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Analysis of fixed and floating structures in random multi-directional waves Nwogu, Okey


Offshore structures have traditionally been designed on the assumption of long-crested or uni-directional incident waves. Realistic sea states are however short-crested or multidirectional with a distribution of wave energy over both frequency and direction. The present thesis investigates the influence of the directional spreading of wave energy on the forces on fixed structures and motions of floating structures. The work is both theoretical and experimental, with the experiments carried out at the multi-directional wave basin of the Hydraulics Laboratory at the National Research Council in Ottawa. Different methods of estimating directional wave spectra are evaluated using numerically synthesized time series of the water surface elevation and horizontal orbital velocities at a single location, and the maximum entropy method is found to provide the best directional resolution. The maximum entropy method is developed further to estimate directional wave spectra from an array of wave probes. Expressions are developed for the spectral densities of the inline and transverse components of the force on a slender cylinder in random multi-directional waves, and for the probability distribution of the peaks of the corresponding resultant force. The former are based on a linearization of the Morison equation, while the latter is based on the assumption of a narrow-band spectrum. Experiments were carried out to measure the forces on a segmented vertical cylinder in random multi-directional waves. The theoretical expressions for the force spectral density and probability distribution match the measured data reasonably well. Reduction factors relating the forces in short-crested seas to the forces in long-crested seas are also presented. Experiments were also carried out with a moored floating barge in regular and random multi-directional waves. The experiments show an increase of the sway, roll, and yaw motions due to directional spreading, and a slight reduction of the pitch and first order surge motions. The second order, low frequency surge motions are however significantly reduced in multi-directional waves. Linear diffraction theory is used to predict the transfer functions for the first order surge, heave, and pitch motions of the barge. Reasonable agreement was obtained between the measured and predicted first order transfer functions. A procedure to compute the spectral density of the second order drift forces in multi-directional waves based on the concept of a bi-frequency, bi-directional quadratic transfer function is also presented.

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