Research On Low Frequency Multi-line Spectrum Vibration Control Of Typical Structure Of Offshore Platform

As the equipment of offshore platforms becomes larger,more complex and diversified,the low-frequency vibration of the equipment such as dynamic positioning and main engine is difficult to attenuate during the propagation,which causes local resonance of the offshore platform structure now and then.The local resonance will aggravate the fatigue damage and affect the comfortableness of the platform seriously.When adopting the method of dynamic vibration absorption to control low-frequency vibration of the platform,it is necessary to analyze the dynamic characteristics of local resonance area.The offshore platform can be regarded as a large and complex structure composed of beams,plates,stiffened plate,revolving column shells and other typical structures.Many scholars have analyzed the vibration characteristics of typical structures of offshore platforms based on numerical and analytical methods.However,there are still some problems in the application process of dynamic vibration absorption when controlling low-frequency multi-line spectrum vibration control of offshore platforms,such as a unified analysis model for coupled dynamic vibration absorption of typical structures under elastic boundary conditions has not been effectively established,the studies about distributed dynamic vibration absorption of typical structures are few,the design process of low-frequency multi-line spectrum vibration control of offshore platforms needs improvement.To this end,the paper aims at the application of dynamic vibration absorption in the low-frequency multi-line spectrum vibration control of offshore platforms,focusing on the mechanism of coupled dynamic vibration absorption of the local structure of offshore platforms,and the research work of this paper is as follows:Taking the beam of offshore platforms as the research object,two vibration analysis models of beam structures are established,and the applicable scope of two beam theories is discussed.Based on the unified analysis model of beam structure vibration,considering the coupling relationship between dynamic vibration absorption and beam structure,the energy functional of beam structure coupling dynamic vibration absorption is constructed,the equivalent mass of controlled mode of the beam can also be solved quickly;Based on the beam structure-dynamic vibration absorption analysis model,the parametric study of dynamic vibration absorption is carried out.By combining with the optimal coherent design method,the design of distributed dynamic vibration absorption of beam structure under elastic boundary conditions is also studied.A unified analysis model of isotropic plate vibration is established,the simulation of arbitrary boundary conditions is realized through the introduction of virtual boundary springs,the applicable thickness range of different plate theories is discussed;On the basis of the analysis model of isotropic plate vibration,considering the coupling connection of dynamic vibration absorption and plate structure,the energy functional of coupling structure is constructed,and the equivalent mass of the mode to be controlled at any position of the plate structure can be quickly obtained;Combined with the optimal coherent design method,the design of single line spectrum dynamic vibration absorption when vibration displacement and vibration acceleration are respectively the control indicators is studied.In addition,the design and calculation of distributed dynamic vibration absorption of plate structure under complex boundary conditions with multi-point excitation are also carried out.On the basis of the unified analysis model of beam and plate,the elastic connection between stiffener and plate is further considered,the energy functional of the plate frame structure subjected to complex boundary conditions is constructed,and a unified formula for the vibration analysis of the stiffened plate frame with arbitrary placement angle and number is proposed;In addition,a parameterized study on the free vibration analysis of the plate frame structure is carried out,the validity of the numerical model is verified by these examples;To further verify the effectiveness of the unified model of plate frame structure,a forced vibration experiment of loop stiffened plate frame structure is also conducted.By considering the influence of the dynamic vibration absorption device,a unified analysis model of the plate frame structure coupling dynamic vibration absorption device under complex boundary conditions is established,a multi-line spectrum vibration control method for plate-frame structures under complex boundaries is proposed.A unified vibration analysis model for cylindrical structures under complex boundary conditions is established based on thin shell theory and energy method,a parameterized study of the free vibration characteristics of cylindrical structures is carried out and the effectiveness of the unified model for cylindrical structures is verified by comparing with related literature.By considering the relative displacement between dynamic vibration absorption and the cylindrical structure,the energy functional of coupling structure is constructed.The dynamic vibration absorption design of cylindrical shell structure under single point excitation and multi-point excitation is carried out,a design method for dynamic vibration absorption of cylindrical structure under complex boundary conditions is proposed.On the basis of the above theoretical research,the low-frequency multi line spectrum vibration control method of typical offshore platform structure is summarized,meanwhile,the low-frequency multi line spectrum vibration control verification experiment of the scaled model of semi submersible platform is carried out based on this method.A numerical equivalent model for vibration analysis of stiffened grillage on upper deck is established.The effectiveness of the model is verified by comparing with the modal test results.The distributed dynamic vibration absorption design scheme for the upper deck of the scaled model is determined quickly,and the effectiveness of the low frequency multi line spectrum vibration control method for the typical structure of the offshore platform in this paper is verified by the test of relevant working conditions.Finally,the paper takes a semi submarine support platform as the research object by combining the low frequency multi line spectrum vibration control method of the offshore platform with the finite element method.The low frequency multi-linear vibration control is carried out for the local resonance area of the upper deck and the cab deck of the semi submarine support platform.The purpose of this paper is to provide basis for engineering application of low frequency multi line spectrum vibration control method for typical offshore platform structures.