GB2626751A

GB2626751A - Moorings for offshore installations

Info

Publication number: GB2626751A
Application number: GB2301398.0A
Authority: GB
Inventors: Hamdan Nawras
Original assignee: Subsea 7 Ltd
Current assignee: Subsea 7 Ltd
Priority date: 2023-01-31
Filing date: 2023-01-31
Publication date: 2024-08-07
Also published as: GB202301398D0; WO2024160903A1

Abstract

A mooring arrangement 32 comprises at least two outer anchors 24, a reaction anchor 12 disposed between the outer anchors and at least two anchor legs 34 that extend outwardly from the reaction anchor to the outer anchors. The reaction anchor has multi-directional effect, being configured to react against outward forces applied by tension in the mutually-opposed anchor legs. Each anchor leg comprises a tensioner 18, an inner line 16 extending from the reaction anchor to the tensioner, and an outer line 22L extending from the tensioner to the outer anchor. A tensioner line, which may be the inner line or the outer line, may be fixed to the tensioner whereas a mooring line 22 may extend through the tensioner. An upper section of the mooring line 22U may extend from the tensioner to a moored floating body 26. A lower section of the mooring line may form part of the anchor leg, as either the inner line or the outer line.

Description

Moorings for offshore installations This invention relates to the challenges of mooring floating bodies in offshore installations, such as those used in the subsea oil and gas and marine energy industries. The invention is particularly concerned with improving the installation and performance of mooring arrangements that require installation of multiple anchors and tensioning of multiple mooring lines.

With oil and gas production and energy generation taking place in increasingly deep waters, it is necessary to moor or to tether floating bodies where the water is too deep to allow corresponding installations to be mounted on the seabed. For example, spread moorings are commonly used to moor an FPSO (floating production, storage and offloading vessel) or a platform used in subsea oil and gas production. Moorings are also common in offshore energy generation where, for example, arrays of floating wind turbines generate power to be conveyed to electrical substations on the seabed.

Mooring lines extend from the floating body down to subsea foundations that may comprise piles such as suction piles or pin piles, or anchors such as drag anchors, gravity anchors or clump weights. Drag anchors are preferred for offshore wind turbines because they are less expensive than pile foundations. Drag anchors depend upon tension in the associated mooring line that pulls the anchor across the seabed and deeper into the seabed soil. Consequently, in that case, appropriate tensioning of a mooring line is also essential to achieve effective engagement of a drag anchor with the seabed.

In many cases, tensioning of mooring lines is performed from the moored floating body, for example using a mooring winch or a 'gypsy wheel' device that is arranged to engage a mooring chain. This ensures that multiple mooring lines each bear an appropriate share of the loads that will be exerted on them by the floating body in service. Most commonly, the floating body is located at the surface but it could instead be located underwater. For example, WO 2013/076461 describes tensioners in the context of a tethered subsea buoy used in hybrid riser systems.

Tensioning from a location at or just below the surface as described above, hence elevated significantly above the seabed, is not suitable for all types of foundations and water depths. Tensioning in that manner works well with suction or pile foundations that are deeply embedded in the seabed soil and so are strongly located against movement relative to the seabed, but works less well with anchors that depend upon a shallow inclination of the mooring line relative to the seabed, such as drag anchors, gravity anchors or clump weights. Where the mooring line is more steeply inclined relative to the seabed, such anchors are prone to lifting from and sliding across the seabed rather than becoming more deeply embedded with increasing pulling force.

For drag anchors, an improvement involves the provision of a reaction point and a tensioner located on the mooring line, as taught by US 4889065. In that disclosure, a second drag anchor is located in opposition to a tensioning vessel to define the reaction point, and a tensioner pulls the mooring line with shallow inclination. However, this method becomes cumbersome when multiple mooring lines have to be used for the same floating body. For example an FPSO or a floating wind turbine may typically require the installation of three to twelve distinct mooring lines in each instance.

US 2020/407021 proposes directly anchoring multiple floating bodies such as wind turbines to a common foundation in the form of a ring embedded into the seabed. Its objective is to limit the number and complexity of subsea foundations but its principle cannot be implanted by a drag anchor, which has a unidirectional effect and so cannot be shared by multiple mooring lines.

