To obtain the optimal design of a dynamic flexible riser it is of crucial importance to understand which parameters govern the complex cross-sectional response. The response of each "cross-sectional layer is dependent upon the local and global geometry of the riser. This complicates the determination of which failure mode is critical since different layers are limiting for different applications. A 15" flexible gas export riser with a design pressure of 178 bar has been analyzed in the ultimate and fatigue limit states. For the ultimate limit state (ULS), the probability of failure" is 1.4*10–6 for a 100 year condition. In the fatigue limit state (FLS), the probability of failure is 2.5*10.3 for a design lifetime of 25 years. Uncertainties in the yield stress of the reinforcing helix and internal pressure contribute most to the probability of failure for the ULS. For the FLS the uncertainties in the motion transfer functions and limit stress are the parameters contributing most to the failure probability. It is shown that the uncertainties in the environmental variables only have a minor effect on the probability of failure. For the ULS the capacity variables account for more than 90% of the failure probability.
During the last decade several offshore oil fields have been developed by application of floating production units. For- these fields flexible riser and pipe solutions are frequently used. Many diffE1rent applications give a wide range of load conditions. In order to obtain an optimal design of a dynamic flexible riser, it is of crucial importance to understand which are the governing parameters of the often complex global dynamic response. The behavior and degree of utilization of each cross-section layer is dependent upon the local cross-section geometry, as well as the global geometry of the riser configuration.