The structural deflection shall be imposed on all the piping supports on the platform.This movement shall be imposed through CNODE in Caesar-II.Sometime you have to provide a loop in order to absorb this relative deflection between the two platform.
There could be some other movements as well like Jacket settlement.Fatique analysis is very common in the offshore analysis.Your piping flexibility specification will tell you about that & code to be followed for the fatique analysis to calculate the allowable stress values.Also very typical in the offshore piping is the blast analysis.That you have to impose on your system as wind force.

Hope it will help you.

Cheers
Saneh