B31.4 Elastic Bend Stress Allowance
B31.4 Elastic Bend Stress Allowance
(OP)
We are laying a buried 72' diameter CS grade x65 with 15.48 mm wall thickness with Cement Mortar Lining for water supply.
Contractor proposes to pull elastic bends to follow the horizontal alignment. I am not familiar with B31.4. Does the standard permit the use f elastic bends, is there any limits on the stresses. Presumably the residual elastic stress will need to be taken into account in the overall stress analysis and there will be a reduction in the allowable pressure rating.
Contractor proposes to pull elastic bends to follow the horizontal alignment. I am not familiar with B31.4. Does the standard permit the use f elastic bends, is there any limits on the stresses. Presumably the residual elastic stress will need to be taken into account in the overall stress analysis and there will be a reduction in the allowable pressure rating.





RE: B31.4 Elastic Bend Stress Allowance
Hoop 0.72
Long 0.80
Tresca, or Von Mises, Combined Stress Limit is 0.90
Field work should obviously not reduce MAOP, so bend radaii must be limited to those elastic stresses that will have not net effect on MAOP.
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RE: B31.4 Elastic Bend Stress Allowance
RE: B31.4 Elastic Bend Stress Allowance
Each is checked independently, until you get to combined stress, but there is a higher Design Factor for Longitudinal and Combined stresses.
Hoop stressa allowables are calulated using design factors of a0.4, 0.5, 0.6 or 0.72.
For example Sh/Sy = 0.499, checked against 0.50 DF * SMYS
right at the limit.
Longitudinal SL/Sy = 0.6731, but checked against 0.80 * SMYS
Combined Sc/Sy = 0.7917, but checked against 0.9 * SMYS
You'd be right, if you checked each type of stress using the same hoop stress factor.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv
RE: B31.4 Elastic Bend Stress Allowance
You have suggested in the past that I should go out and buy my own copy f B31.4 and I succumbed and done just that. (I have to give up trying to get the rest of the world to work to BSs)
The situation appears straight forward - B31.4 - 434.10 covers the assumption that the pipeline in land should be relatively stress free after laying.
if I go to Chapter IX Section A402.3.5, as you suggest, and set the hoop stress to 0.72 Sy and the Longitudinal stress from elastic bending to 0.5 Sy (stress from bending to 500D) then the combined Von Mises stress is 1.06 Sy against an allowable of 0.9 Sy. To be within the 0.9 allowable I have to reduce the hoop stress to 0.53 Sy. My conclusion therefore is that the Elastic Bending pulled by the contractor (without approval) is not acceptable. Furthermore if all of the allowable longitudinal stress is used in elastic bending then there is nothing left for unaccounted residual laying stresses. I.e If the pipe was laid as required by Clause 434.10 to be relatively stress free then we could accommodate unaccounted for longitudinal stress of up to 0.25 Sy in combination with hoop stresses of 0.72 Sy before the combined Von Mises stress exceeded 0.9 Sy.
RE: B31.4 Elastic Bend Stress Allowance
Looks right to me. Close to natural sag under its own weight. The alternative is field cold bending.
I would remove transient pressures from the hoop stress when calculating the combined stress, since you are allowed to go 10% over MAOP with transient pressures, but an equivalent limit increase is not specifically stated for combined stress including transient pressures. After you have done the combined stress calculation without transients, just make sure your transient pressures do not create a hoop stress > 1.10 * MAOP. It might make a small bit of difference.
"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
http://www.youtube.com/watch?v=hpiIWMWWVco
"Being GREEN isn't easy." Kermit
http://virtualpipeline.spaces.liv