GB2204080A

GB2204080A - Method and apparatus for retarding tidal flow

Info

Publication number: GB2204080A
Application number: GB08810302A
Authority: GB
Inventors: Charles Michael Waters
Original assignee: BUKASA Ltd
Current assignee: BUKASA Ltd
Priority date: 1987-05-02
Filing date: 1988-04-29
Publication date: 1988-11-02
Also published as: GB8810302D0

Abstract

Tidal flow is retarded by the use of a plastics net curtain (5) the lower edge of which is anchored to the seabed (2) and the upper edge of which is attached to positive buoyancy means (6, 7) comprising a pair of cylindrical floats (7) which are connected by diamond-shaped frames (6) which function as wave breakers. <IMAGE>

Description

METHOD AND APPARATUS FOR RETARDING TIDAL FLOW Field of the Invention This invention relates to a method and apparatus for retarding tidal flow so as to prolong the period in which a diver can work in an area which is subject to tidal flow.

In many areas where divers work, the ti-dal flow is so strong that a diver can only work in a given area for a very limited period. This period is known as the slack period and lasts for about 2 hours at the time when the direction of the tide changes. In many cases, this short period of working is extremely inconvenient for both the diver and his employer and, in many operations, this introduces a very time consuming and expensive delay in completing a particular job.

Summary of the Invention According to a first aspect of the present c invention there is provided a method of retarding tidal flow in a given area to prolong the slack period, which method comprises locating a tidal flow curtain in a position such that it is upstream of the given area which then lies in the lee of the curtain, the curtain including means disposed at its lower edge to locate the lower edge of tllc curtain relallve to the seabed, said curtain having positive buoyancy as a feature of its general composition or being provided with buoyancy means along its upper edge to maintain the curtain in an upright or semi-upright position, the structure of the curtain being such that it retards the tidal flow.

The use of one curtain in one position with respect to the given area can only extend the time in which a diver can work in an area for one direction of tidal flow and, when the direction of the tide changes, the diver will feel no benefit from the curtain. It is possible to move the curtain to a second position at the other side of the given area when the direction of tidal flow changes. However, it is more convenient to use 2 curtains positioned one on each side of the given area.

The means for stabilizing the location of the bottom edge of the curtain on the seabed may just comprise a block of sufficient density and weight, but preferably the curtain may include means to move positively anchor the curtain in a given position.

The structure of the curtain is preferably perforate and may comprise a sheet of plastic netting or interlocking units.

It should be noted, that in use the curtain may not be vertical but may be at an angle to the vertical due to the tidal flow.

According to a second aspect of the present invention, there is provided a method of retarding tidal flow in a given area, the method comprising the steps of locating a tidal flow curtain in a position upstream of the given area such that the said area lies in the lee of the curtain; holding the upper edge of the curtain in use, at, adjacent, or substantially above, surface level; and holding the lower edge of the curtain in use below surface level in such a way as to tend to maintain the curtain in an upright or semi upright position by anchoring the curtain to the seabed whilst preventing the lower edge of the curtain from contacting the seabed.

The upper edge of the curtain is preferably held at, adjacent or substantially above surface level by positive buoyancy means secured to the curtain.

However in -an alternative embodiment the upper edge of the curtain is secured to a ship or boat from which the curtain is draped over the side. The curtain may be secured with the vessel so that the curtain passes over the edge of the vessel into the water. Alternatively the supper edge of the curtain may be secured to a beam which is suspended from the vessel by a crane. This is of particular use by divers wishing to work in the lee of a vessel on rigs and platforms in the North Sea.

As long as the positional stabiLity of the curtain is maintained by anchoring it to the seabed, its lower edge may be prevented from contacting the seabed by so dimensioning the curtain that, in use-, its lower edge stops short of the seabed.

Alternatively or additionally, the lower edge of the curtain may be anchored to the seabed by anchor lines or the like.

In the case just outlined, the anchor lines may constitute the sole means anchoring the curtain to the seabed.

Alternatively or additionally, anchor lines or the like may run from the sea-surface adjacent area of the curtain to the seabed.

For example, in the case just outlined, anchor lines may run from the positive buoyancy means down to the seabed.

According to a third aspect of the present invention there is provided apparatus for retarding tidal flow comprising a curtain, positive buoyancy means attached to the upper edge of the curtain, and a-nchoring means for holding the lower edge of the curtain in use below surface level in such a way as to tend to maintain the curtain in an upright or semi-upright position, said positive buoyancy means comprising spaced apart buoyancy elements between which wave breakers are located.

It is to be appreciated that the inethod and apparatus for retarding tidal flow can be used for other purposes in addition to extending the period for which a diver can work in a given area.

For example, the apparatus can form an economic and quickly assembled means of producing sheltered and calm water either to facilitate the mooring of vessels or to protect beaches from erosion.

Brief description of the Drawings Examples of apparatus in accordance with the invention together with the installation of apparatus for retarding tidal flow will now be described with reference to the accompanying drawings, in which: Figure 1 shows the installation of apparatus accordance with the third aspect of the invention.

Figure 2 shows in perspective a curtain installed in accordance with a method embodying the second aspect of invention and, Figure 3 shows, in side elevation, two alternative ways of installing curtains by mehtods of embodying the second aspect of in Invention Description of the Preferred Embodiments The apparatus shown in figure 1 includes a plastics net curtain 5 at the lower edge of which there is an elongate spreader beam 4 which is provided at intervals along its length with anchor lines 3 connected to barbed fixings 1 which are driven into the seabed 2 with the lengths of the anchor lines 3 such that the plastics net curtain 5 is maintained at a predetermined spacing from the seabed 2.

