WO2004011327A1

WO2004011327A1 - Sheet anchor

Info

Publication number: WO2004011327A1
Application number: PCT/NL2002/000509
Authority: WO
Inventors: Francois Bernard; Dirk Bauer
Original assignee: Francois Bernard; Dirk Bauer
Priority date: 2002-07-26
Filing date: 2002-07-26
Publication date: 2004-02-05
Also published as: AU2002319960A1

Abstract

Method and device for introducing an anchor (1) in the soil (2), i.e. the seabed, the anchor having a sheet-like resistance element (4), wherein the method comprises the steps of: - providing the anchor (1) with an anchor line, such as an anchor chain (10), - attaching the anchor to a guiding element (3), such as a pile or a pipe, - moving the assembly of the anchor (1) and the guiding element (3) towards a target position, - introducing the assembly of the anchor (1) and the guiding element (3) in the soil (2), by means of a penetration device, such as vibratory hammer, wherein the longitudinal axis of sheet-like element during the penetrating in essence is directed in the direction of travel of the anchor (1), - removing the guiding element (3), and - exerting a pulling force on the sheet-like element (4), in order to change the orientation of the sheet-like element (4) with respect to the penetration path.

Description

SHEET ANCHOR

The present invention relates to a method for introducing an anchor in the soil, i.e. the seabed. The method is especially suitable for introducing a so-called "sheet anchor" in the seabed. Moreover, the invention relates to a sheet anchor suitable for use in the method according to the present invention and to an assembly of such a sheet and a pile or pipe.

Sheet anchors are used in the offshore technology for anchoring floating objects, such as production platforms, to the seabed. According to a typical procedure for positioning the sheet anchor in the soil, the anchor is positioned on the seabed and the anchor is penetrated into the seabed by pulling on the anchor chain. The sheet anchor is designed in order to be orientated with an angle position with respect to the seabed. The angle between the surface of the seabed and the forward end of the sheet anchor will determine the penetration path of the anchor in the soil. That means that a pulling force exerted on the sheet anchor, in essence directed in a horizontal plane, will lead to a translation of the sheet anchor both in a horizontal plane and in a vertical plane. It is clear that the actual displacement of the anchor both in vertical and horizontal direction will be dependent on i.e. the pulling force, and the soil conditions.

A first important drawback of the method according to the prior art is the fact that the final position of the sheet anchor in the soil can not be predicted with a high accuracy. That means that an operator first has to introduce the anchor in the seabed and- thereafter has. to .determine what the actual position of the sheet anchor is.

Especially when the sheet anchor is supposed to withstand an extremely high load, the inaccuracy of the positioning method will lead to unpredictable strains in the anchor chains.

A further important drawback of the method according to the prior art is the fact that the final orientation of the sheet anchor is also a resultant of the soil condition and the pulling force exerted on the sheet anchor. Thereby is it impossible to have the centre line of the sheet anchor to be in essence parallel to the seabed itself. That means that it is impossible to exert substantially vertical pulling force on the sheet anchor, without partially retracting the sheet anchor.

With respect to the above, it is a first object of the present invention to provide a method for introducing an anchor in the soil, i.e. the seabed, wherein the final position and orientation of the sheet anchor can be obtained with a higher accuracy compared to the method according to the prior art.

That object is achieved in that a method for introducing an anchor in the soil, i.e. the seabed, the anchor having a sheet-like resistance element according to the present invention comprises the steps of:

- providing the anchor with an anchor line such as an anchor chain, attaching the anchor to a guiding element, such as a pile or a pipe,

- moving the assembly of the anchor and the guiding element toward a target position, - introducing the assembly of the anchor and the guiding element in the soil, by means of a penetration device, such as a vibratory hammer, wherein the longitudinal axis of the sheet-like resistance element during the penetrating in essence is directed in the direction of travel of the anchor,

- removing the guiding element, and - exerting a pulling force on the sheet-like element, in order to change the orientation of the sheet-like element with respect to the penetration path.

