GB2353016A

GB2353016A - Pile anchor system

Info

Publication number: GB2353016A
Application number: GB0020773A
Authority: GB
Inventors: James William Bunce
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-07-12
Filing date: 2000-08-24
Publication date: 2001-02-14
Anticipated expiration: 2020-08-24
Also published as: GB0020773D0; GB0020895D0; GB2353016B

Abstract

An anchor system comprises an elongate anchor pile 1, one or more elongate tension plates 2 each pivotally attached to said anchor pile at one end and attached to one or more anchor lines 4 at their other end, one or more bridle lines 3 each attached to said anchor pile at one end and to said anchor lines or tension plates at their other end. The system is arranged such that when the anchor lines are under load, tension is transmitted to the anchor pile through both the tension plates and the bridle lines. During installation the tension plates and bridle lines are folded close to said anchor pile (Fig 6) until installed at the required depth, the tension plates are then rotated vertically relative to the pile (Fig 7), cutting through the soil, under anchor line tension until the required vertical angle between said tension plates and the anchor pile is obtained. The anchor system may be used to for mooring floating vessels.

Description

1 2353016 ANCHOR SYSTEM This invention relates to anchor systems for

mooring vessels floating at sea or other similar application where a tension force is required to be resisted at the sea floor, such as anchoring guy cables attached to a guyed offshore structure or the like, or for resisting a force in pulling an object through the water or along or beneath the sea floor.

Vessel mooring anchors are generally only required to resist horizontal loads and the anchor lines attached to them hang from the vessel in a catenary shape with a long length of anchor line lying on the sea floor before the anchor to eliminate any tendency for it to uplift.

In recent times, more efficient 'taut leg' mooring systems have been introduced, where the anchor lines hang from the vessel in the shape of a shallow parabola and connect directly to an anchor with capacity to resist both horizontal and vertical forces, without need for a length of anchor line lying on the sea floor.

A cylindrical pile driven vertically into the sea floor with an anchor line attached is an example of such an anchor. Another example is a large diameter cylindrical suction anchor driven into the sea floor by reducing the water pressure inside the anchor cylinder so that the ambient hydrostatic pressure outside creates an out-of-balance downthrust force. A further example is a plate anchor driven or dragged to the required penetration depth and rotated by lines attached into an orientation that maximises its resistance to the anchor line tension. In some cases, simple gravity anchors relying on their weight on the sea floor can also be used in a taut leg mooring system.

The present invention relates to an anchor pile improved to make it easier to install and to have superior resistance to anchor loads than known anchor pile designs.

2 In an anchor pile the resistance to horizontal loads is obtained by soil bearing pressures acting normal to the pile shaft surface. The resistance to vertical loads is given by the buoyant weight of the pile and 'shaft friction' between the soil and the pile shaft surface. In sands said shaft friction is truly a friction force, in clay soils said shaft friction is actually a cohesive force.

It can be noted as a generality, that development of full soil resistance over the length of an anchor pile is difficult with a single anchor line attachment because of the way the pile flexes and rotates in the soil under single point loading. As a result, the soil contact pressures vary over the length of the pile, being positive in some regions - that is resisting the anchor line force - and zero or negative in other regions. There is thus high soil strength utilisation in some locations and low utilisation in others. This applies no matter what the method of installation of the anchor pile.

One disadvantage of driven anchor piles is that special equipment is needed for the driving and for guiding the pile to keep it vertical. Another is that driving can be a difficult and time- consuming operation in some soil conditions. Furthermore, driving tends to fatigue the pile structure.

An easier way to install an anchor pile is to insert it into a pre-formed hole in the sea floor. This operation is relatively quick, requires no special guide system and does not fatigue the pile.

It may also be noted that anchors will normally be retrieved from the sea floor after use, for redeployment elsewhere. An anchor pile inserted into a pre-formed hole is much easier to retrieve than a driven pile.

The disadvantage of this otherwise attractive installation method is that the pre-formed hole must be oversized, otherwise the pile will not go in easily. By the term 'oversized' is meant 3 the width of the hole is somewhat greater than the width of the pile, for example as would be the case were the hole formed by drilling or waterjetting or other similar method of excavation. As a consequence of this, the lateral resistance of the pile is reduced because some parts of the pile flex out of contact with the side of the hole as the anchor line tension is applied and full mobilisation of the soil resistance cannot be achieved. The vertical shaft friction resistance is also limited by the reduced area of contact between the soil and shaft surface. This loss of load resistance could be overcome or mitigated by injecting grout into the hole between the pile and the soil but this presents other difficulties, not the least in that it requires more equipment and lengthens the installation time.

