Subharmonic Response of Offshore Structures

Abstract

The nonlinear dynamic behavior of flexible offshore structures in waves is studied using both analytical and numerical approaches. In particular, the phenomenon of subharmonic oscillation of order 1/2 is investigated. From the analytical studies, three basic parameters of the coupled wave‐force‐structure system are identified. Ranges of the basic parameters, within which subharmonic responses of the system may occur, are derived. The numerical results obtained by solving in the time domain the nonlinear equation of motion are used to verify the analytical conclusion. Eighteen cases including the combinations of three different linear waves, three structural member sizes, and two values of the drag coefficient, have been analyzed. Numerical results have not only demonstrated the existence of subharmonic components in the responses, but have also confirmed the existence conditions for such components. The main source of nonlinearity that causes the subharmonic response in the system is identified to be the large displacement effect in the wave force calculation.