According to the present invention the sheet anchor is attached to a guiding element. This guiding element for instance is a pile or a pipe. The sheet anchor is attached to a first end of said pile or pipe. At the other side of the pile or pipe a penetration device can be fixed. Such a penetration device for instance is a vibratory hammer. In the prior art is well known to use such a vibratory hammer to introduce a pile -in the seabed. -Because, of.the vibrations of the hammer the sott underneath the assembly of the pile and the anchor will liquify and therefore exert less resistance to the introduction of the assembly. When the sheet anchor is inserted in the soil to a predetermined depth, a guiding element is removed from the anchor. That means that the pile itself will be partly or completely retrieved, leaving the anchor at its position. Thereafter a force is exerted on the anchor itself. This force must be applied on the anchor in that the sheet-like element of the anchor will move towards an orientation wherein the sheet-like element of the anchor is able to provide resistance to pulling forces on the anchor. That means that the orientation of the sheet-like element with respect to the penetration path thereof must be altered. The method according to the present invention provides both a method for accurately introducing the anchor at a target position. Moreover, with the method according to the present invention it is possible to control the orientation of the sheetlike member in that the sheet-like element of the anchor is in essence parallel to the seabed.

According to the present invention it is possible that the anchor line is used for exerting a force on the sheet-like element for changing the orientation thereof with respect to the penetration path.

According to the present invention it is advantageous that the guiding element is attached to the anchor in order to, in essence, coincide with the centre line of the sheetlike element. Because of this measure the forces exerted on the guiding element will be distributed symmetrically to the sheet-like element.

According to the present invention it is possible that the guiding element is attached to the anchor by means of a hydraulic clamping device. Thereby it is possible that the hydraulic clamping device is adapted to be actuated by means of the penetration device.

The method according to the present invention for instance can be performed by using the device according to the earlier application PCT/NL99/00242 (Promaster) in the name of the same applicant. This device is provided with means for accurately approaching a target position on the seabed. Moreover, this device can be coupled to a penetration device, such as a vibratory hammer. That means that inside the device means -for- producing- hydraulic power will be. available. According to the present invention those means for providing hydraulic power can be used for actuating the penetration device. According to the present invention it is possible that the guiding element is adapted to enclose the distal end of the anchor line.

When introducing the sheet anchor according to the present invention, the distal part of the anchor line, such as the anchor chain, must also be introduced in the seabed. When the anchor line would not be controlled, the anchor line would be laying on a seabed. When introducing the sheet anchor in the soil, the anchor line would be pulling on the anchor and thereby altering the position of the sheet anchor during introduction thereof. Therefore, it is advantageous when also the distal part of the anchor line is enclosed in the guiding element, such as the pile or pipe. In order not to risk influence of the anchor chain on the accuracy of anchor- positioning moreover it is advantageous that prior to moving the assembly to the target position, the end part of the anchor line adjacent to the assembly is. provided with at least one buoyancy element. For clarity reasons it is mentioned that with the "distal end" the very end of the anchor line is meant which is closest to the anchor itself. The "end part" of the anchor line adjoins this distal end of the line.

According to the present invention it is possible that the sheet-like element prior to the introduction in the soil thereof is provided with a retrieval line, the line being attached to the sheet-like element for exerting a force in order to move the sheet-like element with respect to the penetration path thereof.

When the anchor is introduced in the soil, it is advantageous to enable retrieval of the anchor from the soil, in order to be able to re-use the anchor. When prior to the introduction of the anchor in the seabed the retrieval line is attached to the sheet-like element, the anchor can be retrieved by first pulling on the sheet-like element by means of the retrieval line, in order to move the sheet-like member towards a position wherein the anchor can be retrieved. This position will resemble the position the sheet anchor had when it was introduced in the soil.

The present invention also relates to a sheet anchor, adapted to be introduced in the soil, i.e. the seabed, having a sheet-like resistance element with a forward end, and a trailing end, the anchor having means for attaching a line, such as an anchor chain. The^~sheet^' anchor according'toihe -present invention is characterised in-that the forward - end of the sheet-like element is provided with a cutting edge, symmetrically shaped with respect to the centre line of the sheet-like element, for receiving a symmetrical distributed resistance force on the anchor when introducing the anchor in the soil and in that the trailing end of the sheet-like element is provided with a cutting edge having a shape in order to receive an asymmetrical distributed resistance force when retracting the sheet anchor from the soil.

