Penetration resistance – existing pipelines

I’ve written before about penetration resistance and the B-factor as specified by AS 2885.1 Appendix M, but that post didn’t distinguish between design of a new pipeline and review of an existing line.  From time to time I see confusion in this area so it seems worth attempting a clarification.

The design task is relatively simple (emphasis on relatively).  For each location along the pipeline:

  1. Identify the maximum credible excavator size and worst-case tooth type (which should be based on a proper land user survey)
  2. Determine the location class
  3. From the location class select a B-factor
  4. Calculate the wall thickness required to resist penetration by the selected excavator and B-factor

All that is fairly well described in Appendix M.

But existing pipelines also need assessment of penetration resistance as part of the safety management study review.  Here the objective is quite different because the wall thickness has already been determined and isn’t going to change (except in truly extraordinary circumstances).  Rather the purpose of the penetration resistance calculations, as I see it, is to provide data that can be used during risk evaluation to support judgements about the likelihood of penetration by various types of equipment.  There are no design criteria as such, it’s just a calculation for information.

My approach is to use the Appendix M equations to work out the sizes of excavator that can penetrate at B = 0.75 and at B = 1.3.  I view the B = 0.75 case as the condition in which an excavator digging normally has a reasonable chance of penetrating if the impact conditions are optimum (which is not the same as saying that penetration is probable).  The B = 1.3 case is a lower bound to excavator size at and below which no machine has any chance of penetrating no matter how aggressive the assault.  In between there is a progressively lower probability of penetration.  I find this provides a useful basis for making judgements about the likelihood of penetration when assessing any given threat – whether it’s a backhoe digging for utility maintenance or huge machine working in a big gravel pit.

I’m sometimes asked why I don’t do the calculation for B = 1.0, as listed in Appendix M.  In the light of the explanation above I hope the answer is apparent:  B = 1.0 is just a point in the middle of the range and has no particular significance.

This approach won’t work if you don’t have a basis for estimating the size and type of machinery that might be digging near the pipeline.  I’ve written about that before.  Having that data is important for both design and operational review.

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3 Responses to Penetration resistance – existing pipelines

  1. Chris Hughes says:

    A very valid point Peter, and one we had to take into account some years back when we performed an SMS on all Gasnet’s high pressure lines in Victoria. When looking at “No Rupture” requirements the magnitude of the threat is probably the most significant aspect – a fact that has been brought home to me recently on a couple of new pipelines where the designers have used resistance to a 55 tonne excavator as their criterion without any consideration of the probability of such a machine ever working near the pipeline.

    This also brings up the question of dealing with the extension of protection a measurement length into a lower class area – say when a pipeline leaves an urban (T1) area and enters an R1 area. The “No Rupture” requirements for the T1 area may only need to consider a 5-15 tonne excavator as the possible threat, whilst the same requirement in the affected R1 area may need to consider a D9 dozer with a ripper – clearly a more damaging threat requiring stronger pipe to resist it (or greater depth of burial or other protection).

    So to emphasise your last point, knowledge of the possible threats is an absolute requirement of safe and cost-effective pipeline design.

  2. Ross Cochrane says:

    Unfortunately it is normally not possible to ascertain the actual penetration resistance threat requirement especially in the preliminary phases of a project in a timely fashion. Ie within project schedule constraints and particularly in Concept and FEED project stages. To obtain threat data from the various property owners usually take months and then the property owners will not necessarily come forth with a relevant reply. For low pressure pipe lines where no rupture pipe is required the penetration resistance will probably be the critical criteria in selecting the pipe wall thickness. So the pipeline engineer must select an arbitatory penetration resistant requirement and in keeping with normal engineering philosophies obviously a fairly conservative is selected. On this basis the 35T requirement has normally been selected based on our experience.
    If the standard could provide a better selection guide this would be helpful in enabling the pipeline engineer to select the required penetration resistance.

  3. petertuft says:

    Ross,
    You are right about the uncertainty in the early stages of design, and the need for a conservative approach. The 35 t excavator is a reasonable conservative assumption. Of course, in an ideal world project proponents would schedule things so that proper engineering and design can be done. If other imperatives drive them to an accelerated schedule then they will have to wear the cost of possible over-engineering. The iron triangle of project management: low budget, fast schedule and high quality but you can never have more than two out of these three (if that).

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