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.