This paper presents the frequency domain analysis method for evaluating the response of the fixed offshore structures in deep water subjected to random wave forces. The study focuses on the investigation of the effect of the fixed-end forces. All the equivalent nodal forces are calculated using the work equivalent principle and the power spectral density of the stress on each element is obtained by summing the stress due to the overall behavior analysis and that due to the fixed-end forces. The flow characteristics in deep water are described by the linear wave theory and are assumed to be zero mean Gaussian processes. The hydrodynamic forces on the structures are computed using the Morison equation. The nonlinear hydrodynamic forces are linearlized by an equivalent linealization technique.
Numerical studies have been carried out on two fixed offshore structures in 500 ft, 700 ft depths of water for three seastate conditions. The power spectral densities of the stresses are obtained at a point near the free surface. The results indicate that the influences of the fixed-end forces increase gradually as the seastate conditions become severe.