Similar problems occur in piles vibrated to the required penetration depth in that the soil in contact with the pile is disturbed and weakened.

Further difficulties are encountered with anchor piles in relation to embedment of the anchor line into the soil adjacent to the pile.

In most cases the point of attachment of the anchor line to the anchor pile needs to be several metres below the top of the anchor pile beneath the sea floor in order to develop satisfactory horizontal resistance of the pile. The embedment of the anchor line is obtained by pulling on the anchor line until sufficient embedment is obtained, with the anchor line taking up an 'inverse catenary' curved profile within the soil.

The embedded profile of the anchor line would ideally be a straight line in a taut leg system, however, a curved profile will be satisfactory if: the tangent angle between the anchor line and the pile axis at the point of attachment is sufficiently large that the local vertical component of the anchor line tension does not increase the uplift force appreciably; the vertical angle between the anchor line and sea floor where the embedded anchor line emerges from the soil matches the design requirement; 4 - the horizontal resistance of the anchor pile satisfies the design requirement; and - the embedded profile under full design anchor tension does not change appreciably from the as- installed profile. (That is, there should not be an appreciable loss of stiffness in the pile/anchor line/soil system as anchor line tension is applied.) If the soil is relatively hard, for example dense sand or stiff clay, the tension required to pull the anchor line into a satisfactory embedded shape may be very large, or impractical to achieve with the equipment available. This difficulty occurs both in driven and vibrated piles and in piles inserted into pre-formed holes. The difficulty may be particularly troublesome for a pile inserted into a pre-formed hole because there will be little resistance to uplift loads to begin with and the action of pulling on the anchor line to embed it may pull the pile out of the hole.

The present invention is of an anchor system wherein an anchor pile is inserted into an oversized pre-formed hole in the sea floor and/or is driven or vibrated to the required penetration depth but which has improved resistance to horizontal and vertical loads compared with known anchor pile designs.

Thus, according to the present invention there is provided an anchor system comprising an elongate anchor pile, one or more elongate tension plates each pivotally attached at their bottom ends to said anchor pile and attached to one or more anchor lines at their top ends, one or more bridle lines each attached to said anchor pile at one end and to said anchor lines or tension plates at the other end such that in final service anchor line tension is transmitted to said anchor pile through said tension plates and bridle lines; wherein said tension plates and bridle lines are sized configured and vertically spaced over the length of said anchor pile so as to flex said anchor pile into positive contact with the soil over its length thus to maximise soil resistance forces against said anchor pile; wherein said tension plates and bridle lines are folded close to said anchor pile during initial installation operations and are configured to rotate vertically relative to said anchor pile so as to cut through the soil under anchor line tension during installation operations until the required vertical angle between said tension plates and anchor pile is obtained.

Said tension plates and bridle lines may be configured such that anchor line tension causes said anchor pile to rotate out of the vertical within the soil to increase resistance to vertical uplift effects.

Said anchor pile may be designed to deform plastically as the ultimate anchor tension is approached such that the deformed shape of said anchor pile produces an increased resistance to load.

Said anchor pile may be metal, for example, a steel tubular or box section or channel section or H-section bent about the major or minor axis, or a combination of such crosssectional shapes, or of reinforced or prestressed concrete construction. The cross-sectional shape of said pile may vary along its length.

Said tension plates are preferably metal but may be of reinforced or prestressed concrete construction. Cross- sectional profile of said tension plates may vary along the length and may be shaped to give a cutting edge. Said tension plates may be joined together to form tension spars to give increased strength and rigidity for cutting through the soil. Said tension plates/spars may incorporate hinged articulations along their lengths so as to allow free rotation at the point of articulation and so relieve bending stresses as plates cut through soil. Said tension plates may incorporate holes to reduce skin friction as plate cuts through soil.

Said anchor lines and bridle lines may be metal chain, wire rope or strand or lightweight synthetic material or a combination thereof.

During installation said tension plates or spars and bridle lines are preferably folded close 6 against said anchor pile such that the folded assemblage can be inserted into a pre-formed hole in the sea floor, or driven into the sea floor by use of pile driving apparatus, or vibrated to the required penetration depth. Said hole may be pre-formed, for example only, by drilling or by water jetting or may be excavated by drilling or water jetting as a combined operation with insertion of the pile assemblage.

