A Study for Statistical Characteristics of Riser Response in Global Dynamic Analysis With Irregular Wave

A study was conducted to have a deeper understanding to the statistical characteristics of response of flexible riser in global dynamic simulation with irregular wave. If consider the numerical simulation model as a system and the input wave train as an excitation to it, the time histories of riser load should be the response of the system to the excitation. In order to look the effect of riser configuration and water depth, the study was conducted with three kinds of configuration: Free-Hanging, Lazy-S and Tethered-Wave, which were in different water depths. In order to examine the stationarity and ergodicity of riser response, 100 simulations were performed. Each simulation was performed with a 3-hours-long storm. Except the seeds used to generate the random phases to the wave components, the 100 irregular wave processes for each riser are completely the same. When the number of wave components is enough large, the input irregular wave train should be a stationary normal process. Since the software used for the dynamic simulation is high nonlinear, however, the time history of riser response may not be perfectly stationary normal process. Then different probability distribution theories were applied to fit these time histories and the most fitting one was found out for different riser responses and for different riser configurations. The ensemble autocorrelation functions and the time autocorrelation functions were also examined for both irregular waves and the riser responses. Then the study indicated that both irregular waves and riser responses as random processes should be ergodic stationary. Finally the cross correlations between the irregular waves and riser responses were also examined and it was found that the irregular wave for each riser should be jointly stationary with each response of the riser.