CFD-assisted linearized frequency-domain analysis of motion and structural loads for floating structures with damping plates

Abstract

Damping plates are widely used on floating offshore structures to reduce the dynamic responses in waves. Linearization of the nonlinear drag loads are needed to solve the floater motions by frequency-domain methods, which, to date, still dominate the analysis of wave-frequency responses of floating structures in engineering practice. While existing studies focus on damping effects on heave motions, there is a lack of understanding on how the drag loads on damping plates should be modeled to predict the angular motions and structural loads of the floaters. Two different configurations of Morison elements are investigated in this paper. The first considers the entire damping plate as a whole, while the second locates Morison elements at the estimated ‘points of acting’ of drag loads for different parts of the damping plate. Computational Fluid Dynamics (CFD) analysis is utilized to obtain the required drag coefficients and the ‘points of acting’ as function of Keulegan–Carpenter number. In comparison to full CFD analysis of the same floater in regular waves, a hybrid approach is shown to give satisfactory wave-frequency results, not only for motion responses, but also bending moments and shear forces on the damping plate. However, non-negligible second-harmonic structural loads that cannot be properly accounted for by the existing Morison-drag formulation are also identified in CFD analysis, calling for special attention to structural assessment under extreme wave conditions. This study also demonstrates that locating the Morison elements at ‘points of acting’ improves the prediction of motion responses and structural loads at resonance.