Recently, coupled analysis of deepwater floating structures has come to the forefront. Briefly, a coupled analysis entails a full model of the platform and the connected moorings and risers, and permits the nonlinear dynamic interaction between the system components to be rigorously captured. Because of the time consuming nature of coupled analysis, usually performed in the time domain, an efficient hybrid time/frequency domain approach has been developed in a prior work. The hybrid method simulates the slow-drift responses in the time domain and analyzes the wave-frequency responses in the frequency domain. The analyses at the two time scales are coupled via a special feedback mechanism. It has been shown that the hybrid method compares well with fully coupled analysis, in terms of predicting the standard deviations of the vessel motions and line tensions. In this paper, the hybrid method is extended for fatigue analysis, which is an important design criterion for deepwater risers. The fatigue damage calculated by the hybrid method is found to compare favorably with the result from fully coupled time domain analysis using the rainflow counting algorithm.
In the design of a floating production system, it has been the norm for many years to perform the dynamic analysis of the platform and the connected moorings and risers separately, and this approach is generally known as an uncoupled analysis. Recently, as the deficiencies of uncoupled analysis are unveiled, coupled analysis has come to the forefront (Ormberg and Larsen, 1998). Briefly, a coupled analysis (usually undertaken in the time domain) involves a simultaneous analysis of the platform and all the attached moorings and risers in order to account for all the dynamic interactions in a consistent manner. Coupled analysis in the time domain is accurate and reliable, yet it is exceedingly time consuming for at least two reasons.
