UBC Theses and Dissertations

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UBC Theses and Dissertations

An experimental study of the seismic forces on submerged structures Pegg, Neil Gordon


In this investigation, the dynamic characteristics of a submerged cylinder were determined by performing vibration tests on a model underwater. These characteristics are expressed in terms of the added mass and damping values of the cylinder. Such quantities are required in the design of offshore structures in seismic zones. Sinusoidal tests were used to determine these values as a function of excitation frequency. The frequency range was varied from 0.5 to 6.0 Hertz, which is the primary range of interest of most earthquakes. The testing was carried out in the Seismic Simulation Laboratory of the Department of Civil Engineering at the University of British Columbia. The experimental values of added mass and damping versus frequency were compared with the values produced using potential flow theory. The experimental and theoretical results were found to agree very closely. The theoretical added mass and damping values were then used to develop the frequency transfer function for the base shear developed in the cylinder as a result of an input acceleration record. To check the validity of this theoretically derived transfer function, the base shear was measured for a given random acceleration input and compared to the results obtained using the theoretical transfer function. The transfer function derived from Fourier transforms of the random test records, as well as the transfer function developed through sinusoidal tests were also compared to the theoretical transfer function; the agreement was good. This study is restricted to structures which fall into the large body or wave diffraction regime. This means that fluid separation does not occur and Laplace's equation for potential flow can be used in solving the problem with the assumption of inviscid fluid and irrotational flow. The theoretical solution used in this work contemplates complete free surface boundary conditions, which account for the production of surface waves in the physical problem. These boundary conditions are usually ignored in other studies of this problem, as they increase the difficulty of the solution. Part of the work for this thesis involved the design and construction of testing apparatus and procedures to be employed in the studies of seismic effects on offshore structures. This aspect of the research is described in some detail. The study reported in this thesis confirms that an existing potential theory wave diffraction program can be used to accurately determine the added mass and added damping values for application in the aseismic design of offshore structures. These parameters can then be applied to evaluate the transfer function for such systems.

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