The response of offshore structures subjected to wave and current is given by the summation of the non-deterministic response to wave forces including current effects and the static response to the steady current force. The comparison of the amplitude of the most probable peak value of bending moment with that of static bending moment of the structural member suggests necessity of the adequate consideration of both dynamic and static current effects on designing the offshore structure.
In designing the offshore structures, knowledge for the structural responses under the action of wave and current is necessary. The subject of combined effects of current and wave on objects immersed in deep water has attracted attention, and the effects of wave-current interactions on statistical properties of fluid force on the objects have been precisely investigated (Tung and Hang, 1973). Umeda, et. al (2001) studied the coefficient of fluid force of immersed cylinder subjected to simultaneous oscillatory and mean flows. However, the interests in these effects on the response properties of offshore structures have been less highlighted.
Taniguchi and Kawano (2000, 2001) presented a general theory for evaluating the response of offshore structures in deep water when they are simultaneously subjected to random wave forces and current force, including nonlinearities arising from hydrodynamic drag forces and wave-current-structure interaction. This theory can calculate response spectrum of offshore structures regardless of the presence of current. The wave characteristics in deep waters are described by linear wave theory and are assumed to be the zero-mean Gaussian processes. On the other hand, the current characteristic is assumed to have steady velocity that uniformly distributes in depth. The hydrodynamic forces on the structure are computed through the Morrison equation including combined effects of current and wave.