Wall Thickness

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Is is really correct that Equation 5.2.5(2) for wall thickness at the extrados of an induction bend allows a thickness less than for straight pipe with the same design factor?

Clause 5.2.5 gives equations for the wall thickness of bends. This is relevant to induction bends in particular because the bending process can result in non-trivial thinning of the wall on the extrados (outer radius) of the bend and thickening on the intrados. However these requirements have sometimes been misunderstood or misinterpreted in two different ways:

Firstly, although bending causes thinning of the pipe at the extrados it also happens (fortunately) that the thickness required for pressure containment is indeed less at the extrados (and increased at the intrados). So even though WT is reduced by the bending process it may (but not necessarily) still be acceptable for pressure containment. This may (but not necessarily) apply even when the extrados thickness is less than required by the pressure design factor for straight pipe.

Secondly, the equations in Cl 5.2.5 have sometimes been interpreted as giving the WT changes that are caused by the induction bending process. This is not correct. The Standard does not address the bending process, it just specifies the requirements that the completed bend must meet.

Engineers specifying linepipe and induction bends may need to liaise with induction bending contractors in order to understand the thinning that may occur and hence to specify the wall thickness of the linepipe to be provided for bending. (Peter Tuft)

Details of the theory behind Clause 5.2.5 can be found in Issue Paper IP 4.24 written in 2003 for the 2007 revision of Part 1.

In Clause 5.2.8, why are tolerances not required to be considered if pipe is manufactured from strip or plate but “may” be considered for seamless pipe? This means when using a design factor of 0.8 for a material such as API 5L ERW (which has a wall thickness tolerance of ± 0.1t), the actual hoop stress in the pipe could potentially be as high as 88% SMYS.

A combination of the API 5L tolerance on pipe weight and the nature of the ERW manufacturing process (from rolled strip) means that uniform 10% underthickness in ERW pipe is not possible. Specifically, API 5L section 9.14 requires that mass per metre of pipe be no less that 98.25% nominal on average and 96.5% for an individual pipe. The ±10% tolerance then deals only with local thickness variation, which inevitably is reinforced by adjacent material and has no effect on integrity.

On the other hand seamless pipe is made by creating a hole through a cylindrical billet and the hole is sometimes not in the exact centre of the billet so that underthickness along the full length of a pipe joint is possible even though the pipe is within the weight tolerance.

It is true that if pipe has uniform 10% underthickness then the hoop stress would be 88% SMYS if the design factor is 0.8. For this reason Clause 3.2.2(a)(iii) (mentioned in the note to 5.2.8) requires special consideration of wall thickness tolerance when design factor exceeds 0.72.

Finally, there is some conservatism in all this because even if uniform 10% underthicknes in 0.8 DF pipe did occur, 88% SMYS is not a stress level that would lead to failure; at least not in straight pipe that is not subject to other stresses such as thermal expansion effects at bends. Rupture will occur only when the stress exceeds not just the yield strength but the tensile strength. Clause 3.5.2 puts a limit on pipe yield/tensile ratio for various reasons including this. (Peter Tuft with advice from John Piper)

If the design factor for a buried pipeline is typically 0.72, what is the recommended design factor for an aboveground pipeline? (from APGA Webinar on Design Factor, 3 Mar 2021)

Design factor is purely for pressure containment and is unaffected by whether the pipe is buried or aboveground. There are many other considerations in selecting wall thickness as nominated in AS 2885.1 Clause 5.2.3. The wall thickness for an aboveground pipeline will also be influenced by consideration of (at least) damage resistance, stress/strain, fracture control, corrosion allowance, etc. The influence of these may vary greatly depending on the installation conditions. These could range, for example, from a high pressure flowline in a remote outback gathering network to a low pressure transfer line associated with a port or terminal in a urban/industrial area. (Peter Tuft)