Research On Suppressing Motions Of FLNG Based On Sloshing In The Liquefied Natural Gas Tanks

In order to exploit the natural gas field in the deep sea,the floating liquefied natural gas(FLNG)facility is designed and constructed gradually.FLNG facility represents a new technology with a promising future in the offshore oil industry,which combines the production,storage,transportation with a liquefied natural gas(LNG)liquefaction unit.FLNG facility will be mooring in the deep sea throughout the design life cycle by the use of single or multi-point mooring.The LNG liquefaction equipment used in FLNG facility will suffer considerable wave-induced motions which appear within a wide range of sea states and lasts for almost the whole design life of the FLNG facility.As a result,the frequent occurrence of the wave-induced motions of the FLNG facility of relatively large amplitude may cause low efficiency of liquefaction process or malfunction of the equipment.Hence,it is of practical interest to reduce excessive wave-induced motions of the FLNG facility.The wave-induced motions of the FLNG facility may cause severe sloshing in a partially filled tank and,in turn,the sloshing in the tank impacts the motions of the FLNG facility.FLNG facility using partially filled tanks for control of roll and pitch motion response to wave-exciting forces is investigated in this paper successively.Firstly,based on the potential flow theory,the equation of ship motion is obtained in frequency domain.Besides,IRF(impulse response function)method is used to convert ship motion from the frequency domain to time domain.Lastly,the equations for sloshing analysis of rectangular tanks under pitch motion excitation are established by treat the LNG tank as a TLDs(tuned liquid dampers),and the coupled pitch equation of FLNG is derived in time domain by assuming that pitch is uncoupled from other modes of vibration.In addition,the roll is similar to pitch.By using state-space model to represent fluid-memory effect,the coupled equation can be transformed to first-order ordinary differential equations which can be solved with sloshing equations simultaneously with the given initial conditions.By using the proposed coupling model for FLNG facility and the liquefied natural gas(LNG)tanks,the performance of partially filled tanks for suppressing motions of FLNG facility is numerically assessed.The parametric studies on the examples show that there is a beneficial filling level by which the motions of FLNG can be considerably reduced.