Global Dynamic Performance of Flexible Risers Employing Coupled Analysis

Flexible risers are inevitable in the operation of floating production units in intermediate water depths in harsh environments. The operation of flexible pipes under these conditions is challenging, and reliable and accurate analysis of the global performance of risers is vital. The main purpose of this paper is to describe an analysis method for the global performance of flexible risers taking advantages from advanced floater motion modelling and comprehensive modelling of the risers. The analysis scheme has been employed on a semi-submersible platform, operated in harsh environments in the North Sea and all its flexible risers and umbilicals have been analyzed. The platform motions are generated by means of a coupled vessel/slender structure model in which the vessel force model is introduced into a finite element model of the slender structures including the mooring lines and all its flexible risers. In this way all relevant coupling effects from damping, restoring and current loading on the slender structures are consistently included in the platform motion predictions. Subsequently each riser is analyzed in separate models with high element mesh resolution. The platform motions, in terms of low frequency and wave frequency motions obtained from the coupled model, are applied consistently with wave and current loading in the detailed riser model. In the detailed riser models, bending stiffeners are modelled with non-linear material behavior. The diffracted wave field is included in the wave loading of the risers. The stick-slip bending of the flexible pipes have been modelled for a selection of risers and the effect of this on the riser hang off loads and the curvature in the bend stiffener region is discussed. The main responses looked into have been the curvature in the bend stiffener region, the hang off loads and interference between risers and platform pontoons. The riser responses obtained from the coupled analysis scheme performed in the present work is compared to more standard analysis schemes where extreme offsets and transfer functions are used to generate platform motions.