In the case where said anchor pile assemblage is penetrated into the sea floor as part of the hole forming operation, said pile may incorporate one or more pipes or ducts through which drill string or water jetting apparatus passes. Said drill string may be disconnectably attached to a cutting tool at its bottom end and said cutting tool may remain embedded in the sea floor below said anchor pile after drill string has been disconnected and retrieved.

Where said anchor pile is inserted into an oversized pre-formed hole, particular advantages are obtained if said anchor pile has a channel or Hsection cross-sectional shape and is arranged to flex such that the flanges penetrate into the side of the hole. In this case, firstly, the flange penetration into the sides of the hole increases the shaft friction resistance; secondly, said tension plates or spars, bridle lines and anchor lines may be partly or fully housed within the cross-sectional profile of the pile while it is transported and inserted into the hole and can thus be carefully arranged and protected in position during these operations but can easily be pulled away from said pile as anchor tension is applied; thirdly, said H-section shape can easily be sized to allow formation of a plastic hinge mechanism where this is required.

Said anchor system may be pretensioned and tested in situ after installation. Said anchor system may incorporate load measurement devices in the form of strain gauges or load cells.

Said anchor system may incorporate corrosion protection means, for example, protective coatings and / or a cathodic protection system.

7 Said anchor system may be used for mooring a vessel or other object floating at the water surface or below it, or for guying an offshore structure or the like to the sea floor, or for resisting a force in pulling an object through the water or along or beneath the sea floor.

Said anchor system may be part of an array of anchors.

An embodiment of the invention will now be described, by way of example only, with reference to Figures I to 9 of the accompanying drawings which show an anchor system comprising anchor pile, tension plates/spars, bridle lines and anchor line.

Figure I shows, on elevation, the anchor system in its final service configuration.

Figure 2 shows an example of the pivot attachment detail between tension plates and anchor pile on cross-sectional plan view, and also shows an example of how said tension plates may be connected together to form a tension spar.

Figure 3 shows an example of the attachment detail between bridle lines and anchor pile on cross-sectional plan view.

Figure 4 shows, on plan view, an example of a hinged articulation detail between tension plates.

Figure 5 shows, in cross section, an example of the connection detail between tension plates to form a tension spar.

Figure 6 shows the anchor system in its initial installation configuration on elevation.

Figure 7 shows the anchor system in an intermediate installation configuration on elevation.

8 Figure 8 shows, on elevation, a further example embodiment of the invention where said anchor system includes a hinged articulation along the length of a tension spar.

Figure 9 shows, on cross-sectional plan view, an example of how a pipe or duct can be incorporated into the anchor pile to accommodate drill string or water jetting apparatus.

The preferred configuration of said anchor pile, bridle lines tension plates/spars and anchor lines will depend on the soil conditions and anchor line forces to be resisted.

Referring to Figure 1, anchor system comprises an elongate anchor pile I of H-section cross-sectional shape, for example, with elongate tension plates 2 pivotally attached. Bridle lines 3 are each attached to said anchor pile I at one end and to said tension plates 2 at the other end. Said tension plates 2 are attached to an anchor line 4 at the end remote from said anchor pile 1 such that anchor line tension is transmitted to said anchor pile I through said tension plates 2 and bridle lines 3. Said tension plates 2 and bridle lines 3 are sized, configured and vertically spaced over the length of said anchor pile I so as to cause said anchor pile 1 to flex into positive contact with the soil over its length and, if required, to cause said anchor pile I to rotate out of the vertical within the soil and, if required, to deform plastically to increase resistance to load.

Referring to Figure 2, pivotal attachment between said anchor pile 1 and tension plates 2 comprises pin joint means 5 between said anchor pile and tension plates. Said tension plates are shown connected together by plates or rods 6 to form a tension spar.

Referring to Figure 3, attachment between said anchor pile I and bridle lines 3 comprises shackles or rope termination socket means 7 affixed to said anchor pile 1.

Referring to Figure 4, said hinged articulation along the length of a tension spar comprises pin joint means 8 between said tension plates 2.

9 Referring to Figure 5, said tension plates 2 are shown connected together by plates or rods 6 to form a tension spar, to give increased strength and rigidity for cutting through the soil.

Referring to Figure 6, said tension plates/spars 2 are shown folded close to said tension pile I during initial stages of installation operations as anchor system penetrates the sea floor. Said bridle lines 3 hang slack during these initial stages. Said anchor system may be inserted into a pre-formed hole in the sea floor or may be driven or vibrated to the required penetration depth, or a combination of these methods of installation.

