Mooring systems for offshore installations

Abstract

A mooring tendon for a bottom-fixed or floating marine structure comprises: an elongate rigid member; a flexible or articulated mooring line that is deployable from the rigid member; and a stopper that can act between the rigid member and the mooring line to lock the mooring line after deployment from the rigid member. The structure is moored by deploying the mooring tendon from the structure; deploying the mooring line from the rigid member; extending and coupling the mooring line to a subsea anchoring point; and then locking the mooring line against further extension from the rigid member. By retracting the mooring line into the rigid member, the mooring tendon is tensioned after coupling the mooring line to the anchoring point.

Description

[0001] Mooring systems for offshore installations

[0002] This invention relates to tensioned moorings for offshore installations, such as floating structures or structures that stand on the seabed.

[0003] Moorings of the invention can be used to moor floaters such as platforms or buoys, especially vertically-elongate floaters such as spar buoys. However, the invention will be exemplified herein by post-tensioned moorings for columnar foundation structures like monopiles that are used to support bottom-fixed offshore wind turbines. In that example, the moorings provide additional stayed or guyed support to a tall column that extends from the seabed to above the surface.

[0004] Taut or tensioned mooring lines, tethers or tendons may comprise chains, cables, ropes or tubes that can be used alone or in end-to-end combination as a hybrid mooring. For example, US 7422394 discloses tubular tendons. Cables or ropes may be of steel wire or of synthetic materials, such as the composite rope tendons disclosed in US 2007 / 0231078.

[0005] Tendons can be used to moor tension-leg platforms (TLPs) as exemplified by EP 0441413, in which vertical tendons act in tension to connect the TLP to a foundation on the seabed. Similar mooring systems can be used for floating wind turbines, as disclosed in CN 217893155. In some applications, tensioned mooring lines or tendons are inclined to the vertical as disclosed in WO 2009 / 005357 and in WO 2023 / 178075.

[0006] In KR 20150045198, short telescopic tubes are pivotally connected to a turret supported by a vessel. The tubes can be deployed vertically to guide respective mooring chains stored on the vessel. The mooring chains slide longitudinally within the guides.

[0007] Tendons are lengthy and heavy components that present challenges of storage and transportation. For example, US 6682266 proposes transporting a tension leg tendon by towing it to an installation site.

[0008] When tendons are used to moor a floater, challenges also arise in view of the installation and tensioning sequence as exemplified in JP 2010030379. Specifically, tendons can be installed before the floater arrives at an offshore installation site and then connected to the floater and tensioned, or the tendons can be installed and tensioned after the floater arrives at the installation site. This involves multiple vessel operations that are costly and susceptible to weather-related delays.

[0009] Irrespective of the order of installation, a critical element being either the floater or at least one of the tendons is left temporarily without a connection at some stage or tension forces may be unbalanced among the tendons. It is therefore important to complete the installation and tensioning process without undue delay and within an available weather window.

[0010] US 4372706 discloses an apparatus to prevent the accidental release of a wire rope. The apparatus discloses a clamp attached to the wire rope which, when the rope is released, hits a stopping ram which presses into wedge-shaped rope grippers. The wedges squeeze the rope and stop against wedge blocks which are attached to an inside portion of the guyed tower.

[0011] GB 2069450 discloses a self-standing production riser that also forms part of the state of the art.

[0012] The invention aims to simplify the installation of such moorings and potentially to reduce the weight of their mooring lines.

[0013] Against this background, the invention resides in a mooring tendon that comprises: an elongate rigid member; a flexible or articulated mooring line that is deployable from the rigid member; and a stopper acting between the rigid member and the mooring line to lock the mooring line after deployment from the rigid member. The stopper may, for example, be a ratchet that is operable after deployment of the mooring line to resist distal movement of the mooring line relative to the rigid member but to permit proximal movement of the mooring line relative to the rigid member.

[0014] Conveniently, the mooring line can extend along the rigid member before its deployment from the rigid member. For example, the rigid member may be tubular, in which case the mooring line can extend within a lumen of the rigid member.