The invention aims to simplify the installation of multiple anchors, especially drag anchors, to anchor one or more floating objects.

Against this background, the invention provides a mooring arrangement that comprises at least two outer anchors and a reaction anchor, such as a caisson, a pile or a ring, disposed between the outer anchors. The outer anchors may comprise at least one drag anchor that is configured to self-embed upon being dragged toward the substantially static reaction anchor.

The reaction anchor is configured to react against outward forces applied by tension in at least two anchor legs that extend outwardly from the reaction anchor to the outer anchors. Each anchor leg comprises a tensioner, an inner line extending from the reaction anchor to the tensioner, and at least one outer line extending from the tensioner to at least one of the outer anchors. Thus, the reaction anchor may comprise at least two lateral connection points for connection of the respective inner lines of the anchor legs.

The outer anchors can be on mutually opposed sides of the reaction anchor, and the anchor legs can extend radially from the reaction anchor in mutually opposed directions. For example, the anchor legs may be diametrically opposed about the reaction anchor as a pair in mutual longitudinal alignment.

In some embodiments, the inner line of each anchor leg is a tensioning line that is coupled to the tensioner and the outer line of each anchor leg is a lower section of a mooring line that extends through the tensioner. In that case, an upper section of the mooring line may converge upwardly with an upright axis that aligns with the reaction anchor. The mooring line can thereby extend upwardly and inwardly to a floating body such as an FPSO that is substantially aligned with that axis, for example to moor the floating body with the mooring lines that extend from the respective anchor legs of the arrangement.

Conversely, in other embodiments, the outer line of each anchor leg is a tensioning line that is coupled to the tensioner and the inner line of each anchor leg is a lower section of a mooring line that extends through the tensioner. In that case, an upper section of the mooring line can diverge upwardly from an upright axis that aligns with the reaction anchor. The upper section of each mooring line can thereby extend upwardly and outwardly to moor a respective one of an array of floating bodies, such as floating wind turbines, that surround the upright axis.

Elegantly, in each case, at least one additional mooring line could extend directly from the or each floating body to the reaction anchor.

Conveniently, the tensioners may be operable to impart tension in the respective anchor legs by pulling the upper section of the mooring line upwardly. Thus, the tensioners may be operable to lock to the mooring line upon pulling the upper section of the mooring line.

At least one of the anchor legs could terminate in a group of two or more of the outer anchors. For example, the outer anchors of the group could be angularly spaced about the reaction anchor or aligned on a common radius that extends from the reaction anchor. At least one of the anchor legs may have at least two outer lines that extend from the tensioner to respective ones of the outer anchors of the group. Also, an outer line of an anchor leg can be divided into two or more auxiliary outer lines that each extend to a respective one of the outer anchors of the group.

Correspondingly, the inventive concept embraces a method of installing a mooring arrangement. That method comprises: fixing a reaction anchor at a seabed location; connecting at least two anchor legs to the reaction anchor, each anchor leg radiating from the reaction anchor across the seabed and terminating outwardly in at least one outer anchor such that the reaction anchor is disposed between the outer anchors; imparting tension in the anchor legs to pull the outer anchors inwardly toward the reaction anchor; and mooring a floating body with at least one mooring line extending upwardly from each anchor leg.

Where each anchor leg comprises a lower section of a respective one of the mooring lines, tension may be imparted in the anchor legs by pulling upper sections of the mooring lines that extend upwardly from the anchor legs. Beneficially, tension can be imparted in the anchor legs in a direction, or along an axis, that is substantially parallel to the seabed.

Embodiments of the invention provide a mooring layout to anchor one or more floating objects, the layout comprising: a reaction anchor for providing a reaction point when tensioning multiple mooring lines; outer anchors spaced apart from the reaction anchor; and at least one mooring line extending from a first anchor to a floating object. The mooring line comprises a tensioner and a tensioning line extending between the tensioner and a second anchor, wherein one of the first anchor and second anchor is the reaction anchor and the other of the first anchor and second anchor is an outer anchor.

The reaction anchor may be, or may comprise, a caisson, a suction pile or a ring. The reaction anchor may have at least two lateral connection points for connection of lines, whether mooring lines or tensioning lines.

The reaction anchor is apt to be installed at a seabed location that is between or directly beneath one or more floating facilities to be moored.

