The consideration of including structural deflections in piping stress analysis is all depends on the scope of piping system to be analysed.

For example for FPSO, there will be some piping which is internal to module. If you see the structural deflections of this module due to hogging & sagging of FPSO, then these will be smaller in magnitude and hence does not really affect piping much. As long as pipe is properly supported by structure, it will also move with structure. This is known as rigid body displacement.

It is a matter of concern when one pipe structure supporting one portion of pipe starts moving (deflecting) with respect to the other pipe strutcure supporting the same pipe.

This will occur in FPSO in piping connecting from one module to another or piping which is running along the length of the FPSO. These loading should be categorised as displacement loading, as the displacements are imposed on piping and hence are computed as the displacement stress range. Since the FPSO will be continuosly moving up & down, we have to take fatigue also into consideration.