[0015] The mooring line can extend along the full length of the rigid member from a proximal end to a distal end, or could diverge from the rigid member at an intermediate position along the length of the rigid member. The rigid member may be longitudinally extensible. For example, the rigid member can comprise two or more sections, at least one of which is movable longitudinally relative to another of said sections. Those sections could be in telescopic relation, and may have limit formations that abut or cooperate to limit longitudinal extension of the rigid member.

[0016] The tendon of the invention may be combined with a tensioner that acts on a portion of the mooring line, or on a messenger line attached to the mooring line, protruding from the rigid member. For example, the tensioner can act on a portion of the mooring line or on a messenger line protruding from a proximal end of the rigid member or laterally divergent from the rigid member. In the latter case especially, the tensioner could be disposed along a length of the rigid member.

[0017] The inventive concept embraces a marine structure comprising at least one mooring tendon of the invention. That structure could be a support column, a monopile or other bottom-fixed structure, or a spar buoy, a platform or other floating structure.

[0018] The rigid member of the or each tendon may be pivotable relative to the structure, preferably about a pivot axis that is fixed relative to the structure. For example, a proximal end of the rigid member can be pivotably attached to the structure.

[0019] The structure may be elongate, in which case the or each tendon can extend along the structure and preferably along at least a majority of a length of the structure.

[0020] Correspondingly, the inventive concept extends to a method of mooring a marine structure. That method comprises: deploying a mooring tendon from the structure; deploying a flexible or articulated mooring line of the mooring tendon from a rigid member of the mooring tendon; extending and coupling the mooring line to a subsea anchoring point; and then locking the mooring line against further extension from the rigid member. The mooring tendon can be tensioned after the mooring line has been coupled to the anchoring point, for example while partially retracting the mooring line into the rigid member during tensioning.

[0021] The mooring tendon can be deployed by pivoting the rigid member from the structure, for example by pivoting the rigid member about an upper end and deploying the mooring line from a lower end of the rigid member. The lower end of the rigid member can be released from the structure to enable deployment of the mooring tendon. The rigid member could be extended in length after deploying the mooring tendon. The deployed mooring tendon can be supported with added buoyancy.

[0022] The method of the invention can further comprise the preliminary steps of transporting and / or upending the structure with the mooring tendon attached to the structure, before deploying the mooring tendon from the structure. Also, the method of the invention can further comprise a preliminary step of landing the structure on the seabed before coupling the mooring line to the anchoring point.

[0023] Thus, the invention provides a hybrid tube / mooring line tendon for a floating or bottom-fixed marine sub-structure. A mooring line made of chain or of a mix of cable, synthetic line and / or chain is pre-stored in a tube alongside a floater or other subsea structure. The tube is connected to an upper level of the structure via a hinged connection or a muti-rotation axis connector to define a top-hinged arm. Once the structure is on site, and possibly landed on the seabed, the arm is deployed away from a parking position alongside the structure and the line is pulled out of the tube to be connected to a preinstalled mooring point subsea. Once connected, the line is tensioned back through the tube. Tensioning can be performed either by a subsea tensioning tool mounted on the tube, for example located at or near the bottom end of the tube, or though the tube from the surface using a tensioning tool at a topside position.

[0024] The invention simplifies the storage and transportation of tendons for floating or fixed structures such as tethered spars, tethered towers or monopiles. Advantageously, the invention allows pre-integration of mooring components with the structure at a yard or factory and enables tensioning to be performed and controlled quickly and accurately.

[0025] Embodiments of the invention provide a mooring system for a floating object, the system comprising: a buoyancy tube; and at least one tendon tube articulated to the buoyancy tube at a first end, and comprising a main tube, which may be made of steel or a polymer composite material, and a mooring line extension which may comprise a chain, a spiral strand cable or a synthetic mooring line. The mooring line extension is completely or partially stored inside the tendon tube in a first configuration and extends from a second end of the tendon tube to the seabed in a second configuration.

