Thesis (Nav.E.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering; and, (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2003. / Vita. / Includes bibliographical references (leaf 60). / This research examines the effect of unsteady motion on the forces and moments experienced by an underwater vehicle in shallow water. The test platform is the REMUS Autonomous Underwater Vehicle developed by the Woods Hole Oceanographic Institution, although the results are made non-dimensional to be applicable to a wide range of similar shaped vehicles. The experimental model was moved in sinusoidal motion at various submergences, speeds, frequencies of oscillation, and amplitudes of oscillation. / by Erik D. Oller. / S.M. / Nav.E.

Abstract not available

A marginal ice zone (MIZ) is characterised by distinct ice floes and its direct exposure to the open ocean. Sea ice is typically described as a continuous material but this description can be inappropriate in an MIZ due to the granular nature of the ice cover and the scale of processes acting on the ice floes. In this thesis, the kinematic behaviour of sea ice in an MIZ modelled as a granular material is investigated with an emphasis on the influence of ocean waves. The kinematic behaviour of a set of ice floes subject to ocean wave forcing was recorded in an experiment conducted in the Ross Sea. Kinematic data were recorded from each ice floe using a GPS receiver, tri-axial accelerometer, and compass. The data show (1) the influence of wave forcing and (2) collisions between neighbouring ice floes. It was also discovered that the GPS receivers were able to resolve the effects of ocean wave forcing despite their poor absolute accuracy. The number density and normalised structure factor (NSF) are introduced to describe the spatial structure of a set of ice floes. Four idealised distributions (in 1D and 2D) are analysed to gain insight into the way that different factors determine the shape of the NSF. It is shown that (1) a significant sinusoidal deviation causes a peak in the NSF, (2) ordered structure dominates low spatial frequencies, and (3) disorder dominates high spatial frequencies. A comparison of the contributions from these different factors is used to estimate the significance of a sinusoidal deviation in the positions of the ice floes. A granular model of an MIZ is developed using a novel set of equations of motion to examine the effect of ocean wave forcing. The equations of motion are derived for small ice floes and allows forcing by multiple waves. These equations predict a transient, wave-induced torque, which can be sustained by the application of a second force to the ice floe. Torque induced by the interaction of two forces on an ice floe may be a general feature of sea ice motion. The number density and NSF are used to analyse the distribution of ice floes in the granular model. At low solids-fractions the number density is correlated at the frequency of the wave forcing. As the solids-fraction is increased this correlation is destroyed by collisions between the ice floes and new correlations are created that are related to the packing structure of the ice floes. When the number density is weighted by the velocity of the ice floes, the correlations between floes are related to the convolution of the wave velocity field and the number density. These correlations may be incorporated into the thickness distribution of large-scale models using the maximum entropy method. The granular model was also examined as a percolating network of contacts and it was found that percolation was more likely to occur along the crest of a wave than in the direction of propagation.

Graduation date: 2006

Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1982. / Vita. Bibliography: leaves 321-326.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 130-134).

Not available / text

The prediction of morphological responses of natural and engineered beaches to the prevailing wave, tide and wind conditions continues to be a challenge due to the wide variability in site conditions and the underlying complexities of the processes involved. For this reason, field measurements, especially with a good spatial and temporal resolution, play a critical role in monitoring the performance of intervention work. This thesis presents the results of a five-year study at Aberdeen beach on the North Sea coast of the UK, with the aim of obtaining and analysing high-quality field measurements of beach dynamics before and following beach nourishment and the installation of nearshore breakwaters. Beach topographic data were collected between June 2006 and July 2011 at approximately monthly intervals using real time kinematic GPS, augmented by Argus video monitoring. An assessment of the horizontal and vertical errors of the Argus system found it to be comparable to other similar installations. The initial response of the area of beach subject to recent intervention works was the formation of stable bays in the lee of the nearshore structures, making this zone morphologically distinct from other areas of the beach. Time-synchronous wave data was collected from nearshore wave buoy measurements, or derived from transformed Met Office model-predicted wave data, and were considered in relation to the timing of beach morphology responses to the prevailing wave conditions. A strong link was observed between the 30-day average wave height and the beach dynamic, with erosion commencing when Hm0 rose above 1 m, and antecedence being an important factor in beach response to successive winter storm events. The data and insights from the present study can be used to develop better predictive models and coastal monitoring strategies. Future studies seeking to further understand beach response would benefit from the ability to capture variation within tidal cycles, and to be able to connect the beach to wider nearshore and offshore bedforms and marine processes.

Accurate evaluation of the non-linear wave-wave energy transfer requires a significant proportion of the computation time of ocean wave models. The Discrete Interaction Approximation (DIA) developed within the first version of the WAM model (WAMDI, 1988) is the only algorithm to be used today in operational wave modelling as it is the only way to calculate the wave-wave interactions rapidly enough. In this study, the Two- Scale Approximation (TSA), a potential successor method to the DIA, was successfully implemented in the third generation operational wave model WAVEWATCH IIITM(WW3). Preliminary results (Perrie and Resio, 2009) showing that it offers improved accuracy are confirmed in this study by the modelling of wave evolution under constant winds. Fetch-growth curves and two-dimensional spectra for energy and non-linear wave-wave interactions obtained using the TSA in these conditions show better agreement to more exact computations of non-linear interactions, than the DIA results.

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.