In this paper, special attention is given to the basic assumption frequently used in riser dynamic analysis. This is that motions of a floating vessel can be calculated independently of long line structure dynamics and then motions of a floating vessel can be used in the riser dynamic analysis. Wave actions on a floating vessel are estimated based on three dimensional potential theory and motions of a floating vessel are predicted. Riser dynamic response is theoretically calculated using finite element techniques where wave loads are estimated from Morisons' type equation. Loads from surface vessel displacements are also considered. Finally, comparison is made between experimental results and theoretical calculations and effects of the low frequency motions on riser dynamics are verified. Results in two directional irregular waves are discussed.