Study On The Nonlinear Coupled Heave-roll-pitch Motion Characteristics Of Spar Platform Based On Wave-excitation And Vortex-excitatioin

Marine oil and gas resource drilling and production activities have been developed from offshore area to deep sea area.As an important kind of floating structure for deep water oil and gas exploitation,Spar platforms get more people's extensive concerning since the date of birth.Spar platform has been applied to the deep-sea oil and gas exploitation due to its favorable adaptability to a wide range of water depth,excellent performance in motions and low cost for constructions.In recent years,the nonlinear motion stability and coupled dynamic response of Spar platforms is a hot frontier topic in ocean engineering around the world,and an important strategy topic for the development of deep water oil and gas exploitation in South China Sea.According to nonlinear dynamics theory,the nonlinear coupled response of Spar platforms is studied in this work by utilizing analytic method,numerical simulation and model tests.The instability mechanism and nonlinear dynamic characteristics of Spar platforms are studied deeply.The main research is as follows:(1)The progress and trend of deep-sea oil and gas exploration and development in the world are summarized.And then the importance of Spar platform in deep sea oil and gas development is explicated.After the introduction of the history of Spar platform,structure and performance of Spar platform,the first chapter summarizes the domestic and foreign research on the key issues of Spar platform.The main content and the key issues of this paper are pointed out at the end.(2)According to the rigid body dynamics theory,the six degrees of freedom coupled dynamics and mathematics model of general floating body is established.Considering the varying wet surface and the nonlinear coupled effects in heave,roll and pitch,the nonlinear coupled hydrostatic restoring stiffness is derived.Ultimately the three degrees of freedom nonlinear coupled equations are established for the spar platform under the effect of first-order wave loads in heave and pitch,and vortex exciting loads in roll.The first wave-excited force transfer functions of heave and pitch mode are solved by the three-dimensional potential theory.The vortex-induced force is approximately simulated by computational fluid dynamics(CFD)method.This chapter lays a foundation for the further theoretical analysis and numerical simulation.(3)The nonlinear coupling internal resonance for heave roll and pitch of Spar platforms is studied when the ratio of the frequencies of heave,roll and pitch are approximately 2:1:1 in heave main resonance.By utilizing the method of multi-scales when the incident wave frequency is close to the heave natural frequency,the first order perturbation solution is obtained analytically and further validated by numerical integration.The stability and local bifurcation characteristic of first order perturbation solution are studied.Sensitivity analysis is performed to understand the influence of the damping and the detuning parameter.The results show that there is a saturation phenomenon associated with heave mode in three degrees of freedom systems and all extra energy is first transferred to pitch and then to roll.It is observed that large-amplitude 1/2 sub-harmonic response occurs in roll and pitch.The energy distribution in roll and pitch is determined by the initial value and damping characteristics of roll and pitch.Due to the influence of the low-frequency vortex-excited force,the response of roll is more complicated than that of pitch.(4)The nonlinear coupling internal resonance for heave roll and pitch of Spar platforms is studied when the vortex-exciting frequency is close to the roll natural frequency.By using the method of multi-scales when the first order perturbation solution is obtained analytically and further validated by numerical integration.According to the stability and local bifurcation characteristic analysis,two cases with internal resonance are obtained.The first case is that no saturation phenomenon exists under small vortex-induced loads.The first order perturbation solution illustrates that only vortex-induced frequency motion in roll and super-harmonic frequency motion in heave are excited.Two degrees of freedom heave-roll nonlinear internal resonance occurs.The second case is that the pitch mode is excited with the vortex-induced frequency when the vortex-induced loads are large enough.The second case is that the vortex-induced loads are large enough to excite the pitch and a saturation phenomenon in the heave mode follows.Extra energy is transferred to roll and pitch which causes large amplitude response in both roll and pitch mode.In this moment,three degrees of freedom heave-roll-pitch nonlinear internal resonance occurs.The mechanism of energy infiltration becomes less effective as the damping and internal detuning increase.(5)The nonlinear combination resonance for heave roll and pitch of Spar platforms is studied when the incident wave frequency is close to the sum of heave and roll natural frequency.With the method of multi-scales when the first order perturbation solution is obtained analytically and further validated by numerical integration.Local bifurcation characteristic and chaos analysis of the first order perturbation solution are studied.It is showed that the increase in pitch wave loads causes the bifurcation phenomenon.Two cases with combination resonance are obtained.The first case is stable heave-pitch two degrees of freedom combination resonance under small pitch wave loads.It is observed that 2/3 sub-harmonic response occurs in heave and large-amplitude 1/3 sub-harmonic response occurs in pitch.The roll mode is not excited.When the heave wave loads are large enough,the Spar platforms exhibit an unstable heave-roll-pitch three degrees of freedom combination resonance.The roll mode is not excited.For these cases chaos occurs and large amplitudes response can be induced in heave roll and pitch mode.Increases in roll and pitch damping can reduce the unstable region and prevent chaos moment response.(6)Model tests about Spar platform nonlinear coupling motion under combination of wave and current are carried out,including the free decay test in still water,the 3 DOF nonlinear motion test under regular wave,the vortex-induced motion test and the non-linear motion test under combined interaction of wave and current.The results of theoretical analysis and numerical analysis are validated by model test,as well as the relationship between nonlinear motion of Spar platform and energy transformation under various resonance conditions.The unstable diagram that lead to instability of the platform are verified by model test.At the same time,the research about influence of vortex induced load under nonlinear coupled motion is carried out.