Damien,
For details of jacketed pipe analysis, particularly for checking the stresses at the weld mentioned by you, in my opinion, a finite element analysis is required.
If you have ANSYS or ABAQUS you can try in any of this way:
1) Use shell element based modelling. This will give you an estimate of the stresses in the model.You can estimate the stresses in the weld by use of FSRF.
2) Use submodelling technique to evaluate the stresses in the weld where 3D Elements will be used for analysis of the location of interest based on displacements taken from remainder of the shell based model or use a shell to solid coupling where you again use 3D elements to model location of interest and shell elements for the remaining part. Both these techniques will minimize the computational effort required for a full 3D model.
3) There are some excellent discussions in a recently concluded webinar by PRG on Fatigue( presenter Chris Hinnat). Look for details of fatigue considerations in this webinar.
Hope this helps.
Regards
_________________________
anindya