Referring to Figure 7, during intermediate stages of installation operations said tension plates 2 are pulled away from said anchor pile 1 by force in anchor line 4 and said tension plates 2 cut through the surrounding soil. Said bridle lines 3 are drawn through path cut by tension plates 2 and start to tighten during these stages. Pulling of said tension plates 2 through the soil continues until bridle lines 3 become fully tight and anchor pile I takes up the required configuration, as shown for example in Figure 1.

Referring to Figure 8, a further embodiment of said anchor system is shown wherein a hinged articulation 8 is included along the length of a tension spar, so as to relieve bending stresses as tension plates cut through the soil. Said hinged articulation detail is shown in Figure 4.

In Figure 9 an example is shown of how cross-sectional shape of said anchor pile 1 may incorporate a pipe or duct 9 to accommodate drill string or water jetting apparatus.

Claims

1. An anchor system comprising an elongate anchor pile, one or more elongate tension plates each pivotally attached at their bottom ends to said anchor pile and attached to one or more anchor lines at their top ends, one or more bridle lines each attached to said anchor pile at one end and to said anchor lines or tension plates at the other end such that in final service anchor line tension is transmitted to said anchor pile through said tension plates and bridle lines; wherein said tension plates and bridle lines are sized configured and vertically spaced over the length of said anchor pile so as to flex said anchor pile into positive contact with the soil over its length thus to maximise soil resistance forces against said anchor pile; wherein said tension plates and bridle lines are folded close to said anchor pile during initial installation operations and are configured to rotate vertically relative to said anchor pile so as to cut through the soil under anchor line tension during installation operations until the required vertical angle between said tension plates and anchor pile is obtained.

2. An anchor system according to claim 1 wherein said tension plates and bridle lines are configured such that anchor line tension causes said anchor pile to rotate out of the vertical within the soil to increase resistance to vertical uplift effects.

3. An anchor system according to claim I or 2 wherein said anchor pile is designed to deform plastically as the ultimate anchor tension is approached such that the deformed shape of said anchor pile produces an increased resistance to load.

4. An anchor system according to claim 1, 2 or 3 wherein said anchor pile has a cylindrical cross-sectional shape, or a box section or channel or Hsection. cross-sectional shape or a combination of such shapes or a shape that varies along the length.

5. An anchor system according to any of claims 1 to 4 wherein said tension plates have a cross-sectional profile shaped to give a cutting edge.

6. An anchor system according to any of claims I to 5 where said tension plates are connected together to form tension spars to obtain increased strength and rigidity for cutting through the soil.

7. An anchor system according to any of claims 1 to 6 where said tension plates incorporate holes to reduce skin friction as plates cut through soil.

8. An anchor system according to any of claims 1 to 7 wherein said tension plates/spars incorporate hinged articulations along their lengths so as to allow free rotation at the point of articulation and so relieve bending stresses as plates cut through soil.

9. An anchor system according to any of the preceding claims wherein said anchor pile and tension plates are of metal or reinforced or prestressed concrete construction.

10. An anchor system according to any of the preceding claims wherein said anchor lines and bridle lines are metal chain, wire rope or strand or lightweight synthetic material or a combination thereof.

11. An anchor system according to any of claims 1 to 10 which is inserted into a preformed hole in the sea floor and wherein said hole is preformed by drilling or by water jetting or is excavated by drilling or water jetting as a combined operation with insertion of the pile assemblage.

12. An anchor system according to claim 11 wherein said anchor pile incorporates one or more pipes or ducts through which drill string or water jetting apparatus passes.

13. An anchor system according to claim I I or 12 wherein said drill string is disconnectably attached to a cutting tool at its bottom end and said cutting tool remains embedded in the sea floor below said anchor pile after drill string has been disconnected 12 and retrieved.

14. An anchor system according to any of claims 1 to 10 which is driven into the sea floor by use of pile driving apparatus or is vibrated to the required penetration depth.

15. An anchor system according to any of the preceding claims which is installed by a combination of insertion into a pre-formed hole in the sea floor and driving or vibration to full penetration depth.

16. An anchor system according to any of the preceding claims which incorporates means of retrieval for reuse.

17. An anchor system according to any of the preceding claims, which incorporates.corrosion protection means in the form of protective coatings and / or a cathodic protection system.

