With most of the world's untouched oil and gas resources offshore and the possibility that hurricanes are becoming more frequent and more intense, the risks associated with offshore oil and gas production are increasing. Therefore, there is an urgent need to improve current understanding of extreme ocean waves and their interaction with structures. This thesis is concerned with the modelling of extreme ocean waves and their diffraction by offshore structures, with the ultimate aim of proposing improved tools for guiding airgap design. The feasibility of using linear and second order diffraction solutions with a suitable incident wave field to predict extreme green water levels beneath multi-column structures is investigated. Such tools, when fully validated, could replace the need to carry out model tests during preliminary design. When contemplating airgap design it is crucially important that consideration is given to the largest waves in a sea state, the so-called freak or rogue waves. This thesis studies the nature of one specific freak wave for which field data is available, namely the Draupner New Year wave. Unique features of this wave are identified, distinguishing it from a typical large wave, and an estimate of the probability of occurrence of the wave is given. Furthermore, a design wave, called NewWave, is proposed as a good model for large ocean waves and is validated against field and experimental data. The diffraction of regular waves and NewWaves by a number of structural configurations is studied. In order to assess the validity of using diffraction solutions for the purposes of airgap design, comparisons are made with measured wave data from a programme of wave tank experiments. Wave data for a real platform configuration are examined to highlight the key issues complicating the validation of diffraction based design tools for real structures. The ability of diffraction theory to reproduce real wave measurements is discussed. The phenomenon of near-trapping is also investigated, allowing guidelines for airgap design to be established.

Thesis (M.S.)--West Virginia University, 1998. / Title from document title page. "December 1998." Document formatted into pages; contains x, 78 p. : ill. Vita. Includes abstract. Includes bibliographical references (p. 57-58).

First-order viscous relativistic hydrodynamics has long been thought to be unsta- ble and acausal. This is not true; it is only with certain definitions of the hydrody- namic variables that the equations of motion display these properties. It is possible to define the hydrodynamic variables such that a fluid is both stable and causal at first order. This thesis does so for both uncharged and charged fluids, mostly for fluids at rest. Work has also been done in limited cases on fluids in motion. A class of stable and causal theories is identified via constraints on transport coefficients derived from linearized perturbations of the equilibrium state. Causality conditions are also derived for the full non-linear hydrodynamic equations. / Graduate

Call number: LD2668 .T4 1984 H82 / Master of Science

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

One of the most significant problems faced in the geothermal power industry is that of scaling due to amorphous silica. The silica can deposit out of super-saturated brine in monomeric form and as colloidal particles. Deposition can occur at problematic rates on pipe surfaces and in the rocks of the re-injection wells. Currently there are a number of methods for controlling deposition but the fundamental mechanisms that govern the transport and attachment of silica are not well understood. Many field experiments on silica scaling have been conducted but, due to differences in brine chemistry and operational conditions, the results are hard to compare. Many laboratory experiments have also been performed but these are difficult to correlate with the field experiments. Previous research has found that hydrodynamics are important for the deposition of colloidal particles and inertial impaction was proposed to be the dominant transport mechanism. These results were analysed and, in contradiction, the dominant transport mechanism of the particles was theoretically expected to be that of diffusion. A series of experiments were planned that could test the effect of hydrodynamics on colloidal silica deposition in cylindrical pipe flow. Three parameters were to be varied in the experiment: particle size (10nm and 100nm), Reynolds number (750 - 23,600) and viscous boundary layer thickness (0.06 - 0.38mm). To perform this experimentation, a Silica Scaling Test Rig was designed, built and commissioned. A method for producing synthetic brine was developed which can provide sols that are stable for at least one month and have a particle size of 10-20nm. Silica deposition has successfully been obtained in three preliminary experiments using the rig. Without the exclusion of air from the rig significant corrosion occurs in the mild steel test piece. Protrusions that were likely to be silica deposits were found to be co-located with the corrosion, suggesting that one process promotes the other. Neither deposition nor corrosion was found on the pipe’s weld seam and heat affected zone. Corrosion was prevented using an oxygen exclusion system and two amorphous silica deposition structures were observed: a flat plate-like structure and a globular structure that consisted of 1-5μm diameter globules that built up on each other. Other field and laboratory experiments have produced globular structures similar to those found in this research. To perform the planned experimentation, future work is required: the silica deposition rate must be increased, colloidal silica sol stability must be improved, and some modifications must be made to the rig.

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No description available.

Approved for public release; distribution is unlimited / Internal solitary waves, or solitons, are often generated in coastal or continental shelf regions when tidal currents advect stratified water over bathymetric relief, creating an internal tide which non-linearly evolves into one or more solitons. A major consequence of solitons in a stratified environment is the vertical displacement of water parcels which can lead to sound speed variability of order 10m/s with spatial scales of order 100 meters and timescales of order minutes. Thus significant variations in sonar performance on both surface based ships and submarines can be expected. An understanding into the nature of acoustic propagation through these waves is vital for future development of sonar prediction systems. This research investigates acoustic normal mode propagation through solitons using a 2D parabolic equation simulation and weak acoustic scattering theory whose primary physics is a single scatter Bragg mechanism. To simplify the theory, a Gaussian soliton model is developed that compares favorably to the results from a traditional sech2 soliton model. The theory of sound through a Gaussian soliton was then tested against the numerical simulation under conditions of various acoustic frequency, source depths, soliton position relative to the source and soliton number. The theoretical results compare favorably with numerical simulations at 75, 150 and 300-Hz. Higher frequencies need to be tested to determine the limits of the first order theory. Higher order theory will then be needed to address even higher frequencies and to deal with weakly excited modes. This research is the first step in moving from a state of observing acoustic propagation through solitons, to one of predicting it. / Outstanding Thesis / Royal Australian Navy author