GB1573625A

GB1573625A - Embeddable anchoring device and methods of embedding such devices

Info

Publication number: GB1573625A
Application number: GB575576A
Authority: GB
Original assignee: QMC Indusrial Research Ltd
Current assignee: QMC Indusrial Research Ltd
Priority date: 1976-12-06
Filing date: 1976-12-06
Publication date: 1980-08-28

Description

(54) IMPROVEMENTS IN AND RELATING TO AN EMBEDDABLE ANCHOR ING DEVICE AND TO METHODS OF EMBEDDING SUCH DEVICES (71) We, Q M C INDUSTRIAL RESEARCH LIMITED, a British Company, of 229 Mile End Road, London El 4AA, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an anchoring device for use in anchoring a structure, and in particular to an anchoring device for use in anchoring a marine platform to the sea bed.

The invention also relates to a method of embedding such a device.

Marine platforms for oil drilling and production are conventionally of the gravity type, that is to say they have a large, heavy base made of reinforced concrete and/or steel, and a deck supported above the sea level by means of a tower or towers fixed to the base. As the depth of the sea increases such a platform becomes very large and immensely wasteful in time and resources, both to construct and to position on the sea bed. Moreover, such platforms require specially-built dry docks and nearby deepwater facilities for their construction.

An alternative to the gravity platform is the tension-leg floating platform, which as its name implies has a floating platform which is held in position by a suitable configuration of ties attached to ground anchors on the sea bed. A further alternative is a cable-stayed gravity platform. Such platforms offer greater potential flexibility in design and operation and, on cost alone, seem likely to replace the conventional heavy base gravity platform as exploration reaches the edge of the continental shelves. Unfortunately, the known type of tension-leg floating and cable-stayed gravity platforms have a number of problems, the major one of which is the method of anchoring the ties and cables to the sea bed.

The present invention provides an embeddable anchoring device comprising, prior to embeddment, a thrust member, a tie rod, and means for anchoring the device in the earth, the thrust member having dimensions in directions along the axis of the tie rod and at right angles to that axis that are considerably greater than the maximum dimension of the tie rod in a direction transverse to said axis such that, when the device is embedded, the thrust member at one end of the tie rod provides resistance to lateral movement of the device while the anchoring means at the other end of the tie rod provides resistance to upward movement of the device.

This anchoring device has the advantage of using a mass of earth material for holding it in place. This contrasts with the known ground anchors of the embedded type which require a large concrete dead weight and extensive excavation of the earth and subsequent implacement. Moreover, the device has the potential to be considerably cheaper than known anchors, is easier to make, handle and install, can be installed faster, and, on some forms, it has the advantage of being recoverable from the earth for re-use.

The thrust member permits the device to be embedded substantially vertically in the earth and yet accommodate uplift forces over a wide range of angles to the surface of the earth. This contrasts with known ground anchors of the embedded type which must be installed to lie along the expected direction of the uplift force. Consequently, this anchoring device is much more versatile, particularly for use on the sea-bed where the angle of an attached cable will vary according to variations in the tide and other environmental conditions such as wind and waves.

The thrust member may be of any suitable shape but it is of advantage if it is frustoconical in form, and is coaxial with the tie rod. This does not restrict the angle of uplift (pull) on the device to a given plane, but permits pull from any direction and over a wide range of angles.

The thrust member may be made from any suitable material, but it is of advantage if it is made from reinforced concrete or sheet material which is preferably metal plate.

The anchoring means may be constituted by an anchor plate extending at right-angles to the tie rod.

The anchor plate may be of any suitable configiguration but it is of advantage if it is disc-shaped.

Advantageously, a plurality of jets are provided on the anchor plate which jets are arranged to pass fluid in a direction away from the thrust member.

Further, it is of advantage if a plurality of jets are provided on the anchor plate which jets are arranged to pass fluid in a direction towards the thrust member.

-An alternative form of anchoring means is constituted by a plurality of flukes which are pivoted to the tie rod and movable between one condition in which they lie substantially parallel to one another and to the tie rod, and another condition in which they diverge from the axis of the tie rod and in direction away from the thrust member.

Where the device is to be used in earth which is non-sandy/gravelly, for example predominantly clayey in nature, the anchor means may comprise a tube having a split or weakened end portion.

Advantageously, to allow the tie rod and thrust member of the device to be removed from the earth, the anchoring means is detachably secured to the tie rod.

