To reduce large motion responses of moored platforms in a harsh environment in deep waters, a thruster-assisted position mooring system can be applied. By applying the system, global dynamic responses can be improved in terms of the mooring line/riser top tensions, operational radii, and the top and bottom angle of the production risers. Kalman filtering as an optimum observer and estimator for stochastic disturbances is implemented in the developed control algorithm to filter out wave frequency responses. Investigation of the performance of thruster-assisted moored offshore platforms was conducted in terms of six-degree-of-freedom motions and mooring line/riser top tensions by means of a fully coupled hull/mooring/riser dynamic analysis program in the time domain and a spectral analysis. The two cases, motion analyses of a platform with thrusters and without thrusters, are extensively compared. The numerical examples illustrate that for deepwater position-keeping of platforms a thruster-assisted moored platform can be an effective solution compared to a conventionally moored platform.