis, and the manager on this project just asked me to read the report. When I read it, I looked at some of the equations that they were using, and they were the standard equations that you might find in ALA (2005), which is the standard industry practice for soil mechanics for pipelines. I realized that these fellows and folks who were doing the stress analysis clearly did not understand the soil inputs. The equations in ALA, for example, will have a term for what’s called the undrained strength, and there’s another term for the effective strength of the soil.
Those two strengths are completely independent from each other and used in different modes of loading, yet the stress analysis folks were intermixing them. I said to the manager of the pipeline project that what you need is a short course or a lunch hour lunch-and-learn soil mechanics 101.
Out of that, I did that workshop for them. From that, it developed first a four-hour workshop, and then it became an eight-hour workshop. Then, in the last couple of years, it’s become a two-day workshop for some clients.
From that came the book. When I started writing the book, [laughs] the long story is that I was stuck on an airplane for eight hours on the tarmac in Frankfurt waiting to fly home. The pilot came on and said, “Well, we’re going to sit here for eight hours before we take off.” I thought, “Well, what am I going to do for these eight hours?” I thought, “Well, I might as well start writing my book.”
All I did was, to start writing the book, I took each of my PowerPoint slides and wrote a story around it. On that trip back from Frankfurt to Calgary, I wrote about a third of the book in those 8 to 14 hours. Now, admittedly, it took two more years to write the other two-thirds, but it got me started.
Russel: There’s two things that are hard about writing anything is starting and finishing. The middle part’s just work.
James: That’s true, because, certainly, when I finished the second edition earlier this year — and the second edition book was published in September — I literally had to put a hard close on it because I was always finding new information to add into the book. New literature was coming out. It’s a real problem, that it’s a very dynamic research area, and there’s new stuff literally coming out monthly. At some point, you have to shut it down and say, “Enough for this right now.”
Russel: Yeah, no doubt. One of the things that I remember from doing what little bit of geotechnical work I did in university, and then following that when I was in the military, is that, when you’re doing structural engineering, and you’re looking at the steel, or the concrete, or whatever you’re using to build your structure, there’s a lot of control in the variability in the material.
You can make some pretty good assumptions about this material all being the same, and yet, when you start talking about geotechnical, by its nature, it’s not the same.
Russel: It’s very different, and it’s very dynamic. There’s a lot of things that impact how soil behaves. I guess one of the questions I would have is how big a challenge is it to get integrity management professionals that are used to working around the steel to understand the geotechnical aspects and what’s going on there? Just getting fundamental knowledge to be able to navigate the complexities, I guess.
James: There’s a couple of points there that you raise, Russel. First, you’re right. In pipeline engineering, the steel properties are known to two or three decimal places. That might not be true, but you get the idea.
The coefficient of variation on the minimum specified yield strength (SMYS) is two or three percent. The coefficient of variation on undrained strength is somewhere between 20 and 50 percent. You get an undrained strength at some location, and the reality is that it could be literally double a few feet or a few meters away, whereas you don’t get that in the steel. That’s certainly one issue.
The other issue, then, is the issue of geotechnical input into pipeline designs. Back in the 1980s, typical geotech input would have been limited to a foundation for a pump station or a compressor station, or perhaps a valve station, something like that.
What’s developed in the last 20 to 30 years is more geotech input. We are now, geotech engineers, are routinely being requested to provide input to routing and identification of geo hazards and things like that. Then also, we are getting much better at providing input into soil-pipe interaction.
Again, a lot of the geotech engineers don’t understand some of the specifics or the details of pipeline engineering, versus other geotechnical engineering projects, such as foundations or bridge abutments and things like that.
Russel: I think that’s an interesting conversation, too, because most geotechnical engineers, their background is going to be in retaining walls, foundations, bridge abutments. They’re going to be soils-supporting structures.
Russel: Versus soil impact on a pipeline.
Russel: To me, knowing very little about geotechnical as it relates to pipelines, it seems to me like that would be a very different kind of analysis.
James: Well, there’s a couple of points there. Most geotech engineers in university, or most university programs in geotech engineering, all deal with what I call high-stress problems. Those are foundations.
Those are building foundations. Those are bridge abutments. Those are embankment dams on the ground and tailing dams, etc. All of those deal with high-stress environments, stress environments in the order of 50 to 200 PSI, or 300 kPa to over 1,000 kPa. Those are high-stress environments.
For pipelines, where the technical depth of burial is three feet or four feet, which is somewhere one and a half meters, for example, that’s a low-stress environment. The stresses at that depth are literally in the order of 5 to 10 PSI, or 20 kPa to 50 kPa.
The whole realm of soil mechanics actually changes, and lots of geotechnical engineers don’t appreciate that change in the soil mechanics. The soil mechanics doesn’t change, but you have to use a different mindset when you are looking at these low-stress environments, such as pipelines.
