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:
- Identify the maximum credible excavator size and worst-case tooth type (which should be based on a proper land user survey)
- Determine the location class
- From the location class select a B-factor
- 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.