In all the devices constructed in accordance with the present invention the tie rod is, with advantage, provided with a longitudinal passage extending between its ends, or is in the form of a hollow shaft or tube.

Thus when the anchoring means is constituted by an anchor plate, it is of advantage if the passage or hollow interior of the tie rod is in fluid communication with the jets Water or air can then be forced down the passage or tube to pass out through those jets which pass fluid in a direction away from the thrust member in order to embed the device into the earth.

Further, when the anchoring means is constituted by a plurality of flukes, a member may be provided in the passage or hollow interior of the tie rod which is operatively linked to the flukes and is axially displaceable to move the flukes from the said one to the said other condition.

When the anchoring means comprises a weakened or splitended tube, an end portion of the tube or shaft constituting the tie rod can itself be used to constitute the weakened split-ended tube.

In order to resist the corrosive effects of sea water, it is preferable for the device to be made of corrosion-resistant steel.

The present invention further provides a method of embedding an anchoring device, constructed in accordance with the invention, in the earth, the method comprising positioning the tie rod substantially vertically above the surface of the earth with the thrust member uppermost, and driving the device into the earth until the thrust member is lodged therein.

The device may be driven into the earth until the thrust member lies substantially flush with the surface of the earth.

The device may be driven into the earth by jetting and/or vibration.

When the device comprises an anchoir plate, the device may, with advantage, be driven into the earth by forcing fluid, for example, air or water, through those jets which pass fluid in a direction away from the thrust member.

An anchoring device constructed with such an anchor plate and positioned by this jetting and/or vibration process is particularly suitable for embedding in the bed of the North Sea which is composed predominantly of sandy/gravelly sediments. Consequently, as the device is driven in by this process, the sediments are disturbed slightly as the anchor plate burrows in, the disturbed sediments then settling above the anchor plate.

The pressure of the sea water above compresses the settled sediment so that the device is firmly embedded. Moreover, this settling and compressing of the sediment occurs quite quickly, so that the device can be put into use soon after installation.

When the device comprises an anchoring means constituted by flukes, the flukes are, advantageously, arranged in the said one condition as the device is driven into the earth, and moved into the said other condition after the device has been driven in.

This embedding process is also suitable for the bed of the North Sea. Moreover, as the fluke-type anchoring means can easily be driven into the sea bed without disturbing the sea bed material significantly, the device can be put into use even sooner after installation than the device using the anchor plate as the anchoring means.

When the anchoring means of the device comprises a weakened or split-ended tube, an explosive charge may, with advantage, be arranged in the tube, which charge is detonated after the device has been driven in so that the end of the tube is splayed out to form diverging prongs or a bulge.

The explosion also provides a cavity in the earth round the prongs into which a binding material, for example, a rapid hardening mortar, can be introduced for anchoring the device. This form of device can also usefully be used in said and in this case, a rapid hardening mortar, or preferably, an epoxy resin could be used to bind the sand together round the prongs. However, in this case although an explosion is still required to form the diverging prongs on the anchor means, the explosion can be smaller since it is not necessary to form a cavity in the sand.

When the tie rod is provided with a longitudinal passage or is constructed by a hollow shaft or tube, then binding material, for example a rapid hardening mortar or an epoxy resin may, with advantage, be forced down the passage or tube after the device has been embedded so that it binds together the material surrounding the anchoring means.

When the anchoring means is in the form of an anchor plate and the passage or hollow interior of the tie rod is in fluid communication with the jets the binding material is forced down the passage or tube and out through the jets.

Three forms of anchoring device, each constructed in accordance with the invention, and embedding processes using these devices, will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side elevation of the first form of anchoring device shown anchored in the sea bed; Figure 2 is a force diagram pertaining to the device of Figure 1; Figure 3 is a schematic side elevation of the second form of anchoring device shown being driven into the sea bed; Figure 4 is a schematic side elevation of the anchoring device of Figure 3 shown anchored in the sea bed; Figure 5 is a schematic side elevation of the third form of anchoring device shown anchored in the sea bed.

Referring to the drawings, Figure 1 shows an anchoring device, indicated generally by the reference numeral 1. The anchoring device 1 comprises a tie rod in the form of a hollow shaft or tube 2 provided at one end with a frustoconical thrust member 3 made up from several plates and, at the other end, with an anchor plate 4 in the form of a disc. A cable (not shown) can be fastened to the tie rod 2 at the point 5. All the parts of the device 1 are made of suitable corrosion resistant steel.

