RK - on your first point... Yes, pressure will provide a load on the pipe that causes extension. In most cases, this is ignored as the pipe is usually very stiff in the longitudinal direction. Codes typically include this effect not through the solution of F=KX but instead by simply adding an additional longitudinal stress to the sustained case as PD/4t.
In very long runs of pipe, stiffness falls and the elongation may cause bending on attached pipe. In this situation, including the structural deformation due to pressure may be significant. To include this in CAESAR II, activate the bourdon effect in translation only. The program includes this effect automatically for FRP pipe (with it's lower axial stiffness) and users should consider this in (long run) pipeline applications.
The rotation effect in CAESAR II's bourdon application is intended for use with bent pipe and not forged bend fittings. Bent pipe ovalizes in the bending process and internal pressure tends to return the cross section circular, thereby opening the bend.