[0026] Embodiments of the invention also implement a method to install the mooring system for a floating object, the method comprising: providing a vertical buoyancy tube, comprising tendon tubes at its periphery, wherein a tendon tube can pivot relative to the buoyancy tube and contains at least one mooring line extension; pivoting a tendon tube at a mooring line angle; pulling the mooring line extension out of the tendon tube; and connecting the distal end of the mooring line extension to a foundation. The buoyancy tube can be separated from the tendon tubes after installation.

[0027] The method may further comprise a subsequent or final step of tensioning the mooring line by pulling it upwards inside the tendon tube and using a stopper mechanism of the tendon tube, such as a ratchet or pawl, to achieve a final tension in the tendon.

[0028] The buoyancy tube may be transported horizontally in an unballasted state and can then be upended by ballasting. The buoyancy tube may be longer than the water depth and can be laid or landed on the seabed before deployment of the tendon tubes.

[0029] The tendon tube may contain a telescopic secondary tendon tube that completely or partially contains the mooring line extension. In that case, the length of the mooring catenary comprises the tendon tube plus an extended portion of the secondary tendon tube plus the mooring line extension.

[0030] The invention could also be used to retrofit an installation by connecting at least one tendon tube to the installation, for example via a padeye or other connection that defines a pivot axis, then pivoting the tendon tube before pulling out the mooring line extension.

[0031] In summary, a mooring tendon of the invention for a bottom-fixed or floating marine structure comprises: an elongate rigid member; a flexible or articulated mooring line that is deployable from the rigid member; and a stopper that can act between the rigid member and the mooring line to lock the mooring line after deployment from the rigid member. The structure is moored by deploying the mooring tendon from the structure; deploying the mooring line from the rigid member; extending and coupling the mooring line to a subsea anchoring point; and then locking the mooring line against further extension from the rigid member. By retracting the mooring line into the rigid member, the mooring tendon can be tensioned after coupling the mooring line to the anchoring point.

[0032] 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: Figure 1 is a schematic side view of a foundation structure fitted with a mooring system of the invention;

[0033] Figure 2 is an enlarged schematic sectional side view corresponding to Detail II of Figure 1 ;

[0034] Figure 3 is a schematic top plan view of the foundation structure of Figure 1 ;

[0035] Figure 4 is a schematic side view showing the foundation structure being landed on the seabed after transportation to an installation site;

[0036] Figure 5 is a schematic side view showing the mooring system in successive stages of deployment from the foundation structure after landing on the seabed;

[0037] Figure 6 is an enlarged schematic sectional side view corresponding to Detail VI of Figure 5;

[0038] Figure 7 is an enlarged schematic sectional side view corresponding to Detail VII of Figure 5;

[0039] Figure 8 is a schematic side view showing a variant of the mooring system in initial stages of deployment from the foundation structure;

[0040] Figure 9 corresponds to Figure 8 but shows the mooring system in a subsequent stage of deployment;

[0041] Figure 10 is an enlarged schematic sectional side view corresponding to Detail X of Figure 8;

[0042] Figure 11 is an enlarged schematic sectional side view corresponding to Detail XI of Figure 9;

[0043] Figure 12 is a schematic side view showing a further variant of the mooring system undergoing tensioning after deployment from the foundation structure; Figure 13 is an enlarged schematic sectional side view corresponding to Detail XIII of Figure 12;

[0044] Figure 14 corresponds to Figure 13 but shows an alternative tensioning approach; and

[0045] Figure 15 is a schematic side view showing a completed bottom-fixed installation of the invention.

[0046] Referring firstly to Figures 1 to 3 of the drawings, a subsea support column 10 of the invention comprises an elongate buoyancy structure or floater 12. In this example, the floater 12 comprises a bundle of parallel tubes 14 that are clustered with equi-angular spacing around a central longitudinal axis 16. The tubes 14 of the floater 12 contain one or more ballasting tanks to adjust its buoyancy and trim. By way of example, depending upon the targeted water depth, the support column 10 could be 100m to 150m long and 10m to 15m wide.

