| TLP(Tension Leg Platform)is an important equipment suitable for deep-sea oil and gas exploitation,but also a platform to be considered in the exploitation of oil and gas fields in the South China sea.In the South China Sea,the deep ocean environment is very complicated,and the TLP is affected by the typhoon and the sea wave,which is prone to different forms of damage.A time domain analysis of the strongly nonlinear floating platform system is available,as well as the establishment of the model of dynamic response of the structures under the extreme marine environment loads and impact loads.The nonlinear dynamic response characteristics and damage mechanism of TLP under random wave are revealed,which has important theoretical and engineering significance.A classic tension leg platform with working depth of 450 m is taken as the research object.The semi-coupled analysis method is used to study the whole platform system,that is,the dynamic response method of floating body motion is adopted and the tension leg mooring system is regarded as a non-linear spring.By considering the set-down motion of the platform,which caused by the horizontal motions including surge,sway and yaw motions,the nonlinear restoring stiffness is established.The force generated by the static displacement of the mooring system is loaded into the platform through the coupling boundary condition of the platform and the mooring system,to realize the dynamic coupling between the platform and the mooring system.Applying this simulation calculation tool developed in this thesis,the relationship between horizontal motions and set-down motion of platform is analyzed.Based on Airy's wave theory,nonlinear dynamic responses of TLP are acquired in both time and frequency domains.The results of the numerical calculation show that the set-down motion will strengthen the heave motion significantly.The response to the wave and the super-harmonic response at the multiple frequencies are included in the heave motion.Surge,sway and yaw motions have important influences on the set-down motion.Considering the extreme sea conditions,the random wave loads is calculated base on the JONSWAP wave spectrum,and the the six-degree-of-freedom motion response of the platform is carried out.The results show that,under random wave the low frequency motion of surge,sway and set-down are significant.Because the set-down motion has the characteristics of quadratic nonlinearity,multiple frequencies motion is included in the set-down motion.In the severe sea condition,heave motion is mainly controlled by set-down motion,and with the increase of sea conditions,the set-down motion becomes more and more significant.A method computing wind load on the topside of the platform is proposed.Wave loads on pontoons and columns are evaluated by using JONSWAP wave spectrum.Based on NPD wind spectrum,wind loads on topside and columns are calculated.Differential equation of motion is solved with a 4th order of Runge-Kutta's integration scheme,and the time histories,mean values and standard deviations of motion amplitudes are obtained.It is shown that wind loads on topside have important influences on the equilibrium positions of surge,sway and heave motions,as well as on the amplitude of heave motion.However,it has little impact on the other degrees of freedom,such as roll,pitch and yaw motions.The motion response of the platform under first-and second-order wave forces is simulated.The second-order wave forces includes the mean-drift force,difference and sum-frequency forces.Based on the Potential Flow Theory,the wave transfer function in frequency domain is calculated by using DNVGL software WADAM,and then the random wave loads are generated according to the hydrodynamic parameters and wave spectrum.Using the analysis model presented in this thesis,the motion response of the platform is calculated.The analysis shows that the second-order wave force has a major impact on motions of the TLP.The second-order difference-frequency wave force has an obvious influence on the motions of surge and sway,and it can cause a large set-down motion,which has an important impact on the heave motion.Besides,the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch.However,little influence of second-order wave force is found on the yaw motion.Dynamic response of tension leg platform in freak wave is studied.The time series of freak wave was generated by the Random Frequency Components Selection Phase Modulation Method.The first and second-order difference frequency and sum frequency wave loads on platform are calculated under the freak wave.The 6-DOF motions of platform were calculated by numerical method.According to the simulation results,both the first-and second-order wave loads are affected by the freak wave,but the increments of each components are different on 6-DOF.The low frequency motion of TLP,such as surge,is most affected by the second-order difference frequency load.The high frequency motions of TLP,such as heave and pitch,are most affected by second-order sum frequency load.Under the effect of second-order difference frequency load,the set-down motion which is caused by the horizontal motion of TLP is increased significantly,and led to the obvious increment of heave motion.Therefore,heave motion in freak wave is affected by both second-order difference and sum frequency motion.Besides,because of the impact characteristic of freak wave,the amplitudes in each DOF appear at different times.The model test of the 6-DOF motion response of the classic TLP with the scale ratio of 1:120 are carried out in the test pool,and the test results are compared with the numerical simulation results.This model test is aimed to simulate the set-down motion caused by the horizontal motion of the platform,when the axial stiffness of the tension leg is infinite.The response and spectral characteristics of the horizontal motion and set-down motion are analyzed.14 groups of test conditions are selected to measure the motion response of the TLP.RAOs of surge and set-down motions are given and compared with the numerical simulation results.The results show that the amplitude of surge motion firstly decreases,then increases with the incease of the wave period and the set-down amplitude always increases with the increase of the wave period.Five groups of test conditions with different wave heights and the same wave period are also carried out.The test results show that with the increase of the wave height,the amplitude of the surge and set-down motion show an increasing trend,and the curves of the model test and numerical results are consistent.Under the the regular wave,the surge response spectrum appears an obvious peak at the wave frequency,that is,the surge motion is dominated by the wave frequency motion.The response to the wave and the super-harmonic response at the multiple frequencies are included in the set-down motion,and the test results agree well with the numerical results. |
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