Russel: I can visualize that in my mind’s eye. I think about something like clay soils and how they behave under very high load, versus how they behave under a low load, and it’s completely different. The soil properties are the same, but the way that the soil behaves under those different load levels is quite distinct.
Russel: It’s interesting.
James: The geotechnical engineers who are listening, they will understand the concept of the Mohr-Coulomb envelope. What that is is shear stress versus normal load. In typical geotech engineering for most projects, we assume that that’s a straight line.
The slope of that line represents the friction angle of the soil. Well, at low stresses, in most soils gravels, clays, peats, just the whole range of soils at low stresses, that Mohr-Coulomb envelope, that slope, is not a straight line, but in fact, is curved.
The result of that, it curves down towards zero, so that, in the end, at low stresses, you would have a case of low effective cohesion, and probably zero cohesion, but you would have a friction angle that is considerably higher than you would if you were dealing with a building foundation or a bridge abutment, for example.
Whereas you might assign a friction angle of 30 degrees for a particular soil if you were doing a foundation, if you were doing the same soil for a pipeline, that friction angle might be 38, 39, or 40 degrees. That impacts the soil resistances that are acting on the pipeline in a soil/pipeline interaction problem.
Russel: I guess your book is really designed to orient pipeline engineers to the geotechnical issues, but also, to orient geotechnical engineers to those things that are unique or special considerations in pipeline.
James: Right, yeah. Originally, the book was written with the whole idea of educating mechanical engineers and pipeline engineers who didn’t understand or weren’t taught about soil mechanics. As I say, that’s the original intent.
What I’ve found since writing the first edition in 2016 and in the last five years, was that a lot of geotech engineers are now picking up the book to look at it, because they want to get tuned up about what are the important issues for pipelines that are not being taught in universities in their classical soil mechanics and foundation engineer classes.
Russel: Yeah, exactly. I think a university education is often just getting you to the point you have the ability to learn what you’re going to need to learn once you take on your career.
James: Yep, absolutely.
Russel: When I was in university, we had a program where we had working engineers come and talk, and he told a whole roomful of engineers within six months of graduating that, well, whatever you think you’re going to be doing five years from now, you’re going to be doing something different. We all smirked and laugh and thought he was crazy, but he was absolutely right.
Russel: What you realize is that there’s a whole lot of vertical specialization in every field of engineering that you don’t find until you get out and go to work.
James: Absolutely. Even in writing this book, I admit clearly that I learned an awful lot from writing this book. This was not all my knowledge that I just simply put down into a computer. This was ideas that I had but didn’t understand, and by going and doing lots of reading and studying, I learned this.
In fact, I did a presentation at the University of Calgary recently. Afterward, I was talking to the professors who were teaching the course. One of them had said, “If you really want to learn a subject, either write a book or teach a course.” That’s certainly true for this.
Russel: Oh, yeah. Being a guy who’s taught a lot of different kinds of courses and written a lot of technical articles, you always learn. You always learn because you have to. The other thing you do, I think, too, is when you write a book or you teach a class, you organize that learning in a different way.
You have to get it organized in a way that you can teach it to people, and you have to understand, “Here’s where you start, and this is how you build on the idea. Here’s where you’re headed.” There’s a lot of work that goes into that.
James: Absolutely. As I say, when the book grew out of the workshops that I was teaching on the same topic, and getting the questions and answers from the participants in the workshop clearly demonstrated, in some cases, that my presentation was lacking in clarity and needed more work on my part. That really helped me in building the text around a concept, such that the pipeline engineers could understand it and then be able to apply it.
Russel: I think one of the questions it’d be interesting to ask is to what extent is geotechnical issues impacting pipeline incidents, and is this a growing thing or a shrinking thing? Where are we in the life cycle of understanding pipeline incidents and how geotechnical factors contribute?
James: I think there is probably a couple of different answers to that, Russel. Some of our input now is certainly to help create better designs. For example, our geotech input in terms of geohazards are clearly making better, more economical designs.
We are able to identify a landslide, for example, where the pipeline route wants to go, and we would say, “Look, if you move your pipeline from this location to this location, you’re going to avoid a landslide that, if you don’t avoid it, you’re just going to have a headache during operations of ongoing maintenance, mitigations, and interventions. Whereas, if you put it over here, it’s a much more stable location, and yes, it’s going to add some distance to your pipeline, and so as a capital cost, it will have an implication, but the operating cost of that pipeline will be much lower, because you’re now avoiding some of those issues.”
That’s one thing. On the soil stress and soil pipe interaction side, geotech input has generally been conservative. We don’t see problems with soil pipe interaction or stress analysis coming up with problems during the design, because traditionally, the geotech engineers are being conservative.
