GB2057364A - Buoys for use with sea currents - Google Patents

Abstract

A float 22 supports a string of drogues 25, 26 and 27 and a sinker 28. The drogues are acted upon by sea currents at their respective depths. Alternatively one long drogue is used for the entire depth. The net result is that the effect of currents at different depths is integrated; while not giving precise scientific data about current at any particular depth, such integration of current over depth can have two practical uses. One use is to provide a measure of the effect of current of a large floating body, such as an iceberg, in order to estimate the effects of other forces (e.g. towing or wind) or the body. The other use envisaged is to provide a buoy which is substantially stationary over the sea bed in spite of sea currents. In this case a vector triangle must be devised so that currents at different depths cancel. <IMAGE>

Description

SPECIFICATION Buoys for use with sea currents The present invention relates to buoys for use with sea currents. The surface currents of the oceans are fairly well known, and it is also known that there are other currents which flow beneath the surface currents and in different directions. The extent, the direction, the speed, the seasonal variation and other characteristics of these deeper currents are less well known than the equivalent characteristics of the surface currents.

A buoy comprising a float, a sinker suspended below the float by a cable, and a drogue deployed along a length of the cable could be used to measure the current at various depths by fixing the drogue at the depth to be measured, and then observing the drift of the buoy.

Such an instrument is fairly crude for scientific measurement of a current in that the drogue needs to be large enough to swamp the drift effects of wind and waves on the float, but if the drogue is too large it will be subject to currents at different depths acting in different directions.

The present invention avoids this drawback by realising that, in addition to the purely scientific interest in mapping sea currents at various depths, there are at least two practical applications in which advantage can be taken of different currents at different depths without necessarily distinguishing accurately between one current and another.

The present invention provides a buoy for use with sea currents, the buoy comprising a float, a sinker suspended below the float by a cable and having drogue means deployed at different depths between the float and the sinker.

The drogue means may either comprise a single drogue extending over substantially the entire depth between the float and the sinker or alternatively it may comprise a plurality of drogues deployed at spot depths.

In the case of the single drogue of considerable vertical extent the drogue may be of sufficiently strong material to support the sinker and thereby serve as the cable in addition to constituting the drogue means.

The first application for such a buoy is in determining the total effect of ocean currents on a large floating object. In this case the desired information is the integrated effect of all currents from the surface down to some predetermined depth. Thus the requirement is for a buoy whose drogue or drogues extend downwards to the pre-determined depth to present a substantially uniform resistance to the currents regardless of the direction from which they act. Such buoys could be particularly useful for teams engaged in moving tabular icebergs from the Antarctic to arid regions in need of fresh water e.g. the Arabian peninsula or Australia.

This application may be refined by suspending drogues at different depths or by using drogues of different sizes, or by a combination of such techniques. The object of such refinement is to take account of the fact that some large floating objects to not offer a uniform cross-section to currents in all direction or at all depths. For example an iceberg which is twice as long as it is wide will offer a greater resistance to tranverse currents than to longitudinal currents; while a semi-submersible off-shore oil rig offers most resistance at some particular depth with very much less resistance between that depth and the surface.

Whether a refined or a simple version of such a buoy is used, the buoy can be used for two principal purposes: a first purpose is to map the current-induced drift to be expected of an iceberg over large regions of the oceans, i.e. it is a preparatory or exploratory purpose; while the second purpose is for use during an iceberg manoeuvering operation as a guide to indicate what effect the wind, waves, and the applied towing effort is having on an iceberg. The combined effect of these forces can be seen by watching how the buoy drifts away from the iceberg.

A second application of such a buoy, and closely related to the "refined" version of the first application described above, requires a somewhat better knowledge of the distribution of the currents at a particular point. The idea is to deploy various areas of drogue at differing depths so that the net drift of the buoy over the sea bed is substantially nil. Clearly this can be exactly achieved, at least in theory, provided a vector triangle of currents is "available" at a particular point. However, it often happens that two vertically adjacent currents are in substantially opposite directions, in which case the resultant drift on balancing just two such currents against each other can be very small.

The purpose of such a "fixed point" application is to provide a reference point in the high seas to be able to see effectively how much progress is being made, e.g. when towing an iceberg. Such a fixed point may be easier to work from than the output of a satellite-based navigational system, and whatever its virtues from the strictly navigational point of view, it could well be worthwhile simply from a psychological point of view.

Moving an iceberg will seem to be an endless task having no visible effect for weeks on end, so a disappearing "fixed" buoy will at least provide an encouraging visible effect forthe team involved.

Clearly there are other uses for such quasi-stationary or low-drift buoys. A use which may turn out to be of considerable importance is for collecting weather data. A simple meteorological station which has a satellite transponder and which stays in substantially the same point over the surface of the globe for several weeks or even months at a time, could be most valuable.

