| Due to the rapid increase in demand for oil and gas resources,offshore oil&gas industry is continuously expanded to deeper seas.As a key facility in the deep-water oil&gas field,various floating production units(FPUs)like Floating Production Storage and Offloading(FPSO)unit,Semi-Submersible Platform(SSP),Tensioned Leg Platform(TLP)and Spar Platform,have been and will be planned to be installed over the world.An FPU is typically subjected to multiple degrees of freedom(DOF)of motions,including pitch,roll,yaw,heave,surge and sway,arising from wind,waves and currents under varying weather conditions.However,most of main production separators installed on the FPU are horizontal three-phase separators,which work on the gravity settling principle.They require quiescent operating conditions to separate the original well fluids into gas,oil and water.Obviously,significant liquid sloshing excited by the FPU motions will disturb this condition and make a great difficulty for achieving the desired separation performance.However,there is still lack of specific guidelines on how to properly control it.This paper is aimed to study the sloshing characteristics of oil-and water-phases in horizontal three-phase separators installed on FPUs,and to propose effective sloshing suppression measures.The achievements provide useful guidance in the anti-sloshing design for FPU-based three-phase separators.In this paper,the research is studied as follows:(1)The modal analysis model based on the linear potential theory is developed to predict natural sloshing frequencies and the corresponding mode shapes in three-phase separators.The effectiveness of the model is validated with the analytical solution and the experimental data.The impact factors of natural sloshing frequencies in three-phase separators are investigated,including separator's length-to-diameter ratio,the liquid level and the type of liquid phase.The results show that the resonance modes could be avoided by decreasing the length-to-diameter ratio of separators or increasing the liquid level.This is due to the fact that the natural sloshing frequencies,especially the 1 st natural frequency,would be increased and thus moved away from the external exciting frequencies in this manner.(2)The continuity and Navier-Stokes equations based on a non-inertial reference frame moved with the FPU-based separator are deduced.The momentum source terms with respect to a specific cell inside the fluid domain under multi-DOF motions are obtained.A computational fluid dynamics(CFD)model based on the Volume of Fluid(VOF)method is developed to simulate the dynamic sloshing response to external excitations.The accuracy of the model is also validated by data from other literature.(3)The sloshing response characteristics in FPU-based three-phase separators under multi-DOF motions are investigated.The results show that the resonance modes would be incurred if the external exciting frequencies are within the range of 0.8 ?1 and 1.2 ?1(?1 is the 1st natural sloshing frequency in a separator vessel).Thus,the 1st natural sloshing frequency should be controlled to below 1.25 ?(? represents the external exciting frequency),There is an approximately linear-increasing relationship between the maximum sloshing peak and the external exciting amplitude under non-resonant modes.In contrast,there is a rapidly increasing relationship under resonant modes.In addition,the coupling effects among various DOF motions are inconsistent with each other.Generally,all external excitations should be taken into consideration to obtain more reliable prediction for sloshing response.(4)The effects of separator location,length-to-diameter ratio,oil-and water-level on the sloshing response are evaluated quantitatively.The results show that there is an approximately linearly-increasing relationship between the maximum displacement peak and the distance from the vessel position to the center of rotation(COR)of FPUs,either along longitudinal or transversal placement.Thus,the separators should be located as close as possible to the COR of FPUs.The increase rate of the maximum sloshing displacement is relatively slow for L/D?3.The 1st natural frequency increases with the decrease of L/D.Thus,the length-to-diameter ratio of FPU-based horizontal three-phase separators is recommended to be no greater than 3.In addition,there is an approximately linearly-decreasing relationship between the maximum sloshing displacement peak at the oil/gas interface and the liquid level.Once the upper space occupied by the gas phase is large enough to satisfy the maximum gas velocity requirement,the liquid level should be set as high as possible.The depth ratio of water-to oil-layer has a more significant impact on the sloshing amplitude at the oil/water interface than that at the oil/gas interface.Under the same liquid level,the maximum sloshing displacement at the oil/water interface increases with the depth of water layer.Thus,the water level in a three-phase separator should be lowered as far as possible,if the fluids could be separated to approach to their respective interfaces.(5)The porous media model is developed to approximately simulate the damping effects of perforated baffles,and the accuracy of the model is validated against the results from the 3D model.The results show that the anti-sloshing performance of perforated baffles can be improved up to 50%when the opening area ratio is below 0.4.However,the anti-sloshing performance of perforated baffles decreases with the increase of the opening area ratio.The anti-sloshing performance of perforated baffles with the opening area ratio of 0.2,0.4,0.6 and 0.8,is deteriorated rapidly when the baffle spacing is above 12 m,8 m,4 m and 4 m,respectively.(6)The anti-sloshing characteristics of longitudinal solid baffles have also been investigated by accounting for different number and height of baffle(s).The results show that the longitudinal solid baffles are not suitable for FPU-based horizontal three-phase separators from the anti-sloshing view of point. |
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