Torque Transfer System for offshore Turret Production System
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Turret production systems allow Floating Production Storage and Offloading (FPSO) units to weathervane and thus significantly reduce environmental loads on the mooring system. There are several different variants of such mooring systems, and some can be disconnected in case of ice bergs or cyclones coming into proximity of the FPSO. BW Offshore have developed a new solution, called the torque transfer system, for connecting and transferring rotational movement from the disconnectable buoy/turret to the production system inside the FPSO. The torque transfer system shall also transfer frictional torque from the production system, release all other Degrees of Freedom (DoF) and accommodate all inherent tolerances. This thesis aimed to determine if structural integrity is fulfilled based on the applicable standard, considering Ultimate Limit State (ULS), Fatigue Limit State (FLS) and Accidental Limit State (ALS) conditions. Comprehensive principal and technical description of the system were conducted to fully comprehend the functionality. Static assessment of the system formed the basis for the analyses. Components, system activation, redundancy system, maintenance and allowable positional adjustments, are also described in detail. Through Finite Element Analysis (FEA) and hand calculations, the initial torque transfer system was found to fulfill ULS condition. However, two regions with stress concentration close to material strength capacity were identified. Hence, implementing geometry optimization was proposed that proved to reduce the stress concentrations. This optimized geometry was consequently used in the rest of the thesis work, as ULS forms the basis for FLS, ALS and sensitivity study. FLS condition was also fulfilled and fatigue fracture shall not occur during operational lifetime of the system. The system was found to withstand maximum ALS loading of 1.5 times the ULS design torque. The sensitivity study proved that positional adjustment of the components, leading to an unsymmetrical system, still resulted in ULS condition to be fulfilled. As a result of the analyses, a few improvements to the system were recommenced. And it is believed that the optimized torque transfer system will fulfill all applicable demands for offshore steel structures and ease operation of turret production systems in FPSO units.
Master's thesis in Mechanical engineering