Identification of the excitation and reaction forces on offshore platforms using the random decrement technique

This paper describes a method to identify the parameters of the dynamic model of a fixed offshore platform subjected to wind-generated random waves using its stationary response. The structure is modeled as a single degree of freedom system. The parameters identified are the damping coefficient, the natural frequency, and the excitation. In addition, the moment and force acting on the foundation are also identified. The method uses the random decrement signature as a tool to identify the parameters in the equation of motion. Excellent agreements were obtained between the predicted and actual values of the parameters as well as for the reaction and moment at the platform's foundation. The method can be applied without any interruption to the operation of the offshore structure. The method is easy to apply, and uses inexpensive motion measurement instruments. The estimated force and moment can be used as a tool for an on-line foundation check.