Hi all.

I hope this makes sense and that I haven't got myself totally confused (quite possibly)...

A vibration survey has been carried out on a line, at a number of locations in the system there are noticed vibrations/displacements. Therefore, a fatigue assessment has been requested.

Obviously, the system vibration/displacements were measured at hot conditions. Therefore, how do you model this in CAESAR II? I know that as soon as a displacement is applied in the input it constrains the specific nodes in those degrees of freedom for all loadcases regardless of whether the displacment is called into the loadcase or not.

What I'm asking is if from ambient to hot position the pipe has a moved to say a position 10mm, 10mm, 10mm (dx, dy and dz respectively) and the vibration displacement is noticed to be 5mm and 6mm (dx and dy respectively) by entering the 5mm and 6mm in the displacement box, at the specific node location, results in restraining the pipe to that position whereas in reality would it not be the following?

dx = 10mm +/- 5mm giving 15mm or 5mm
dy = 10mm +/- 6mm giving 16mm or 4mm
dz = 10mm

I believe I should consider the fatigue at the hot position as that would give largest displacement but am now doubting myself as cant seem to do it.

Unless I have to undertake multiple runs as follows:

1 - normal run no applied disps to give system movements.

2 - apply system movements as D1 loadcase and add the vibration displacements as D2 loadcase and build loadcases to add the D1 +/-D2

W+T+P+D1 - hopefully will give similar results to run 1 results
W+T+P+D1+D2
W+T+P+D1-D2

Then do relevant loadcase subtraction to do the FATigue loadcase.

Any insight/help very much appreciated as scratching my head here, its a large model so the above approach could be problematic.

Many thanks.