As I said previously, that if a geotech engineer is using a high-stress mindset, he’s going to suggest a friction angle of 30 degrees, which is, for most cases, in pipeline soil pipe interaction, is a conservative assumption for soil resistances. In fact, the actual friction angle that’s being mobilized is probably 38 or 39 degrees.
Traditionally, the absence of good geotech engineering in pipeline stress analysis has led to conservative designs. What one of the functions of my book and the discussions that I have with pipeline engineers is not necessarily to reduce the…Well, I guess I’ll say it is to reduce the conservatism, but make better designs.
There are cases, for example, where we could avoid some mitigations that would otherwise be applied because of some conservatism. Overall, that makes a better design, and it makes it more economical and good for everybody.
Russel: No, absolutely.
James: If I can give you one example, several years ago, I was asked to review a soil pipe interaction problem for a pipeline coming out of an HDD in Louisiana. The soil pipe interaction folks had asked me to provide some input to that.
“What was the drilling strength of the drilling mud? Or sorry, what was the undrained strength of the drilling mud that they would use in their soil pipe interaction?” I gave them a number, and frankly, there’s very little research on this topic. But, I had given them a number, and then they had gone and did their analysis, and everything’s fine. Then the owner decided that they wanted an independent review of the report, which is perfectly fine. The independent reviewer had said, “Well, where did you get that undrained strength for the drilling mud?”
After some discussions, we more or less agreed that it was a value that did not have a firm basis in research, but was more engineering judgment. The conclusion, then, was, well, if you can’t prove that that is a particular specific number by research, then let’s assume that that drilling mud strength is zero.
As a result of that, when they ran that in the analysis, of course, you’ve got this pipeline with a large thermal expansion effect coming into play during the operation. The pipeline was literally coming out of the ground at the overbends from the HDD, because there was no actual restraint of the pipeline within the HDD bore. As a result of that, the owner had to apply mitigation in terms of screw anchors and some other mitigation that was relatively costly, because of this conservatism.
What I’m trying to advocate with this book and my discussions is that we can be better. We can do better geotech engineering than just assuming zero strength.
Russel: Yeah. [laughs] That is a very interesting discussion to me, Jim, when you start getting into this conversation about engineering judgment because engineering judgment is a blunt instrument. It’s not precise, and it really goes to your confidence in an individual and their technical ability and their analysis.
It’s a combination of those things that you’re looking at. What I will say is one of the things I certainly learned, and I probably learned this in geotechnical more than anything else, is that ultimately, it all comes down to engineering judgment.
Russel: No matter how sharp of a knife you have, you’re still cutting a brick. You have to use some judgment. What you’re getting at is the better the tools we provide people, the better their engineering judgment can be.
James: Absolutely. Another way to look at that is, when your coefficient of variation of undrained strength is 20 to 50 percent, the ability to use a precise number and a precise model is frankly nonexistent. You really do need to use some judgment as to deciding which values are most appropriate for your problem. That goes into another problem of soil parameter selection. You can have the same parameter for one stress analysis that is conservative, but a different stress analysis for a different problem would be unconservative.
A classic example would be, say, comparing a side bend, thermal expansion of a side bend, and pipeline soil interaction on a landslide. Soil parameters that are weaker are generally considered to be conservative for thermal expansion into a side bend.
The same conservative, i.e., low, values that you would use in a stress analysis for a side bend turn out to be generally non-conservative for a stress analysis related to a landslide. Low numbers in one case, low soil values in one case, are conservative, and high soil values in another case, or a different loading scenario are conservative.
Russel: [laughs] What that reminds me of is…I don’t know how much this is off-topic, and I want to ask you a little bit about how this technology’s been evolving in recent years. What this really boils down to me, and I think this is important for any, particularly a young engineer to understand.
You go to engineering school, and you get these problems. They all start with given and then required, and then you’ve got to do a solution. In university, 80 percent of your effort is understanding how to do the analysis and come up with a solution.
Then you get out into the real world, 80 percent of your analysis is figuring out what the given is. What is the loading? What is the conditions? What is the soil? How is this pipeline going to operate? And on and on and on. That’s the real engineering work is figuring out the given.
James: Yeah, the analysis is the easy part. It’s developing the input parameters and the boundary conditions, and understanding the front end of the problem. That’s where…
Russel: Yeah, it’s all the problem definition.
James: Yep, exactly.
Russel: Then, making sure you have that problem definition well-documented so that somebody can come back and check your analysis. What rarely happens is the analysis is wrong. What generally happens is some new piece of data enters the puzzle, and now you’ve got to recheck your problem definition.
James: Yes, exactly.
