<p> The isotopic composition and concentration of dissolved helium have been measured in over 250 samples of Atlantic and Pacific seawater. The helium isotope ratios were measured using a double collection "static" mass spectrometer specifically designed and constructed for this purpose. The helium and neon contents of 118 of these samples were determined using the isotope dilution technique, and the helium contents of the remaining samples were determined by peak-height comparison with standard air aliquots. Krypton and xenon concentrations were measured by isotope dilution for the Pacific samples. </p> <p> Two sources of excess nonatmospheric ³He are discerned in the Atlantic Ocean: a primordial component and a component produced by in situ decay of bomb-produced tritium. The former component occurs in three distinct features; two emanating from the south at 1000 m and greater than 4000 m depths, and one emanating from the north at about 3000 m depth. The spatial properties of these features are studied in the framework of simple models. The latter, or "tritiugenic" component is coupled with tritium concentrations measured by others to obtain Tritium-helium Ages, the characteristics of which are compared with hydrographic features. For example, the spreading velocity of the Mediterranean Water is determined to be ⪝ 1.5cm sec⁻¹ in the North-Western Atlantic. </p> <p> The helium and neon concentrations measured in Atlantic Waters indicate that significant variations occur for helium contents, both as a function of depth and latitude. The Antarctic Bottom Water appears to be a source of excess helium. </p> <p> In the Pacific, the distribution of excess ³He is shown to be consistent with injection of primordial ³He into the Deep and Bottom Waters from the East Pacific Rise. The characteristic mid-depth maximum seen in the helium isotope ratio anomaly profiles is shown to be a circulational feature. Upper and lower limits on the amount of excess nonatmospheric ⁴He are set at 5.5 and 2.6% respectively. Application of the simple one-dimensional diffusive-advective model indicate an upward flux of 6±2 and 1.1 ± 0.5 × 10⁶ atoms/cm²/sec for ³He and ⁴He respectively. </p> Pacific seawater is, on the average, 16 and 31% supersaturated in krypton and xenon respectively. Although there is no known process that is capable of such enrichments, the spatial distribution of the krypton and xenon concentrations indicate that a significant amount of the enrichment occurs in situ in Pacific Deep Waters. </p> / Thesis / Doctor of Philosophy (PhD)

As developers seek to convert the energy of the tides into electricity, sub-sea turbines must be designed to perform well in increasingly harsh conditions. Such energetic seas have historically been avoided, hence measurements taken below the surface in strong tidal currents and large waves are relatively few, and the theory behind these interactions is underdeveloped. This thesis compares measurements of subsurface velocity taken in the field, at a UK site proposed for development, to the velocity outputs of a model capable of combining waves and currents in a number of ways. In particular the interaction between waves and currents is investigated. The methodology incorporates a novel virtual velocity measurement instrument to measure the model flow, replicating the physical instruments used at sea, such that direct comparisons can be made between the two data-sets. Model and field velocities show good agreement across a range of current speeds and wave heights, with a range of metrics used to demonstrate the suitability of the model, based on linear wave-current theory, for this site. The wave-current interaction module is calibrated, with linear superposition of wave and current velocities proving a suitable representation of field velocities. Calculation of a dispersion relationship affected by mean current velocity marginally improves calibration with field data. Analysis of other sites using the tools developed will further validate this type of model, which in combination with blade element momentum theory, is able to predict pre-construction site specific loads on tidal turbines. Doppler Current Profilers (DCPs) are able to measure subsurface water particle kinematics and sea surface elevation simultaneously, however assumptions made by these instruments jeopardise detail when recording in highly energetic seas, particularly where waves and turbulent tidal currents combine. Models developed to optimise the design of tidal turbines require correct site specific inputs to accurately reflect the conditions that a turbine may encounter through its lifetime, moreover, the kinematics of these models must be accurately validated. To overcome the limitations in DCP measurements a 'Virtual' Doppler Current Profiler (VDCP) is developed (Crossley et al. 2017), enabling quantification of error in site characteristics, and 'like for like' comparisons of field and model kinematics that has never previously been documented. The numerical model developed incorporates tidal currents, waves and turbulence combined linearly to output subsurface velocity based on conditions from the field which have been averaged over ten minute intervals. The inputs are simple, time averaged characteristics (current magnitude, direction, and profile; wave height, period and direction, turbulence intensity and turbulence length-scale) and the model outputs velocities over a two dimensional grid that develops with time. The VDCP samples this flow as if it were the very instrument in the field that recorded the data used for validation. Taking into account the heading, pitch and roll of the instrument a data set directly comparable to that measured in the field is generated. The VDCP is initially used in quantifying error in wave and turbulence statistics, demonstrating a phase dependency of velocity measurements averaged between beams and providing a theoretical error for wave and turbulence characteristics sampled under a range of conditions, in order to improve tidal site characterisation. Spectral moments of the subsurface longitudinal wave orbital velocities recorded by the VDCP can be between 0.1 and 9 times those measured at a point for certain turbulent current conditions, turbulence intensity measurements may vary between 0.2 and 1.5 times the input value in low wave conditions and turbulence length scale calculations can vary by over ten times the input value, dependent on both current and wave conditions. The methodology can be used to determine a theoretical error in any site characterisation parameter for any set of wave, current and turbulence conditions. Results of the model validation using the VDCP show that the tidal flow model, and in particular the newly developed wave-current interaction module, is effective in simulating field subsurface velocities over a range of depths, for waves of up to 3m significant wave height and currents of up to 3.5ms-1. The model is effective in reproducing the wave climate using both measured and modelled surface elevation spectra, and tests, with marginal improvements, the effect of modifying the dispersion equation to account for currents. Field and model velocities compare well over the frequency range dominated by waves, showing only small underestimations in model standard deviations with respect to those from field data, at depths close to the sea surface. At the low frequency end of the modelled spectra, where large turbulent eddies dominate, there is some deviation in model accuracy, particularly during the ebb tide where recorded turbulence parameters are extremely variable, creating uncertainty due to a relatively small sample size. Between field and model velocity maxima, some scatter is observed, potentially providing uncertainty in the estimation of ultimate loads. Model and field damage equivalent velocities, used in the determination of fatigue loads, agree well. Results suggest that a linear wave-current representation of subsurface velocities at this particular tidal site is applicable. Care should be taken when interpreting this result due to the relatively small sample size, and the possibility of site specific nuances, and as such further studies are proposed. The Virtual DCP model is a novel development which has proven its usefulness in the work contained in this thesis and in the analysis of commercial field data. It is extremely versatile, adapting to a range of configurations and set up criteria such that it can be used in the quantification of DCP measurement error for a range of flow characteristics. This information is useful in the design of tidal turbines (and other sub-sea structures) as well as for oceanographic and biological processes. The tidal flow model developed extends beyond the capability of similar numerical models with the capability to model the interaction between waves and currents according to a number of different options. Combined with the VDCP, which samples from the model flow field, a system is created that can be effectively calibrated to find the best model solution to replicate flows at a tidal site measured by a 'real' DCP over a broad range of sea conditions and water depths. The purpose is to ensure that models used to predict the sub surface velocities in the field are suitable and a key question was to understand whether the linear super-position of linear wave models and a turbulent current flow provides a realistic model of the particle kinematics with a view to undertaking loads analysis of a tidal stream turbine. Comparisons of this kind have not previously been documented, and this thesis lays out the path to improved site characterisation.

The topic of this diploma thesis is design of a personal submarine for scientific purposes. The main goal is to solve analysed problems, meet the technical requirements, adequate ergonomic solution and morphology for the environment in which the vessel will be employed.