Method for installing a tension leg platform based floating body - Patents.com

Abstract

A method for installing a TLP-based floating body at anchor points, the floating body comprising a central body and several buoyancy assemblies positioned around the central body in a horizontal plane, each buoyancy assembly being connected to the central body and connectable to one of the anchor points, the method comprising: attaching a mooring leg with at least one mooring line to each anchor point; for each anchor point, connecting a pull-down line between the buoyancy assembly and the anchor point; tensioning the pull-down line such that the floating body is lowered to an installation level below the operational level; for each anchor point, connecting the mooring leg with the buoyancy assembly in slack mode; and after connecting the mooring line with the buoyancy assembly, paying out the pull-down line such that the floating body is raised upwards from the installation level to the operational level where the mooring legs are tensioned.

Description

[Technical field]

[0001] The present invention relates to a method for installing a tension leg platform based floating body according to the preamble of claim 1. [Background technology]

[0002] In the prior art, the installation of Tension Leg Platform (TLP) based floating bodies is conventionally performed by ballasting the hull, connecting the tendons and deballasting to transfer the loads to the tendons. According to the prior art, the tensioning system is coupled with the tendons or mooring lines in such a way that the retraction operation uses the mooring lines.

[0003] Such a method is known from US Patent No. 7,452,162 which discloses a method and system for compensating for inherent hull instability during diving, providing motion arrest and aiding station keeping. The system includes tensioning devices mounted on the TLP, usually one per tendon. Each tensioning device is equipped with a pull-down line connected to a corresponding tendon. The hull of the TLP is submerged to a lock-off draft by tensioning the pull-down line connected to the top of the tensioner, or by a combination of tensioning the pull-down line and ballasting the hull. As the tensioners take up the pull-down line, the hull sinks, i.e. the draft increases.

[0004] This type of operation has the disadvantage that the TLP stabilizes during the installation of the mooring lines. In particular, floating bodies equipped with superstructures that protrude relatively high above the water surface can only be installed under (very) calm sea conditions to avoid the risk of instability of the floating body.

[0005] It is an object of the present invention to overcome or alleviate the problems of the prior art. Summary of the Invention

[0006] The object is achieved by a method for installing a tension leg platform based floating body at anchor points at sea using an installation vessel and a remotely operated underwater vehicle (ROV), the floating body comprising a central body and at least three buoyancy assemblies positioned around the central body in a horizontal plane, each of the buoyancy assemblies being at least connected to the central body and adapted to be connected to an associated one of the anchor points; The method comprises: attaching a mooring leg with at least one mooring line to each anchor point; for each anchor point, connecting a pull-down line between each buoyancy assembly and its associated anchor point; tensioning the pull-down line such that the floating object is lowered relative to the water surface to an installed lifting level below the operational lifting level; for each anchor point, connecting the mooring leg with an associated buoyancy assembly while the mooring leg is in a slack mode; and after connecting the mooring line with the buoyancy assembly, paying out the pull-down line to transfer a load to the mooring leg such that the floating object is raised upward from the installed lifting level to the operational lifting level where the mooring leg is in a tensioned mode.

[0007] The lowering operation from the seabed anchor point below the floater will result in better stability performance. Moreover, the lowering operation of the TLP is decoupled from the installation of the mooring legs, which simplifies the operation of connecting the mooring legs to the floating body.

[0008] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings which are schematic in nature and therefore not necessarily drawn to scale, in which identical or similar elements are indicated by the same reference symbols. [Brief description of the drawings]

[0009] [Figure 1] 2 shows a schematic top view of an anchoring location on the seabed during an initial method step according to an embodiment of the present invention; [Diagram 2] FIG. 1 shows a schematic top view of anchor points with a hydraulic tensioner device installed on each anchor point. [Diagram 3] 1 shows a schematic top view of an anchor point with tensioning lines attached to each tensioner device. [Figure 4] 1 shows a schematic side view of a floating object. [Diagram 5] 13A and 13B show schematic steps for connecting the tensioning line and the auxiliary line. [Figure 6] 13 shows diagrammatically the steps of tensioning the connected tensioning and auxiliary lines and lowering the floating body to a mounting level below sea level; [Figure 7] FIG. 13 shows a perspective view of tensioning the connected tensioning and auxiliary lines and lowering the floating body to an installation level below sea level according to a further embodiment. [Figure 8] 13A-13C show diagrammatically the steps of connecting a mooring leg in slack mode to a floating body; [Figure 9] 13 shows a schematic diagram of a step of tensioning a mooring leg; [Figure 10] 1 shows a schematic diagram of a floating body with tensioned mooring legs between the floating body and anchor points after installation according to an embodiment of the present invention; [Figure 11] 13 shows a perspective view of a floating body after installation with tensioned mooring legs between the floating body and anchor points according to a further embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The method described below relates to the installation of a Tension Leg Platform (TLP) based floating body in a lake or ocean location.

