| Floating Liquefied Natural Gas(FLNG)is a maritime platform settled near the gas field on the ocean.It’s equipped with natural gas pretreatment,liquefaction,storage and offloading system.With all these systems,FLNG is expected to replace onshore liquefaction factory and starts a brand new modal for the exploitation of maritime natural gas.This new modal can save the investment of subsea pipeline and the construction time of onshore factory.After the exploitation of one gas field,it can be shipped to another one without any further infrastructure.These advantages make FLNG become the future direction of exploiting maritime resources.But as a high-tech maritime platform,there are still some scientific and engineering problems to be solved,particularly the adaptability and the reliability of onshore equipment working in an ocean environment.In this work,computational fluid dynamics methods,such as Volume-of-Fluid,fluid control equations are used to simulate the fluid sloshing in a membrane FLNG storage tank and the thermodynamic flow in a tubular heat exchanger.The influence from tank shape,environmental disturbance has been studied to get a better understanding of pressure distribution inside the tank,liquid behavior under different excitations and thermal response of heat exchanger in different working conditions.Major outcomes are elucidated as follows.(1)Evaluation model of sloshing behavior in a partly filled tank.Averaged peak value is applied to evaluate the liquid sloshing in a FLNG tank.Based on this quantitative evaluation,natural frequency and sloshing characteristic can be analyzed under different excitation conditions.Peak value can be the local maximum value of fluid depth,fluid displacement or hydraulic pressure on the wall.By calculating the average peak value of one selected parameter,liquid behavior during the whole sloshing can be described.Bigger value means the liquid sloshing is more violent and the excitation frequency is much closer to the natural frequency.As the value selection is not limited to simulation parameters,this method has a good compatibility in both numerical and experimental analysis.(2)Study on FLNG storage equipment:Based on Volume-of-Fluid method and independent grid building,numerical model of liquid sloshing in a FLNG membrane tank has been built.Sinusoidal excitation is applied to the tank to study the liquid behavior.With a lower filling rate,natural frequency of sloshing will decrease with the growing of bottom slopes.Bigger slope comes out with lower hydraulic pressure on the bottom.But with a higher filling rate,this affection from bottom slope is weaken and the peak pressure on the bottom is barely changed by enlarging the slopes.Coupled rotate-heave excitation is added to the VOF model of tank sloshing to study the liquid behavior in this complicated disturbance.Frequency ratio between heave and rotate excitations works like balance parameter.When the frequency ratio is small,liquid behavior is dominated by rotate excitation.In a bigger ratio,liquid flow pattern is more likely controlled by heave excitation.The balance value of frequency ratio is 2,where the rotate sloshing pattern increases to the largest amplitude.Impact pressure from liquid can be several times larger than the steady liquid.This balance pattern can be found in all filling rates.(3)Study on FLNG production equipment:Heat exchanger,as a key equipment in the production process,is used to transfer thermal energy between different working fluids.Thermal distribution and response of heat exchanger are studied under different situation.Reading from the simulation results,outlet temperature of heat exchanger is a one-way corresponding parameter to the mass flow rate of shell-side flow.Outlet temperature is more sensitive in lower mass flow rates and therefore more stable in high mass flow conditions.Temperature cross between tube and shell outlets will be found when the mass flow is smaller than a specific value,which means the heat exchanger doesn’t match with the working condition.After this,inclination condition is added to the heat exchanger to study the influence from inclination angle and location.It’s found that longitude inclination has a larger thermodynamic affection than the radial inclination.Heat exchanger working in lower mass flow rate is easier to be affected by spatial inclinations.(4)Experimental validation and application in industry:Analysis results from numerical simulation are validated by some similar experiments and then applied to the design of skid-mounted BOG reliquefaction system for the maritime platform.All these performance studies and loading calculations are used to optimize the land-factory equipment,so that they can adapt to the maritime environments by a shorter developing process. |
