External Interference Protection
References: AS2885.1 Section 5.5 AS2885.1 Appendix E AS2885.1 Section 5.4.5(b)(ii)
Q: In regards to industry testing of penetration resistance, what effect would having the pipe under pressure have, versus testing on an empty pipe?
(Peter Tuft) Any effect of internal pressure is insignificant. Given the huge uncertainty in these calculations it would be very much a secondary influence. The resistance to penetration is provided by the strength and bending stiffness of the steel, and the failure mode is largely bending and shear. In contrast, puncturing something highly flexible like a balloon involves stretching the membrane until it fails in tension. Hence the tension from high internal pressure is additive to the external load and much more significant than for a stiff structure like a pipe wall.
Q: The standard again takes account of the use of excavators and impacts on pipelines; has HDD damage also received the same attention?
(Peter Tuft) HDD damage has been recognised as needing the same attention as excavators, but the information necessary to write rules in the Standard is simply not yet available. The EPCRC has a research project in this area. We can hope that good guidance information will be available for the next revision (and any such guidance can be used as soon as it becomes available, without waiting for the Standard to be revised).
Q: Are plastic slabs recognised as a means of physical protection?
(Peter Tuft) They are recognised but in quite a subtle way (i.e. not named as such). See Part 1 Clause 5.4.5(b)(ii). Paragraph (D) says other barriers may be used, and a later paragraph was written specifically with plastic slabs in mind: "Where barriers comprise discrete segments, they may not be counted as a PHYSICAL CONTROL unless the segments are large enough, or sufficiently interconnected, that the design THREAT cannot remove a segment and leave the pipe exposed. Barriers other than concrete encasements or coatings should have a separation of at least 300 mm from the pipeline."
Q: Has anyone tested or reviewed the adequacy of plastic overpipe protection slabbing?
(Jeff Jones) Yes, there has been work done by APA and SEAGas, refer to presentations available on the APGA website. Be cautious if trying to claim a Physical Control versus Procedural Control. Refer specific Part 1 wording for use of non-concrete slabbing in the design case Part 1 S 5.4.5 (b) (ii). Application of plastic slabs for existing pipelines as part of achieving no-rupture ALARP assessment is a no-brainer, especially if concrete barrier would not be cost-justified by the owner.