[0011] Such a floating body is typically a floating structure comprising a central body and at least three buoyancy assemblies positioned around the central body in a horizontal plane, each of the buoyancy assemblies being at least connected to the central body.

[0012] The present invention will be described with reference to a floating wind turbine as an example of a TLP-based floating body, it will be appreciated that the invention is not limited to this example and may also relate to other types of TLP-based floating bodies.

[0013] The method relates to the installation of floating bodies such as floating wind turbines (or offshore wind turbines) that are to be installed offshore at mooring locations having water depths that do not allow the wind turbine to be mounted directly on the seabed by masts.

[0014] In such a situation with floating wind turbines, the tower of the wind turbine is located on a floating structure.

[0015] FIG. 1 shows a schematic top view of an anchoring location on the seabed during an initial method step according to an embodiment of the invention.

[0016] On the seabed, a mooring location L is determined where the TLP floater will be installed.

[0017] According to the invention, an anchor point 2 is installed on or in the seabed of the mooring location. The anchor point may be selected from an anchor, an anchoring pile and an anchoring body.

[0018] The number of anchor points typically corresponds to the number of buoyancy assemblies that are part of the floating wind turbine, for example if the floating wind turbine comprises three buoyancy assemblies then three anchor points will be installed.

[0019] The buoyant assemblies are each provided with one or more mooring line connectors (depending on the number of mooring lines between the anchor point and the associated buoyant assembly).

[0020] The installation of anchor points is carried out by installation vessels or other facilities equipped for installing anchor points.

[0021] In a next step, on each anchor point 2, a mooring leg 4 with at least one mooring line is attached to one end of the respective anchor point. Each of the mooring legs is laid on the seabed with a recovery loop so that two or more of the mooring legs 4 do not cross or become entangled on the seabed. Optionally, the mooring legs 4 are attached to the anchor point by a line connector.

[0022] In a further embodiment, the TLP body is designed with two mooring lines in each mooring leg 4 between each anchor point 2 and its associated buoyancy assembly. Again, each mooring line is laid on the seabed with a recovery loop to prevent two or more of the mooring legs 4 from crossing on the seabed.

[0023] FIG. 2 shows a schematic top view of anchor points with a hydraulic tensioner device installed on each anchor point.

[0024] The next step in the method is to install a subsea tensioner 6 on each anchor point. Each subsea tensioner is preferably pre-oriented with an upward tensioning direction to match the slope of the mooring leg or mooring line.

[0025] In addition, a hydraulic support system 8 is installed on the subsea tensioner 6 for driving the subsea tensioner.

[0026] The hydraulic support system 8 comprises hydraulic hoses configured to connect to a hydraulic pump located, for example, on board the installation vessel or another vessel.

[0027] Installation is performed with assistance from a remotely operated underwater vehicle (ROV), as is known in the art.

[0028] The tensioner uses the tension line to generate tension for the tensioning operation, which is illustrated in more detail with reference to FIG.

[0029] In an alternative embodiment, tensioning is provided from the floating wind turbine, in which case the tensioner and hydraulic support system are mounted on the floating wind turbine, or more specifically on the buoyancy assembly.

[0030] FIG. 3 shows a schematic top view of an anchor point with tensioning lines attached to each tensioner device.

[0031] In a subsequent step, each tensioner has a tensioning line 10 coupled to one end. The free end of each tensioning line 10 is laid on the seabed such that two or more of the tensioning lines 10 do not cross over, and no crossings should occur with the mooring legs 4 laid over the seabed.

