Browsing by Author "Ghosh, Rupak"
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Item Dynamic response of force-excited systems(1999) Ghosh, Rupak; Veletsos, Anestis S.The elastic and inelastic responses of the force-excited single-degree-of-freedom (SDOF) systems are investigated from the gross characteristics of forcing functions such as the frequency content, the integrals and the amplitudes. The studies are carried out initially with simple version of force-excitation and then, using progressively complicated version of forcing functions having more than one frequency. The forces are chosen in such a way that they resemble the simple version of wave-induced forces, thus, paying a special attention to compliant systems. The responses due to actual wave-induced forces are also studied considering the surface record from the Gulf of Mexico during Camille hurricane (August, 1969). The responses are displayed in tripartite response spectra and explained from the characteristics of the forces. The effect of the frequency content in the forces and their integrals are addressed while interpreting the results. The studies are also made based on the parameters such as damping of the systems and the periodicity of forcing functions. An attempt is made to approximate the elastic responses from the characteristics of the integrals of forcing functions in low and low-medium frequency region. The inelastic responses are studied based on the gross characteristics of forcing function and they are compared with the elastic responses. Low-frequency systems of high ductility and low ductility are given a special attention due to current interest in compliant systems.Item Effect of top tensioned risers (TTR) on the spar responses: Time domain and frequency domain approaches(2005) Ghosh, Rupak; Spanos, Pol D.The effect of top tension risers on the spar marine structure responses is studied by using time domain as well as frequency domain approaches. A simplified model of the combined dynamic system consisting of a spar, a mooring line system, and top tension risers is established. The top tension of a riser is provided by a buoyancy-can that is laterally supported by a preloaded compliant guide. The overall system is nonlinear due to the mooring line restoring force, the friction between the buoyancy-can and the compliant guides, and the friction between the risers and guides at the spar keel. The friction between the spar and riser system is represented via a continuous Coulomb model. The simplified model is numerically integrated using input time histories of force and moment that are compatible with a random representation (Jonswap spectrum) of a typical extreme environmental loading in the Gulf of Mexico. For validation, the dynamic responses of the simplified system are compared with the related responses of the combined system. The effect of the top tensioned risers on the spar responses is studied by using nonlinear time domain analysis. The responses of the simplified model are also determined by using a frequency domain approach based on the statistical linearization technique. Several parametric studies are conducted to investigate the sensitivity of the spar responses to different design parameters.