Influence of structural flexibility and wave interference on dynamic behavior of idealized offshore platform

An approximate method is presented to estimate the hydrodynamic loading and structural response of an idealized offshore platform subjected to a regular train of linear surface waves. The platform is taken to consist of four bottom-mounted, flexible, circular cylinders supporting a rigid deck and is assumed to be aligned parallel to the incident wave direction. The response of each column is assumed to be one-dimensional and to be governed by linear beam theory. The solution technique for the fluid velocity potential involves replacing scattered waves by equivalent plane waves together with non-planar, first-correction terms, and can be shown to be a large spacing approximation.Numerical results are presented which show the effect of hydrodynamic interference and structural flexibility on the platform response.     

Uptake of plutonium from seawater and sediment by a marine polychaete worm

The results of the uptake of plutonium 239 from sediment by a marine polychaete wormNereis diversicolor is reported in the present paper. A comparison is made of the relative importance of two possible uptake pathways, sediment and seawater by which this worm may obtain its plutonium body-burden. Under the laboratory conditions used for the comparison of those two routes it would appear thatNereis obtains greater than 98% of its body activity from seawater.     

Evaporative weathering of petroleum residues: The age of pelagic tar

The rate of evaporative weathering of a component of crude oil residue is assumed to be proportional to its equilibrium vapor pressure and to the amount remaining in the sample. This model is tested using literature data on normal paraffin profiles of crude oil weathered artificially and gas chromatographic data on crude oil residues weathered on rocky shores. Semiquantitative agreement over the time range from 1 to 400 days is demonstrated. Application of the model to gas chromatograms of pelagic tar lumps collected at Station S near Bermuda in 1972 implies that most of these lumps were formed by fragmentation of much larger and older masses, some within less than one day of the time they were collected. The weathering of pelagic tar thus involves a great deal more physical fragmentation than was previously supposed.     

A pockmark field in the Patras Gulf (Greece) and its activation during the 14/7/93 seismic event

During a recent oceanographical-geophysical survey carried out in the southeastern part of the Gulf of Patras in Western Greece for the construction of an outfall, an active pockmark field was found. The pockmark field was formed in soft layered Holocene silts. The pockmarks are associated with acoustic anomalies attributed to gas-charged sediments. The pockmarks vary in size and shape from 25 to 250 m in diameter and from 0.5 to 15 m in depth and are among the largest and deepest observed in the world.

On July 14th, 1993, during the survey, a major earthquake of magnitude 5.4 on the Richter scale occurred in the area. During the 24 hour period prior to the earthquake the bottom water temperature anomalously increased on three occasions, whilst for a few days after the earthquake it was noted that the majority of the pockmarks were venting gas bublles.

It is considered that the three abrupt sea-water temperature increases were probably the result of upward migrating high-temperature gas bubbles in the water column. It is further suggested that the earthquake was the triggering mechanism and that the gas expulsion was caused by the reduction in the pore volume in the sediments resulting from changes in the stress regime prior to the earthquake. Therefore, it can be suggested that in seismic areas adjacent to pockmark fields, earthquake prediction may be achieved by monitoring the water temperature and/or the rate of gas venting in the pockmark field.

Our analysis indicates that the pockmark field in the Patras Gulf has formed slowly during the Holocene by continuous gas venting, which is periodically being interrupted by short-duration events of enhanced gas seepage triggered by earthquakes.     

Joint Soviet-Turkish expedition in the Black Sea

The hydrological and hydrochemical structures of the upper 300 m water column of the Black Sea in autumn 1988 have been studied. Regularities in the distribution of the physico-chemical characteristics in the zone of interaction between aerobic and anaerobic waters as well as the topography of the H₂S zone boundary and its connection with a certain density gradient have been found.Translated by Mikhail M. Trufanov.     

Hypothesis for a submarine landslide and cohesionless sediment flows resulting from a 17th century earthquake-triggered landslide in Quebec,Canada

Textural isotopic and microfossil data from two gravity cores obtained in Saguenay Fjord, Quebec, suggest that a distinctive sandy clay bed was deposited as the result of a major landslide in the Saguenay River basin. Pb-210 dating of the cores indicate that the bed is of similar age to the magnitude 7 earthquake of February 5, 1663. The slide involved sensitive marine clays and may have occurred in two stages. Slide sediments carried into the Saguenay River channel were probably reworked and subsequently transported down the Fjord basin as two distinct cohesionless mass flows. Fine clay laminae that overlie the older mass flow bed record the modulation of depositional processes by tidal currents for several weeks after this event.     

Experimental study of the fine structure of the wind field near a rough surface

The air-flow velocity field near the water surface is studied in the zone of wind-wave intensification. Caused by a periodic separation of eddies, a nonzero time-averaged value of the wind velocity in the near-water streamline is detected at the leading slope of the wave. The distribution of pressure along the wave with allowance for the vertical velocity shear and disturbances produced by eddies and a periodic deceleration of the viscous layer was calculated with the aid of the Cauchy-Lagrange integral. This procedure made it possible to calculate the growth rate of the wave amplitude, whose value was found to be close to its experimental value at the initial stage of acceleration.     

On the form of crests in limiting gravity waves

Properties of surface singularities and the form of wave crests of limiting gravity waves in steady-state flows of an ideal liquid are considered by analyzing the kinematic boundary condition. It is shown that, for rotational waves, the angle at the crest can have any value from 0° to 180°, while it has the only value 90° in the case of irrotational waves. Two inferences are made from Bernoulli’s integral and the properties of singularities: (i) the Stokes wave is a rotational wave and (ii) no angular points can appear on the profiles of capillary-gravity and capillary waves.     

Adaptive optimal control of an autonomous underwater vehicle in the dive plane using dorsal fins

In this paper, adaptive control of low speed bio-robotic autonomous underwater vehicles (BAUVs) in the dive plane using dorsal fins is considered. It is assumed that the model parameters are completely unknown and only the depth of the vehicle is measured for feedback. Two dorsal fins are mounted in the horizontal plane on either side of the BAUV. The normal force produced by the fins, when cambered, is used for the maneuvering. The BAUV model considered here is non-minimum phase. An indirect adaptive control system is designed for the depth control using the dorsal fins. The control system consists of a gradient based identifier for online parameter estimation, an observer for state estimation, and an optimal controller. Simulation results are presented which show that the adaptive control system accomplishes precise depth control of the BAUV using dorsal fins in spite of large uncertainties in the system parameters.