Operational modal analysis and fluid-structure interaction

Operational modal analysis (OMA) has in the last decade shown its potential in the field of offshore structures such as oilrigs and wind turbines etc. Typically, the estimated modal parameters will be used in conjunction with a finite element (FE) model. However since bottom-fixed offshore structures typically will be semi-submerged this affects the modal parameters by fluid-structure interaction. Fluid surrounding a structure is known to retard the response and locally introduce an increase in mass and damping. These local changes yield a complex alteration of the modal parameters and this paper highlights these effects by a wave flume experiment conducted on a cantilever structure. The structure is a scaled offshore structure and its modal parameters are examined at different water levels ranging from dry state to 60 % submerged.