Ph.D. - Ocean Engineering

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https://hdl.handle.net/10125/2123

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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.
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    Nearshore circulation and sediment transport
    ( 1993) Wang, Nengjia
    The project of "Sand for Hawaiian Beaches" is briefly described in the introduction. Mathematical models are developed to simulate 3-D coastal current structure and to predict the fate of suspended sediments due to offshore sand dredging and dumping operations, thus providing an effective tool for environmental impact assessments. A 2-DH hydrodynamics model provides necessary input and boundary conditions to the 3-D flow model. The effects of waves are incorporated in these models through the wave radiation stresses and thus the models possess the capability of predicting the wave induced longshore and rip currents. A wave transformation model is applied to calculate the radiation stresses and the energy dissipation due to wave breaking and bottom friction. Wind effects are also included through the 2-DH model. The 3-D flow model gives the current field for the 3-D suspended sediment transport model, which deals with fine sand transport due to dredging and dumping operations. The sediment transport model predicts the suspended sediment concentrations and the accumulation of sand on the bottom. The finite volume method as described by Patankar is employed for the 2-D water circulation models as well as for the 3-D suspended sediment transport model. For the 3-D circulation model, a time split technique is applied in order to handle different physical mechanisms separately and a vertical coordinate transformation is employed to obtain convenient finite difference schemes. Effects of the dredging pit on the stability of the bottom profile and of the beach are studied using Ballard's sediment transport model. The filling and the erosion related to the dredging pit are estimated with a 2-DV hydrodynamics model, a k-e turbulence model, and the SUTRENCH sediment transport model. The models are applied to the coastal region off Waikiki and the computational results show that the dredging operation at the Halekulani sand channel has no noticeable adverse effects on the bottom profiles and on the beach. A 24-hour continuous dredging operation will result in a sand deposition of 0.1mm on the nearest live coral area under normal South Swell wave conditions and the assumed critical deposition and erosion shear stresses.
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    Nonlinear forces and response of floating platforms in regular and random waves
    ( 1992) Chitrapu, Srinivasamurthy
    Floating platforms in the ocean are subjected to mean and low-frequency drift forces, and high-frequency springing forces in addition to the first-order forces in the wave-frequency range. These forces, which are caused by both potential and viscous effects, can induce large amplitude, resonant response of platforms due to the near absence of damping at such frequencies. In this study, a frequency-domain method based on the relative velocity formulation of Morison's equation is presented to compute viscous drift forces and moments in regular and random waves. The method, which is applied to a semisubmersible and tension-leg platforms, indicates significant mean forces and moments in surge, pitch and yaw modes of the platform motion. The combined effect of waves and current, variable submergence of platform members and computation of forces in the displaced location of the platform appear to have a pronounced effect on the computed drift forces and moments. It is shown that the viscous drift forces are important in the long-period range and hence must be considered under design wave conditions. A time-domain model which uses Morison's equation method for force computations is developed to simulate platform motions. This model can include most of the nonlinearities such as the nonlinear drag force, effect of finite wave elevation, nonlinear restoration of the positioning system and nonlinearities in the equations of motion of the platform. The viscous drift forces and response obtained from the frequency-domain method are compared with results from time-domain simulations. Good agreement has been found for the forces and response, both in regular and random waves. This frequency-domain method can be used to predict viscous drift forces and response in the preliminary design stage and for parametric studies due to its superior computational efficiency as compared to the time-domain simulations. The effect of the nonlinear drag force, in inducing higher-harmonic forces and tether-tension response, has been studied using the time-domain simulation results together with power spectral methods. For the wave and current conditions used in this study, second- and higher-harmonic drag force and tether-tension response are observed in regular, bi-chromatic and random waves and current. Inclusion of current is shown to affect the nonlinear response of the platform. Another theoretical model, based on the application of linear potential theory in the time domain, to simulate large amplitude nonlinear motions of platforms is also presented. The theory is based on the combination of potential and viscous flow effects in the time domain to determine forces acting on the platform. Hydrodynamic coefficients and wave excitation forces, obtained a priori from the linear, three-dimensional potential theory, are included in the nonlinear, large amplitude simulation model for platform motions. First-order memory effects are included through velocity based convolution integrals. The results obtained from this simulation model are compared with those obtained using Morison's equation model and the agreement is found to be good. It is believed that this method, due to its ability to model both potential and viscous-flow effects accurately in a large amplitude motion simulation model, will give better predictions to the various nonlinear effects mentioned above.
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    Development of a solar pond system design computer model
    ( 1991) Rezachek, David A.
    The purpose of this dissertation was the development of a solar pond system design computer model which incorporates a number of unique features which might help to improve the validity of simulation results and the design and performance of solar ponds. The computer model developed for this study extends previous treatments of internal reflection by: (1) dividing the spectrum into a finite umber of spectral wavelength bands (five), assuming an average extinction coefficient for each wavelength band and separately evaluating the effects of internal reflection and absorption on each wavelength band; (2) separately evaluating diffuse and direct radiation components; and (3) extending equations developed for a single-fluid solar pond to two- and three-fluid solar ponds. This latter feature, when combined with appropriate heat transfer and stability relationships, allows the comparison of simulations of various multi-fluid solar ponds (e.g., gel or immiscible solar ponds) to that of a salt gradient solar pond. Furthermore, through manipulation of these governing equations, the performance of membrane stratified solar ponds, or ponds with four, or more, fluids can also be simulated and compared to other types. A second unique feature of this computer model is its ability to evaluate various methods of augmenting the performance of solar ponds. Methods of augmentation include the use of deep, cold seawater as a heat sink and auxiliary heat sources such as high temperature solar, waste heat, and fossil or biomass fuels. A number of simulation runs were conducted. Among the major conclusions are: (l) the computer model developed provides reasonable, but more conservative projections of solar pond performance; (2) the effects of wind mixing, convective overturn and internal reflection are significant and may have to be controlled; (3) solar pond performance can be significantly improved by augmentation, with use of cold, deep sea water and superheating of working fluid vapor showing considerable promise; and (4) further work is required to more effectively use the equations developed for multi-fluid solar ponds. In particular, suitable fluids need to be identified and their radiation transmission and other fluid properties and heat transfer mechanisms need to be better characterized