I am analyzing a 30" diameter FRP system at 100°F and 140 psig. I am getting really high loads from thrust due to pressure. From hand calcs using the formulas in the mfr's catalog, the loads should be opposite what CAESAR is telling me. In other words, the pipe should effectively contract (pressure expanding the pipe radially would contract the pipe axially). COADE's stance is that the program implements Eq. 7.3 of BS 7159, but disagree as to whether the equation itself is correct (Did I state this correctly, Richard). It turns out that elongation due to pressure (Bourdon effect) is always larger than contraction due to pressure (radial expansion), according to Eq. 7.3 of BS 7159.

Basically, I'm getting conflicting results from CAESAR and mfr data.

Do any of the wizards out there have similar experiences they'd like to share?

TIA,

It is possible to design FRP pipe to contract under biaxial pressure loading (i.e. simultaneous hoop and axial loading), but no commercially available FRP pressure pipe that I am aware of is designed to do so. FRP pipe, like pipe made from any other material, will expand axially under biaxial pressure loading.
If the pipe is exposed to hoop pressure loading only, as could be the case for axially-unrestrained joints (eg. O-ring joints, un-tied bellows, etc.), the pipe will contract due to the Poisson effect (radial expansion). This is true of any piping material, not just FRP.

Bruce Hebb
Chief Engineer
Reinforced Plastic Systems Inc.

The words of Bruce Hebb reminded me of one of the results of modeling expansion joints. CAESARII applies an expansion joint pressure thrust as a force at the ends of the expansion joint element -not at the ends of the pipe run (elbow, tee, cap). In reality the pressure thrust at the pipe ends will result in tensile axial stress with an expansion joint, but with CAESARII forces applied at ends of the expansion joint element, the pipe's axial stress would be computed to be compressive when pipe elbows are restrained.
Since the size 30" pipe area and 140 psi produces 100k thrust loads, I would suggest that tie rods be used for thrust restraint ( between elbow pairs ; tie-backs to vessel ), instead of designing structural steel to restrain the high loads. If the size 30" FRP is routed underground, then concrete thrust blocks at the changes in direction could be best restraint.

I don't think tie-rods are practical for 15'+ runs of fiberglass pipe. And this is a seawater lift pump discharge system on a SPAR in the GoM.

But thanks for the input. If anyone else has any comments, I'd love to hear them.