[0032] At each anchor point, a mooring leg 4 and a tensioning line 10 are connected as shown in Figure 3. The free ends of the mooring legs and tensioning lines are laid on the seabed. The lines are positioned so that no tangling occurs when one or more of the free ends are raised at a later stage during installation.

[0033] FIG. 4 shows a schematic side view of a floating wind turbine.

[0034] In an embodiment of the invention, the floating body is a floating wind turbine as described above: a floating structure 12 comprising a central body 14 and at least three buoyancy assemblies 16 positioned around the central body in a horizontal plane, each of the buoyancy assemblies 16 being at least connected to the central body 14 by beams 18.

[0035] According to the method, an auxiliary line 20 is installed on each of the buoyancy assemblies 16 with one end connected to a connector (not shown) on the respective buoyancy assembly 16. The other end may be secured on top of the buoyancy module during towing operations.

[0036] In a further embodiment, the method comprises disposing a passive shock absorber (not shown) between the connector on the buoyancy assembly and one end of the auxiliary line. This option provides that the shock absorber can prevent high dynamic peak loads during the initial stages of the tensioning operation as described below with reference to FIG.

[0037] During installation, when the floating wind turbine 12 arrives at the mooring location L, the method comprises releasing the auxiliary lines 20 such that the auxiliary lines 20 hang freely from the floating wind turbine 12. The releasing step may be performed remotely.

[0038] FIG. 5 shows a schematic diagram of the steps for connecting the tensioning line and the auxiliary line.

[0039] During this step, the free end of the tensioned line 10 connected to one of the anchor points is raised by the installation vessel and brought close to the free hanging end of the auxiliary line 20 connected to the associated buoyancy assembly. With the assistance of the ROV, the free end of the tensioned line is then mechanically coupled to the free hanging end of the auxiliary line to form a pull-down line 22 between the anchor point 2 and the associated buoyancy assembly 16. The installation vessel's crane is used to raise the free end of the tensioned line from the seabed and bring it close to the free hanging end of the auxiliary line. The ROV is then used to complete the coupling.

[0040] This step is repeated for each of the anchor points.

[0041] During this step, the formed down line 22 is placed in a slack mode, ie, not tensioned, to avoid destabilizing the floating wind turbine 12 .

[0042] In a further embodiment, the step of joining the free end of the tensioning line 10 and the free end of the auxiliary line 20 comprises mechanically connecting the free end of the auxiliary line to a first end of an impact stabilizer element (not shown) and mechanically connecting a second end of the impact stabilizer element to the free end of the associated tensioning line.

[0043] According to this further embodiment, the down-haul line 22 is thus provided with an impact stabiliser element intermediate the tensioning line 10 and the auxiliary line 20. Advantageously, the impact stabiliser element provides that high dynamic peak loads are prevented during the initial stages of tensioning of the down-haul line, when the floating wind turbine has not yet been stabilized by the down-haul line.

[0044] FIG. 6 shows diagrammatically the steps of tensioning the connected tensioning lines 10 and auxiliary lines 20 and lowering the floating wind turbine 12 to a mounting lifting level 24 below the water surface 26 .

[0045] After each downhaul line 22 is connected at one end to its associated anchor point 2 and at the other end to the associated buoyancy assembly 16 of the floating wind turbine 12, in a subsequent step the downhaul lines are simultaneously tensioned by subsea tensioners 6. The tensioners 6 are arranged to control the tension in each of the downhaul lines 22 to substantially keep the floating wind turbine in balance during this operation. To this end, the tensioners 6 are hydraulic devices actuated / driven by a synchronized hydraulic system.

[0046] During the tensioning operation, the floating wind turbine 12 is lowered to an installation lift level 24 relative to the water surface 26. For example, the floating wind turbine is lowered to about 35 meters.

[0047] By lowering the floating wind turbine 12 to the water surface 26, the floating wind turbine 12 is stabilized and placed in a "storm safe" condition well before the mooring legs 4 are attached between the anchor points 2 and the buoyancy assemblies 16.

[0048] FIG. 7 shows a perspective view of tensioning the connected tensioning and auxiliary lines and lowering the floating body to a installation level below sea level according to a further embodiment.

