Multi-Dimensional Seismic Response Analysis And Vibration Control Of Offshore Platform Under Ambient Excitation

Both theoretical research and earthquake damage analysis indicate that earthquake excitations are complex multi-component movements including three translational motion components and three rotational motion components.Most researchers have been devoted to the dynamic response analysis and response control for offshore platform only considering one directional earthquake acceleration acting along one axis of the platform.Moreover, different from land environmental conditions,the particularity issues in ocean ambient environment are needed to consider in seismic analysis for offshore platform.Therefore,it is necessary to implement research on multi-dimensional seismic response analysis and vibration control of offshore platform under ambient excitation.The six aspects of work in this thesis are listed as follows.(1) The influence of combined actions for earthquake and wave on the dynamic response of platform has been studied.The small dimension piles,symmetric platforms and asymmetric platforms are selected as research objects.The parameters which will affect the platform response are analyzed,such as seismic intensity,wind wave state,the site conditions, period characteristic of platform and the eccentricity of asymmetric platform.Results show that the effects of added mass of sea water and fluid-structure coupling on the pile responses are necessary to consider for the combined action analysis.When the seismic intensity is low and wind wave state is moderate or rough,or for the long-period platform in different ground conditions,considering the combination of seismic action and wave action is necessary for the seismic analysis of offshore platforms,while for the short-period platform,such combined action effect could be neglected.(2) The influence of single direction input,bi-direction input and different incidence of earthquake action on the dynamic response of platforms is studied.Then a wavelet energy method is presented to search the critical incidence of earthquake excitation in multi-dimensional seismic response of offshore platforms.The total effective energy and the instantaneous effective energy input rate of an earthquake can be obtained by wavelet transformation to predict the critical incidence of the earthquake and then the maximum dynamic response of the platform can be calculated.The critical incidence is determined using this method for an actual platform and a group of one-storey platform systems. Numerical results show that the application of wavelet transform in multi-dimensional seismic response of structures is convenient and reliable.(3) The torsion coupling seismic response of asymmetric platforms considering the combined action effect of earthquake and wave is studied and the vibration equations and its dimensionless form considering earthquake rotational component are derived for different eccentricity forms.The influence factors on torsion coupling response,such as eccentricity forms,earthquake rotational component,occasional eccentricity,incidence of wave action, the ground site conditions and the period characteristic of platform are analyzed through numerical studies.The results indicate that the influence caused by different eccentricity forms is nonnegligible and the torsion component of earthquake motion has some influences on the seismic response of the asymmetric structure.Accidental eccentricity may increase platform's rotation response and it will be more severe for bi-directional eccentricity case. Such rotation coupling effects do have relation with the period characteristic of platforms. However there is not obvious rule for the ground types.(4) The effectiveness under optimal design of a Tuned Liquid Column Damper(TLCD) in suppressing environmental excitation-induced vibrations of offshore platform is studied in this paper.The problem is analyzed in the frequency domain with white noise excitation, wave spectrum excitation and earthquake spectrum excitation as three kinds of action models in this study.Optimum parameters of the TLCD for maximum reduction of peak structural response to the three models are calculated and compared.The numerical analysis results show that for the wave-induced vibration better control performance can be obtained when wave spectrum model is chosen instead of white noise excitation.Directional wave spectrum which can describe the full nature of wind-induced wave is first addressed in the analysis so that the directional distribution can be considered.In addition,from the view of optimum control effect,it is better for earthquake-induced vibration than for wave-induced vibration.(5) Based on the classical viscoelastic damper,a brand-new damper is designed by the change of simple construction to implement vibration control for both translational vibration and rotational vibration simultaneously.Theoretic analysis has been carried out on the restoring force model and the control parameters.Two improved models are presented to obtain high simulation precision.The influence of the size,shape of the viscoelastic material, the ambient temperature,the response frequency and the optimal number and its arrangement of damper on the vibration control effect is analyzed.Then reasonable explanation on the control effects is made by the response spectrum method.(6) Experiments on the mechanical behaviors of the new viscoelastic dampers and its vibration control for steel frame structure are performed.Excitation frequency,strain amplitude and ambient temperature are considered to investigate their effects on the mechanical parameters of the damper.Then,shaking table tests on a group of one-storey platform systems(symmetric platform,asymmetric platform with different eccentricity forms) are performed to investigate the multi-dimensional control effect.The experimental results show that the restoring force model has a high precision for the pure shear test and has a satisfied precision for the rotation and the complex deformation.The dampers installed in the steel frame absorbing much vibration energy as well as increase the structural initial rigidity. The damper has a significant control effect for both directional vibration of the symmetric and asymmetric structure.However,such control effect is not obvious for the structure direction in which its vibration is very weak while the vibration in the other direction is very strong. When the shaking table is filled with water,the control effect is relatively lower than the corresponding case in air.