As can be seen from Figure 1, the thrust member has dimensions that are consider ably greater in the directions of the axis of the tie rod and at right angles to that axis, than the diameter of the tie rod.

In order to anchor the device 1 in the sea bed 6, the device is positioned substantially vertically above the sea bed and then driven into the sea bed by a jetting and/or vibra tional process. A plurality of downwardly directed jets (not shown) are provided on the lower surface of the anchor plate 4, these jets being activated, when the device 1 is correctly positioned, to throw out jets of water or air. The jets are connected to the hollow interior of the tie rod 2 so that water or air can be forced down the rod and out through the jets. The action of these jets enables the device to sink into the sea bed 6, and the burrowing of the device into the sea bed material is assisted by vibrating the device 1. This vibration is carried out by any suitable vibrator (not shown).This device 1 is particularly useful for anchoring in the bed of the North Sea as here the sea bed material is predominantly sand/gravel sediment. Thus, the jetting and vibration of the device 1 into place, causes only slight disturbance of the sea bed material and this rapidly re-settles over the device and is subsequently compressed by the weight of the sea water above to provide a firm anchorage.

When the device 1 is anchored in position, a cable fastened to the point 5 can be tensioned. When the cable is at an angle to the vertical (that is to say to the axis of the tie rod 2) of up to 900, say angle a as shown in Figure 1, an uplift force P along the cable is resisted in part by soil thrust acting on the thrust member 3 and in part by the resistance R to upward movement offered by the anchor plate 4. A typical force diagram is shown in Figure 2. As can be inferred from Figure 2, the thrust member 3 provides resistance to lateral movement of the device when embedded whilst the anchor plate 4 provides resistance to upward movement.

The device 1 can be retrieved from the sea bed, for use elsewhere, by a vibratory pull combined with reverse Jetting, using a plurality of upwardly directed jets (not shown) provided on the upper surface of the anchor plate 4.

Figures 3 and 4 show an alternative form of anchoring device 11, also suitable for use in the bed of the North Sea. This device 11 comprises a tie rod in the form of a hollow shaft or tube 12 provided at one end with a frusto-conical thrust member 13 and, at the other end, with a plurality of flukes 14 hinged to the tie rod. A runner (not shown) which may be in the form of a rod, is arranged within, and can be moved longitudinally relative to, the tie rod 12. Each of the flukes is pin connected to the runner through longitudinal slots (not shown) provided in that part of the tie rod 12 of smaller diameter. All the parts of the device 11 are made of suitable corrosion-resistant steel.

Again, as in the case of the embodiment of Figure 1, the thrust member 13 is of considerably greater dimension than the diameter of the tie rod 11, in the directions of the axis of the tie rod and at right angles to that axis.

In order to embed the device 11 in the sea bed 16, the device is positioned substantially vertically above the sea bed with flukes 14 in their closed position (see Figure 3). The device 11 is then driven into the sea bed by any suitable means, the pointed end 12a of that part of the tie rod of smaller diameter assisting with this process. When the thrust member 13 lies substantially flush with the surface of the sea bed 16, the driving-in process is stopped and the flukes 14 are forced outwardly to anchor the device 11 (see Figure 4). Outward movement of the flukes 14 is effected by upward movement, relative to the tie rod 12, of the runner. As very little sea bed material is disturbed by this driving-in process, the device 11 is ready for use very quickly, a cable 17 (see Figure 4) being attached to the tie rod 12 at the point 15 and tensioned if desired.This device 11 has similar resistance to pull-out as the device 1, and the force diagram to Figure 2 also applies.

If desired the device 11 can be removed from the sea bed by returning the flukes 14 to their closed position and applying a vibratory pull.

Figure 5 shows a device 21 suitable for embedding in clay. The device 21, which is shown anchored in position, comprises a tie rod 22 in the form of a hollow shaft, a frustoconical thrust member 23, and a plurality of prongs 24.

The frusto-conical thrust member 23 is of considerably greater dimension than the diameter of the tie rod 22, in the directions of the axis of the tie rod and at right angles to that axis.

All the parts are again made of a corrosive-resistant steel. Prior to anchorage the tie rod is in the form of a split-ended tube or a tube having a plurality of lines of weakening, the prongs 24 being formed, as described below, after the rod has been driven into place. The device is embedded in a clay sea bed 26 by driving it vertically into the sea bed. When the thrust member 23 lies substantially flush with the surface of the sea bed 26. an explosive charge, which can already be arranged in position or which is passed down the hollow-shafted tie rod 22 until it is positioned at the split or weakened end thereof. The charge is then detonated so that the split or weakened ends splay out to form the diverging prongs 24 and so that a cavity 27 is formed around the prongs 24.

