Coupled Analysis Of Offshore Floating Systems

Traditionally, quite different numerical approaches have been required in the dynamic analysis of complete offshore floating systems, including the motions of a floater as well as dynamic responses in moorings and risers. Thus, while the hydrodynamic forces on the floater are usually modelled by a large-volume diffraction-radiation analysis based on potential theory with a rigid body, the responses on lines and risers are obtained from slender-body analysis with elastic elements. Also, the actual time scales needed in the numerical implementations may be quite different. Therefore, the total (or global) analysis has traditionally been carried out in two steps, by the so-called de-coupled approach where the forces from the lines/riser on the floater are first modelled as simplified spring elements and empirical damping coefficients. Then the slenderbody responses are analysed using the predicted floater motions as input. This approach is widely in use and is often a valuable tool if properly calibrated and validated. However, dynamic coupling effects between the slender line/riser system and the rigid hull are not directly modelled and must be approximated in one way or another, depending on the type of floater system. A more direct modelling, called coupled analysis, has been developed during the last decade, where the coupling effects are accounted for by including the