[0047] The top and bottom ends of the support column 10 comprise end plates 18 that join and close the top and bottom ends of the tubes 14. The end plates 18 lie in parallel planes that are orthogonal to the central longitudinal axis 16. The upper end plate 18 at the top end of the support column 10 serves as a transition piece on which an offshore wind turbine or other above-surface structure may be placed, or assembled, after the support column 10 has been installed on the seabed.

[0048] The support column 10 further comprises an array of elongate tendon arms 20 that extend substantially parallel to the tubes 14 of the floater 12, with equi-angular spacing around the central longitudinal axis 16. In this example, each tendon arm 20 is in angular alignment with, and outboard of, a respective one of the tubes 14, although such mutual alignment is not essential. The tendon arms 20 extend along most, indeed along nearly all, of the length of the floater 12.

[0049] At its upper or proximal end, each tendon arm 20 is pivotably connected to the floater 12 directly or via the upper end plate 18 by a respective pivot mount 22. Near its lower or distal free end, each tendon arm 20 is secured to the floater 12 via a bumper 24 to which the tendon arm 20 is releasably attached while the support column 10 is being transported to, and upended at, an installation site. When released from their bumpers 24 for deployment subsea, the lower ends of the tendon arms 20 can swing away from the floater 12 while their upper ends pivot about the pivot mounts 22.

[0050] The pivot mounts 22 are equi-angularly spaced around the floater 12 at an upwardly-offset position located toward the upper end of the support column 10. The pivot mounts 22 may be hinges or spherical joints that permit two-axis rotation of the tendon arms 20 relative to the floater 12. This allows the tendon arms 20 to extend radially from the floater 12 in plan view, in planes that converge on and contain the central longitudinal axis 16 of the support column 10. The tendon arms 20 splay downwardly and outwardly from the pivot mounts 22 with mutually-opposed inclination in a frusto-conical arrangement at an angle of, for example, between 30° and 75° to the vertical.

[0051] As can be seen in Figure 2, each tendon arm 20 has an upper part that comprises a tendon tube 26 of steel or polymer composite and contains a mooring line 28. The tendon tube 26 is a substantially rigid member, albeit potentially capable of bending elastically along its length, whereas the mooring line 28 is of flexible or articulated construction. The mooring line 28 is stored within the tendon tube 26 while the support column 10 is being transported to, and upended at, an installation site and is then deployable by being pulled telescopically in a downward or distal direction out of the lower end of the tendon tube 26.

[0052] The mooring line 28 is exemplified here by a chain but could instead comprise a rope or a cable. The mooring line 28 extends centrally along the lumen of the tendon tube 26. At its lower or distal end, the mooring line 28 terminates in a subsea connector 30 such as a shackle, which may protrude from the tendon tube 26 as shown here or may be stored within the lower end of the tendon tube 26 until the mooring line 28 is deployed. Conversely, an upper end portion of the mooring line 28 exits the tendon tube 26, for example through an upper end of the tendon tube 26 as shown here, to extend to a storage drum 32 via a tensioner 34 such as a strand jack. In this example, the storage drum 32 and the tensioner 34 may conveniently be mounted on the upper end plate 18 of the support column 10, or on another structure atop the upper end plate 18.

[0053] During deployment, the mooring line 28 is payed out from the storage drum 32 and then passes down the tendon tube 26 in a distal direction. Once coupled to a subsea foundation via the connector 30, the mooring line 28 is then tensioned by operating the tensioner 34 to pull or retract a previously slack portion of the mooring line proximally, back up into the tendon tube 26. Simultaneously, an upper portion of the mooring line 28 extending from the tendon tube 26 is wound back onto the storage drum 32.

[0054] The storage drum 32 and the tensioner 34 need not act directly on an upper end portion of the mooring line 28 but could instead act on a messenger line that is attached to the upper end of the mooring line 28.