The plastics net curtain 5 is provided at its upper end with a positive buoyancy means which comprises a pair of elongate cylindrical floats 7 interconnected by a series of diamond.-shape frames 6 which are connected bY fixing lines to the upper edge of the curtain 5. The series of diamond-shaped frames 6 function as wave breakers and the number of frames 6 will depend on the length of the curtain. Individual frames 6 can be fastened to individual struts which are connected to the buoyancy floats 7 which extend parallel to one another, one on each side of the curtain 5. The buoyancy floats 7 anchor the individual. frames 6 can be joined together in groups to form a single elongate unit.It is to be noted that the diamond-shaped frames 6 are of -open-work construction and are positioned so that diagonals of the diamonds. are at surface level. The open spaces within the diamond-shaped frames 6 allow the energy of the waves to be dissipated.

As shown in figure 1- of the drawings, the direction of wave travel and of tidal flow is from right to left and that, although there is an area of turbulent wave formation 8 to the right of the curtain, there is an area 9 of sheltered water to the left of the curtain. At the same time, the strong tidal currents 10 to the right of the curtain 5 result, because of the retarding effect of the plastics net curtain 5, in much reduced currents 10A to the left of the. curtain 5.

The apparatus can be installed adjacent a beach-head to resist erosion. It can also be used to produce sheltered water to facilitate the mooring of vessels. It can also be used to extend the period for which a diver can word under water. In such- circum stances, one curtain will normally be positioned on each side of the area in which the diver. is to work.

The tidal flow retarding curtains shown in figures 2 and 3 of the drawings are perforate and comprise sheets of plastics netting, weighted along theirs respective lower edges, and with positive buoyancy means in the form of buoys spaced in the case of one curtain. along. the upper edge of the curtain, and in the. case of a second curtain along the length of the curtain between the upper edge and the lower edge.

In use, referring initially to figure 2, a plastics net curtain 11 with buoys 12 spaced along its upper edge 13 has a line of weights generally 14 running along its lower edge; and, in use, is installed in the sea so that its upper edge 13 floats at sea surface level and the curtain itself floats suspended in the sea with its lower edge 14 some way short of the seabed 15 as shown in figure 3.

Lines 16 anchor the curtain to the seabed 15. In this particular embodiment, the lines run from the lower edge 14 of the curtain to the seabed. Lines such as the lines 16 could, alternatively or additionally, run from the upper edge 13 - for example, from each of the buoys 12 - to the seabed 15 to -maintain the positional stability of the curtain 11 in use.

The second embodiment shown in figure 3 is installed to work in a similar way to that of the embodiment of figure 2. Here however the positive buoyancy means 12 are spaced along the curtain at approximately its mid-height. In use, therefore, as figure 3 shows, as much of the curtain 11 protrudes above sea surface level as the floats below the surface level. Again, the lower edge 14 of the curtain is weighted and the curtain is anchored to the seabed 15 by lines 16 which run from the weighted lower edge 14 of the curtain.

In this second embodiment, the above-surface protruding portion of the curtain 11 need not necessarily be perforate.

In use, the curtains are illustrated as installed adjacent a beachhead 17 to resist erosion. The curtains could comprise segmental blankets of the kind currently marketed under the name LINKLOK by Hydrofax Limited of Monument House, South Quay, GREAT YARMOUTH, Norfolk, United Kingdom. In any embodiment of the invention, of course, the construction of curtain must be such that the curtain has a net positive buoyancy when installed in the sea.

Claims

1. A method of retarding tidal flow in a given area to prolong the slack period, which method comprises locating a tidal flow curtain in a position such that it is upstream of the given area which then lies in the lee of the curtain, the curtain including means disposed at its lower edge to locate the lower edge of the curtain relative to the seabed, said curtain having positive buoyancy as a feature of its composition or being provided with buoyancy means along its upper edge to maintain the curtain in an upright or semi-upright position, the structure of the curtain being such that it retards the tidal flow.

2. A method according to Claim 1, which includes the use of 2 curtains, located one on each side of the givers area.

3. A method of- retarding tidal flow in a given area, the method comprising the steps of locating a tidal flow curtain in a position upstream of the given area such that the said area lies in the lee of the curtain; holding the upper edge of the curtain in use, at, adjacent, or substantially above, surface level and holding the lower edge of the curtain in use below surface level in such a way as to tend to maintain the curtain in an upright or semi-upright position by anchoring the curtain to the seabed whilst preventing the lower side of the curtain from contacting the seabed.

4. A method according to Claim 3, wherein the upper edge of the curtain is in use held at, adjacent or substantially below surface level by positive buoyancy means secured to the curtain.

5. A method according to Claim 3 or 4 wherein the lower edge of the curtain is anchored to the seabed by anchor lines.

6. Apparatus for retarding tidal flow comprising a curtain, positive buoyancy means attached to the upper edge of the curtain, and anchoring means for holding the lower edge of the curtain, in use, below surface level in such a way as to tend to maintain the curtain in an upright or semi-upright position, said positive buoyancy means comprising spaced apart buoyancy elements between which wave breakers are located.

7. Apparatus according to Claim 6, wherein said wave breakers comprise open diamond-shaped frames.

8. Apparatus according to Claim 7, wherein the curtain is attached to the wave breakers.

9. Apparatus according to Claim 6, wherein anchor lines are provided extending from the lower edge of the curtain for anchoring the curtain to the seabed.

10. A method of retarding tidal flow according to Claim 1 or Claim 3 and substantially as hereinbefore described.

11. Apparatus for retarding tidal flow as claimed in Claim 6 and substantially as hereinbefore described with reference to and as illustrated in figure 1 of the accompanying drawings.