In the present text the wording "sheet anchor" is meant to relate to any anchor having a sheet-like resistance element.

Because of the specific design of the sheet anchor according to the present invention the sheet-like element will enter the soil in a straight line. And after positioning the sheet anchor in the soil a pulling force is exerted on the sheet element, automatically an asymmetrical force will be exerted on the sheet element, in order to move the orientation of the sheet element with respect to the penetration path thereof.

This movement can be achieved in that the trailing end is provided with a cutting edge asymmetrical shaped with respect to the centre line of the sheet anchor. Alternatively it is possible that the trailing end is provided with a cutting edge having a cutting element, moveably attached to the sheet anchor.

In order to further improve the features for introducing the sheet-like anchor according to the present invention in the soil it is advantageous that the sheet-like element of the sheet anchor in the direction of the forward end is tapered, in that the side edges of the sheet-like element include an angle with respect to the forward end larger than 90°. Thereby it is possible that the trailing end of the sheet-like element is tapered, in that the side edges of the sheet-like element include an angle with respect to the trailing end which is larger than 90°.

According to the present invention it is possible that the sheet-like element is provided with means for receiving a pile or pipe. Thereby it is possible that the means for receiving a pile or pipe are adapted for positioning the pile or pipe in substantially the centre of the sheet anchor.

Further it is advantageous that the trailing end of the sheet-like element is provided with a retrieval line for retracting the sheet anchor when introduced in the soil. Moreover it is possible that the retrieval line is provided with a buoyancy element, such as a ball or balloon.

-According - to— the present, invention the .sheet anchor comprises stiffening members. The stiffening elements, for instance ribs, are used to enforce the sheet anchor against bending. Furthermore the present invention relates to an assembly of a sheet anchor according to one of the claims 9-18 and a pile or pipe attached to such anchor. The assembly according to the present invention is characterised in that the pile is positioned in substantially the centre line of the anchor. Furthermore it is advantageous that the connecting means for connecting the pile or pipe to the anchor are provided with cylinders.

It is possible that the means for connecting the pile or pipe towards the anchor are provided with means for transferring a translational movement into a rotational movement. It is for instance possible to move the connecting means using the cylinders. Connecting means thereby are translated in a first direction. The connecting means then can run in a groove, such as a helical groove. The presence of these helical grooves will make connecting means rotate with respect to the sheet anchor. Via this rotational movement the connecting means can reach a final clamping position. According to the present invention it is possible that the connecting means are provided with a ball and an element for fixing the end of the pile or pipe, the connecting element being connected to the ball. Thereby it is possible that the pile or pipe is provided with a slot for allowing to pass shackles of an anchor chain through the wall of the pile or pipe. In order to improve the assembly according to the present invention it is possible that the slot in the pile or pipe is provided with elements for providing rigidity to the pile or pipe.

The invention will now be described making reference to the figures, wherein:

Figure 1 shows the assembly of a sheet anchor according to the present invention and a pile, introduced in the soil. Figure 2 shows the removal of the pile, leaving the sheet anchor in the soil.

Figure 3 shows the altering of the orientation of the sheet anchor with respect to the penetration path thereof.

Figure 4 shows the sheet anchor according to the present invention in frontal view. Figure 5 shows the sheet anchor according to the present invention in side view.

Figure 6 shows the sheet anchor according to figure 4 in crosssectional view according-to-the line-NI-VL-

Figure 7 shows a first embodiment of a clamping mechanism for clamping a pile or pipe to a ball-connecting element assembly, which ball is connected to the sheet anchor.

Figure 8 an embodiment of a clamping mechanism for clamping the pile or pipe to a padeye plate fixed on the sheet anchor.

Figure 1 shows the sheet anchor 1 according to the present invention which has been introduced in the soil 2, i.e. the seabed, by means of a pile 3. The assembly of the sheet anchor 1 and the pile 3 will be introduced into the soil 2 by means of a penetration device, such as a vibratory hammer (not shown). The vibrations applied to the assembly of sheet anchor 1 and pile 3 will liquidity the soil 2 underneath the assembly in order to decrease the resistance of the soil. In order to be able to penetrate the assembly in the soil 2 an optimal connection between the pile 3 and the sheet anchor 1 is essential. If such a connection would not be present, the pile would vibrate not only with respect to the soil 2 but also with respect to the sheet anchor 1. In that case the assembly of sheet anchor 1 and pile 3 could not be introduced in the soil 2. Specific examples of the connection between the pile 3 and the sheet anchor 1 will be described with reference to the figures 7 and 8.