The outer anchors may comprise at least one drag anchor that is dragged toward the reaction anchor during tensioning of an associated mooring line.

Conveniently, the reaction anchor itself can also be used as an anchoring point after the mooring lines have been tensioned to the outer anchors. For this purpose, an extra mooring line can be pulled directly between the reaction anchor and the floating facility.

When used to install drag anchors in symmetrical mooring patterns, a tensioner unit can apply tension to mutually opposed anchor legs at the same time. In effect, each drag anchor then serves as a reaction anchor for the other drag anchor of a pair. Whilst this has the advantage of reducing the number of tensioning operations required, a drag anchor only has a unidirectional anchoring effect. Consequently, unlike the present invention, a drag anchor cannot serve as a multi-directional or omni-directional static reaction anchor from which angularly-spaced tensile lines extend toward outer movable anchors disposed on mutually-opposed sides of the reaction anchor. Nor can a drag anchor facilitate connection of a secondary mooring line that lies at an angle to the horizontal substantially greater than that of another line also extending from the same drag anchor.

In summary, a mooring arrangement of the invention comprises at least two outer anchors, a static reaction anchor disposed between the outer anchors, and at least two anchor legs that extend outwardly from the reaction anchor to the outer anchors. The reaction anchor has multi-directional effect, being configured to react against outward forces applied by tension in the mutually-opposed anchor legs.

Each anchor leg comprises a tensioner, an inner line extending from the reaction anchor to the tensioner, and an outer line extending from the tensioner to the outer anchor. A tensioner line, which may be the inner line or the outer line, is fixed to the tensioner whereas a mooring line extends through the tensioner. An upper section of the mooring line extends from the tensioner to a moored floating body. A lower section of the mooring line forms part of the anchor leg, as either the inner line or the outer line.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figures 1 and 2 are schematic side views showing the operation of a tensioning system of the invention; Figures 3 and 4 are schematic side views of mooring arrangements of the invention installed by the tensioning system shown in Figures 1 and 2; Figure 5 is a schematic side view showing the operation of another tensioning system of the invention Figure 6 is a schematic side view of a mooring arrangements of the invention installed by the tensioning system shown in Figure 5; and Figures 7 and 8 are schematic top plan views of mooring arrangements of the invention Referring firstly to Figures 1 and 2 of the drawings, a tensioning system 10 of the invention is shown here in operation when installing subsea anchor foundations for a spread-moored offshore installation to be shown in Figures 3 and 4. That installation may, for example, be part of a production system for producing hydrocarbons from subsea wells.

The tensioning system 10 shown in Figures 1 and 2 comprises a centrally-positioned multi-directional fixed reaction anchor 12 on the seabed 14. In this example, the reaction anchor 12 is a cylindrical suction pile that is embedded in the seabed 14 in an upright orientation. The reaction anchor 12 is multi-directional in its anchoring effect because it provides substantially equal resistance to horizontal or lateral loads applied by lines disposed on mutually-opposed sides of the reaction anchor 12. This is to be distinguished from a drag anchor that has only unidirectional effect and so would fail if pulled from opposite directions.

In plan view, two or more tensioning lines 16 extend radially outwardly with mutual angular spacing from the reaction anchor 12 to respective subsea tensioner units 18, only one of which is shown in these drawings. For this purpose, the suction pile serving as the reaction anchor 12 comprises attachment points 20, such as padeyes, to which the tensioning lines 16 are coupled. In plan view, the attachment points 20 are spaced apart angularly, preferably equi-spaced, around the circumference of the tubular skirt of the suction pile.

As is conventional, the attachment points 20 on the reaction anchor 12 are offset toward a lower end of the skirt so that a radially inner portion of each tensioning line 16 is buried in the seabed 14. This mitigates a risk of the reaction anchor 12 tilting away from a vertical axis, or overturning, under tension applied through the tensioning lines 16 in use.

Each tensioner unit 18 acts on a respective mooring line 22 that extends radially inwardly, in plan view, from a respective outer anchor 24 spaced radially across the seabed 14 from the reaction anchor 12. The outer anchors 24 are distributed with mutual angular spacing in an approximately circular array that is centred on the reaction anchor 12.

