Dynamic fluid-structure interaction analysis of floating platforms

Thangam-Babu, Potti V. (1981) Dynamic fluid-structure interaction analysis of floating platforms. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The thesis presents finite element analyses of dynamic fluid-structure interaction effects on the response of floating structures subjected to earthquake forces. -- The transmission of seismic accelerations from the sea bottom to the surface through the water medium is studied using a system of lumped masses, springs and dashpots. Horizontal accelerations are not transmitted to the surface since the water is a shear-free medium. Depending on the water depth, vertical accelerations transmitted to the bottom of the floating structure are found to be amplified to about 30 to 40 times compared to those at the sea-bottom. Cavitation, a non-linear effect of the water medium wherein the fluid detaches from the structure, is a possibility for greater depths and higher accelerations. -- The coupled fluid-structure interaction is studied with finite element modelling. The resulting unsymmetric coupled equations of motion incorporate surface wave effects, radiation damping effects, fluid-structure interface hydrodynamic interaction, and the structural flexibility. -- A new numerical integration scheme, based on the Wilson-O method, to solve the coupled unsymmetric equations of motion is discussed in detail. The procedure is illustrated for a floating nuclear plant (FNP) and a liquid petroleum gas (LPG) storage facility subjected to amplified earthquake accelerations. The results are compared with those obtained by using approximate techniques. -- As an alternative approach, since the principle of superposition is valid for linear analysis, the structure and the fluid are isolated and analyzed separately. The analysis discussed in the previous paragraph yields the hydrodynamic pressures acting at the interface of the floating structure. Thus, the floating platform itself is modelled as a thick plate resting on an elastic foundation; the platform is discretised with a newly developed high precision triangular thick plate bending element resting on an elastic foundation. The hydrodynamic pressures are simulated as external forces acting on the fluid-structure interface, in addition to the earthquake forces, and the response is evaluated using the standard Wilson-O method in the time domain; the frequency domain analysis is also carried out. Two illustrative numerical examples (the floating nuclear plant and the liquid petroleum gas storage facility) are provided. -- A computer program, FLUSIN, has been developed to perform all the above analyses described. Its organization is similar to NONSAP, and is very flexible to adapt for other types of offshore structures.

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