The sheet anchor 1 comprises a sheet-like body 4 having a forward edge 5 and a trailing edge 6. The underside of the sheet-like element 4 is tapered, in order to facilitate the introduction of the sheet anchor in the soil. The specific form of the sheet anchor will be clarified with reference to figures 4, 5 and 6. On the sheet-like element 4 reinforcement elements, such as ribs, are added. These ribs 7 enhance the resistance of the sheet anchor 1 against bending. At the sheet-like body 4 a dome-shaped element 8 is present. In this element a ball can be received. The ball will be connected to the distal part of an anchor chain 10. From figure 1 it is clear that the shackles, forming the distal part of the chain 10, are enclosed within the body of the pile 3. That means that the distal part is guided with the sheet anchor when introducing the assembly of the sheet anchor and the pile into the soil 2. hi order to be able to remove the pile after introduction of the assembly in the soil, the pile is provided with an elongate aperture 11. This elongate aperture 11 will weaken the pile 3. In order to minimize this effect, additional plates 12 are fixed over the aperture 11. The plates 12 are strong enough to increase the stiffness and the strength of the pile 3, and are attached to the pile in order to break loose when the pile is removed from' the soil.

In order to further minimize the influence of the anchor chain during the introduction of the assembly of the anchor and the pile in the soil 2 buoyancy elements 13 are attached to part of the anchor chain 10 that adjoins the anchor chain part that is enclosed in the pile 3.

The trailing end 6 of the sheet-like element 4 of the sheet anchor 1 is provided with a retrieval line 14, at the end thereof provided with a buoyancy element 15. The function of this retrieval line 14 will be clarified with respect to figure 3. In figure 2 it is shown that the pile 3 is removed from the soil 2, leaving the sheet anchor 1 at its position. In figure 2 it is shown that the plates 12 will break loose in order to allow the removal of the pile 3. When the pile 3 is completely removed the sheet anchor 1 is positioned at a predetermined depth in the seabed. Since the sheet-like element 4 is still in essence in line with the penetration path of the sheet anchor 1, the anchor 1 is not able to withstand substantial forces. In order to be able to function properly, the orientation of the sheet-like element 4 must be altered with respect to the penetration path. This is shown in figure 3. In figure 3 it is shown that a force is exerted on the sheet anchor 1 by means of the anchor chain 10. Because of this force, the sheet anchor will be displaced upwards. Because of the specific shape of the sheet-like element 4 during this displacement the sheet-like element 4 will rotate with respect to the seabed 2. When the sheet-like element is in essence parallel to the seabed 2, the position of the sheet anchor 1 is optimal for withstanding forces exerted on the anchor chain 10.

When the anchor is introduced in the soil the orientation of the sheet-like element 4 has been altered according to figure 3, it is advantageous to be able to retrieve the anchor from the soil. This means that the anchor can be re-used. The retrieval is made possible by exerting a force on the retrieval line 14, which is attached to the trailing end 6 of the sheet-like element 4. By exerting a pulling force on the sheet-like element, this element will return to its initial position, meaning the position which has been shown in figure 1. Thereafter the anchor can be retrieved from the soil.

In figures 4, 5 and 6 a specific embodiment is shown of the sheet anchor 1 according to the present invention. Figure 4 shows a frontal view of the sheet anchor, h figure 4 it is clear that the sheet-like element 4 at the under side thereof is tapered. Because of the presence of the tapered side edges 20 and 21 towards the forward end 5 of the sheet-like-element .4.the-penetration of the sheet anchor 1 in the soil is facilitated. The specific angle between the forward end 5 and the side edges 20, 21 will be dependent on specific soil conditions. Also the total surface area of the element 4 will be dependent on soil conditions. As a rule of thumb, the surface area of the sheet-like element 4 will be smaller, the harder the material of the soil.