Alternatively the weakened ends can form a bulge after the charge is detonated. A rapid hardening mortar 28 is then introduced into the cavity 27 via the interior of the hollow tie rod 22. When this mortar 28 solidifies, a cable (not shown) attached to the tie rod at point 25 can be tensioned if desired.

Alternatively. the device 21 could be used in sandy/gravelly material. In this case it is not necessary to form a cavity and only a small explosion would be necessary to form the prongs 24. An epoxy resin is then pumped down the hollow tie rod 22 to bind together the material surrounding the prongs.

By making the prong section 24 as a detachable part of the device 21, the remainder of the device could be removed from the sea bed, for re-use elsewhere, by detaching the prongs and applying a vibrating pull to the tie rod 22. Here again, the provision of the frustoconical thrust member enables the device 21 to take up uplift forces over a wide range of angles.

It will be apparent that a plurality of each of the devices 1, 11 and 21 described above could be used to anchor a marine platform with the aid of a suitable configuration of ties. Such an arrangement provides a secure anchorage and enables accepted requirements for a mooring system to be satisfied, namely: - (i) the moored object (marine plat form) is kept in its station within certain acceptable limits; (ii) the moored object is sufficiently stable when subjected to a wide range of environmental forces: (iii) it results in a sufficiently permanent anchorage in an aggressive chemical environment.

(iv) it is commercially viable from an economic standpoint.

Moreover, the movements of the platform owing to the variation in tides and/or the effect of large waves, winds etc. can be restricted, using the anchoring devices described above, to little more than that resulting from the elastic deformation of the ties brought about by increased tension.

These platform movements will be very much less than for the presently known semi-submersible platforms that are secured with conventional anchors. On this type of platform, work is not possible, owing to heave, when the wave height exceeds 15 metres.

Although the anchoring devices described above are specifically intended for use in fairly deep waters, it will be apparent that they may also be used in shallow waters and even on dry land. Thus, they may be used at a range of water depths for mooring construction barges, tankers, ships, light ships, weather stations, pipe-laying platforms, semi-submersibles, buoys, underwater buoyant bridges, sea wave energy converters, subsea systems containing people, materials, fish or other species, floating and submerged reservoirs, pipelines which are floating or suspended or fixed to sea bed, floating or suspended instrument modules, breakwaters, flood barrages, Jetties, floating dry docks. On dry land they may be used for anchoring towers, for preventing bridge pontoons sliding, for anchoring cable roofs, for preventing uplift of such structures as dry docks, for preventing sliding of walls and similar structures, for prevention or adjust ment of foundation heave, for anchoring reservoirs, dams, balloons and airships.

It will be obvious that a number of modifications could be made to the anchoring devices described above. Thus, they could be made from other materials than steel and other materials than mortar and epoxy resin could be used for anchoring the device of Figure 5. Further, because the tie rods in the Figure 1 and Figure 3 constructions are hollow, binding material such as mortar or epoxy resin can, if required, be passed down these rods to bind together the material surrounding the plate 4 or the flukes 14 respectively.

WHAT WE CLAIM IS: 1. An embeddable anchoring device comprising, prior to embeddment, a thrust member, a tie rod, and means for anchoring the device in the earth, the thrust member having dimensions in directions along the axis of the tie rod and at right angles to that axis that are considerably greater than the maximum dimension of the tie rod in a direction transverse to said axis such that, when the device is embedded, the thrust member at one end of the tie rod provides resistance to lateral movement of the device while the anchoring means at the other end of the tie rod provides resistance to upward movement of the device.

2. A device as claimed in claim 1, in which the thrust member is made from reinforced concrete.

3. A device as claimed in claim 1, in which the thrust member is made from sheet material.

4. A device as claimed in claim 3, in which the material is metal plate.

5. A device as claimed in any of claims 1 to 4, in which the thrust member is frustoconical in form and is coaxial with the tie rod.

6. A device as claimed in any one of claims 1 to 5. in which the anchoring means is in the form of an anchor plate extending substantially at right angles to the tie rod.

7. A device as claimed in claim 6, in which the anchor plate is of disc-shaped form.

8. A device as claimed in claim 6 or claim 7. in which a plurality of Jets are provided on the anchor plate which jets are arranged to pass fluid in a direction away from the thrust member.