[0049] According to this embodiment, separate rigging lines 30 are provided between one downhaul line 22 attached to one anchor point 2 and two or more other downhaul lines 22 each attached to another anchor point 2 such that a first series of horizontal links by the rigging lines are created between the downhaul lines.

[0050] The attachment of the rigging lines 30 to the lowering line can be performed on the tensioning lines 10 as part of the lowering line during the stage where the tensioning lines are laid on the seabed. Alternatively, the rigging lines can be connected to the auxiliary lines 20 as part of the lowering line before or during the stage where the auxiliary lines are in a slack mode hanging freely below the floating body. For example, the rigging lines 30 can be installed on the auxiliary lines when the floating body sets sail or when the auxiliary lines are lowered from the floating body.

[0051] The rigging lines 30 are positioned at substantially the same first predetermined level, and a first series of horizontal links between the downhaul lines provides increased stiffness to the downhaul mooring system (i.e., the arrangement of the downhaul lines between the anchor points and the buoyancy assembly).

[0052] Typically, the rigging lines are configured in lengths such that, while tensioning the down lines, the rigging lines are tensioned between each pair of down lines to which they are connected. This causes the rigging lines to exert a horizontal tensile force Fh (indicated by the horizontal arrows) on the down lines which causes the down lines to be pulled somewhat horizontally towards each other when in tension mode (force Fv, indicated by the vertical arrows).

[0053] In an embodiment, the rigging lines in the first series of horizontal links are arranged to form a simple polygon when viewed in horizontal projection. Taking the arrangement of Figure 7 as an example, there are three anchor points and associated down lines, so the connections of the rigging lines form a triangle with the down lines (or anchor points) at the corners of the polygon.

[0054] According to the present invention, there is no limit to the number of anchor points, mooring legs and down lines. For example, with four anchor points or down lines, the rigging lines would form a rectangle.

[0055] In further embodiments, the rigging lines may be provided at several predetermined levels to form a plurality of horizontal links between each downhaul line attached to one anchor point and at least two other downhaul lines each attached to a respective other anchor point, each of the plurality of horizontal links being formed above or below the first predetermined level, similar to the first horizontal link described above.

[0056] The rigging lines are made from ropes or cables of any suitable material known to those skilled in the art, such as steel or synthetic, and are sized to generate and withstand the horizontal forces required by the rigging line construction.

[0057] The rigging line arrangement connecting the haul lines horizontally has the effect of relatively increasing the stiffness of the overall connection between the body and the anchor points during the "hauling" phase, and extreme loads on each of the haul lines are damped by the rigging line arrangement.

[0058] The rigging line arrangement offers the advantage of damping the mooring loads on the down lines in a robust manner by not using any mechanically active components, thus providing a passive alternative to active systems such as hydraulic or mechanical load dampers.

[0059] FIG. 8 shows diagrammatically the steps of connecting the mooring leg 4 in slack mode to the floating wind turbine 12.

[0060] The mooring legs 4 (each comprising at least one mooring line) are then each connected between a respective anchor point 2 and an associated buoyancy assembly 16. In an illustrative embodiment, a pair of mooring lines are connected between each anchor point 2 and an associated buoyancy assembly 16. It will be appreciated that according to the present invention, the number of mooring lines between each anchor point 2 and an associated buoyancy assembly 16 is at least one, depending on design and installation requirements.

[0061] The free end of each mooring leg 4 is then raised from the seabed by the installation vessel's crane with the assistance of the ROV and brought close to the mooring line connector on the buoyancy assembly 16 associated with the mooring leg 4. The free end is then mechanically coupled to the mooring line connector of the associated buoyancy assembly. This step is repeated for each of the mooring lines on each buoyancy assembly.

[0062] During this step, the mooring legs are arranged to be in slack mode, ie, not tensioned.

[0063] FIG. 9 shows diagrammatically the step of tensioning the mooring leg 4 .

[0064] After the mooring leg 4 has been positioned between the anchor point 2 and the buoyancy assembly 16, the method comprises tensioning the mooring lines.

