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. Using the reliability index, this study highlights effects of current on the safety margin of the structural member. The results suggest that the non-deterministic response performs primary role in evaluating the safety margin.
The subject of combined effects of current and wave on objects immersed in deep water has attracted attention, and the effects of wavecurrent interactions on statistical properties of fluid force on the objects have been precisely investigated (Tung and Hang, 1973). 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 simultaneously subjected to random wave forces and current force, including nonlinearities arising from hydrodynamic drag forces and wave-current-structure interaction. The wave characteristics in deep waters are described by linear wave theory and are assumed to be 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. The consequent nonlinearities in the system are eliminated by a modified equivalent linearization technique, the essence of which is the alteration of damping and drag coefficients in an optimal manner by minimizing the mean-square errors introduced. The procedures also suggested that the structural response to the steady current force should be separately computed from the dynamical system and was given by the static response to steady current force.
