Thankyou for your response CraigB. I agree bourdon effect has to do with the tendency of elbows to straighten however this is the term Caesar uses for both rotational and translational pressure elongation effects so therefore i have used it here.

As i see it, the pressure will cause the pipe to elongate a certain amount according to the pressure applied and generate displacements in the piping. Once this displacement reaches its set amount then it will stop whether or not the pipe has yielded and therefore the stresses due to the elongation are self limiting. Of course you would still need to also consider the other effects of pressure that are not self limiting seperately.

The tee that is causing problems is at the pig launcher just afer the pipeline comes above ground. The only practicle place for an anchor is below ground and as you point out the loads for a pipe this size are astronmical (over 600tonne). Even with this anchor modelled and allowing for some movement of the anchor, the displacements at the tee in question are sufficient to cause overstress in the sustained case when the pressure elongation effect is modelled in caesar. I have calculated pressure elongation to be equivalent to approx 32degC temperature rise. It seems over conservative to require the displacement stresses due to pressure elongation be incorporated into the sustained stress requirements.