9. A device as claimed in any one of claims 6 to 8, in which a plurality of Jets are provided on the anchor plate, which Jets are arranged to pass fluid in a direction towards the thrust member.

10. A device as claimed in any one of claims 1 to 5, in which the anchoring means is constructed by a plurality of flukes which are pivoted to the tie rod and movable between one condition in which they lie substantially parallel to one another and to the tie rod, and another condition in which they diverge from the axis of the tie rod and in direction away from the thrust member.

11. A device as claimed in any one of claims 1 to 5, in which the anchoring means is in the form of a tube having a split or weakened end portion.

12. A device as claimed in any one of claims 1 to 11, in which the anchoring means is detachably secured to the tie rod.

13. A device as claimed in any one of claims 1 to 12, in which the tie rod is provided with a longitudinal passage extending between its ends.

14. A device as claimed in any one of claims 1 to 12, in which the tie rod is in the form of a hollow shaft or tube.

15. A device as claimed in claim 13 or claim 14 when appendant to claim 8 or claim 9, in which the passage or hollow interior of the tie rod is in fluid communication with the jets.

16. A device as claimed in claim 13 or claim 14 when appendant to claim 10, in which a member is provided in the passage or hollow interior of the tie rod which is operatively linked to the flukes and is axially displaceable to move the flukes from the said one to the said other condition.

17. A device as claimed in claim 14 when appendant to claim 11, in which an end portion of the tube or shaft constituting the tie rod itself constitutes the weakened or splitended tube.

18. A device as claimed in any one of claims 1 to 17, in which the device is made of corrosion-resistant steel.

19. An anchoring device substantially as hereinbefore described with reference to, and as illustrated by, Figure 1, Figures 3 and 4, or Figure 5 of the accompanying drawings.

20. A method of embedding an anchoring device as claimed in any one of claims 1 to 19 in the earth, the method comprising positioning the tie rod substantially vertically above the surface of the earth with the thrust member uppermost, and driving the device into the earth until the thrust member is lodged therein.

21. A method as claimed in claim 20, in which the device is driven into the earth until the thrust member lies substantially flush with the surface of the earth.

22. A method as claimed in claim 20 or claim 21, in which the device is driven into the earth by Jetting and/or vibration.

23. A method as claimed in claim 22, in which the device is as claimed in claim 8 and is driven into the earth by forcing fluid through the jets.

24. A method as claimed in any one of claims 20 to 22, in which the device is as claimed in claim 10, the flukes being arranged in the said one condition as the device is driven into the earth, and being

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ment of foundation heave, for anchoring reservoirs, dams, balloons and airships. It will be obvious that a number of modifications could be made to the anchoring devices described above. Thus, they could be made from other materials than steel and other materials than mortar and epoxy resin could be used for anchoring the device of Figure 5. Further, because the tie rods in the Figure 1 and Figure 3 constructions are hollow, binding material such as mortar or epoxy resin can, if required, be passed down these rods to bind together the material surrounding the plate 4 or the flukes 14 respectively. WHAT WE CLAIM IS:

1. An embeddable anchoring device comprising, prior to embeddment, a thrust member, a tie rod, and means for anchoring the device in the earth, the thrust member having dimensions in directions along the axis of the tie rod and at right angles to that axis that are considerably greater than the maximum dimension of the tie rod in a direction transverse to said axis such that, when the device is embedded, the thrust member at one end of the tie rod provides resistance to lateral movement of the device while the anchoring means at the other end of the tie rod provides resistance to upward movement of the device.

4. A device as claimed in claim 3, in which the material is metal plate.

moved into the said other condition after the device has been driven in.

25. A method as claimed in any one of claims 20 to 22, in which the device is as claimed in claim 11, an explosive charge being arranged in the tube, which charge is detonated after the device has been driven in so that the end of the tube is splayed out to form diverging prongs or a bulge.

26. A method as claimed in any one of claims 20 to 25, in which the device is as claimed in claim 13 or claim 14, and binding material is passed down the passage or tube after the device has been driven into place so that it binds together the material surrounding the anchoring means.

27. A method as claimed in claim 26, in which the device is as claimed in claim 15 and the binding material is forced down the passage or tube and out through the wets.

28. A method as claimed in claim 26 or claim 27 in which the binding material is an epoxy resin.

29. A method as claimed in claim 26 or claim 27, in which the binding material is mortar.

30. A method of embedding an anchor device as claimed in any one of claims 1 to 19, the method being substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.