An embodiment of the invention is described by way of example with reference to the accompanying drawings, in which: Figure 1 is a side view of a buoy in accordance with the invention; and Figure 2 is a detail perspective view of an alternative type of drogue.

In Figure 1, a buoy 10 comprises a cylindrical float 12 having a vertical axis, with a pole 14 projecting above the float to hold a radar reflector 16 at a height of about 5 m above the surface of the sea 18. The float 12 is ballasted by a length of chain 20 suspended from the bottom of the float 12 with a second float 22 connected to the lower end of the chain. A cable 24 is suspended from the second float 22 and supports three drogues 25,26 and 27 spaced out along its length with a sinker 28 at the bottom.

There are thus essentially two parts to the buoy: there is the upper part above the second float 22 and subject to the action of waves and swell; and there is the lower part from the second float 22 downwards subject to the action of sea currents at various depths. The second float 22 is designed to have just insufficient buoyancy to support the weight of the elements suspended below it, with somewhat less than 10% of this weight being supported by the first float 12. In operation the first float is fairly stable vertically since it is long and thin vertically, but in any case the chain 20 provides considerable additional isolation of the lower portion from the effects of surface waves and swell. While the first float 12 is in a wave trough, the chain 20 will go slack and the lower portion will start to sink, but it will not move fast since it has nearly neutral buoyancy.In the following wave crest the chain will be pulled taut but thereafter the drag and inertia of the lower part will slow the rise of the first float. If the waves are regular there will be a tendency for the entire lower part to rise slightly above its calm water equilibrium depth and for it to move only slightly thereafter in the vertical direction.

The lower portion is shown with three drogues 25, 26 and 27, and for the sake of example they can be considered to be at depths of 10 metres, 20 metres, and 200 metres from the equilibrium surface of the sea 18. Each drogue is in the form of a rectangular sheet of sail material, typically 2 mx 1,4 m, with two horizontal spars, one at its upper edge and the other at its lower edge. Each spar is connected via a bridle to a length of the cable 24. The sinker 28 is attached to the lowermost bridle, and the entire lower part may typically have a mass of 50 kg. As shown, the drogues are placed in between separate lengths of the cable 24.

An alternative would be to have a single cable extending over the entire vertical distance with suitable drogue-fixing means disposed along its length. In either case the depth, the size and the number of drogues used depends on the particular use intended for the buoys, as discussed in the introduction above.

An alternative form of drogue shown in Figure 2 comprises a hollow cylinder 30 with an expanded plastics outer wall 31. Such cylinders are used, for example, in swimming pools on the floating lines which separate different lanes during competitive event. They are threaded on the line between floats and serve to dampen wave interference between adjacent swimmers.

Atypical size is 30 cm long, cm diameter with a lattice-work outer wail 31 made of two substantially orthogonal sets of 4 mm thick bars e.g. 37 and 38 which spiral around the cylinder along helixes and which leave diamond-shaped openings 39 of about 4 mm a side. The effect of the expanded outer wall, when used as a drogue, is to produce approximately twice as much drag as is produced by a smooth cylinder of the same size. Such drogue elements can thus be strung out along the cable 24 in any desired configuration (up to a continuous run), and held in place by top and bottom knots 32,34. An alternative method of holding the cylinders in place is to use a cable having a thick plastics sheath which is too large to pass through the end holes of the cylinders. The cable is then covered from its bottom to at least the topmost cylinder with alternating lengths of sheath and cylinder.

The cylindrical form of drogue has the added advantage of symmetry, i.e. of presenting the same resistance to currents from any horizontal direction.

LIST OF REFERENCE NUMERALS 10 buoy 12 cylindrical float 14 pole 16 radar reflector 18 sea 20 chain 22 second float 24 cable 25,26,27 drogues 28 sinker 30 hollow cylinder 31 expanded lattice-workouterwall 32,34 knots 37,38 orthogonal helical bars of lattice 39 hole

Claims (6)

1. A buoy for use with sea currents, the buoy comprising a float, a sinker suspended below the float by a cable and having drogue means deployed at different depths between the float and the sinker.

2. A buoy according to claim 1, wherein the drogue means comprises a single drogue extending over substantially the entire depth between the float and the sinker.

3. A buoy according to claim 1, wherein the drogue means comprises a plurality of drogues deployed at different spot depths between the float and the sinker.

4. A buoy according to claim 2, wherein the drogue has sufficient mechanical strength to support the sinker and thereby serve as the cable in addition to serving as the drogue means.

5. A buoy according to any preceding claim, wherein the float supporting the drogue means is of insufficient buoyancy to support the drogue means and the sinker, and is itself supported by a float floating at the sea surface, thereby providing a degree of isolation between the drogue means and any waves or swell.

6. A buoy for use with sea currents, substantially as herein described, with reference to and as illustrated in either of the accompanying drawings.