In the design of SPM systems the environmental parameters are of prime importance. In operational condition both the magnitude of the characteristics of wave spectra and the wind and current velocities and their mutual directions may vary. By combining operational metocean parameters in a systematical way insight in the extreme design hawser loads and tanker responses may be gained. In this paper a systematic number of combinations of wave spectrum, wind and current with regard to mutual directions and different current velocities were applied to a loaded and ballasted tanker moored to a CALM buoy. With these combinations of weather conditions the design hawser loads and tanker responses were studied. The results were obtained by means of time domain computer simulations. The results clearly indicate the most severe combinations of the considered sea states with regard to the extreme design loads. Some model tests were carried out to support the reliability of the computations.