Study On Mechanical Properties Of New Anti-seismic Jacket

Jacket is widely used in Marine engineering as the supporting structure of offshore platform and offshore wind turbine.In a complex Marine environment,jacket structures must not only withstand wind,wave,current,ice and other environmental loads,but also have the ability to resist earthquake damage.Because of the extremely strong earthquake destructive force,many offshore platforms and offshore wind farms are located in the seismically active areas of Bohai Sea,Yellow Sea and China's South-East coastal areas,so it is necessary to improve the seismic resistance of jacket.In order to improve the seismic performance of jacket structure,this paper proposes a new eccentric jacket with completely overlapped joint.By combining theoretical analysis and numerical simulation,this paper takes the traditional N-type jacket as the comparative analysis object,and conducts the following research on the strength,earthquake resistance and fatigue of the new jacket.In this paper,two methods of building the multi-scale finite element model of jacket are proposed,and the FEA model is built based on ANSYS.Through static calculation,the structural strength and ultimate bearing capacity of the new type jacket and the N-type jacket under the action of wave and current are compared and analyzed.The results show that two jackets are equivalent,but the strength of the new jacket can be improved by adjusting the wall thickness of the chords and through braces under the condition of reducing the weight of the structure.The stress distribution and high-stress zone of the two joints are different.Compared with the T/Y type joints,the stress of the chords and the overlapped braces of the completely overlapped joint decreases by 25% on average,while that of the through braces increases by 25% on average.For completely overlapped joints there is no high stress zone on chord under normal operation condition,and all plastic strain occurs on the through brace in the limit state.The seismic response of a new jacket was studied by spectral earthquake analysis and time history earthquake analysis.The natural frequencies and modes of the jacket were calculated by modal analysis,and the design response spectrum was determined according to the construction and inspection specifications of the fixed platform in shallow sea.The strength,displacement and reaction force of the two kinds of jacket were obtained by spectral earthquake analysis,and the results do not show the difference.Therefore,under the guidance of the code of architectural seismic design,the displacement,acceleration and bottom shear response of the structure under the action of EI Centro wave,Tangshan wave and artificial seismic wave are calculated by the transient dynamic method,and the results show that the new jacket has better earthquake resistance.The fatigue life of the new jacket was analyzed.The jacket modeling wizard was used to build the model.In order to avoid the need for the stress concentration coefficient to calculate the hot spot stress,and there was no standardized formula for the completely overlapped joint,the tool of Joint Mesher was used to convert the joint into a three-dimensional finite element model.After calculating the dynamic characteristics of the jacket structure,the transfer function was generated,and the fatigue damage of the new jacket in the design fatigue life was calculated by spectral fatigue analysis based on the wave statistical data of the China North Sea.The results of this paper show that the new jacket can improve the anti-seismic performance of jacket and realize the lightweight structure,which verifies the feasibility of the design,provides reference for the design and calculation of new jacket and promotes its engineering development.