Residual stresses in roped bends

[Hurricanes]

We have an above ground oil line made from linepipe. Along the pipelines run there are multiple roped bends. One of the roped bends has a bend radius smaller than the others. When the stress in the pipe due to bending the pipe into place is calculated, it is found to be significant.

I am assessing this pipeline to AS2885.1 and B31.4. The strain in the bend is less than the maximum recommended by AS2885. Section 5.7.5 (b) in AS2885.1 states that for unrestrained pipe the combination of the stress in the bend plus either the operating stresses or hydrostatic stresses should not lead to uncontrolled strain.

So I take that as meaning simply check the pipe to make sure it won’t yield during hydrotesting or operation at the bend.

Another engineer here argues this section in AS2885.1 means the residual stress in the bend should be combined with the sustained stress and assessed to the sustained limit of 0.54% of SMYS. As the pipe is threaded and we therefore have SIFs of 2.3 along the bend, it fails this criterion.

If we were going to combine the bend stress and assess it to an allowable, I would have thought combining it with the expansion stresses and assessing it to 72% of SMYS would be more sensible, but 5.7.5 (b) does say check with operating stresses.

Does anyone have any thoughts on this? Code interpretations to B31.4 are welcome.

 

[BigInch]

Residual stresses are usually ignored in design of ductile materials for static loads. This is true for B31.4 except as stated in A402.3.4, where they are the consequence of a deliberate action during the erection procedure. That puts them in the 0.8 SMYS limitation on the sum of sustained and operational longitudinal stress (for restrained or unrestrained pipe).

Independent events are seldomly independent.

 

[Hurricanes]

Thanks. I note that buckling seems to be the main concern in A402.3.4, and an allowable strain limit may be used - 0.5% as stated in AS2885.1 seems a good limit to use.

 

[BigInch]

I'm not looking at it, but I think the general strain limit in 31.4 is 1%

Independent events are seldomly independent.