[0055] The tendon tube 26 comprises a stopper 36 that is operable to act on the mooring line 28 during tensioning. The stopper 36 is exemplified here by a ratchet mechanism comprising jaws in mutual opposition about the mooring line 28. During deployment, the stopper 36 remains disengaged from the mooring line 28 to allow the mooring line 28 to be pulled downwardly or distally out of the tendon tube 26. Conversely, during tensioning, the stopper 36 engages the mooring line 28 to permit only unidirectional movement of the mooring line 28 relative to the tendon tube 26, that being upward or proximal movement of the mooring line 28 through the tendon tube 26. Activation of the stopper 36 can be controlled remotely from the surface or via an ROV.

[0056] Thus, during and after tensioning, the stopper 36 engages the mooring line 28 to block downward movement of the mooring line 28 through the tendon tube 26. This transfers tensile load from the mooring line 28 to the tendon tube 26 as the mooring line 28 is tensioned. The tendon tube 26 and the mooring line 28 therefore act together in series as a tensioned tendon to tether the upright support column 10.

[0057] In this example, the floater 12 comprises three tubes 14 and three tendon arms 20 in an equilateral triangular arrangement in plan view as shown in Figure 3. The end plates 18 have a similarly triangular shape, with the tubes 14 and the tendon arms 20 aligned with the vertices of the triangle. In other examples, the floater 12 could have more or fewer tubes 14 and the end plates 18 could have other shapes. In one such example, the floater 12 could comprise an elongate monotube 14 of circular cross-section, whose side wall is rotationally symmetrical around the central longitudinal axis 16 as shown by a dashed line in Figure 3. Similarly, the support column 10 could have more or fewer tendon arms 20, and there could be more or fewer tendon arms 20 than the number of tubes 14 of the floater 12.

[0058] The support column 10 can be fabricated at a shipyard using techniques that are familiar in the art. Conveniently, the support column 10 can be fabricated in a horizontal orientation; moreover, to minimise its draft to suit shallow inshore water, the support column 10 can also be launched into and transported through water in that orientation. Thus, there is no need for deep-water port or fjord to construct and transport the support column 10.

[0059] In this respect, the left side of Figure 4 shows the support column 10 at the surface 38 of a body of water in a horizontal orientation, whereby a tug (not shown) can conveniently tow the support column 10 from a fabrication site to an installation site. On arrival at the installation site as shown centrally in Figure 4, the support column 10 is upended by adding or transferring ballast 40 toward the lower end of the floater 12 as shown. For example, water may be admitted into the lower end of the floater 12 by controlled pumping or by free flooding. The support column 10 therefore contains a positively-buoyant upper section 42, which may contain air, atop a negatively buoyant lower section containing ballast 30. The result of this downwardly-offset ballasting is that the support column 10 eventually lies stably in an upright orientation assured by its centre of gravity being held beneath its centre of buoyancy.

[0060] The length of the support column 10 exceeds the water depth at the installation site. Initially, however, enough buoyancy is maintained in the floater 12 for the bottom of the upright support column 10 to be held suspended clear of the seabed 44 like an unmoored spar buoy. This allows fine control of the position of the support column 10 in a horizontal plane to ensure that the support column 10 will be installed accurately.

[0061] Once correctly positioned, the support column 10 is ballasted further, for example by admitting more water ballast 30 as shown on the right side of Figure 4. This causes the support column 10 to sink until its bottom end plate 18 encounters and comes to rest on the seabed 44, leaving only an upper portion of the support column 10 still breaking the surface 38. The pivot mounts 22 are also above the surface 38 in this example. The lower end of the support column 10 may sink into and become embedded in the soil of the seabed 44 to a minor extent.

[0062] On landing the support column 10 in this way, frictional and mechanical engagement of its bottom end plate 18 with the seabed 44 then resists lateral movement of the support column 10 relative to the seabed 44. Additional ballast 30, which may include solid ballast that is denser than water, can be added beneath the centre of buoyancy to increase stability. Moreover, residual positive buoyancy in the upper section 42 of the support column 10 provides uprighting support. This keeps the support column 10 self-stable in a substantially vertical orientation despite horizontal forces of currents, waves and winds acting on the support column 10 at levels below, at and above the surface 38.

[0063] The inherent stability of the ballasted support column 10 facilitates subsequent installation operations. The residual buoyancy of the support column 10 also supports much of its weight, such that the apparent weight load exerted on the seabed 44 is substantially less than the actual weight of the support column 10 including its ballast.

