Dynamic response of an offshore structure to measured and simulated stochastic wave forces Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3j333498n

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  • The dynamic response of an offshore structure to measured and simulated stochastic wave forces is investigated. The theoretical nondeterministic analysis is performed in the frequency domain for a two-dimensional model. The ocean wave system is assumed to be a zero mean, stationary, ergodic Gaussian process described by a one-dimensional wave energy density spectrum. Hydrodynamic velocities and accelerations are computed from the wave energy density spectrum by the use of linear wave theory. The simulated stochastic wave forces are computed via the Morison equation which is modified to account both for the effects of wave-structure interaction and for the equivalent linearization of the drag component in the time average mean square sense. The dynamic equations of motion for the wave-structure system are solved by the normal mode-superposition technique. The coupled generalized damping matrix is diagonalized through an optimization procedure. The solution process is iterative both in the determination of the equivalently linearized drag term and in uncoupling the generalized damping matrix. The theoretical method is compared to responses computed from measured data collected in Wave Project II (WP II). Measured wave energy density spectra are computed from the wave elevation records by a finite Fourier transform algorithm. Pierson-Moskowitz, Bretschneider and Scott theoretical wave energy density spectra are fitted to the variance and to the best least-squares estimate for the peak frequency of the measured spectra. The Scott spectrum shows a better assimulation of the measured wave energy spectrum in the magnitude of the peak frequency value. The WP II prototype structure is modeled by the lumped mass procedure and a linear Winkler foundation model is included. The computer program SAP IV is employed to obtain the natural frequencies and modal shapes of the wavestructure- soil model. The structural spectral displacement response is found to be represented largely by only the first mode of free vibration. The empirical equivalently linearized drag coefficient is determined by comparison of the measured and theoretical spectral characteristics of the generalized wave forces. The structural displacement spectra and frequency response functions are computed for the analyses of the structural response to measured and theoretical wave forces. A non-interaction theoretical analysis shows that a near resonant condition results in the structural displacement density spectra. A theoretical analysis, which includes the effects of wave-structure interaction, takes into account the hydrodynamic damping in the determination of the frequency response function. In the theoretical analysis, the spectral characteristics of the structural displacements are shown to be dependent on the characteristics of the wave energy density spectrum. The structural displacement density spectra for all analyses are found to be narrow banded when measured by both the Lonquet- Higgins and Vanmarke bandwidth parameters.
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