Investigation on the Performance of the Universal Buoyancy System
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- Institutt for marin teknikk 
Future increase in the market demand for liquefied natural gas (LNG) will pose new challenges to the existing infrastructure for LNG distribution. The Universal Buoyancy System (UBS) seeks to contribute to the solution of these challenges by offering a new, cost-efficient, and safe solution for small scale LNG transfer, as an alternative to land based distribution. A critical component in the UBS is the floating cryogenic pipeline, connecting an onshore LNG terminal to an inshore loading buoy. The loading buoy acting as an interface between the UBS system and the LNG-carrier, being a floating structure secured to the ship-hull with permitted motions in heave, pitch, and roll.To determine the operational performance of the bonded flexible pipeline when incorporated into the UBS, a global dynamic finite element model has been established. The model seeks to accurately display the response characteristics of a bonded flexible pipeline subjected to critical environmental conditions, with special considerations on curvature and displacements for varying levels of pre-tension and hang-off angles. The configuration chosen is that of a taut flexible pipeline, floating at the surface and attached to the loading buoy with a hang-off angle. The configuration allows for translation of the pipe's shore-end in the longitudinal direction, while applying a fixed boundary condition at the interface between the pipe and buoy. Buoy motions in six degrees of freedom (DOF) have been introduced to ensure accurate modelling of the pipe and buoy interaction. This, as response amplitude operators (RAO) for coupled buoy response where not available at the time of the analyses. Three different pre-tension levels have been applied for two different hang-off angles to evaluate the pipeline's response, seeking to determine combination of maximum hang-off angle and minimum pre-tension that will keep the curvature within the design criteria. Thus, not compromising the integrity of the pipe and ensuring safe transfer operations. Furthermore, model tests of the independent and coupled response for the UBS loading buoy has been performed to validate numerical RAOs, investigate the functionality of the connection system, and obtain data for verification of a multi-body dynamic analysis developed by Fæhn. The experimental RAOs have been found to correspond well with numerical estimates, generally displaying values of lower response. Thus, extreme responses from the global pipeline analysis are believed to be conservative with regards to the design limits. Additionally, the connection system was observed to perform above expectations, but should be re-designed to allow for transverse displacements on one of the connection arms to reduce stresses from pinching. Nonetheless, maximum forces in the transverse and normal directions of the connection system were found to comply with the present-day design criteria; reaching extreme values of 43 [kN] and 35 [kN], respectively. The model test was deemed a success, providing important information for the continuing design process. Results from the global analysis indicate that the implementation of a bonded flexible pipe into the Universal Buoyancy System is highly feasible. It was discovered that a pre-tension of 15 [kN] or more needs to be applied for the pipe to comply the design criteria at a maximum hang-off angle of 30 [deg]. This was found to be the largest angle that could safely be applied without increasing the bending stiffness towards the termination point. For the 15 [kN] pre-tension, a maximum curvature of 0.34 [1/m] was observed, corresponding to a minimum bend radius (MBR) of 2.94 [m] at the tie-in between the loading buoy and the pipeline. In relation to the curvature design limit of 0.37 [1/m], this was found to be acceptable and indicates that the structural integrity of the pipe will not be compromised throughout the service life. It should be noted that the design limits were not exceeded for tension in any case applied. To establish the accumulated fatigue damage throughout the service life of the pipeline, a local stress analysis has been performed. A simplified approach based on application of formulations from structural mechanics is presented, where it is assumed that the internal steel helices will be the critical cross-sectional component with regards to fatigue accumulation. Through use of Miner summation and S-N curves, the consummation of fatigue resistance was found to correspond to 4.1 % of the allowable limit over the expected service life of the system. This is acceptable and in accordance with API standards. It is recommended that the bending stiffness should be increased towards the termination point between the loading buoy and pipeline to allow for a larger hang-off angle. This will reduce the pipeline-buoy interaction, and should be accomplished by modifying the integrated bending stiffener or by restricting the curvature with other means. A follow-up study will therefore be required to obtain the new optimal combination of pre-tension and hang-off angle, as these are dependent on each other. It is further recommended that a pre-tension level of 20 [kN] is applied for the analysed configuration, this as the benefits with regards to curvature and stresses are found to exceed the disadvantages of an additional 5 [kN] increase in pre-tension.