Due to the ongoing rather intense development of deep water gas and oil fields, the technical community has been increasingly focusing its attention to the dynamic behavior of Spar floating structures. Spar dynamics exhibits a highly nonlinear behavior due to the presence of various components such as mooring lines, moonpool, and risers (Spanos et al., 2005). In this regard Gupta et al, 2008a, have studied the reduction of the heave response in a single degree-of-freedom spar model due to the oscillations of water entrapped in the moonpool through the partially closed bottom plates. In this paper a novel coupled six-degree-of-freedom analytical model of a Spar system tensioned by TTR risers is proposed. The model accounts for the interactions among spar hull motions (heave, surge and pitch), the riser motion (heave and surge), and the moonpool. This model involves six coupled nonlinear differential equations comprising nonlinearity terms associated not only with stiffness and damping but also with inertia terms. A dynamic analysis is performed by subjecting the model to JONSWAP ocean wave spectrum compatible extreme forces (corresponding to the 100 yr wave); and to moments applied to the center of gravity computed by means of standard motion program. Both numerical and semi-analytical techniques (equivalent linearization including inertial terms) are used for the determination of the response of the proposed dynamic model both in the time and frequency domains. Some parameter study results are reported, including ones pertaining to the dependence of the spar motion on the open guide plates.