It is assumed that a frequency domain analysis is carried out when the analyzed structure system is at its equilibrium location, which should either be imported from the previous estimation (discussed in Equilibrium Estimation and Stability Analysis of Structure System) or directly defined by the user. Mooring line stiffness, hydrostatic stiffness and the stiffness due to reaction forces at articulations are re-estimated at this equilibrium location. An Aqwa frequency domain evaluation of the system response does not directly include system nonlinearities in its analysis. Some nonlinearities, such as wind drag force, the drag force on Morison elements, or dynamic cables, are firstly linearized and then applied to the motion response calculations.