Underwater Leveller

This invention relates to underwater levelling.

The traditional method of levelling underwater is to take soundings and relate them to the water surface and then to a datum. The greatest accuracy which might be achieved by this method would be in the region of +_ 150 mm, the sounder beam or lead is indiscriminate and accuracy decreases with depth.

Where greater accuracy has been necessary, for example for lighthouse foundations, spirit levels mounted on long beams and cable stayed to a centre column have been used. This operation is very costly, time consuming and difficult, particularly in deeper water, and inaccuracies result from beam and cable deflections. It is unlikely that accuracies of greater than +_ 50 mm, over a very short distance, may be obtained by this method.

An apparatus has also been used in nautical archaeology, having a flexible tube to the sea at one end and to a large inverted air reservoir at the other (Bill St.John Wilkes, Nautical Archaeology, 1971). This apparatus has had several serious drawbacks, in particular from the point of view of accuracy as the reservoir has not provided a constant fixed datum from which the measure, being affected by the amount of air in the tube and by fluctuations in ambient

sea pressure.

According to the present invention there is provided an underwater levelling device comprising a flexible tube having first and second openings and an inlet for fluid intermediate the openings, the inlet communicating with means for introducing a continuous flow of fluid into the tube.

Further according to the invention there is provided a method of underwater levelling comprising introducing a continuous flow of fluid into a flexible tube having first and second openings, allowing the fluid to escape through one of the openings so as to provide a fluid/water interfac in the tube, and recording the position of the interface relative to its surroundings.

Preferably the fluid is gas, most preferably air, and the means for its continuous introduction may be a compressed gas cylinder or other pressurised container. A valve is preferably provided to regulate the flow of gas and thereby to give a supply at constant pressure. The valve may be a single-stage reduction valve.

V/hen the fluid is a gas the tube centre section floats in use so that the openings are downward-facing. Gas then emerges continuously from the first opening so long as the second opening is at a lower level; a non-return valve is preferably provided at the first opening to prevent water entering the tube and becoming trapped if the second openin is raised above the level of the first opening.

The tube inlet is preferably close to the first opening, an at least the portions of the tube adjacent the openings are preferably of the same cross-sectional area.

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A flotation chamber is preferably provided for filling with air or with water to provide buoyancy and ballast respectively for the device in use.

To minimise the effect of ambient pressure fluctuations damping means may be present at least at the second opening. The damping means may for example take the form of constrictions of the tube at the openings, either by insertion of internal collars or by physical constriction of the tube itself.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram of a device of this invention.

The device of this embodiment has a flexible tube 1 of air- impervious transparent plastics material, for example polythene or PVC, in one end portion 2 of which is a non¬ return valve 3 arranged to allow only outflow from the tube 1. The other end 4 of the tube is open to sea water in which the device is submerged. Both end portions 2, 4 of the tube are constricted by means of plastics sleeves 5 disposed within the tube 1.

The tube 1 has a branch 6 leading into it from a compressed air cylinder 7. A single-stage valve 8 is present between the cylinder 7 and the branch 6, the valve 8 being capable of maintaining a pressure of air in the branch 6 and tube 1 . dependent on the ambient water pressure. The branch 6 also contains a series of flow reducers (not shown) for restricting the flow rate of air through the branch 6, and a flow adjustment valve 9 for manual control of the flow rate and pressure.

The cylinder 7, branch 6 and end portion 2 of the tube 1 are mounted on a stand 10, the branch 6 and end portion 2 being

held in a bracket 11.

A graduated scale 12 which reads in both directions is provided at the end portion 4 of the tube 1, the scale 12 being connected to the tube 1 through a pair of connection 13 slidable along the scale 12.

In use, the stand 10 is placed in a first position on the bed, the air s'ipply from the cylinder 7 is started and the end portion 2 of the tube blocked off to ensure that the branch 6 and tube 1 fill with air and no water is trapped therein. The single stage valve 8 is preset at a pressur for example, 100 psi above ambient water pressure, and the end portion 2 of the tube is unblocked. The end portion of the tube 1 is maintained as far as possible at a lower level than the end portion 2, whereby air from the cylinde and branch 6 escapes from the end portion2 of the tube 1 a steady stream of bubbles. At the other end portion 4 the water enters the tube 1 up to approximately the same level the end portion 2 from ^' which the air is escaping, and this level remains virtually constant so long as the stand 10 i not moved. An air/water interface 14 indicates this leve When a suitable site for the stand 10 has been found, therefore, levels can be taken quickly and very easily at points within the range of the tube 1 by reading the heigh of the interface 14 off the scale 12.

When the device is used near the surface of the sea, sligh movement of the interface 14 up and down the tube 1 may sometimes be encountered; this is due to transient pressu variations produced by waves on the sea surface, causing slight pressure differences between the endportions 2 and of the tube 1. This movement of the interface 14 is damp by the presence of the sleeves 5 constricting the tube end and the greater the depth of the device the less the movement.

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Should the end portion 4 be raised to a level higher than the end portion 2 water will tend to enter the tube 1 at the end portion 2, but this is prevented by the non-return valve 3 which immediately closes; any air escaping through the end portion 4 is replenished in the tube 1 from the cylinder 7 and branch 6.

Some advantages of the device of this embodiment of the invention are:

1. Accuracy. In a water depth of 13 , three seabed stations, 9 m apart, were levelled in varying sea conditions. The test was carried out in open water and nine sets of readings were taken. The average standard error of these readings was +_ 2.5 mm and the average standard error of the mean was _+ 0.9 mm. That is, the results indicate that the stations were levelled to within + 1 mm.

2. The effect of wave action is relatively small, i.e., pressure variations reduce rapidly from the surface and accuracy increases with depth.

3. The device can be used in poor visibility.

4. It is possible to level through and over obstructions.

5. Only one operation is required.

6. The device may be set up in a few minutes and levels quickly taken, thus reducing diving time.

7. The device is cheap to make, robust, easily checked and needs a minimum of maintenance.

8. The air tube floats and makes snagging on obstructions unlikely.

9. The datum can be set at a predetermined level, for example a desired depth to which an area should be dredged, and physical checking of

actual levels relative to this can be taken very quickly and easily.

10. The device is compact and self-contained.

11. Only a small amount of air is used. A single 60 ft cylinder may last many hours.

The area covered or the distance between change points is governed solely by the amount of tubing that can be comfortably handled by a diver. 20m seems about the optimum. This gives a distance between change points of 30m or sea bed coverage from one set-up of the device o

2 about 700 m .

The accuracy which can be obtained with this device is comparable, for local underwater levelling, with that whic is acceptable for engineering work on land.

In some environments supply of gas from a compressed air bottle may be impractical, for example in deep water where the ambient pressure is high and the bottle would empty in a short time, and in these cases it is better to have a constant flow from another source, for example f a compressor at the surface or from a diving bell. The length of time during whuch the device can be in operation without interruption can thus be increased.

Modifications and improvements can be made without departing from the scope of the invention.