The upper part of the sheet-like element 4 is provided with a circular channel 22 which more clearly is shown in figure 6. This channel 22 is present in order to enclose the distal part of a pile or pipe to be inserted in the channel. According to figures 4 and 5 the sheet-like element is provided with a dome-shaped element 8 which element is adapted to receive a ball-shaped rotating element. This ball-shaped element will be connected to a connecting element which forms the most distal part of the anchor chain 10 (see figures 1 and 2). Figure 4 also shows that the upper part of the sheet-like element 4 is tapered. That means that the two sides 23 and 24 enclose an angle which is larger than 90° with the trailing end 6 of the sheet-like element 4. The angle between respective sides 23, 24 and 6 will also be dependent on soil conditions. An important aspect of the sheet anchor 1 according to the present invention is the fact that the cutting edge at the forward end 5 of the sheet anchor 1 is symmetrically shaped. This is shown in figure 5. That means that when introducing the sheet anchor in the soil the resistance force of the soil on this cutting edge will be evenly distributed. That means that it is possible to introduce the sheet anchor 1 according to a straight line into the soil. The cutting edge of the trailing end 6 of the sheet anchor 1 is, contrary to the cutting edge 5, asymmetrically formed. That means that when retracting the sheet anchor 1 from the soil, for instance by pulling on the anchor chain 10, an asymmetrical distributed resistance force will be executed on this cutting end 6. Because of this asymmetrical force 6 during the displacement of the sheet anchor with respect to the soil, the sheet anchor will rotate. During this rotation the orientation of the sheet-like element 4 with respect to the penetration path will alter. This rotation has been shown in figure 3.

As mentioned above the sheet-like element 4 is provided with reinforcement elements, such as ribs 7. These ribs can clearly be seen for instance in figure 6.

As mentioned above it is essential to have an optimal connection between the pile 3 with the sheet anchor 1. In figure 7 a first embodiment is shown for connecting the distal end of the pile 3 to a connecting element 31 which is connected to a ball 32. The ball 32 will -be enclosed -in the dome .8 which can be. seen in figure _4. The connecting element 31 will be fixed to the most distal shackle 33 of the anchor chain 10. For clarity reasons the other shackles of the anchor chain are not shown. In order to connect the pile 3 to the connecting element 31 in the pile 3 cylinders 35 are present. Those cylinders can be driven by means of hydraulics. When the cylinders are activated a sleeve 36 is moved with respect to the outside of the pile 3. The sleeve 36 comprises cams 37 and a projection 38. The connecting element 31, at the outside thereof, is provided with cam slots 39. During the displacement of the sleeve 36 the cams 37 will be received in the cam slot 39. Because of the fact that the cam slots are spiral shaped, the connecting element 31 will be rotated with respect to the pile 3, when the cams proceed through the cam slots. When the sleeve 36 has been displaced over a predetermined length with respect to the connecting element 31, the projection 38 will enter the aperture 40 in the connecting element 41. The aperture 40 is L-shaped. As can be seen in figure 7 the projection 38 is shaped in order to be fixed in the lower part of the projection 40. By reversing the operation of the cylinder 35 the locking of the pile on top of the connecting element 31 can be disconnected. A further method for fixing the pile 3 on the sheet anchor 1 is shown in figure 8.

According to figure 8 the sheet anchor 1 is provided with a padeye plate 50. According to the embodiments as shown in figure 8 inside the pile 3 a cylinder assembly 51 is present at the end thereof provided with clamping shoes 52. These clamping shoes, when displaced towards the distal end of the pile 3, will be guided by a tapered end side 53 of the pile 3. By means of the clamping shoes 52 the pile can be clamped on top of the padeye plate 50.

Claims

1. Method for introducing an anchor in the soil, i.e. the seabed, the anchor having a sheet-like resistance element, wherein the method comprises the steps of: - providing the anchor with an anchor line, such as an anchor chain,

- attaching the anchor to a guiding element, such as a pile or a pipe,

- moving the assembly of the anchor and the guiding element towards a target position,

- introducing the assembly of the anchor and the guiding element in the soil, by means of a penetration device, such as a vibratory hammer, wherein the longitudinal axis of the sheet-like element during the penetrating in essence is directed in the direction of travel of the anchor,

- removing the guiding element, and

- exerting a pulling force on the sheet-like element, in order to change the orientation of the sheet-like element with respect to the penetration path.

