Dynamic response analysis of a floating offshore structure subjected to the hydrodynamic pressures induced from seaquakes

• A seismic analysis method for a floating offshore structure is developed.
• The offshore structure is subjected to the hydrodynamic pressures from seaquakes.
• Compressible fluid–structure interaction is considered rigorously.
• The compressibility of fluid influences seismic behavior of the structure greatly.

A seismic analysis method for a floating offshore structure subjected to the hydrodynamic pressures induced from seaquakes is developed. The hydrodynamic pressure exerted on the structure by the ocean water, modeled as compressible non-viscous ideal fluid, is calculated taking into account the fluid–structure interaction, the energy absorption by the seabed, and the energy radiation into infinity. From the hydrodynamic pressure, the added mass and the effective load, resulting from the hydrodynamic pressure, are obtained. They are combined with the finite element model of the structure, resulting in a numerical model for the entire coupled system. For validation of the numerical model, the hydrodynamic pressures induced by the vertical motion of the seabed at free field, and due to the vertical vibration of a floating massless rigid disk, are calculated and compared with exact analytical solutions. The developed method is applied to seismic analysis of a simplified support structure for floating offshore wind turbines subjected to the hydrodynamic pressures induced from seaquakes. Analysis results show that dynamic response of a floating offshore structure induced by the vertical seismic motion of the ocean bed can be greatly influenced by the compressibility of sea water and the energy absorption capacity of the seabed.