Numerical And Experimental Study On Hydrodynamics Of An Flng System

With growing demands for clean energy, liquefied natural gas (LNG) attracts moreand more attentions, making the exploitation of the gas fields located in deep watersbecomes increasingly attractive and desirable. Deep waters together with the remotelocations from the onshore infrastructures are impeding the exploitation of theseoffshore gas fields. To overcome these difficulties and facilitate the exploitation ofthese gas fields, floating liquefied natural gas (FLNG), a new concept of floatingoffshore platform, has been proposed recently.A typical FLNG will theoretically produce, liquefy, store and transfer LNG at sea.Serving as an economical and effective program, FLNG technology will significantlychange the conventional mode of offshore natural gas exploitation. Furthermore, thechallenges in laying pipelines will be avoided, making it possible to exploit thestranded gas fields in deep waters. Research has supported FLNG as a feasible andprospective alternative of exploiting stranded gas fields. Natural gas is usually cooledto-162℃(in form of LNG) for the storage. Super-low temperature is a big challengefor offloading operations. Furthermore, the strong nonlinear sloshing would result inhighly localized impact pressures on the hold walls, which may cause structuraldamages and may even induce sufficient moments to capsize the vessel. Thesecharacteristics distinguish the hydrodynamics of FLNG systems from those of FPSOs.Thus, establishing a systematic procedure to examine the hydrodynamics of an FLNGsystem is essential.Hydrodynamics of an FLNG system is one of the research focuses in oceanengineering. Based on numerical simulations and experimental results, hydrodynamicperformances of an FLNG system have been examined, with a focus on thehydrodynamics of an FLNG vessel, the coupling between vessel motions and mooringdynamics, and the coupling among multi-bodies, mooring systerms and connectionsystems. Based on systemical investigations, the factors that signinficantly affect thehydrodynamics of an FLNG system have been further studied, such as the effects of the internal sloshing on global motions of FLNG vessel. In particular, the nonlinearcoupling effect and mechanism between internal sloshing and global vessel motionsare comprehensive examined.Based on a3-D potential theory, hydrodynamics of a reference FLNG vessel areexamined in frequency domain. Furthermore, short-term response analysis has beenconducted to estimate the hydrodynamics of the FLNG vessel in waves of100-yearreturn period sea condition in South China Sea. Using the Cummins theory, coupledanalyses considering vessel motions and mooring dynamics are conducted by anindirect time domain method. The standing ability of the reference FLNG system inextreme sea state in South China Sea is estimated. Go one setp further, thehydrodynamics of a multi-body system in side-by-side and tandem configurations areinvestigated. The numerical models are setup considering vessel motions, mooringdynamics, dynamics of hawsers and fenders, and the mechanical and hydrodynamiccoupling effects between multiple bodies. Moreover, sensitivity studies are conductedto clarify contributions from pretension, stiffness, length and also the arrangement ofthe connection hawsers. To verify the reliablility of the numerical model, a series ofmodel tests have been conducted, such as decay tests in still water, white noiseirregular wave tests and irregular wave tests including wind and current. Based on thenumerical and experimental results, some useful proposals have been provided on thedesign of FLNG system and its offloading configurations.Besides the systematical study on hydrodyanmics of the FLNG system, the factorthat can signinficantly affect the global motions of an FLNG system have beenstudied, such as internal sloshing. White noise wave tests have been conducted, inwhich the model is ballasted with water and steel weights, respectively. It is observedthat the roll is most pronouncedly affected compared to other degrees of freedom.Go one step further, the coupling effect and mechanism between ship motions andnonlinear sloshing have been examined. A2-D coupled numerical code has beendeveloped considering nonlinear sloshing and linear ship motions based on aboundary element method, and be verified by a series of corresponding model tests.Different filling conditions of the reference FLNG section are designed to examinethe influence of filling levels and natural frequencies. As for the internal sloshingproblem, a semi-Lagrangian approach is adopted to capture the strong nonlinear freesurface inside the tank; The boundary value problem is built based on the concept ofacceleration velocity potential, to accurately calculate the internal pressures on thehold walls. As for the vessel motion problem, it is solved through an indirect time domain method. Runge-Kutta4th-order time-integration scheme is used for the timemarching of the coupled problem. Based on the validated numerical model, theperformances of the nonlinear sloshing are investigated, together with the influencesof the incident wave height and wave frequency. The coupling mechanism betweenship motions and nonlinear sloshing is preliminarily examined.Aimed at providing a better understanding of the hydrodynamics of an FLNGsystem, both numerical and experimental studies are conducted, constructing asystematic prediction program. Hydrodynamics and the feasibility of an FLNGsystem in South China Sea are examined. Improvement program for offloadingconfigurations has been proposed. Overcoming difficulties in capturing and updatingof the strong nonlinear free surface, the calculation theory has been improved.Furthermore, a time-domain numerical code has been developed, fulfilling the reliablepredition of coupling response between nonlinear sloshing and ship motion. With thetime-saving character and reliability, this code exhibits great value in industry.Outcomes of this study are expected to provide usefull information for the ongoingprojects of FLNG system, in the viewpoint of design, production and offloadingoperations.