[0064] The right side of Figure 4 also shows the possibility of landing the support column 10 on a previously prepared area of the seabed 44. The seabed 44 itself can be prepared to bear the downward load of the support column 10, for example by rock dumping. However, in this example, the seabed 44 is prepared by pre-installing a subsea foundation 46 on which the support column 10 will rest, with the foundation 46 then being interposed between the support column 10 and the seabed 44. By way of example, the foundation 46 could be a rigid raft or template structure lying on or embedded into the seabed 44, or a mudmat.

[0065] Turning next to Figure 5, this drawing shows one of the tendon arms 20 being deployed from the floater 12 of the landed support column 10. Successive stages of deployment are shown here from left to right. First, as shown to the left of Figure 5, the lower end of the tendon arm 20 is released from its bumper 24, either remotely from the surface or by an ROV or a diver. Next, using a line 48 coupled to the connector 30, a vessel 50 moving across the surface 38 pulls the free lower end of the mooring line 28 out of the tendon tube 26. In so doing, the vessel 50 also pulls the lower end of the tendon tube 26 further away from the floater 12 as the tendon tube 26 swings away from the vertical about the pivot mount 22.

[0066] While the mooring line 28 is being deployed, the jaws of the stopper 36 within the tendon tube 26 remain open as shown in Figure 6. This allows the mooring line 28 to be pulled downwardly through and out of the tendon tube 26. Eventually, as shown to the right of Figure 5, the connector 30 at the free end of the mooring line 28 is pulled across the seabed 44 to engage the mooring line 28 with an anchoring foundation 52 such as a suction pile, a pin pile, a clump weight or a drag anchor. The anchoring formations 52 for the tendon arms 20 are embedded in or lie on the seabed 44 at locations spaced horizontally from the support column 10 and from each other.

[0067] When the mooring line 28 has been engaged with the anchoring foundation 52, the jaws of the stopper 36 are closed as shown in Figure 7 so that tensioning can begin. The mooring line 28 is then pulled upwardly into and through the tendon tube 26, passing through the jaws of the stopper 36 acting as a unidirectional ratchet. The initially slack portion of the mooring line 28 protruding from the lower end of the tendon tube 26, shown in Figure 5 as a dashed line, straightens when tensioned as shown in a solid line. The tension applied to the tendon arm 20 substantially exceeds that arising from its self-weight.

[0068] Whilst the tensioned tendon arm 20 comprising the tendon tube 26 and the mooring line 28 is shown schematically as being straight in the drawings, in practice the tendon arm 28 will adopt a shallow catenary shape under self-weight. As noted above, the nominally rigid tendon tube 26 is capable of elastic deflection along its length for this purpose.

[0069] Moving on now to Figures 8 to 11, these drawings show a variant of the invention in which the tendon tube 26 is extensible in length. For this purpose, the tendon tube 26 comprises concentric outer and inner tendon tubes 26A, 26B in telescopic relation. The mooring line 28 extends along the lumen of the inner tendon tube 26B, which also supports the stopper 36.

[0070] As shown in Figure 8, a vessel 50 moving across the surface 38 uses a line 48 coupled to the connector 30 to pull the free lower end of the mooring line 28 out of the inner tendon tube 26B. Then, with continued movement away from the support column 10, the vessel 50 pulls the inner tendon tube 26B out of the outer tendon tube 26A as shown in Figure 9. The inner tendon tube 26B and the protruding portion of the mooring line 28 provide the extra length that is necessary for the deployed tendon arm 20 to reach the anchoring foundation 52.

[0071] Figure 10 shows that the inner tendon tube 26B may contain a relatively short length of mooring line 28 such as a chain, with a lighter messenger line 54 at an upper end of the mooring line 28 extending to a tensioner. In this example, a short messenger line 56 also protrudes from a lower end of the inner tendon tube 26B for connection to the line 48 that is suspended from the vessel 50. Figure 10 shows the jaws of the stopper 36 open to allow the mooring line 28 to be pulled out of the inner tendon tube 26B before the tendon arm 20 is tensioned.

