B-factor and penetration resistance

There is sometimes confusion (or unwarranted conservatism) over factor B in the penetration resistance calculations required by AS 2885 and described in Appendix M.  I look at it this way:

  • B = 0.75 provides the borderline between penetration / no penetration if the excavator is being operated in a normal responsible manner, and is fine for most situations.
  • B = 1.3 is the limiting condition for the worst possible case; penetration should be impossible regardless of how hard the excavator operator tries.

Table M5 in AS 2885 uses different words but says much the same thing.

The paragraphs following Table M5 explain where the B values came from.  I was involved in the second of the field trials mentioned, when a 36 t excavator assaulted a DN 500 x 12.7 mm WT pipe – quite entertaining.  Watching it gave me confidence that a 36 t excavator cannot penetrate 12.7 mm pipe, because I saw the vigour of the attack and the numerous attempts from different angles.  That gave me an appreciation that doesn’t come from reading the dry descriptions in Table M5.  Unfortunately only a few of us shared that experience, but I hope that by reiterating it here other pipeline engineers will overcome the fear that any B value less than 1.3 will result in punctured pipes all over the place.  Even B = 0.75 provides a great deal of resistance to penetration, it just isn’t quite absolute.

Here are a few pictures from those trials (12.7 mm WT X42, 36 t excavator):

The excavator pressed a penetration tooth down on the pipe until the front of its tracks lifted off the ground then dragged the tooth along. The gouge is moderately serious (a couple of mm) but not remotely close to penetration.

This is what happened when the penetration tooth was hammered onto the pipe as hard as the operator was prepared to hit without risking his machine. A very serious dent but the actual gouges are only 5 mm deep, not even halfway through the 12.7 mm wall.

This is typical of the worst that a tiger tooth could do. In this case the bucket was wedged against the back wall of the excavation and levered against the pipe. Again a very long way short of penetration.  (A single point did not penetrate either.)

 

It’s not easy to put a hole in thick pipe.

 

 

Advertisements
This entry was posted in External interference, Pipeline design, Standards. Bookmark the permalink.

8 Responses to B-factor and penetration resistance

  1. Chris Hughes says:

    The two factors that I find cropping up most often in SMS workshops are:
    what size excavator will you actually see operating in the vicinity of the pipeline, and
    what type of teeth are likely to be fitted to the bucket?
    Around T1 and T2 (“No Rupture Pipe”) areas it is extremely unlikely that you will find excavators greater than 10-15 tonne working outside of major construction sites, and most of these will have GP teeth rather than tiger or single point, and yet I come across designs which are based on resisting puncture from 35 tonne excavators with single point teeth.
    The greatest threat to a pipeline in these areas is probably not an excavator at all but a mini HDD rig installing a new service (power, telecom) across the pipeline: it is generally accepted that if the HDD operator doesn’t realise he’s hit the pipeline he can keep drilling until the pilot makes a hole in the pipeline, and there is no form of physical protection which can prevent that.

  2. petertuft says:

    Chris,

    Until recently I might have agreed with you about the frequency of excavators larger than 10-15 t in urban areas but lately I have noticed that machines in the 30 tonne class seem to be the tool of choice for demolition of houses in my neighbourhood (where there is a trend to replace tasteful post-war double-brick houses with pretentious brick-veneer monsters, but I digress). Admittedly those machines are not digging in the street but they are surprisingly common. And the 36 t machine that did the damage shown in the photos was from a contractor who specialises in installation of urban services.

    Then there is the question of teeth. I get different answers in every workshop I run. Sometimes the ground conditions in the area require tiger teeth. Other times, even in sandy Perth for instance, people familiar with the local construction practices insist that they cannot be ruled out because (for example) they were fitted to the machine for its last job and the contractor didn’t bother changing them. Much as we would like to dismiss the possibility of tiger teeth, sometimes that is hard to justify.

    I definitely agree with you about the mini HDD threat. It bothers me considerably. Apart from anything else the rig might be set up a long way from the pipeline so even daily patrol will have trouble spotting them unless they explore up every side street along the pipeline route. There are some very preliminary discussions in the Energy Pipelines CRC about research into rotary impact equipment (HDDs, augers, boring rigs) analogous to the excavator research underlying Appendix M.

  3. darrenholt says:

    Hi Peter,
    Interesting post thankyou.
    I assume these tests were done with an empty pipe. It strikes me (pardon the pun) that the outcome of the same impacts to a pipe operating at close to MAOP with significant hoop stress would be noticably different. I haven’t considered the mechanics of penetration in great detail, but it is intuitive to me that a reasonable hoop stress would reduce the energy required for penetration and favour penetration over denting.
    This is a thought that has dogged me regarding the approach presented in appendix M of the standard as it doesn’t seem to have been considered in empirical results of testing.

    I’m not really sure if it is signficant or indeed valid – do you know if it has been considered in any supporting research?

    • petertuft says:

      The original work at UWA did consider internal pressure and found it to be a secondary effect. If one thinks in terms of a tensioned membrane like a balloon then internal pressure is obviously relevant. However a steel pipe is a pretty stiff structure. The loads applied by internal pressures even as high as 10-15 MPa are relatively small compared to the forces required to deform and penetrate the steel wall. There probably is an effect of internal pressure, but it’s hardly worth worrying about given the gross uncertainty in so many other factors that affect whether a pipe will be penetrated.

  4. Phil Venton says:

    Just a caution – puncture resistance is not everything.
    A dent and gouge combination is very dangerous – there is a synergistic combination that can result in pipe failure at a significantly lower pressure than would occur if either the dent or the gouge was present by itself. For this reason Part 1 requires dent-gouge combinations to be cut out.
    Should a dent-gouge combination be found the pressure should be immediately reduced to a safe level while plans are made to remove the defect, or to secure it with reinforcement.
    Note that a 10-15% pressure reduction may not be sufficient to reduce the failure risk sufficiently to allow the defect to be assessed.
    Phil Venton

  5. Lynndon Harnell says:

    This subject should be sorted out well before SMS, and has been a hotly debated topic where I work.

    AS2885.1 Appendix M contains data sufficient to predict puncture resistance for excavators up to and including 55t, however there is at least one hire company in Australia that will hire a 75t excavator. General rural earthworks contractors predominantly have excavators ranging up to 30t with a few 35t machines available. This size is I understand the largest you can get on a float without a police escort making them inconvenient for general useage. More specialised contractors (such as this that might construct a highway, dam or GPF) commonly have machines up to 45t. A 55t excavator is a fairly rare beast.

    The effect of both GP and Tiger teeth can be mitigated by the mandating of cutter bars or bucket without teeth within an operational area such as CSG. AS2885 excavator teeth calculation methods are effectively silent on this technique. Within a CSG operational area, tiger teeth can be banned excepting under rigorously controlled circumstances.

    Only by analysis of data collected by land management can you get a realistic definition of the size of machinery in the area.

    • petertuft says:

      Agree strongly with your first and last paragraphs – all the data needs to be gathered in a rigorous way and the calculations done before the workshop. (And I would not argue with your other comments either.)

  6. Pingback: Penetration resistance – existing pipelines | Pipelines OZ

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s