As floating production systems extend to deeper waters, the effects of mooring and risers become increasingly significant when predicting the response of the floater. For these water depths, the viscous damping, inertial mass, current loading and restoring effects from both the mooring and riser system must be adequately modelled in order to accurately predict the system motion response. The main design factor for the Spar platform is the large draft that reduces the first order heave wave excitation sufficiently. But other hydrodynamic aspects must be considered carefully. Examples are low-frequency second order wave excitation for vertical modes, the effect of the water in the moonpool, the behaviour of the risers within the moonpool, the effect of wind and current excitation, viscous hull- and mooring damping. Coupling effects due to the mooring and riser system will typically tend to reduce the low frequency motion of Spar platforms compared to the traditional de-coupled approach, Colby (2000). The ability for more accurate prediction of the low frequency Spar motions may consequently contribute to a smaller and less expensive mooring and riser system and hence a lighter Spar platform through a reduction in payload requirements. Given the limitations of current model basins for deepwater Spar systems, quantification of the coupling effects from the mooring and riser system on the Spar global response can best be done through careful mathematical modelling and numerical simulations. This paper describes recent efforts to predict and quantify these effects.
The present study covers a general SPAR structure typical for the Gulf of Mexico environment and the conclusions and findings made are in general only applicable to this specific structure and environmental condition (Astrup et.al. (1999)). Throughout the paper the (uncoupled) analysis are used. • Fully Coupled analysis:
