Geometrical aspects of a sonar-based navigation system for underwater vehicles

A novel self-contained navigation system has been devised for underwater vehicles operating in and around offshore installations. This system matches data from a sector-scanning sonar device to a computer model of the installation. The paper begins by highlighting the existing approaches to subsea navigation before outlining the main features of the proposed system. It then concentrates on a key component of this system which is a method for calculating the position and heading of an underwater vehicle navigating in the vincinity of tubular steel structures. An iterative solution method is presented which incorporates six degree of freedom vehicle motions and this is verified in a series of laboratory experiments with various arrangements of structural members and using a commercial sonar device. The key features, applications and performance of this method are discussed. The main conclusion is that the proposed method for calculating the position and heading of an underwater vehicle contributes towards achieving an accurate and reliable subsea navigation capability.     

Measurement of the redox speciation of iron in seawater by catalytic cathodic stripping voltammetry

Catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complexes of 1-nitroso-2-napthol (NN) can be used to determine iron in seawater. It is shown here that iron(II) is effectively masked in the presence of 2,2-dipyridyl (Dp) so that iron(III) is measured selectively. The concentration of iron(II) is then calculated as the difference between the concentrations of reactive iron (Fe_R) in the absence and presence of 2 μM Dp, Fe_R being defined as that which was complexed by 20 μM NN at pH 6.9 in the presence of 1.8 mM H₂O₂ and 5 ppm sodium dodecyl sulphate. A 30 min reaction time was allowed for Dp to react with iron(II) in seawater prior to the determination of reactive iron(III) using the same conditions as used for Fe_R. Detection limits of 0.08 nM, 0.077 nM and 0.12 nM were obtained for Fe_R, iron(III) and iron(II), respectively, using a 60 s deposition time.The method was utilised to determine the redox speciation of iron in the northern North Sea. Concentrations of Fe_R ranged between 0.8 and 3.5 nM with nutrient-like depth profiles. Iron(II) was found to be present at concentrations up to 1.2 nM, the highest concentrations occurring in the upper 20 m of the water column.     

Risk of dike failure due to flow slides

Flow slides may affect the stability of dikes. A flow slide is an instability of a submerged slope caused by liquefaction of loose, (medium) fine sand. Whether a flow slide will occur depends on the properties of the sand, which are a function of its density, and the geometry of the slope, as determined by wave and current induced scour and sedimentation. The influence of sand properties and the geometry parameters on the risk of flow slides are discussed. The application of a flow slide prediction method to an example and a risk analysis is briefly discussed.     

A comparative study of some directional sea models

Current specification of the ocean wave environment for the design of offshore platforms does not adequately describe the directional nature of a real seaway. The strong wave frequency dependent nature of the directional behavior of observed seas is often over-simplified for design. A general formulation encompassing a wide range of directional sea models is presented. Parameter values used in some of the more popular directional sea models are examined. Approximate expressions for the two frequency dependent parameters in a modified Longuet-Higgins cosine wave spreading model are presented. A general procedure which allows an engineer to estimate parameters for alternate wave spreading models is discussed. To illustrate this procedure an empirically based modified cosine spreading model is used as the basis to estimate frequency dependent parameters for circular normal and wrapped Gaussian wave spreading models. A comparison of the contours of the various directional sea models and the prediction of the root-mean-square velocity distribution is presented.     

Morphodynamics of a bar-trough surf zone

A field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar-trough topography (the “longshore-bar-trough” and “rhytmic-bar-and-beach” states as described by Wright and Short, 1984). Characteristic features of the morphology include a shallow bar with a steep shoreward face, a deep trough, and a steep beach face. This morphology, which is favored by moderate breaker heights and small tidal ranges, strongly controls the coupled suite of hydrodynamic processes. In contrast to fully dissipative surf zones, the bar-trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach. The degree of incident wave groupiness does not change appreciably across the surf zone. Infragravity standing waves which, in dissipative surf zones, dominate the inshore energy, remain energetically secondary and occur at higher frequencies in the bar trough surf zone. Analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal—antinodal positions over observed surf-zone profiles, indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough. Standing waves which would have nodes over the bar are suppressed. Sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress. In addition, the extra near-bottom eddy viscosity provided by the reformed, non-breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current. Whereas the bar is highly mobile in terms of onshore—offshore migration rates, the beach face and inner regions of the trough are remarkably stable over time.     

Wave drift enhancement effects in multi column structures

A theoretical assessment is made of mean wave drift forces on groups of vertical circular cylinders, such as the columns of a floating offshore platform. A complete analytical solution is obtained for two cylinders extending from seabed to free surface, and a long wave approximation is found to provide reliable predictions of the drift force in line with the waves at low frequencies. For moderate separation between the two cylinders, this force is found to tend at low frequencies to a value four times the force on an isolated cylinder.A numerical method is employed to study two surface piercing cylinders truncated below the free surface, and an arrangement of four vertical cylinders characteristic of a floating offshore platform. The mean vertical drift force is found to be reasonably well approximated, over the frequency range of practical interest, by the force on an individual cylinder considered in isolation multiplied by the number of cylinders in the group. Interaction effects, however, have a profound influence on the total horizontal drift force. At low frequencies this force is found to tend to the force on an isolated cylinder multiplied by the squate of the number of cylinders in the group.     

An experimental investigation of plunging breaker boundary layers in the transformation zone

Experimental investigation is made on the boundary layers of the transformation zone (i.e. the region between the last symmetrical wave profile depth and the breaking point) of plunging breakers propagating on a smooth beach with 1/12 uniform slope. Using a laser anemometer, the particle velocities are measured at four verticals along the transformation zone for three different steepnesses of waves within the plunging breaker range. The boundary layer flow in the transformation zone is found mostly of turbulent character and vertical distribution of particle velocities does not seem to conform to the classical law of the wall distribution given for steady-flow boundary layers. The results show that free-stream particle velocities, in the boundary layer of the breaker under the crest phase, increase considerably as the wave progresses towards the breaking point. The boundary layer thickness, defined as the velocity-affected region, remains constant throughout the transformation zone but it decreases with increasing deep-water wave steepness for the particular beach slope tested.     

Design of a caisson plate under wave impact

From the laboratory experiments and field studies it has been shown that when a wave breaks directly on a vertical wall, impact pressures of high magnitude and short duration, are produced. Despite the recent advances made in collecting data on impact pressure histories and their spatial distributions, analyses on the structural behaviour of the walls loaded by the impact forces do not seem adequate. In the present study the theoretical analysis of the response characteristics of a caisson plate, having different aspect ratios, under the wave impact loading is investigated. Numerical results for the dynamic values of moments and transverse displacements are obtained by the method of finite elements. Some prerequisite experimental data for wave breaking and resulting impact pressures are provided. The static results for moments and deflections are also computed for comparing them with the dynamic values. The dynamic results are found significantly greater than the static values. The ratio between the dynamic and static values is called “Dynamic magnification factor” that varies with plate aspect ratio. Based on this factor a procedure is proposed which may have practical consequences in the design of caisson plates.     

Circulation

Low-frequency current and temperature variability on the southeast US continental shelf during summer conditions of weak wind forcing and vertical stratification was found to be similar in many aspects to previous findings for winter, when stronger wind forcing and vertical homogeneity prevails. Subtidal variability in the outer shelf is dominated by the weekly occurrence of Gulf Stream frontal eddies and meanders. These baroclinic events strongly affect the balance of momentum in the outer shelf, but not at mid-shelf. A negative alongshore sea level slope of order −10⁻⁷ is required to balance mean along-shelf momentum at the shelf edge, similar to oceanic estimates, and can contribute to the observed northward mean flow over the shelf.Low-frequency flow at mid-shelf and coastal sea level fluctuations appear to occur as a forced wave response to local alongshore wind stress events that are coherent over the shelf domain. Momentum balances indicate a trapped wave response similar to the arrested topographic wave found in the mid-Atlantic Bight (CSANADY, 1978). Density driven currents from river discharge do not appear to be significant at mid-shelf. Cold, subsurface intrusions of deeper, nutrient rich Gulf Stream waters can occasionally penetrate to mid- and inner-shelf regions north of Cape Canaveral, causing strong phytoplankton and zooplankton responses. These events were observed following the simultaneous occurrence of upwellings from northward winds and Gulf Stream frontal eddies at the shelf break during periods when the Stream was in an onshore position. Subsurface Gulf Stream intrusions to mid-shelf occur only during the summer, when the shelf is vertically stratified and cross-shelf density gradients do not present a barrier as in winter.     

Upwelling in the Black Sea by the results of numerical experiments and satellite data

The paper analyses results of the numerical simulation of upwelling events in the north-western part of the Black Sea, mostly near the South Crimea. The calculations were performed using a numerical model based on primitive hydrodynamics equations. Emphasis is laid on the case when a salinity front simulating the Black Sea rim current is prescribed in the initial conditions. The interaction of the Black Sea rim current's stream with the coastline and bottom topography leads to the development of an upwelling near the Crimea's coast, even in the absence of wind forcing. The paper discusses the structure of the three-dimensional circulation of waters in the shelf area of the NW Black Sea. Numerical modelling results are matched up with the satellite data obtained by the HRPT receiving station. Translated by Vladimir A. Puchkin.