Reference: AS/NZS 2885.1 Appendix E Resistance to Penetration
Should destructive testing for puncture resistance be mandated for new pipelines?
While destructive testing is highly desirable it is too onerous to mandate for all new pipelines, especially those that are entirely in remote areas, are similar to existing pipelines for which data exists, or are very minor pipelines with a small budget. Nevertheless anyone building a significant new pipeline is strongly encouraged to include penetration resistance testing to add to the industry data. AS 2885.1 requirements for penetration resistance have been validated by only two actual tests (as at Feb 2020). (Peter Tuft)
In calculating penetration resistance Equation E3(3) uses the required wall thickness (tw). This seems counterintuitive in that no credit is given for additional thickness if the actual thickness exceeds the required thickness.
Part 1 is a design standard, so for the purpose of design it is quite reasonable to use tw for penetration resistance calculations. This partly because the nominal WT (tN) may not have fully been determined at an early stage of design and partly because penetration resistance calculations should not take credit for things like corrosion allowance that may be lost over time.
If the penetration resistance calculations are for an existing pipeline then it would indeed be reasonable to use tN less any sacrificial allowances.