I am involved in FPSO project and it is a requirement that aside from ship accelerations the structural deflections due to its effect be included in the analysis. Hogging and sagging are also included but only if piping is running from one module to another.

We are having a problem for we are encountering huge horizontal structural deflections due to structural group design criteria of H/300. We requested them to decrease the deflections but the minimum they can provide is still significant for piping analysis. It became very problematic to qualify nozzle loads most particularly rotating equipments without resorting to excessive flexibilty or using flexible joint and since it is FPSO piping, natural frequency will be another issue.

In the past, usual FPSO piping analysis doesn't consider such anymore. I would like to know if you have encountered such requirements and how did you deal with such cases. I feel that we are going over conservative with this approach considering that our part is mostly seawater piping.

Hi,
General comment: -
1. Consider these deflection as a fatigue type.
2. Consider the stiffness of the structure in your calculation.
3. If possible then consider the equipment supporting structure stiffness as well.
4. You can refer ASME appendix W (high pressure fatigue assesment of piping system) and also take a note that when performing fatigue analysis as per DNV-RP-C203, all SIF shall be replace by SCF.

It would be greate if you can flash some light on the type of load cases used for the calculation and how you are combining the displacement.

Any commnets are welcome.

Regadrs,
R.K.