The effect of coupled dynamic analysis of floater motions on the mooring and riser system response of a FPSO

Abstract

Floating unit, mooring lines and risers comprise an integrated dynamic system that responds to environmental loading due to wind, waves and currents in a complex way. The sea current forces acting on the moorings/risers and the resulting damping effects can be an important contribution for the floating unit response. For the integrity assessment of the riser system, one of the traditional methodologies employed is to consider wave frequency (WF) floater motion as dynamic excitation through the vessel Response Amplitude Operator (RAO), while low frequency (LF) floater motions are accounted for by an additional static offset. In deep waters, especially with a large number of slender structures supported by the vessel, it becomes increasingly important to consider the coupling effect between the floating unit, mooring lines and risers, since this can affect the motions of the platform and consequently the riser system response. In this paper, the coupled vs. de-coupled approaches for assessment of the riser system are evaluated by case studies of a deep water, spread-moored Floating, Production, Storage and Offloading unit (FPSO) operating in offshore Brazil. Analyses for floating unit response coupled with moorings/risers and the top connection response of the slender structures are presented and the two approaches are compared.