| dc.description.abstract | In deep water with severe environmental conditions, the Free Standing Hybrid Riser (FSHR) is suitable for high motion vessels like FPSOs (Floating Production Storage and Offloading). As the FSHR is capable of decoupling from vessel induced motion, the riser exhibits exceptional motional and structural performances. In this study, the motional and structural characteristics of the FSHR were investigated under 100- and 500-yr hurricane conditions in time domain by using a coupled hull/mooring/riser dynamic analysis program, CHARMD3D. Then, the estimated short-term fatigue damage was compared with SCRs and LWSCRs. In addition, fixed-fairlead analysis of the FPSO uncoupled FSHR was conducted separately, and the results were used to compare with those of the moving-fairlead analysis to check the effects of the environmental loads against the riser system.
In the moving-fairlead analysis, the FPSO coupled FSHR was investigated for 100-and 500-yr hurricane conditions with parallel winds, waves and currents. Firstly, the motion analysis was conducted with respect to the submerged structures and the flexible jumper. Regarding to the behavioral characteristics of the flexible jumper, the FPSO-induced motions were dissipated along the jumper due to the decoupling effect. Secondly, structural strength analysis was conducted for the grades X65/X70/X80 using normalized von Mises stress, Method 1 and Design Membrane Loading Utilization (DMLU). The grade X65 was shown to be near failure limitation in DMLU considering ultimate limit state in both 100 and 500-yr hurricane conditions. Finally, the short-term fatigue damage was estimated for the given material. The grade X65 with the high class safety factor did not meet the targeted product service life of 25 years in 500-yr hurricane conditions. In comparison with SCRs and LWSCRs, the FSHR had outstanding lower maximum fatigue damage of 4.78E-08 in such extreme conditions.
In conclusion, for the riser system the difference of both the motion responses and the stress evaluation results were minor, regardless of the environmental conditions. The results definitely indicate that the flexible jumper decouples the overall riser system from FPSO induced motions. Therefore, the variations of the flexible jumper length or the underwater-location of the submerged structures is carefully analyzed. In addition, with respect to the structural analysis results, the maximum value of X65 in DMLU approached limit state, and the estimated fatigue service life was below the targeted level. These results prove that using low strength material X65 for the vertical riser material is completely hazardous considering survival condition. Therefore, the grade X70 or the high strength material X80 should be considered in order to prevent structural and fatigue failures in advance. | en |
