Road crossings and high design factor

The rules for allowable stress at road crossings are given in Clause 5.7.3(c) of AS 2885.1.  Recently I had an interesting query which illustrated a bit of a gap in these rules.

Basically the combined equivalent stress should be calculated according to API RP 1102 and must not exceed 72% SMYS at formed road and track crossings but may be up to 90% SMYS at informal crossings where (for example) a farmer may drive a vehicle across the pipeline on infrequent occasions.  All of that is fine for design of a new pipeline.

The problem arises when someone wants to put a road or track over an existing pipeline at a location where the wall thickness corresponds to a design factor of 0.8.  There is just no way to achieve strict compliance with the stress rules – the hoop stress alone may be up to 80% SMYS so a combined equivalent stress of only 72% SMYS is clearly impossible.   The pragmatic thing to do is to build a bridging slab over the pipeline to isolate it from the vehicle loads (and it may also provide external interference protection).  If the slab is properly designed to distribute the load away from the pipe that will be a perfectly satisfactory way of protecting the pipe against high stresses.

But it still won’t comply with the current words in the Standard.  It’s a legalistic problem, not a practical problem.   The combined stress will exceed the 72% limit set by the Standard for a formed road crossing, even though the pipe will experience no greater load than in the adjacent paddock.

Now that we have increasing numbers of pipelines built with design factor of 0.8 this is a question that might arise more often.  Common sense should prevail – put in a bridging slab and satisfy yourself that the pipe stress state will not be adversely affected.  For anyone who is worried about black-letter compliance  I suggest you refer to Clause 1.6.2 of Part 0 which deals with departures from the Standard.

A final comment:  It’s fair to say that the principle behind these rules for transverse external loads is that there is a wealth of history of successful operation with pipelines designed for up to 72% SMYS at road crossings.  Operating at 80% SMYS obviously reduces the margin for additional loads.  It just seems prudent to set a lower stress level for situations (such as road and rail crossings) where the pipe is in a more complex stress state.  At busy road and rail crossings there is the added complication of possible fatigue.  The increased limit of 90% for informal crossings is consistent with the higher stresses usually tolerated for occasional loads, and the level of comfort with this is increased by  recognition that even under gross overload the most serious failure mode is likely to be ovalling of the pipe, not loss of containment.  There is a little more on this in an earlier post.

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2 Responses to Road crossings and high design factor

  1. Chris Hughes says:

    This is another example of where strict adherence to the wording of the Standard makes a practical solution to a problem difficult to achieve: it is all very well to refer to Clause 1.6.2 of Part O but using this clause requires the Licensee to take responsibility for approving the change and I know how difficult it can be to persuade an owner that the intention of the Standard is more important than the actual words written in it. We had a similar situation a couple of years ago which we addressed by the use of a concrete bridging slab, but in that case the Licensee understood the problem and was happy to approve the design.

    Another example of this is cased crossings. RP1102 assumes that the steel casing pipe takes all the load from the crossing and the carrier pipe is centralised in the casing using insulating spacers: however current practice frequently calls for casing pipes to be either steel or concrete and for the annulus between the carrier and the casing to be pressure grouted to maintain a pathway for the cathodic protection. How do we deal with this? Obviously the traffic loads will be transmitted in part to the carrier pipe, but a proportion will be taken by the casing and the grout and it is only by considerable use of FEA that we could determine what the actual carrier pipe stresses will be. Usually we design these (with regard to pipe thickness) as if they were uncased with the assumption that this is conservative design, but is this correct?

  2. petertuft says:

    Chris,
    Some of us are having a parallel off-line discussion about the issue of strict compliance with the standard. It was never intended to be absolute gospel on all matters, because there will always be situations (such as this one and your casing example) that were not contemplated by the members of the committee (who are only human); I’m sure you know that, but the problem is the people who don’t.

    AS 2885 has tried hard to make clear that it is a not a cookbook and that its use requires judgement and experience. Relevant bits are Clause 1.1 of Part 1 and Clause 1.3 of Part 3 (2nd para). But it is expressed most clearly in the last paragraph of Clause 1.3 in Part 0:

    “The Standard is not an instruction manual for untrained persons or a detailed specification. Although certain sections of the Standard contain specific requirements, these requirements do not replace the need for appropriate experience and competent engineering judgement.”

    I think I need to frame that and put it above my desk so I can quote it more easily.

    Getting back to the technical side, what is the failure mode in the grouted cased crossing? Is there one? As I see it the pipe is subject to a displacement-controlled loading, not a force-controlled loading, so I would argue that provided the casing/grout combination has strength to resist collapse there is no conceivable failure mode for the carrier pipe, and hence consideration of stresses is irrelevant. Refer back to “The Standard is not an instruction manual …”

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