The outer anchors 24 are exemplified here by drag anchors that embed into, and hence engage with, the seabed 14 by being pulled by the mooring lines 22 radially inwardly toward the tensioner units 18 and the reaction anchor 12. The reaction anchor 12 is apt to be pre-installed in the seabed 14 before the outer anchors 24 are lowered to the seabed 14 around the reaction anchor 12.

When the mooring line 22 is being tensioned as shown in these drawings, the mooring line 22 extends through the tensioner unit 18 to a floater 26 at the surface 28. An upper section 22U of the mooring line 22 extends from the tensioner unit 18 to the floater 26, whereas a lower section 22L of the mooring line 22 extends from the tensioner unit 18 to the outer anchor 24.

The floater 26 may, for example, be an installation vessel, a production vessel such as an FPSO, a platform or a buoy. Where the floater 26 is to be a permanent part of the offshore installation that is intended to remain on station above the anchor foundations throughout its operational life, such as an FPSO or a platform, the mooring line 22 can serve to moor the floater 26. Conversely, where the floater 26 is an installation vessel whose purpose is to embed and test the load capacity of the outer anchor 24, the mooring line 22 can then be transferred from the installation vessel to another floater that is to be moored permanently.

The tensioning lines 16 and the mooring lines 22 may each comprise one or more lengths of chain, wire or rope, including metallic or synthetic wire or rope. For example, each mooring line 22 may comprise a bottom chain adjoining the outer anchor 24, a top chain extending to the floater 26 and an intermediate section of wire or rope disposed between the bottom chain and the top chain. In that case, each tensioner unit 18 may conveniently act on the bottom chain of the associated mooring line 22. Conversely, the shorter tensioning line 16 may comprise a chain along its full length or at least along its radially inner portion that adjoins the reaction anchor 12 and is buried in the seabed 14.

The upper end of the mooring line 22 is supported by a crane 30 of the floater 26, from which the upper section 22U of the mooring line 22 is suspended as a catenary extending down to the tensioner unit 18. The upper end of the mooring line 22 could instead be supported by other lifting or tensioning equipment aboard the floater 26, such as a winch.

In Figures 1 and 2, each tensioner unit 18 is exemplified by a Stevtensioner® chain-shortening clutch as offered by Vryhof of the Netherlands. The mooring line 22 passes through a body of the tensioner unit 18 as an active line and the tensioning line 16 is coupled to the body of the tensioner unit 18 as a passive line. Optionally, the tensioning line 16 is coupled to the body of the tensioner unit 18 via a load pin to measure tension in the lower section 22L of the mooring line 22 that extends from the tensioner unit 18 to the outer anchor 24.

Tensioner units 18 can work in various ways but a Stevtensioner® exploits the principle that applying a vertical load to a taut, generally horizontal mooring line 22 generates a substantially greater horizontal load in the mooring line 22. Specifically, when the upper section 22U of the mooring line 22 is pulled upwardly toward the floater 26 as shown in Figure 2, the body of the tensioner unit 18 pivots to latch to the mooring line 22 between the upper and lower sections 22U, 22L.

Lifting the upper section 22U of the mooring line 22 lifts the tensioner unit 18 latched to it, and therefore also the tensioning line 16 and the lower section 22L of the mooring line 22, away from the seabed 14. This generates amplified tension in the tensioning line 16 and in the lower section 22L of the mooring line 22, causing the outer anchor 24 to penetrate deeper into the seabed 14 as it is pulled toward the reaction anchor 12 from the outward starting position shown in Figure 1 to the inward position shown in Figure 2.

In the case of a Stevtensioner®, an unlocking wire extending from the floater 26 is also coupled to the body of the tensioner unit 18 although that wire is omitted from these simplified drawings. The tensioner unit 18 can be unlatched from the mooring line 22 by pulling the unlocking wire upwardly to pivot the body of the tensioner unit 18 relative to the mooring line 22. Once unlatched, the tensioner unit 18 can then be lowered back along the mooring line 22 over the length gained by the preceding lifting step, returning to the lower level shown in Figure 1 to ready the tensioning system 10 for another lifting step.

