The determination of the motions of a moored floating structure system in response to environmental forces (i.e. wind, waves, and current) and structure control mechanisms is a complex procedure, which may include system nonlinearities (such as mooring line nonlinearities) and position-dependent environmental loads, as well as any motion control mechanisms. For a systematic parametric study, given the large number of possible combinations of environmental conditions, a time domain analysis would be prohibitively time-consuming; however, a frequency domain evaluation of the system response can provide a simple and fast tool to fulfill this requirement.