External Loads on Pipelines

(Another quick post.  I still intend to continue the series on location classification but it might be a while yet.)

As I talk to pipeline engineers in different operating companies I frequently encounter misunderstanding about the significance of external loads on pipelines, such when someone wants to drive a heavy vehicle over the line.  In fact the misunderstanding seemed to be so widespread that in late 2009 I decided it would be a good topic for a talk at a POG Seminar (Pipeline Operators Group). For various reasons it wasn’t an appropriate topic for the 2010 seminar but at the 2011 seminar last week I finally got to give the talk.  So this has had a long gestation.

The essence of my message was this:

Do external loads matter?

Won’t cause failure (loss of containment)

  • Except in truly extreme circumstances
  • Worst case might be ovalling (pigging problems)
  • May cause fatigue if repeated very often

Don’t solve a problem that might not exist

  • May create unnecessary difficulty for both pipeline operator and third parties
  • Installing unnecessary protection may increase risk

A PDF version of the full presentation is here.  It includes a number of example calculations that show the very minor contribution to total stress from even large external loads.

I certainly don’t want to trivialise issues around external loads.  But I think it is important that pipeline engineers have a proper understanding of what’s important.  Don’t assume the worst – do the calculations.  They are well-defined in API RP 1102 (referenced by AS 2885.1) and they are not hard.

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12 Responses to External Loads on Pipelines

  1. Markus Seitz says:

    Peter,

    A few valid points. I agree we sometimes make more of vehicle crossings than we need to. I would like to see some examples that help us to deal with crossings outside the scope of API RP 1102: In Western Australia we often deal with non-Road legal vehicles (haul trucks) and large tracked vehicles. AS2885.3 allows “conversions” to equivalent loads in this case and the only recourse in that respect that I know is to apply Boussinesq equations for scaling.

    I would be happy to extend this topic with anyone else.

    regards,
    Markus

    • petertuft says:

      Tracked vehicles are rarely a problem, because their very nature is that they exert a low bearing pressure. Even though the total weight may be very large it is also very well spread.

      Non-highway wheeled vehicles should just fit into the API 1102 calculations I think. The calculation method does make assumptions about wheel spacing, but haul trucks often have a wider track than highway vehicles so the calculation will be conservative. If you calculated on this basis and found an acceptable result than that’s the end of the story. You’d only have an analysis difficulty if the calculated stresses were unacceptable.

      In that case, or if you are concerned about a tracked vehicle, then the Boussinesq method is probably your best option. The standard doesn’t attempt to define how to do this – it’s a matter of getting doing the right engineering to understand the loads and stress field.

      • mhseitz says:

        Peter,
        What you say about tracked vehicles spreading loads is intuitively correct. If you look at the way API1102 treats rail loads however, it would appear that widely distributed loads (even though surface pressure is lower) have an adverse sub-surface distribution: The stiffness factors for rail loads (=distributed) are far more severe than those for wheel (=point?) loads. If you treat the load exerted by a tracked vehicle like a rail load, medium-sized excavtors (Cat D7, D8) fail the analysis where I think they shouldn’t. The dilemma we face is that the load profile of a rail crossing more closely represents that of a tracked vehicle and therefore it is more prudent to use those (adverse) factors in API1102. We then revert to methods suggested by ACPA and ALA. Their disadvantage is that they don’t analyse longitudinal stresses (as API does). API’s other advantage is the fatigue analysis. That, however, is hardly applicable to occasional crossings by tracked vehicles 😉

  2. Pingback: Risk-based design: normal and abnormal loads | Pipelines OZ

  3. Terry Hobgood, PE says:

    The first-hand experience I had for equipment pipeline crossings was on a 36-inch in Turkey. Zero internal pressure. When a caliper pig was run through a test section, it showed that the pipe was deformed at a location. When my boss calmed down, he had blamed it on the lowering-in crew, then we found out that NATO had had a group of equipment cross over the freshly backfilled location during a night exercise.
    When we excavated the location, the pipe suddenly sprung back into its original shape (you could hear it spring too).