The lifting/latching and unlatching/lowering actions are then repeated as the tensioner unit 18 undergoes a vertically reciprocating yo-yo movement, ratchefing the outer anchor 24 inwardly in stepwise fashion with each successive lift of the upper section 22 of the mooring line 22. This reciprocation of the tensioner unit 18 continues for as many cycles as are necessary to impart a desired degree of tension in the lower section 22L of the mooring line 22 and thereby to engage the outer anchor 24 in the seabed 14 to a desired extent.

Once tensioning is complete, the tensioner unit 18 can be left in situ as part of a mooring arrangement of the invention. In this respect, Figure 3 shows a resulting mooring arrangement 32 that could be installed by the tensioning system 10 shown in Figures 1 and 2. This shows anchor legs 34 in mutual opposition about a suction pile that serves as a central reaction anchor 12.

Each anchor leg 34 comprises a generally horizontal tensioning line 16 that extends, under tension, from the reaction anchor 12 to a tensioner unit 18 acting on a respective mooring line 22. A lower section 22L of the mooring line 22 extends generally horizontally, also under tension, from the tensioner unit 18 to a drag anchor serving as the outer anchor 24. Conversely, an upper section 22U of the mooring line 22 extends to a floater 26 at the surface 28, exemplified here by an FPSO.

In an alternative approach, a tensioner unit 18 can be recovered by detaching the tensioning line 16, for example using a remote-controlled coupling or an ROV. An unlocking wire can then be pulled to disengage the tensioner unit 18 from the mooring line 22 and then the tensioner unit 18 can be slid up along the mooring line 22 to the surface 28. The tensioner unit 18 can then be used for a subsequent tensioning operation on another mooring line 22 and outer anchor 24, or eventually on another project.

In this respect, Figure 4 shows another mooring arrangement 36 that is apt to be installed by the tensioning system 10 shown in Figures 1 and 2. Again, this shows anchor legs 34 in mutual opposition about a central reaction anchor 12 but in this case the tensioner units 18 have been removed after facilitating installation of the respective outer anchors 24. Consequently, each mooring line 22 now extends directly as a catenary from the respective outer anchor 24 to the floater 26. The tensioning lines 16 have been removed from this view for clarity but, in practice, they could simply be left on the seabed 14 once detached from the tensioner unit 18.

It will be noted from Figures 3 and 4 that the floater 26 is disposed at a central location above and between the outer anchors 24 of the anchor legs 34, generally in alignment with the reaction anchor 12. Conveniently therefore, as shown in dashed lines, an optional auxiliary mooring line 38 can extend from the reaction anchor 12 to the floater 26 without a risk of clashing with the primary mooring lines 22 of the anchor legs 34.

Thus, the reaction anchor 12 is useful not only while the outer anchors 24 are being installed but also as a secondary anchor in ongoing use of the offshore installation.

In these examples, the auxiliary mooring line 38 is a taut line that follows a vertical axis intersecting the reaction anchor 12. However, the auxiliary mooring line 38 could hang as a catenary or other shape, and need not follow that vertical axis.

Moving on now to Figures 5 and 6, these drawings show that the principles of the invention can also be applied to a reversed arrangement in which mooring lines 22 radiate outwardly from a central reaction anchor 12 such as a suction pile surrounded by an array of outer anchors 24 such as drag anchors. In this case the reaction anchor 12 serves as a common primary anchor for an array of moored floaters 26 such as floating wind turbines as shown in Figure 6. Conversely, the tensioning line 16 of each anchor leg 34 extends radially inwardly from a respective outer anchor 24 to a respective tensioner unit 18.

Each tensioner unit 18 acts on the associated mooring line 22 in the same way as shown in Figures 1 to 3 but is simply reversed with respect to the central reaction anchor 12. Again, tension generated by the tensioner units 18 in the lower sections of the mooring lines 22 promotes engagement of the outer anchors 24 with the seabed 14 by dragging them in convergent radially-inward directions toward the reaction anchor 12. In this case, however, that inward load is applied to the outer anchors 24 by the tensioning lines 16 and not by the mooring lines 22.

The mooring arrangements shown in the preceding embodiments share the feature of an inner line that extends from the reaction anchor 12 to the tensioner unit 18, and at least one outer line that extends from the tensioner unit 18 to at least one of the outer anchors 24. In Figures 1 to 3, the inner line is the tensioning line 16 and the outer line is the lower section 22L of the mooring line 22. Conversely, in Figures 5 and 6, the inner line is the lower section 22L of the mooring line 22 and the outer line is the tensioning line 16.

