Study On The Nonlinear Motion Characteristics Of Heave And Pitch Of The Sandglass-type FPSO

The Floating Production Storage and Offloading Unit(FPSO)is one of the vital equipment for the implementation of offshore oil&gas exploitation and production operations.Owning to the inherent defects of the ship-type floating body and the cylindrical floating body under severe sea conditions,the traditional ship-type FPSO and the new cylindrical FPSO suffer from limitations in practical engineering application.By contrast,the excellent hydrodynamic performance of the sandglass-type floating body makes it have the potential for dry tree oil production.Therefore,the proposal of the sandglass-type FPSO can achieve the integration of oil production,storage and offloading,which can be seen as the breakthrough in the study of high-end equipment of the offshore oil&gas exploitation.Thus,the sandglass-type FPSO possesses definite practical application prospects and important academic value.As the core superiority of the sandglass-type FPSO,the fulfilment of the dry tree oil production are mainly affected by the heave and pitch motion of the floating body.In this paper,the factors affecting the heave and pitch motions of the sandglass-type floating body are studied in depth.The main work of this thesis can be concluded as:(1)Study on the heave motion characteristic and design methodology of the shape parameters of the sandglass-type floating body.Firstly,through analyzing the variations of the heave motion and wave excitation force Response Amplitude Operators(RAOs),radiation damping coefficients and radiation wave field with the change of the frequency,it is found that the heave motion has wave-free characteristic.Additionally,when the natural frequency of heave motion is small,the frequency of the wave-free phenomenon and the natural frequency of heave motion are close to each other.Based on this,inspired by the two dimensional strip method,the relational model between the wave-free frequency and the shape parameters is derived.Moreover,considering the influence of the nonlinearity of the first-order wave loads and second-order slowly-varying loads on the wave-free frequency,a design method of the shape parameters of the floating body based on wave-free characteristic to optimize the heave motion is established.(2)Study on the nonlinear dynamic response analysis method and the nonlinear heave and pitch motion characteristics of the of the sandglass-type floating body.Owning to the inclined structure with external expansion,significant change in the instantaneous wet surface will happen when the sandglass-type floating body is subject to severe wave action.Thus,the nonlinear FK force and hydrostatic force can only be obtained by discrete summation instead of analytical expression.In order to cope with the problem,the discrete Fourier transformation is introduced in the incremental balance method.Meantime,with modifying the mass and damping terms.the nonlinear dynamic analysis method based on the instantaneous wet surface and the memory effect of the fluid motion is formed and the computational efficienc)is improved as well.The results indicate that the heave and pitch motion of the sandglass-type floating body both have hardening-type nonlinearity.The coupling of heave and pitch motions makes the heave motion complex in the lower frequency band affected by the hardening-type nonlinearity of pitch motion,while the pitch motion increases significantly in the larger frequency band affected by the hardening-type nonlinearity of heave motion.Besides,it should be noted that large heave motion can cause obvious increase in the pitch amplitude.But the pitch motion still belongs to period-1 motion,which is not caused by the parameter instability of the pitch motion.In addition,a smaller upper angle is not conducive to improving the heave and pitch motion performance of the sandglass-type floating body.(3)Study on the second-order slowly-varying characteristic and statistical method of the vertical plane motion of the sandglass-type floating body.Firstly,by analyzing the relative vertical motion,it is found that both first-order and obvious second-order low-frequency components exist in the motion spectrum.Consequently,the traditional statistical method based on Rayleigh distribution,which only considers the first-order wave loads cannot give reasonable prediction of green water occurrence for the sandglass-type floating body.Accordingly,based on the characteristic function and cumulant-generation function,a combined first-and second-order theory for the prediction of green water occurrence is established.Due to the existence of the second-order low-frequency factor,the vertical motion of the sandglass-type floating body exhibits more obvious non-Gaussian nonlinearity than traditional floating body.(4)Based on the above studies,the overall design process of the shape parameters for the sandglass-type FPSO is proposed.Based on this,one sandglass-type FPSO is designed to meet the dry tree oil production requirements under the 100-year return period sea conditions of one area in South China Sea.Therein,with considering the nonlinear FK and hydrostatic force and the second-order slowly varying force,a time-domain method is developed to analyze the hydrodynamic performance of the sandglass-type FPSO.It is found that the dry tree production system can be employed in the sandglass-type FPSO under the 100-year return period sea conditions of one area in South China Sea.Moreover,by analyzing the results,the validity and rationality of the design process for the shape parameters of the sandglass-type FPSO are proved.