(Getting very busy again, hence it’s been a while since my last post. But I have a few things I want to write about as time permits.)
After the previous post on penetration teeth I received some feedback from Phil Venton, who knows as much about this as anyone because through the APIA Research and Standards Committee he initiated and guided the research project at Uni of WA that eventually yielded the guidelines in Appendix M of AS 2885.1. He was also largely responsible for the survey of tooth dimensions that makes up Table M3.
His main point is that the information about teeth in the Standard is only for guidance – a comment that applies to a great many things in AS 2885. On this particular subject, pipeline engineers have a responsibility to do sufficient research to determine what threats apply to a pipeline in each location. This includes identifying what excavators are operated, and what teeth are used.
For a new pipeline this information (and other threat identification) should be obtained through a land user survey. Someone on the project team must talk to landowners, land occupiers, councils, road authorities, utilities, contractors, etc to develop a comprehensive understanding of the type and size of threats to the pipeline. In my experience this is a neglected task on too many projects, although to be fair I should also note that some companies do it well.
Failure to gather the this information may lead to an under- or overestimate of the threats to the pipeline. Overestimating it may result in over-design and unnecessary cost (eg. thicker pipe than really required). Underestimating it results in increased risk if the pipeline design and its operation are not appropriate for the actual threats. I recall one project in a remote part of northern Australia where we were very surprised to find that one pastoralist had a large fleet of very heavy earthmoving equipment, including D9 rippers, used for turning savannah woodland into pasture. Without the land user survey we would have assumed that there was nothing much more than 4WDs and small tractors in the area.
Getting back to penetration teeth, whatever data the designer decides to use (information from the standard or elsewhere), they should be able to demonstrate that it is in fact valid for their specific pipeline. Of course, on a pipeline of any length there may be different threats at different locations, so different designs may be appropriate too.
To illustrate the variability of this sort of data, there were significant differences between the tip dimensions of the teeth used in the original Uni of WA research and those used in the full scale tests whose photos were included in the previous post – the latter were sharper. And all those teeth were new but the tip profiles change dramatically with wear.
So my closing comment is to repeat a common theme in this blog: Don’t get too obsessed about precision in the calculations – their purpose is to provide a sound and justifiable basis for making informed judgements. The judgements won’t be sound if the calculations are precise but based on guessed input values.