    • petertuft says:

      The zero pressure situation is probably of more concern than a pressurised pipe for exactly the reason demonstrated by your example – thanks for raising it. A long time ago before API 1102 was revised to include the GRI research we used to use alternative methods that also considered ovalling under heavy load with zero pressure, but I suspect that many people (including myself!) tend to overlook that now.

  4. Alan Pugh says:

    Some good points here but during a recent HAZID for a pipeline crossing one of our 3rd party engineers stated that tracked vehicles had a more detrimental effect on buried services than wheeled vehicles. It seemed to go against everything I have used in the past and he didn’t understand it himself. Anyone else heard this?

    • Terry Hobgood says:

      Exce[t for the NATO incident I experienced and commented on earlier, I know of no other buried pipelines buried at least 1 meter over top of pipe being deformed by any type vehicle. The NATO tracked vehicles’ excercise was an isolated case over a freshly buried pipeline. Pipelines are usually pigged. when new and later on a regular schedule, with intelligent pigs and I do not know of any reported damages of the deformed type. I have 35 years in pipeline construction. The key words are “”buried at least 1 meter deep over top of pipe”. Dirt roads have a deeper depth of burial of 4 feet or more as a specific specification requires and may also be cased with a larger OD pipe..Smaller pipes such as 4-inch above ground oil well flow lines, are sometimes driven over by wheeled vehicles and this is discouraged.

  5. petertuft says:

    Alan,

    This is quite interesting. I’ve just done some calcs myself and found that with the A160/M1600 axle load for road vehicles (which is rather extreme) the pipe stresses calculated with API 1102 are marginally unacceptable at 1.2 m cover. A Cooper E80 railway load has roughly the same bearing pressure as a D9 dozer of 50 t (but the rail case has a much bigger total load of about 145 t spread over a bigger footprint). For this load and 1.8 m cover the pipe stress is higher and 10-20% above the allowable value (varies depending on soil type). For the record my sample pipe was DN 300, 9.53 mm WT, Grade B, 10.2 MPa MAOP, hoop stress 72% SMYS which allows negligible margin for the additional loads.

    There seems to be something not quite right here. The vehicle load is 160 kN on two small concentrated footprints of about 0.4 x 0.4 m each (500 kPa). The dozer is about 500 kN on two tracks of about 3.5 x 0.6 m each (120 kPa). The dozer load is substantially greater than the wheel load, but very well spread, and the total load is much less than the rail load that was assumed to be equivalent. Perhaps the difficulty is in working out how to shoehorn the dozer load into the railway calculation. Spreading the dozer load over its total footprint (3 x 3.5 m) rather than just its two tracks reduces the bearing pressure to about 50 kPa and the API 1102 effective stress is reduced but still a bit higher than the worst-case vehicle load.

    Maybe the AS 2885 suggestion of treating tracked vehicles as equivalent to rail loads is not really appropriate (mea culpa). I take comfort from Terry Hopgood’s observations that actual deformation of the pipe is extremely rare. Also, I formed the opinion a long time ago that the research underlying API 1102 never seemed to look at possible failure modes, it just adopted stress limits without question. The stress limits are perhaps pretty arbitrary. None of which is much comfort to an engineer struggling to demonstrate compliance with the Standard.

    • mhseitz says:

      Always glad to see discussion here. May I suggest that the impact factor (1.5) used in the rail calc is over-conservative for a dozer: Dozers walk. Trains run. I have suggested to my colleagues to use less conservative impact factors when applying the API 1102 rail calc to dozers.

  6. petertuft says:

    Good point – in my hasty calculations the other day I didn’t even think about the impact factor, just took the default values. No time to rework the calcs now but it would seem appropriate to use an impact factor of 1.0 for slow-moving machinery.

  7. Pingback: Road crossings and high design factor | Pipelines OZ

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