[0065] During this operation, the hauling lines 22 are simultaneously paid out, which causes the floating wind turbine 12 to rise towards the water surface 26. At the same time, the mooring legs 4 are pulled tighter by the rising floating wind turbine. In this way, the load on the hauling lines 22 is transferred to the mooring legs 4. During the payout operation, the floating wind turbine 12 is allowed to rise from the installation lifting level 24 to an operating lifting level 28 below the water surface. The hauling lines 22 are paid out until they are fully slack and the load is fully on the mooring legs 4.

[0066] In an illustrative, non-limiting example, the operational lift level 28 is about 20 to about 40 meters below the water surface. The installation lift level 24 is about 5 to about 10 meters below the operational lift level. Thus, the floating wind turbine 12 will be elevated about 5 to 10 meters during tensioning of the mooring legs.

[0067] Upon reaching the operational lift level 28 , the floating wind turbine 12 is fully moored and stabilized by the mooring legs 4 .

[0068] FIG. 10 shows a schematic of an installed floating wind turbine with tensioned mooring legs 4 between the floating wind turbine 12 and an anchor point 2 according to an embodiment of the present invention.

[0069] After installation, the down line 22, rigging line 30 (if previously installed), tensioner 6, and hydraulic system 8 can be retrieved and reused during the installation of the next floating wind turbine.

[0070] In an embodiment, the method includes, after the mooring leg 4 is installed and brought into tensioning mode: - disconnecting each of the down-drawing lines; removing the tensioner device; retrieving the tensioner device by the installation vessel; can be executed.

[0071] According to a further embodiment, the disconnection of each down line comprises: - disconnecting the auxiliary line from the tensioning line; - disconnecting the auxiliary line from the buoyancy assembly; disconnecting the tensioning line from the tensioner device; - Retrieving at least one of the tensioning lines and / or the auxiliary lines by the installation vessel; Prepare.

[0072] FIG. 11 shows a perspective view of an installed floating body with tensioned mooring legs between the floating body and anchor points according to a further embodiment of the invention.

[0073] According to this embodiment, separate rigging lines 32 are provided between one mooring leg attached to one anchor point and two or more other mooring legs each attached to another anchor point such that a third series of horizontal links by the rigging lines are created between the mooring lines. The placement of the rigging lines 32 on the mooring legs is preferably performed during the phase in which the mooring legs are laid on the seabed. Optionally, the rigging lines can be connected to the mooring legs during the phase in which they are in slack mode while connected to the buoyancy assemblies.

[0074] The rigging lines 32 are positioned at substantially the same predetermined level such that a third series of horizontal links between the mooring legs increases the stiffness of the mooring system (i.e. the arrangement of the mooring legs between the anchor point and the buoyancy assembly).

[0075] Typically, the rigging lines are configured in such a length that, during payout of the down line, the rigging lines are tensioned between the mooring legs to which they are connected, which causes them to exert a horizontal tensile force Fh on the mooring lines, which causes the mooring lines to be pulled somewhat horizontally towards each other when in tension mode (shown by force Fv).

[0076] In an embodiment, the rigging lines in the third series of horizontal links are arranged to form a simple polygon when viewed in horizontal projection. In the example of Figure 11, there are three anchor points and associated mooring lines so that the rigging line connections form a triangle with the mooring legs at its corners.

[0077] There is no limit to the number of anchor points or mooring legs, for example six anchor points would form a hexagon of rigging lines.

[0078] In a further embodiment, separate rigging lines may be provided at several predetermined levels below the water surface to form a series of multiple horizontal links between each mooring leg attached to one anchor point and each of at least two other mooring legs attached to respective other anchor points, each of the multiple horizontal links being formed similarly to the first horizontal link described above.

[0079] The rigging lines are made from ropes or cables of any suitable material known to those skilled in the art, such as steel or synthetic.

[0080] The horizontal rigging line arrangement connecting the mooring legs has the effect of relatively increasing the stiffness of the overall connection between the body and the anchor point, and extreme loads on each of the mooring legs are damped by the rigging line arrangement.

[0081] The rigging line arrangement in this embodiment, as opposed to an active system, can be permanently integrated into the mooring system if desired, and may be more economical than active systems such as a complex mechanical heave compensation system.

