Nonlinear Random Vibration Analysis Of Mooring-floating Structures Subjected To Wave Loads

With the exhaustion of gas and oil storage distributed in the shallow sea,human begin to focus on exploitations in deep sea area.The frequency response analysis of float structures,as one of the technical challenges for exploitations in the deep sea,has the following four major difficulties:(i)the dynamical behavior of moored float structure in deep sea area turns out to be much more notable;(ii)frequency responses of the mooring and the float lie in different time scale;(iii)the second order wave force can cause obvious low-frequency vibration;(iv)the low frequency and the wave frequency are nonlinearly coupled.A fully coupled frequency-domain analysis is required to guarantee the reliability and accuracy of the dynamic analysis of moored float systems.However,traditional fully coupled time-domain analysis may result in extremely high computational cost,and traditional fully coupled frequency-domain analysis is ineffective for transient or strongly nonlinear problems.To solve these difficulties,we analyze moored float systems based on equivalent linearization techniques.First,knowledges related to moored float structures are introduced.The necessary of fully coupled analysis for moored float structures are discussed.Then,we analyze the stationary random vibration of multi degree-of-freedom Duffing system based on equivalent linearization techniques and the pseudo excitation method,and computational results is in good agreement with that obtained by using the Monte-Carlo method.Furthermore,we analyze the stationary random vibration of hysteretic systems such as Bouc-Wen hysteretic system.After that,we analyze major coupling relationships in moored float structures and simplified dynamic models under the conditions of average,low frequency and wave frequency are established.The model is linearized with the help of equivalent linearization techniques and then solved in the frequency-domain by the pseudo excitation method.Based on the computational results,the arisen of nonlinearity is investigated.Moreover,the discussion offers a better understanding of nonlinear moored cable drag forces and the vital influence of low frequency loads on moored float structures.Finally,we conduct fully coupled time-domain analysis on the finite element model of a certain moored float structure under multi load case by using AQWA-line and the most dangerous load case is determined.Meanwhile,computational results suggest that low frequency play a leading role in the motion of moored float structures,which is in accordance with the results obtained by fully coupled frequency-domain analysis based on equivalent linearization techniques.