Graduate Thesis Or Dissertation
 

Response of guyed offshore towers to stochastic loads in the presence of steady current

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  • The validity of the widely used stochastic linearization method for analysing the response of guyed towers to stochastic loads is investigated. The governing equations of a guyed tower have two main sources of nonlinearities, fluid-structure interaction and the restoring force of the cables. In this study, two models are considered. First, a linearized single degree of freedom (SDOF) model of a guyed tower is developed using the stochastic linearization approach. It is solved in the frequency domain, giving the statistical response of the guyed tower to random waves and earthquake loading. The results are compared to the response statistics of a time simulation of a single degree of freedom system that fully incorporates the nonlinearities of the cable system and the fluid-structure interaction. Second, the dynamic response of a multiple degree of freedom (MDOF) model of a guyed offshore tower to stochastic earthquake loads and steady uniform current is investigated. The nonlinear cable stiffness and the fluid-structure interaction were again linearized using the stochastic linearization method. Numerical results for several load cases are presented and discussed. The displacement statistics from the MDOF linear analysis were compared to the statistics of the SDOF time simulation that fully incorporated the nonlinearities of the structure. The increased damping of the structure with increasing current was found to significantly reduce the stochastic forces and moments of the structure. The SDOF model is adequate for predicting statistics of displacements, but a MDOF model is needed to accurately predict statistics of maximum moments in the tower. It is found that the results from the linearized model agree reasonably well with the results from the time simulation of the fully nonlinear system. The computer time using the stochastic linearization approach is several order of magnitude less than for a conventional time simulation method. It is therefore justifiable to use this method in the early design stages of guyed towers.
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