A Study for Worst Periods and Load Cases Selection in Dynamic Analysis of Flexible Riser

A study was conducted to examine the effectiveness of the methodology which is generally used in screening for the worst wave period and load-case for dynamic analysis of flexible riser. For the purpose of conservatism, it is often required to assume a large load-case matrix by considering all probable loading conditions with different wave and current directions, different wave heights and periods and different vessel orientations with respect to the riser system. The screening for the worst load-cases is performed to reduce the total number of load-cases to be simulated given a limited time-frame and in order to focus attention on the most critical loading conditions. A popular method for the screening is to calculate the vertical motion of the riser hang-off for each load-case with the RAO data of the vessel on which the riser is to be hung. The idea is in that the larger the vertical motion, the worse the dynamic behaviors of the riser. With this method, three criteria should be used during the selection, looking for 1) the largest vertical displacement, 2) the largest vertical velocity and 3) the largest vertical acceleration of the riser hang-off. In this studyconclusions are drawn about the effectiveness of the method for some extreme loading conditions (e.g. the maximum tension and compression), and assessments are made of the relative strengths of influence that the three criteria have on the outcome of the analysis.