Study On Dynamic Response Of FPSO With Overwater Soft Yoke Mooring System

Floating production storage and offloading has developed into an important offshore oil and gas facilities,especially in the development of shallow water and marginal fields.At present,there are some FPSOs with soft yoke mooring working in our country,what's more,soft yoke mooring can also be used in the FPSO-LNG mooring.Accordingly,it gets more and more attention that the ansylsis of its hydrodynamics response in shallow water and the design of soft yoke mooring.Based on the three dimensional potential flow theory and the weathervane effect of single point mooring,this paper only considers three degrees of freedom movement of the first-order motion(surge?pitching and heaving)and the surge of second-order motion within the scope of the frequency domain.Compared the far field and near field method solving the second-order mean drift force,the results show that near field method can achieve the calculation precision of far field method.By using the depth of the water and the draft as a variable,the linear and second order response characteristics of the shallow FPSO are discussed.Studies have shown that the shallow water effect of FPSO is very obvious,and can not be ignored.The first-order motion of the three degrees freedom is sensitive to water depth and draught in the low frequency region.the decrease of water depth will cause the motion peak close to low frequency region,and the movement of the floating body will be aggravated.By using of Full QTF method to analysis the second order difference frequency wave force,the obvious effects of depth of the water is on difference frequency transfer function in the low frequency area.The effect of draft on the second order the average recursive function depends on the influence of operation depth.Quasi static method is used to analysis the static stiffness characteristics of the soft yoke mooring system.According to the static force and moment balance,the program is compiled in order to compare the double chamber and two bottles of type horizontal static stiffness characteristics of the two.Research shows that,under the condition of same size,no matter when FPSO is far from or close to the turret,the box of static stiffness is greater than the bottle.Then the main mooring structure size are discussed to assess its effect to the horizontal of static stiffness,including the length of leg and yoke,ballast water weight of the yoke and the height between leg and single point.The length of leg is the most important part,the second is the length of the yoke.Once the two parameters are determined,we prefer not to change it any more.In turn,the ballast water weight and difference height could be adjusted continuously.Those above can provide a reference for preliminary design of soft yoke mooring.Combination of the frequency domain linear results of FPSO and quasi static method,the effects of water depth and draught surge to the system's horizontal stiffness.Studies have shown that low frequency surge displacement is the main part of the movement outside of the constant deviation in the shallow water,and may even close to constant offset value.Based on the method of Kane multibody dynamics,considering the environment load and system parameter as a variable,the paper use time domain method to analyze the characteristic of the single point system of mooring force.Studies show that the system force is small when wind?wave and flow collinear,otherwise,when the depth of water decreases,horizontal mooring force will increase significantly.The increase of statistics height of wave and spectrum peak period can also increase the mooring force,and the increasing trend is more obvious when wind?wave and flow under the condition of not collinear.The weight of ballast water in Yoke?the tide height and the draft of FPSO can also affect the system's motions response in different degrees,the rules above can be used for the repeated use of FPSO.By comparing the methods between quasi static and dynamic coupling,the paper can provide certain reference for the preliminary design for soft yoke.