This paper presents a multiphysics approach for characterizing flow-induced vibrations (FIVs) in a subsea jumper subject to internal production flow, downstream slug, and ocean current. In the present study, the physical properties of production fluids and associated slugging behavior were characterized by pvtsim and olga programs under real subsea condition. Outcomes of the flow assurance studies were then taken as inputs of a full-scale two-way fluid–structure interaction (FSI) analysis to quantify the vibration response. To prevent onset of resonant risk, a detailed modal analysis has also be carried out to determine the modal shapes and natural frequencies. Such a multiphysics approach actually integrated the best practices currently available in flow assurance (olga and pvtsim), computational fluid dynamics (CFD), finite element analysis (FEA), and modal analysis, and hence provided a comprehensive solution to the FSI involved in a subsea jumper. The corresponding results indicate that both the internal production flow, downstream slugs, and the ocean current would induce vibration response in the subsea jumper. Compared to the vortex-induced vibration (VIV) due to the ocean current and the FIV due to the internal production flow, pressure fluctuation due to the downstream slug plays a dominant role in generating excessive vibration response and potential fatigue failure in the subsea jumper. Although the present study was mainly focused on the subsea jumper, the same approach can be applied to other subsea components, like subsea flowline, subsea riser, and other subsea production equipment.
Bechtel has been contracted for and is in the process of executing multiple onshore Liquefied Natural Gas (LNG) Engineering, Procurement, and Construction (EPC) projects utilizing the modular construction strategy. Modules and associated pieces of equipment have to be shipped to the job site from various manufacturing and fabrication facilities across open oceans. Naval Architects play a key role to assure safe and effective module ocean transportation. Primary naval architectural work consists of a routing study, module design criteria definition, ballasting and stability analysis, grillage and seafastening design, transportation vessel selection and support for module load out and load in. The main challenge is to make the modules, which are originally designed for onshore assembly, sound for ocean transportation. Therefore, module design criteria related to ocean transportation become crucial. Among these criteria, the wave induced inertia loads and vessel deflection have great impact on designed module structure integrity. In order to define inertia loads and deflection appropriately, the interface between vessel and module becomes a main concern. It raises the question of whether the transport vessel and module should be fully integrated. It also increases complexity of the hydrodynamic interaction, which has been demonstrated by widely divergent methods used to address the interface issue in offshore industry. More importantly, whether or not the interface is thoroughly taken into account is critical to successful module design and fully meeting the Marine Warranty requirements. In order to achieve safe and economic module design, a direct method of integrating vessel and module is considered preferable in analysis and design when the inertia effects and structure hydrodynamic response are significant. This paper will provide an overview of integrated vessel and module interaction analysis for the module ocean transportation and focus on the method and procedure of how Bechtel performs analyses: i) spectral motion analysis with a fully coupled constitutive model and ii) vessel and module interaction analysis utilizing an integrated 3D model with fully hydrodynamic loads transferred. In order to determine realistic extreme load case, the equivalent design wave selection will be addressed as well.
This paper presents a multiphysics approach for characterizing flow-induced vibrations in a subsea jumper subject to pressure fluctuation due to downstream slugging and external vortex shedding effects due to ocean current. In this study the associated fluid properties, phase behavior, and slugging dynamics were all characterized at subsea condition using PVTSIM and OLGA programs, respectively; the outcomes were then applied to a two-way fluid-structure interaction analysis (FSI) to quantify the vibration response. To mitigate the resonant phenomenon, detailed modal analysis was also conducted to check the modal shapes and natural frequencies. Therefore, this study integrated the best practices in flow assurance study (OLGA and PVTSim), computational fluid dynamics simulation (CFD), and computational structure analysis (FEA), and provided a complete solution to the fluid-structure interaction involved in a subsea jumper. It is revealed that both the slugging flow and the external ocean current induce vibration response in a subsea jumper. Compared to the vortex-induced vibration due to the external current and the flow-induced vibration due to the internal flow, the pressure fluctuation due to the slug plays a dominant role in generating excessive vibration and fatigue failure of a subsea jumper. Although this study focused on a subsea jumper only, the same approach can be applied to subsea flowline, subsea riser, and other subsea structures.
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