2. Method according to claim 1, wherein the anchor line is used for exerting a force on the sheet-like element for changing the orientation thereof with respect to the penetration path.

3. Method according to claim 1 or 2, wherein the guiding element is attached to the anchor in order to, in essence, go inside with the centre line of the sheet-like element.

^~4: Method-according to claim -1-3, wherein-the-guiding elementis attached, to. the_., anchor by means of a hydraulic clamping device.

5. Method according to claim 4, wherein the hydraulic clamping device is adapted to be actuated by means of the penetration device.

6. Method according to one of the preceding claims, wherein the guiding element is adapted to enclose the distal end of the anchor line.

7. Method according to one of the preceding claims, wherein prior to moving the assembly to the target position, the end part of the anchor line adjacent to the assembly is provided with at least one buoyancy element.

8. Method according to one of the preceding claims, wherein the sheet-like element prior to the introduction in the soil thereof is provided with a retrieval line, the

Claims

line being attached to the sheet-like element for exerting a force in order to move the sheet-like element with respect to the penetration path thereof.

9. Sheet anchor, adapted to be introduced in the soil, i.e. the seabed, having a sheet-like resistance element with a forward end, and a trailing end, the anchor having means for attaching a line, such as an anchor chain, characterized in that, the forward end of the sheet-like element is provided with a cutting edge, symmetrically shaped with respect to the centre line of the sheet-like element, for receiving a symmetrical distributed resistance force on the sheet anchor when introducing the sheet anchor in the soil and in that the trailing end of the sheet-like element is provided with a cutting edge having a shape in order to receive an asymmetrical distributed resistance force when retracting the sheet anchor from the soil.

10. Sheet anchor according to claim 9, wherein the trailing end of the sheet-like element is provided with a cutting edge asymmetrical shaped with respect to the centre line of the sheet anchor. 11. Sheet anchor according to claim 9, wherein the trailing end of the sheet-like element is provided with a cutting edge having a cutting element, moveably attached to the sheet anchor.

12. Sheet anchor according to one of the claims 9-11, wherein the sheet-like element of the sheet anchor in the direction of the forward end is tapered, in that the side edges of the sheet-like element include an angle with respect to the forward end larger than 90°.

13.- Sheet- anchor according .to^one of_. the__claims _.9-12, characterized in that the trailing end of the sheet-like element is tapered, in that the side edges of the sheet-like element include an angle with respect to the trailing end which is larger than 90°. 14. Sheet anchor according to one of the claims 9-13, wherein the sheet anchor is provided with means for receiving a pile or pipe.

15. Sheet anchor according to claim 14, wherein the means for receiving a pile or pipe are adapted for positioning the pile or pipe in substantially the centre of the sheet anchor. 16. Sheet anchor according to one of the claims 9-15, wherein the trailing end of the sheet-like element is provided with a retrieval line for retracting the sheet anchor when introduced in the soil.

17. Sheet anchor according to claim 16, wherein the retrieval line is provided with a buoyancy element, such as a ball or balloon.

18. Sheet anchor according to one of the claims 9-17, wherein the sheet anchor comprises stiffening members. 19. Assembly of a sheet anchor according to one of the claims 9-18 and a pile or pipe attached to said anchor.

20. Assembly according to claim 19, wherein the pile or pipe is positioned in substantially the centre line of the anchor.

21. Assembly according to claim 19 or 20, wherein the connecting means for connecting the pile or the pipe to the anchor are provided with cylinders.

22. Assembly according to claim 21, wherein the means for connecting the pile or pipe to the anchor are provided with means for transferring a translational movement into a rotational movement.

23. Assembly according to claim 21, wherein the connecting means are provided with a ball and a connecting element for attaching the end of the pile or pipe, the connecting element being connected to the ball.

24. Assembly according to claim 19-23, wherein the pile or pipe is provided with a slot for allowing to pass shackles of the anchor chain through the wall of the pipe.

25. Assembly according to claim 24, wherein the slot in the pile or pipe is provided with elements for providing rigidity to the pile or pipe.