Finally, Figures 7 and 8 are plan views that exemplify mooring arrangements 40, 42 made possible by the invention. In each case, anchor legs 34 radiate outwardly from a central reaction anchor 12 to terminate, in each case, in one or more outer anchors 24 that are engaged with the seabed 14. The upper sections 22U of the mooring lines 22 have been omitted from these views for clarity.

The mooring arrangements 40, 42 of Figures 7 and 8 each depict an even number of anchor legs 34 that are apt, as shown, to be grouped into pairs, each pair comprising anchor legs 34 that are in mutual longitudinal alignment and in diametric opposition about the reaction anchor 12. Thus, in these examples, the anchor legs 34 are equiangularly spaced about the reaction anchor 12. Equi-angular spacing is also possible in other mooring arrangements of the invention that have an odd number of anchor legs 34, such as three or five anchor legs 34. In general, pairing or equi-angular spacing of anchor legs 34 may be advantageous to balance forces on the system but is not essential.

In the mooring arrangement 40 of Figure 7, six anchor legs 34 radiate outwardly from the central reaction anchor 12, thus being spaced apart by nominally 60° of arc. The anchor legs 34 are therefore grouped into three pairs of mutually-opposed anchor legs 34. In this example, which is apt to moor a centrally-positioned floater such as an FPSO as shown in Figure 3, the lower sections 22L of the mooring lines 22 extend outboard of the tensioner units 18 to the respective outer anchors 24, whereas the tensioning lines 16 extend inboard of the tensioner units 18 to the reaction anchor 12.

In the mooring arrangement 42 of Figure 8, four anchor legs 34 radiate outwardly from the central reaction anchor 12, thus being spaced apart by nominally 900 of arc. The anchor legs 34 are therefore grouped into two pairs of mutually-opposed anchor legs 34. In this example, which is apt to moor a surrounding array of floaters such as floating wind turbines as shown in Figure 6, the tensioning lines 16 extend outboard of the tensioner units 18 to the respective outer anchors 24, whereas the lower sections 22L of the mooring lines 22 extend inboard of the tensioner units 18 to the reaction anchor 12.

Figure 8 also shows that it is possible for each anchor leg 34 of a mooring arrangement to comprise more than one outer anchor 24. For example, outer anchors 24 are depicted in series in the anchor leg 34 shown to the left in Figure 8. Here, an auxiliary tensioning line 44 extends radially outwardly from the tensioner unit 18 to an outboard outer anchor 24 of the series. Conversely, the anchor leg 34 shown to the right in Figure 8 has outer anchors 24 acting in parallel. In that example, the outer anchors 24 are arranged on axes that converge in a radially inward direction along respective auxiliary tensioning lines 44 coupled to a tri-plate 46 that couples them, in turn, to a common tensioning line 16 extending to the tensioner unit 18. In that sense, the tensioning line 16 is divided to form the auxiliary tensioning lines 44. In another variant, it may instead be possible to couple auxiliary tensioning lines 44 of parallel-acting outer anchors 24 directly to the tensioner unit 18.

Many other variations are possible within the inventive concept. For example, in principle, a moored floating body could be positioned in the water column beneath the surface, such as a subsea buoy. Also, the reaction anchor could be a different subsea structure in fixed relation to the seabed, such as a hammer-driven pin pile, another form of subsea foundation, a subsea electrical substation or a frame or housing that supports and protects subsea equipment.

Moored floaters such as the wind turbines of Figure 6 could have additional mooring lines that extend to other subsea foundations. Those other foundations could also employ mooring arrangements of the invention, such as those illustrated in Figure 6, that provide the additional mooring lines.

