Ph.D. - Ocean Engineering

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    Evaluation of flexible hull types for very large floating structures
    ( 1995) Wang, Suqin
    In this study, Very Large Floating Structures (VLFS) of different hull forms (semisubmersible and mat-like) are evaluated on the basis of their hull motions and structural responses. Some suggestions and recommendations are provided for selecting a configuration. The theory of linear hydroelasticity is applied to the analysis. The success of such an analysis of VLFS by means of available computers rests on the development of three efficient hydroelastic analysis methods that significantly reduce the CPU time and the required computational storage. The first method employs the modified Morison's equation and linear structural dynamic theory. The hydrodynamic coefficients in the modified Morison's equation, are obtained using the extended MacCamy & Fuchs' method for the columns and the strip theory for the pontoons, respectively. The method predicts better results at higher wave frequencies than does the Morison's equation method. In the second method, the simplified zero-draft Green function is employed in the hydrodynamic analysis and in the structural analysis a mat-like floating body is modeled as an equivalent floating plate. These two efforts result in significant CPU savings. The mathematical model of the last method employs a three-dimensional hydroelasticity theory. Two techniques are introduced to increase the computational efficiency of this method. One is related to the convergency of the Green function and the other involves the use of an iterative sparse solver for the linear system of equations. This method is especially efficient for the analysis of a VLFS in terms of CPU and storage. Hence, it has been possible to analyze the hydroelastic response of a VLFS with the available computer resources.
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    System design of a high data rate oceanographic telemetry buoy
    ( 1995) Clark, Andrew Malcolm
    A full-scale prototype of a small (2.3-m diameter) high data rate telemetry buoy is designed, built and tested. A unique hybrid configuration consisting of a toroidal disc and spar configuration is developed through an iterative design process which includes both numerical and experimental techniques. Full-scale ocean tests are conducted with the system instrumented to measure buoy dynamics. Environmental conditions including current and wind speed and direction as well as wave height and direction are measured and recorded. The buoy is demonstrated to exhibit dynamics which permit 2-way communications to a geostationary satellite from an inertially stabilized antenna in conditions though sea state 4. The buoy's displacement, dimensions, mass and mass distribution are all varied both analytically and through experiments to arrive at a configuration which, prior to ocean testing, appears to exhibit the desired attribute of minimizing roll and pitch motions. A frequency domain analysis of the buoy/mooring system is used as a design tool in developing the full-scale prototype. Data collected during the sea trials is reduced for comparison with the predicted motions.
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    Sea level rise and coastal erosion in the Hawaiian Islands
    ( 1995) Jeon, Dongchull
    Time series and the power spectral distributions of relative sea levels are analyzed at selected tide-gauge stations in the western and central North Pacific between equator and about 30°N, in association with different time scales of motions. Coastal response to these sea-level dynamics is discussed in detail, based on the aerial photographs of shoreline changes. Wave climate around the Hawaiian Islands as well as surf conditions on Oahu are examined for simulating cross-shore beach erosion processes with an energetics-based sediment transport model. Long-term trend of relative sea-level rise during the past several decades (+1 to +5 cm/decade at most of the tide-gauge stations) is primarily affected by the local tectonism such as volcanic loading, plate movement and reef evolution, and subduction at the plate boundaries. Continual volcanic loading at Kilauea, Hawaii results in consequential subsidence of the Hawaiian Islands. Secondary reason for sea-level rise is the thermal expansion of sea surface waters due to global warming by increasing greenhouse gases, which may be potentially more significant in the near future. Interannual sea-level fluctuations, associated with ENSO (El Nino Southern Oscillation) phenomena, seem to be the primary factor to cause serious beach erosion (up to 10 times the long-term trend). Mean annual cycle of sea level (H ≈ 10 cm) and alternate annual wave conditions are the main causes of the cross-shore oscillation of sediment transport, although there is still some loss of sediments to deep-water region. Short-term change of beach profiles is basically caused by incoming wave conditions as well as sea-level height, sediment characteristics, and underlying geology. Simulations by a cross-shore sediment transport model show that higher waves result in faster offshore transport and deeper depth of active profile change, and that beach recovery process is usually much slower than the erosion process, especially after a storm surge. Deep erosion during a storm surge can not be recovered for much longer duration by mild post-storm waves, but may be partly recovered by non-breaking long waves such as longer-period swells.
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    Design and performance evaluation of a wave-driven artificial upwelling device
    ( 1993) Chen, Xiaohua
    A wave-driven artificial upwelling device, consisting of a floating buoy, an inner water chamber, a long tail pipe, and two flow-controlling valves, was developed for this research. Hydrodynamic performance of the device to pump up nutrient rich deep ocean water is evaluated by mathematical modeling analysis and hydraulic laboratory experiments. The mathematical model of the device is made up of four simultaneous differential equations. The first three equations, which describe the motion of upwelled water inside the device, were formulated based on momentum and mass conservation principles. The fourth equation is the equation of motion of the device in ambient waves. The model is solved numerically by the fourth order Runge-Kutta method. The equation of motion of the device in ambient waves contains several parameters, including added mass ,damping coefficient, wave exciting force and restoring coefficient. Values of these parameters must be determined before the model equations can be solved. In order to determine these variables, a hydrodynamic problem of wave-device interactions must be solved. The boundary element method is used to solve this hydrodynamic problem of radiation and diffraction. Modeling results are verified by a series of hydraulic experiments conducted in a wave basin in the James K. K. Look Laboratory of Oceanographic Engineering at the University of Hawaii. Comparing analytical and experimental results yields some useful information concerning hydrodynamic coefficients under waves of large amplitude. The mathematical model developed in this study was then used to evaluate the effects of five configuration variables on the rate of upwelling flow at the design wave conditions, and to establish design criteria.