Flexible Pipeline for LNG Offloading System
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- Institutt for marin teknikk 
A theoretical description of the flexible riser has been presented with emphasis on cryogenic inner bore fluid flow. A description of choice of material and cross-sectional design with respect to cryogenic flexible pipes has been discussed.Reel LNG is an inshore alternative to the land based infrastructure for distributing LNG. The critical component of the Reel LNG concept is the floating flexible cryogenic pipeline connecting the inshore docking station to the onshore LNG terminal. The system?s interface to the LNGC is a loading-buoy temporarily moored to the ship side with permitted motion in six degrees of freedom. Global dynamic finite element models of floating flexible pipelines have been established in order to determine the operational performance of the Reel LNG system. Two different flexible pipeline models have been subjected to critical environment conditions. The principal difference between the two case studies is the difference in boundary conditions at the shore-side end-termination. The first pipeline configuration ?Case 1? has simply-supported boundary conditions with an imbedded overlength in the pipeline configuration. The risk of pipeline compression is prominent for large offsets of the loading buoy. The second pipeline configuration ?Case 2? has a free translational degree of freedom in the direction perpendicular to an ideal straight shoreline. The pipeline is subjected to a constant pre-tension, which implies an onshore tensioner mechanism is required.Results from the global analysis indicate that only Case 2 meets the design requirements related to dynamic behavior. With the specified pre-tension, a maximum relative angle equal to 25 degrees was obtained, resulting in a preliminary minimum bending stiffener length equal to 4.2 meters. The curvature did not exceed 0.1 [1/m] during the length of the analysis.Local stress analyses have been performed in order to establish the fatigue damage accumulation during the lifetime of the flexible pipeline. A simplified approach based on the principle of virtual work is presented. It is assumed that the corrugated pipe is the critical cross-sectional component with respect to fatigue damage accumulation in the flexible pipeline. Only Case 2 is relevant for a fatigue analysis as Case 1 failed to meet the requirements in the global verification process. By application of the Miner-rule, the consummation of fatigue resistance corresponded to five percent of the expected lifetime of the system, which is acceptable according to API standards.