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Abstract This work describes a procedure to include dynamic loads of catenary shaped risers to the roll motion response calculation of a Floating Production Storage and Offloading (FPSO) unit in the frequency domain. The procedure involves the application of an asymptotic solution for the dynamic tension at the top of risers based on the work of Aranha & Pinto 1. It is assumed, as an approximation, that the riser dynamic tensions are sinusoidal when the FPSO is under the action of regular waves (this hypothesis is related to the assumption that the quadratic hydrodynamic damping is linearized in the chosen formulation). Since the dynamic tension expression is amplitude dependent, the described procedure is iterative. This work is restricted to the heave and roll motions, but focused on the roll response analyses. The roll response was analyzed for the case study FPSO under beam regular waves coming from portside and starboard, with 60 catenary shaped flexible risers connected. The roll RAOs calculated with the described procedure have presented a good agreement with the roll motions calculated with a coupled time-domain model in OrcaFlex™. From the hull “point of view”, the effect of the riser loads can be considered as equivalent to additional inertia, damping and stiffness effects, which are quantified for the case study of this work.
Chads et al. (Sun,) studied this question.