Russel: What’s going on in the industry recently as we’ve begun to evolve our understanding of geotechnical input for pipeline stress analysis? What are some of the new things people are understanding and developing?
James: Certainly, there’s a couple. One of the things that has given me a really good sense of direction and competency is the increased input of geotech engineering into pipelines. One of the things that I really like is that Mike McSweeny, a geotech pipeline engineer with BP in England — he’s now retired — but a number of years ago, he wrote a really good paper comparing the historical application of geotech engineering to pipelines.
The historical practice, what he did is the metric he used was pipeline ruptures per thousand kilometers per year. The historical practice would have a pipeline rupture rate of somewhere around 2 or 2.1.
Then, he looked at more recent practices where some geotech engineering was provided during operations. That frequency rate dropped from 2 or 2.1 down to about 0.56. Then, with modern practice, with geotech input provided at the design stage and through operations, that ratio dropped from 0.56 to 0.17. In modern practice, with expert geotech engineering and geohazards input during design, construction, operations, etc., that incident rate is approaching zero.
There’s clearly value to be added by geotech and geohazard engineers to pipeline design and to their operations. It’s a good statement on the value that geotech engineers add to pipeline design and operations.
Russel: Fundamentally, this is all about safety effectiveness, so zero incidents and managing resources. As we develop our understanding of all these technologies, and we get better tools and so forth, that’s what we do. We’re able to make better decisions. We’re able to implement appropriate mitigations, and we’re able to do that at better price points. Fundamentally, that’s the game we’re playing, and there’s no end to that.
James: Yeah, and it increases the engineer’s ability to provide good judgment calls when needed.
Russel: Absolutely. Jim, what do you think all pipeline operators — regardless of whether they’re doing integrity management or geotechnical — what should all pipeline operators understand about this conversation we’re having? What should they take away?
James: The first and the most important thing is to have a discussion with the geotech engineers. If they’re doing pipe stress analysis, talk to the geotech engineers. Tell them what their problem is.
If you just say, “Well, give me the soil parameters, because I’m doing a stress analysis,” then the geotech engineers won’t know whether you’re doing a side bend analysis, where you need generally lower values soil input values or you’re doing a problem such as a slope instability issue, and you would benefit from having higher soil input values.
Having that conversation with the geotech is one thing for soil/pipe interaction. For design and early front-end engineering work, for example, engage the geotech engineers for routing. Help the design team identify geological hazards that could negatively impact the design and the operation of the pipeline.
There’s two examples, but having that input is going to be important, and having that conservation is critical. To be honest, there will also be lots of geotech engineers who don’t get pipelines. Having your pipeline engineers explain what their constraints are and what their parameters are is very helpful.
A classic example would be doing terrain modeling or terrain assessments to identify the soil conditions and the terrain units along a pipeline route. Often, geotech engineers will use a standard of terrain mapping that has a split in, say, slope gradients from zero…
Zero would be flat, and then zero percent to five percent would be one category. 5 to 10 percent would be another category. The problem for pipeline engineers is that often the need for grading starts with a side slope of somewhere around three percent.
If the geotech engineers and the terrain mappers are doing a slope category of zero to five percent, that’s not helpful for the pipeline project, because they don’t know whether or not that grade is one percent, which means no grading is necessary, or that gradient is five percent, which would require some gradient. In that case, get the pipeline designers and the project team to be talking to the geotech engineers and saying, “Here are our constraints. Here are our requirements, and help us achieve them.”
Russel: I think that comment’s probably true across a whole lot of disciplines. I think it’s really important for pipeliners to understand that everybody really needs to understand what is the operating condition we’re dealing with, and what is the objective we’re trying to accomplish before we go away to do our analysis? All that’s material to those decisions.
Well, Jim, listen, this has been awesome. I have to say, you’ve made my head hurt a little bit, and that’s a good thing. I appreciate your time, and let me ask this as a last question. How would people find your book if they’re interested in getting a copy?
James: Well, it’s available on Amazon. Alternatively, you can contact me through my website. My consulting company is called Naviq, and the website, then, would be Naviq.ca. There’s a link there to contact me, and also a link to the Amazon websites. Either way.
Russel: Awesome. We will definitely link that up, so that people can go to the Pipeliners Podcast website and find the episode, go to the show notes, and find links to find that stuff. Listen, I appreciate you coming on, and we’ll have to get you back when you publish another book.
James: Right, okay. Well, thank you, Russel. It was a treat.
Russel: I hope you enjoyed this week’s episode of the Pipeliners Podcast and our conversation with Jim. Just a reminder before you go, you should register to win our customized Pipeliners Podcast YETI tumbler. Simply visit pipelinerspodcast.com/win to enter yourself in the drawing.
Thanks for listening. I’ll talk to you next week.
Transcription by CastingWords