[0072] In this example, the outer tendon tube 26A is hinged to the floater 12 via a pivot mount 22 and has an inwardly-projecting flange 58 at its lower end as shown in Figures 10 and 11. The flange 58 bears against an outwardly-projecting flange 60 of the inner tendon tube 26B to limit extension of the tendon tube 26 when the inner tendon tube 26B is fully extended from the outer tendon tube 26A. Thus, the flanges 58, 60 serve as cooperable interlocking limit formations of the tendon tube 26. When the jaws of the stopper 36 are closed around the mooring line 28, tensile loads in the tensioned tendon arm 20 are thereby transferred from the mooring line 28 into the interlocked outer and inner tendon tubes 26A, 26B.

[0073] Figures 12 to 14 show further variants of the invention. These variants are illustrated in the context of a telescopically-extensible tendon tube 26 like that of Figures 8 to 11 but could also be applied to the fixed-length tendon tube 26 of Figures 1 to 7. One such variant is the addition of temporary buoyancy 62 that may be pre-connected to the tendon tube 26 to support the lengthy tendon arm 20 during deployment. Another variant is that the connector 30 couples the mooring line 28 to a partially-embedded pre-installed pile chain 64 extending from a skirt of a suction pile that serves as an anchoring foundation 52.

[0074] Whilst Figure 13 shows an arrangement in which the mooring line 28 extends to an upper end of the tendon tube 26 to be tensioned topside as previously, Figure 14 shows an alternative arrangement in which a shoe or wheel 66 diverts the mooring line 28 through a side wall of the tendon tube 26 to be tensioned subsea. Thus, the mooring line 28 diverges from the tendon tube 26 at an intermediate position along the length of the tendon tube 26. A tensioner 34 is shown mounted to the tendon tube 26 for this purpose. The tensioner 34 can be mounted to the tendon tube 26 permanently or temporarily to be removed when tensioning is complete.

[0075] Finally, Figure 15 shows a completed offshore installation in which the support column 10 is stayed or guyed by tensioned tendon arms 20 in mutual opposition about the support column 10. The tension in the tendon arms 20 applies horizontal and vertical force components to the support column 10. The horizontal force component resists or compensates for tilting of the support column 10 away from the vertical; conversely, the vertical force component pulls the support column 10 down against the seabed 44.

[0076] The landed and tethered support column 10 is then ready for a wind turbine or other structure to be assembled atop the upper portion of the support column 10 that projects above the surface 38. The downward force exerted by the support column 10 on the seabed 44 can be adjusted by adjusting the ballasting or buoyancy of the support column 10 and / or by adjusting the tension in the tendon arms 20. Further adjustments of ballasting, buoyancy or tension can be made when a wind turbine or other structure has been erected atop the support column 10. Many variations are possible within the inventive concept. For example, the rigid upper part of the tendon arm that is pivotable relative to the floater could have a non-tubular structure, such as a channel section that serves as a guide for longitudinal movement of the mooring line. Similarly, if the rigid upper part of the tendon arm is extensible, it need not necessarily extend telescopically but more generally can have any arrangement in which one rigid section is movable longitudinally relative to another rigid section.

[0077] A tendon of the invention could be transported separately to and retrofitted to an existing preinstalled offshore structure such as a monopile or a spar buoy, and then deployed from the structure, anchored and tensioned in the various ways described above. This would allow pre-existing moorings to be upgraded, supplemented or replaced without having to move the structure.

[0078] A non-pivoted, vertical tendon of the invention could be advantageous in some cases, especially for retrofitting existing structures. Tendons of the invention can similarly be used for anchoring a tension-leg platform (TLP), as a vertical tendon, which entails no or little pivoting.

Claims

Claims1. A mooring tendon, comprising: an elongate rigid member comprising a pivot mount at a proximal end; a flexible or articulated mooring line that is deployable from the rigid member; and a stopper acting between the rigid member and the mooring line to lock the mooring line after deployment from the rigid member.