Claims

Claims 1. A mooring arrangement, comprising: at least two outer anchors; and a reaction anchor disposed between the outer anchors and configured to react against outward forces applied by tension in at least two anchor legs that extend outwardly from the reaction anchor to the outer anchors, wherein each anchor leg comprises a tensioner, an inner line extending from the reaction anchor to the tensioner, and at least one outer line extending from the tensioner to at least one of the outer anchors.
2. The arrangement of Claim 1, wherein the outer anchors are on mutually opposed sides of the reaction anchor.
3. The arrangement of Claim 1 or Claim 2, wherein the anchor legs extend radially from the reaction anchor in mutually opposed directions.
4. The arrangement of Claim 3, wherein the anchor legs are diametrically opposed about the reaction anchor in mutual longitudinal alignment.
5. The arrangement of any preceding claim, wherein the inner line of each anchor leg is a tensioning line coupled to the tensioner and the outer line of each anchor leg is a lower section of a mooring line extending through the tensioner.
6. The arrangement of Claim 5, wherein an upper section of the mooring line converges upwardly with an upright axis that aligns with the reaction anchor.
7. The arrangement of Claim 6, wherein the mooring line extends upwardly to a floating body that is substantially aligned with said upright axis.
8. The arrangement of Claim 7, wherein the floating body is moored by the mooring lines that extend from the respective anchor legs of the arrangement.
9. The arrangement of Claim 7 or Claim 8, further comprising an additional mooring line that extends directly from the floating body to the reaction anchor.
10. The arrangement of any of Claims 1 to 4, wherein the outer line of each anchor leg is a tensioning line coupled to the tensioner and the inner line of each anchor leg is a lower section of a mooring line extending through the tensioner.
11. The arrangement of Claim 10, wherein an upper section of the mooring line diverges upwardly from an upright axis that aligns with the reaction anchor.
12. The arrangement of Claim 10, wherein the upper section of each mooring line extends to a respective one of an array of floating bodies surrounding the upright axis.
13. The arrangement of Claim 6 or Claim 11, wherein the tensioners are operable to impart tension in the respective anchor legs by pulling the upper section of the mooring line upwardly.
14. The arrangement of Claim 13, wherein the tensioners are operable to lock to the mooring line upon pulling the upper section of the mooring line.
15. The arrangement of any preceding claim, wherein at least one of the anchor legs terminates in a group of two or more of the outer anchors.
16. The arrangement of Claim 15, wherein the outer anchors of the group are angularly spaced about the reaction anchor.
17. The arrangement of Claim 15, wherein the outer anchors of the group are aligned on a common radius extending from the reaction anchor.
18. The arrangement of Claim 16 or Claim 17, wherein said at least one of the anchor legs comprises at least two outer lines extending from the tensioner to respective ones of the outer anchors of the group.
19. The arrangement of Claim 16 or Claim 17, wherein the least one outer line of said at least one of the anchor legs is divided into two or more auxiliary outer lines each extending to a respective one of the outer anchors of the group.
20. The arrangement of any preceding claim, wherein the reaction anchor is a caisson, a pile or a ring.
21. The arrangement of any preceding claim, wherein the reaction anchor comprises at least two lateral connection points for connection of the respective inner lines of the anchor legs.
22. The arrangement of any preceding claim, wherein the outer anchors comprise at least one drag anchor that is configured to self-embed upon being dragged toward the reaction anchor.
23. A method of installing a mooring arrangement, the method comprising: fixing a reaction anchor at a seabed location; connecting at least two anchor legs to the reaction anchor, each anchor leg radiating from the reaction anchor across the seabed and terminating outwardly in at least one outer anchor such that the reaction anchor is disposed between the outer anchors, imparting tension in the anchor legs to pull the outer anchors inwardly toward the reaction anchor; and mooring a floating body with at least one mooring line extending upwardly from each anchor leg.
24. The method of Claim 23, wherein each anchor leg comprises a lower section of a respective one of the mooring lines and tension is imparted in the anchor legs by pulling upper sections of the mooring lines that extend upwardly from the anchor legs.
25. The method of Claim 23 or Claim 24, comprising imparting tension in the anchor legs in a direction substantially parallel to the seabed.
26. The method of Claim 24 or Claim 25, comprising pulling the upper section of each mooring line cyclically in a succession of pulls, each pull locking the upper section relative to the anchor leg.
27. The method of any of Claims 23 to 26, comprising mooring the floating body with two or more mooring lines that converge upwardly from the anchor legs.
28. The method of any of Claims 23 to 26, comprising mooring two or more floating bodies with mooring lines that diverge upwardly from the anchor legs.