Implementation and Application of Modal Analysis During Time Domain Dynamic Simulation of Floating Offshore Systems

This paper presents the implementation and application of modal analysis during nonlinear time-domain dynamic simulations of floating offshore systems. The simulations are performed by a fully coupled nonlinear time-domain analysis methodology, which considers the interaction between the hydrodynamic behavior of the hull and the structural/hydrodynamic behavior of the mooring lines and risers. Considering the nonlinear variation of the stiffness and added mass of the floating system with time, the objective is to assess the variation of the natural periods of vibration for the 6-DOF of the floating system (surge, sway, heave, roll, pitch and yaw). To accomplish this goal, the generalized eigenvalue problem associated to the system is assembled, and the Generalized Jacobi Method is employed to solve this problem and determine natural periods of the system, at selected time intervals during the dynamic simulation. Case studies are selected to assess the variation with time of the natural periods, considering two different types of floating systems: the ITTC semi-submersible platform, for which experimental results are available; and a CALM monobuoy system. The results obtained stresses the importance of the calculation of natural periods in different positions of the system: due to the marked nonlinear behavior of the mooring lines and risers, the natural periods can show considerable variations, for instance, from the neutral design position to an equilibrium position under action of current.