2. The tendon of Claim 1, wherein the mooring line extends along the rigid member before its deployment from the rigid member.

3. The tendon of Claim 1 or Claim 2, wherein the rigid member is tubular and the mooring line extends within a lumen of the rigid member.

4. The tendon of any preceding claim, wherein the mooring line extends along a full length of the rigid member from the proximal end of the rigid member to a distal end of the rigid member.

5. The tendon of any of Claims 1 to 3, wherein the mooring line diverges from the rigid member at an intermediate position along a length of the rigid member.

6. The tendon of any preceding claim, wherein the stopper is a ratchet that is operable after deployment of the mooring line to resist distal movement of the mooring line relative to the rigid member but to permit proximal movement of the mooring line relative to the rigid member.

7. The tendon of any preceding claim, wherein the rigid member is longitudinally extensible.

8. The tendon of Claim 7, wherein the rigid member comprises two or more sections, at least one of which is movable longitudinally relative to another of said sections.

9. The tendon of Claim 8, wherein the sections are in telescopic relation.

10. The tendon of Claim 8 or Claim 9, wherein the sections have limit formations that are cooperable to limit longitudinal extension of the rigid member.

11. The tendon of any preceding claim, further comprising a tensioner acting on a portion of the mooring line or on a messenger line attached to the mooring line, that portion of the mooring line or the messenger line protruding from the rigid member.

12. The tendon of Claim 11 when dependent on Claim 4, wherein the tensioner acts on a portion of the mooring line or on the messenger line protruding from a proximal end of the rigid member.

13. The tendon of Claim 11 when dependent on Claim 5, wherein the tensioner acts on a portion of the mooring line that is divergent from the rigid member.

14. The tendon of Claim 13, wherein the tensioner is disposed along a length of the rigid member.

15. A marine structure comprising at least one mooring tendon of any preceding claim.

16. The structure of Claim 15, wherein the rigid member of the at least one tendon is pivotable relative to the structure.

17. The structure of Claim 16, wherein a proximal end of the rigid member is pivotably attached to the structure via the pivot mount.

18. The structure of Claim 16 or Claim 17, wherein the rigid member is hinged about a pivot axis that is fixed relative to the structure.

19. The structure of any of Claims 15 to 18, wherein the structure is elongate and the at least one tendon extends along the structure.

20. The structure of Claim 19, wherein the at least one tendon extends along at least a majority of a length of the structure.

21. The structure of any of Claims 15 to 20, being a support column, a monopile or other bottom-fixed structure.

22. The structure of any of Claims 15 to 20, being a spar buoy, a platform or other floating structure.

23. A method of mooring a marine structure, the method comprising: deploying a mooring tendon from the structure by pivoting a rigid member of the mooring tendon from the structure; deploying a flexible or articulated mooring line of the mooring tendon from the rigid member; extending and coupling the mooring line to a subsea anchoring point; and then locking the mooring line against further extension from the rigid member.

24. The method of Claim 23, comprising tensioning the mooring tendon after coupling the mooring line to the anchoring point.

25. The method of Claim 24, comprising partially retracting the mooring line into the rigid member during tensioning.

26. The method of any of Claims 23 to 25, comprising pivoting the rigid member about an upper end and deploying the mooring line from a lower end of the rigid member.

27. The method of Claim 26, comprising releasing the lower end of the rigid member from the structure to enable deployment of the mooring tendon.

28. The method of any of Claims 23 to 27, comprising extending the rigid member after deploying the mooring tendon.

29. The method of any of Claims 23 to 28, further comprising a preliminary step of transporting the structure with the mooring tendon attached to the structure, before deploying the mooring tendon from the structure.

30. The method of Claim 29, further comprising a preliminary step of upending the structure with the mooring tendon attached to the structure, before deploying the mooring tendon from the structure.

31. The method of any of Claims 23 to 30, further comprising a preliminary step of landing the structure on a seabed before coupling the mooring line to the anchoring point.

32. The method of any of Claims 23 to 31, comprising supporting the deployed mooring tendon with added buoyancy.

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