B31.4 Elastic Bend Stress Allowance

B31.4 Logic assumes that installation loads for land pipelines do not control, so installation limits are not specifically stated for land works. Elastic bends are allowed; no yielding is implied with elastic bending, so there is nothing unusual to worry about there. That being said, there obvioulsly must be some limitations and fortunately limitations for offshore pipelines are specifically stated, so I also use those as limits for onshore pipeline installations too, simply to avoid any doubts of the adequacy of the installation method in meeting the pipeline's intended purpose.

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.

"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
"Being GREEN isn't easy." Kermit

Thanks for the response - I have now seen the calculation - bend radius for the elastic bend is being limited such that the stress at installation is limited to 50% of yield stress, but no reduction is being made to the MAOP. Obviously if this permanent stress is added into the combined stress there must be some reduction in MAOP. The designer has already had to increase wall thickness to accommodate surge pressures which exceed the 10% allowed under B31.4. (i.e the pipe is already designed to the limits before adding in elastic bend stresses.

Not necessarily. You can have a hoop stress that is at limits and, if axial stress is lower than its limit and combined stress is lower than its limit, you can increase axial stress with no penalty towards hoop stress and the resulting MAOP.

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
"Being GREEN isn't easy." Kermit

Biginch Thanks for the reply - sorry for the tardy acknowledgement.

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.

Ya. In Texas its absolutely prohibited to use anything with the letters "BS" in it.

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
"Being GREEN isn't easy." Kermit