I S. An anchor system according to any of the preceding claims, which incorporates load measurement devices in the form of strain gauges or load cells.

0 = 19. An anchor system according to any of the preceding claims, which incorporates pretensioning and load testing as part of the installation operation.

1 20. An anchor system comprising a plurality of anchor systems according to any of the preceding claims.

2 1. An anchor system according to any of the preceding claims which is used for mooring a vessel or other object floating at the water surface or below it or for guying an offshore structure or the like to the sea floor, or for resisting a force in pulling an object through the water or along or beneath the sea floor.

I 1-1 22. An anchor system substantially as described herein and with reference to the accompanying drawings.

17. An anchor system according to any of the preceding claims, which incorporates corrosion protection means in the form of protective coatings and / or a cathodic protection system.

18. An anchor system according to any of the preceding claims, which incorporates load measurement devices in the form of strain gauges or load cells.

19. An anchor system according to any of the preceding claims, which incorporates pretensioning and load testing as part of the installation operation.

20. An anchor system comprising a plurality of anchor systems according to any of the preceding claims.

21. An anchor system according to any of the preceding claims which is used for mooring a vessel or other object floating at the water surface or below it, or for guying an offshore structure or the like to the sea floor, or for resisting a force in pulling an object through the water or along or beneath the sea floor.

13 22. An anchor system substantially as described herein and with reference to the accompanying drawings.

Amendments to the claims have been filed as follows 1. An anchor system comprising a substantially vertical elongate anchor pile with one or more elon(lyate tension plates vertically inclined to its axis and pivotally attached at their bottom ends to said anchor pile and attached to one or more anchor lines at their top ends, one or more bridle lines above said tension plates each attached to said anchor pile at one end and to said anchor lines or tension plates at the other end such that in final service anchor line tension is transmitted to said anchor pile through said tension plates and bridle lines; wherein said tension plates are folded close to said anchor pile during initial installation operations with said bridle lines slack and said tension plates configured to rotate vertically relative to said anchor pile so as to cut through the soil under anchor line tension during installation operations until the required vertical inclination of said tension,. Plates to anchor pile axis is obtained and said bridle lines become tight.

2. A_n anchor system according to claim I wherein said tension plates and bridle lines are configured such that anchor line tension causes said anchor pile to rotate out of the vertical within the soil to increase resistance to vertical uplift effects.

An anchor system according to claim 1 or 2 wherein said anchor pile is desi C'ned to deform plastically as the ultimate anchor tension is approached such that the deformed shape of said anchor pile produces an increased resistance to load.

4. An anchor system according to claim 1, 2 or 3) wherein said anchor pile has a cylindrical cross-sectional shape, or a box section or channel or H-section cross-sectional shape or a combination of such shapes or a shape that varies along the length.

5. An anchor system according to any of claims I to 4 wherein said tension plates have a cross-sectional profile shaped to give a cutting edge.

6. An anchor system according to any of claims 1 to 5 where said tension plates are connected together to form tension spars to obtain increased strength and rigidity for cutting ID through the soil.

7. An anchor system according to any of claims 1 to 6 where said tension plates incorporate Z:I holes to reduce skin friction as plates cut through soil.

8. An anchor system according to any of claims I to 7 wherein said tension plates/spars incorporate hinged articulations along their lengths so as to allow free rotation at the point of articulation and so relieve bending stresses as plates cut through soil.

9. An anchor system according to any of the preceding claims wherein said anchor pile and %tension plates are of metal or reinforced or prestressed concrete construction.

10. An anchor system according to any of the preceding claims wherein said anchor lines and bridle lines are metal chain, wire rope or strand or lightweight synthetic ma terial or a combination thereof 11. An anchor system according to any of claims I to 10 which is inserted into a preformed hole in the sea floor and wherein said hole is pre- formed by drilling or by water jetting or is excavated by drilling or waterjetting as a combined operation with insertion of the pile assemblage.

An anchor system according to claim 12 wherein said drill string is disconnectably M - attached to a cuttinc, tool at its bottom end and said cuttina tool remains embedded in the sea floor below said anchor pile after drill string has been disconnected and retrieved.

14. An anchor system according to any of claims I to 10 which is driven into the sea floor by use of pile driving apparatus or is vibrated to the required penetration depth.

M 15. An anchor system according to any of the preceding claims which is installed by a combination of insertion into a pre-formed hole in the sea floor and driving or vibration to full penetration depth.