Study On Oil-water Sloshing And Mass/heat Transfer In Storage Tank Of New Spar FPSO Platform

With the increasing demand and rising price for global energy,and the gradually exhausting resources and energy on land,oil and gas exploration and development is turning to the ocean,and transforming from shallow water,semi-shallow water to deep water.Conventional deep-water oil and gas exploitation needs at least two floating units,drilling unit and the producing unit,the cost of which is heavy.Spar FPSO is a multifunction floating platform,which has the integral function of drilling,production,storage and offloading.The concept of Spar FPSO,combining Spar with FPSO,was proposed in 2000 s.The world's first Spar FPSO platform is the Aasta Hansteen,which is designed by Technip-USA and currently being built by Hyundai Heavy Industries in South Korea.The Aasta Hansteen Spar FPSO platform capable of producing 23 million cubic meters of gas a day and storing 160,000 barrels of condensate.The oil storage mode employed by Aasta Hansteen is dry storage.The dry storage technology greatly limit the storage capacity of underwater storage system and the operating costs is high.With this in mind,a new concept Spar FPSO platform has been proposed by Wang Jin.Wet storage,i.e.storing oil over seawater,is chosen as the method to store the produced crude oil in the storage tank in the new Spar FPSO platform.This drilling production storage offloading platform is excellent in storage capacity,economic and reliability,it improves the flexibility of technical solution and significantly decreases the overall development cost for deepwater oil production,particularly suitable for marginal deepwater oil fields.The patents for new concept Spar FPSO have been issued in China and USA,and the relevant research works has received strong support from China National Offshore Oil Corporation(CNOOC).Oil-water displacement technology has over 50 years of successful application experience in overseas and is widely used in underwater oil storage tank and gravity based structure.One of the most striking differences between the wet storage system of the new Spar FPSO platform and dozens of foreign wet storage system is that the working condition of the former is unstable,this characteristic brings many difficulties and challenges to the process design and structural design of the storage system.At present,the drilling and production technology associated with Spar platform is relatively mature,therefore,the dynamic wet storage technology is key technology of the new Spar FPSO platform.This thesis as an important component of CNOOC research subject “Conceptual design of Spar drilling production storage offloading platform” and “Scheme of oil-water displacement experiment of new Spar floating storage unit”,aims to deeply study the phenomenon of oil-water interface sloshing and oil-water heat/mass transfer in the wet storage system of the new Spar FPSO platform.The main content of this thesis are as follows:The development of oil-water displacement technology is presented,the basic information of 26 wet storage structures installed in foreign offshore oil fields are summarized.The potential problems of wet storage system of the new Spar FPSO platform are elaborated.The research status of single-layer-liquid sloshing,muti-layerliquid sloshing and oil-water displacement technology are introduced in detail.Based on the conceptual designs and similarity principle,the important parameter of model test storage system of the new Spar FPSO platform are confirmed.Then,a 40:1 experimental setup of oil storage and offloading is designed and constructed.The sloshing motion simulator designed by present study could roll in different angles and frequencies,and could simulate the freedom of the platform movement in the practical sea conditions.Several test cases are designed by combination of platform motion(static condition,dynamic condition caused by 1-year sea condition and 100-year sea condition),crude oil physical property(medium crude oil and heavy crude oil),environment temperature(summer,autumn and winter),oil storage and offloading rate(design flow rate and double flow rate).In the process of experiment,the oil-water mass transfer law is investigated by the analysis of oil content in the displaced water and oilwater emulsification;the oil-water heat transfer law is investigated by the analysis of temperature filed of liquid in the storage tank.To obtain a high efficient and accurate numerical method,the main numerical parameters associated with liquid sloshing simulation,such as grid spacing,time step size,discrete method,difference scheme,interface capturing method,surface tension model and turbulence modelling strategy,are systematically evaluated based on the experimental results of single-layer liquid sloshing in a rectangular tank.Two interface capturing methods,the volume of fluid(VOF)and the coupled level set(LS)and VOF(CLSVOF)and four turbulence modelling strategies,laminar flow assumption,RANS k–? model,very large eddy simulation(VLES)and large eddy simulation(LES)are considered.The CLSVOF method,which combined the LS and VOF methods,was proposed by Sussman and Puckett.The new VLES model is developed in this thesis to overcome some of the apparent deficiencies of RANS and LES.The influence of interface capturing method and turbulence model on sloshing simulation are discussed by the qualitative and quantitative comparison of the computational velocity fields,wave profiles and pressure data.Subsequently,the multiphase flow simulation method obtained by previous applicability evaluation is employed to investigate the two-layerliquid sloshing and three-layer-liquid sloshing in rectangular tanks.The applicability of the present numerical method is further evaluated by the comparison between simulated and experimental results.In addition,the Kelvin-Helmholtz instability at the liquidliquid interface and the associated occurrence mechanism are investigated.The oil-water interface sloshing in the oil storage tank of the Spar FPSO platform is investigated by numerical simulation.The formula for natural frequency of the system of a circular cylindrical tank with two-layer liquids is derived,then the accuracy of the formula is verified by the numerical free decay test.According to the computational results of natural frequency of oil-water interface in oil storage tank,18 simulation cases of oil-water sloshing are confirmed.The influence of external excitation frequency and amplitude on the motion of oil-water interface are discussed.This study focuses on oil-water sloshing when external excitation frequencies close to the natural frequency of oil-water interface.Finally,an optimization scheme for oil storage tank is proposed to suppress the oil-water interface sloshing.The effect of width of the vertical baffles on the sloshing amplitudes,profiles and velocity amplitudes of oil-water interface are discussed.The effectiveness of the optimization scheme is verified and the mechanism of sloshing suppression is revealed.In summary,based on the applications of oil-water displacement technology in Spar FPSO platform,the inherent characteristics of the liquid-liquid interface sloshing and oil-water heat/mass transfer in wet storage system of floating offshore platform are studied,and several important conclusions are drawn.This thesis provides some useful references for mastering the core technology of new Spar FPSO platform,and some of the new findings and results have filled the blank in the field of liquid tank sloshing.