For the last few weeks my all posts here have been about pipeline incidents and organisational safety. Time to get back to some technical stuff …
Discussion with a client last week drew my attention to an ambiguity in the AS 2885 information on excavator teeth, required as input to penetration resistance calculations. This isn’t a definitive clarification but it might help those who need to do these calcs. (This post assumes you are familiar with the AS 2885 penetration resistance calculations.)
Appendix M, and particularly Table M3, talk about general purpose (GP) teeth, penetration (P) teeth and “tiger” (T) teeth. They look like this:
(Front and side views of typical excavator teeth. Image from report Pipeline Resistance to External Interference Phase III, UWA, November 2004.)
More photos of penetration and tiger teeth from a 36 t excavator, taken at the time of the penetration resistance field trials used to validate the formulae that later appeared in Appendix M:
In both sets of images there is a substantial difference between the tip dimensions of penetration and tiger teeth. However Table M3 in AS 2885.1 suggests that they have the same dimensions.
So what? The significance is a bit subtle, but important. It’s fairly easy for a single tiger tooth point to penetrate thinner pipes. However the single point doesn’t make a very big hole (not much bigger than the tip), and is highly unlikely to result in a full bore rupture. Penetrating with both points of a tiger tooth might make a hole large enough to rupture, but it’s much harder to get both points to penetrate. Either way, a tiger tooth has a low likelihood of causing rupture in most cases (exceptions will be very thin pipes with high hoop stress).
Penetration teeth have a tapered shape which means that if they penetrate a pipe the hole may be large and rupture may be quite likely; Table M3 has estimated hole dimensions. The same table gives penetration teeth the same tip dimensions as tiger teeth, which means such a tooth may fairly easily penetrate AND create a large hole, and that makes it hard to achieve a “no rupture” design. That can be a problem.
The concern I have is that the penetration teeth in common use look like those in the photos and do not have the very sharp tip indicated by Table M3. Diabolically pointed teeth do exist, as shown in this clip from an Esco web page:
However such very sharp teeth seem to be used very rarely. The UWA study that produced the first of the tooth images above also surveyed their frequency in Australia, and came up with this summary:
Note that the occurrence of penetration teeth is only 1%, almost too low to appear on the graph, and presumably refers to the more wedge-shaped teeth of the earlier photos since the graph came from the same report. That implies that the really sharp single points are even less common. Personally I’ve never seen one, but then I don’t spend a lot of time looking at excavators.
What’s a pipeline engineer to do? I ignore the very sharply pointed penetration teeth. I do the penetration and rupture calculations for one and both points of a tiger tooth, and for a general purpose tooth (which will almost never penetrate). In the light of this recent thinking I might also consider the very sharp single pointed tooth in special cases where there is evidence that might be used.
The risk assessment process can help. If you’ve estimated the frequency of an excavator strike on a pipeline, then the graph above suggests that the frequency of a strike by a machine with penetration teeth is going to be only 1% of that, which moves you down one step on the frequency scale.
Discussion on this topic would be good, especially from people who have more direct experience that I do with the actual equipment in use in the general construction industry.