[0082] In an alternative embodiment of the method for installing a TLP-based floating wind turbine, the method involves the tensioning lines functioning as pull-down lines without the need for auxiliary lines. In this embodiment, the tensioning lines are installed on the anchor points in the same manner as described above. As the tensioning lines are pulled up from the seabed, the free end of each tensioning line is mechanically connected directly to the associated buoyancy assembly while the tensioning line is slack. Each tensioning line now functions as a pull-down line as described above. The tensioning operation for the tensioning lines is the same as described above with reference to FIG. 6.

[0083] It should further be noted that in yet a further alternative embodiment, the tensioner device can be installed on the floating wind turbine instead of the anchor point, and the hydraulic system in this embodiment is installed on the floating wind turbine.

[0084] The present invention has the following advantages: The installation of the floating object can be carried out without any movement of personnel or equipment on the floater, thus improving the safety and maneuverability of the hook-up (higher sea states can be assumed, which substantially benefits the whole installation scheme); -Subsea tensioners can be fitted with shock absorbers to increase maneuverability and minimise dynamic peak loads; The hook-up is performed without any equipment required to be installed on the floating structure during onshore integration (thus reducing the impact on the fabrication critical path) and therefore without any equipment being removed after hook-up, which improves safety for personnel and material transfer and reduces the collision risk between the installation vessel and the floating structure; - not installing heavy equipment and accessories on the floating object is favorable for stability during towing to the site; - the connection of the permanent mooring legs is made without tensioning the legs, allowing simpler connectors to be planned and therefore it is envisaged to design and use connectors without a pull-through requirement; -Once hook-up is completed, the installation equipment is retrieved by the installation vessel and deployed on the next set of anchorage points, which is an offshore activity that can be planned during times when the installation vessel is not occupied, thus improving overall project planning as overburdened work onshore (installation preparation work) is transferred to the now available installation vessel.

[0085] The present invention has been described with reference to several embodiments. Obvious modifications and alterations will occur to others upon reading and understanding the foregoing detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims.

[0086] [Reference sign] Anchor Point 2 Mooring Leg 4 Location L Subsea tensioner 6 Hydraulic Support System 8 Tensioning Line 10 Floating Wind Turbines 12 Central body 14 Buoyancy Assembly 16 beam 18 Auxiliary Line 20 Reduction Line 22 Installation lift level 24 Sea surface (water surface) 26 Operational lift level: 28 Forces Fh, Fv Rigging lines 30, 32 The invention as originally claimed in the present application is set forth below. [1] A method for installing a tension leg platform-based floating structure at multiple anchor points at sea using an installation vessel and a remotely operated underwater vehicle (ROV), comprising: the floating body comprises a central body and at least three buoyant assemblies positioned around the central body in a horizontal plane, each of the buoyant assemblies being at least connected to the central body and adapted to be connected to an associated one of the anchor points; The method comprises: - attaching a mooring leg with at least one mooring line to each anchor point; for each anchor point, connecting a pull-down line between each buoyancy assembly and its associated anchor point; - tensioning the lowering line so that the floating object is lowered relative to the water surface to an installation lifting level below an operational lifting level; for each anchor point, connecting said mooring legs with said associated buoyancy assembly while said mooring legs are in slack mode; after connecting the mooring lines with the buoyancy assemblies, paying out the hauling lines to transfer a load to the mooring legs such that the floating body is raised upward from the installed lifting level to the operational lifting level where the mooring legs are in a tensioned mode; A method comprising: [2] The method of [1], further comprising adding at least a first series of links between the down lines by connecting separate rigging lines between the down line attached to one of the anchor points and two or more selected down lines from the other down lines each attached to another of the anchor points, so that each down line is connected to a respective one of two or more of the other down lines by a respective separate rigging line. [3] The method described in [2], wherein in the tensioning mode of the lowering line, the rigging lines connected by adding the first series of links are substantially horizontal at a first predetermined level between the anchor point and the floating body. [4] The method of [2] or [3], wherein, in a horizontal projection, the rigging line in the first series of links forms a simple polygon with a number of polygonal sides equal to the number of anchor points when the haul line is in the tensioned mode. [5] The method according to any one of [2] to [4], further comprising adding at least a second series of links between the down line attached to one of the anchor points and two or more down lines each attached to another of the anchor points, so that in the second series of links, each down line is connected to two or more of the other down lines by a respective separate rigging line, the second series of links being at a second predetermined level below or above the first predetermined level. [6] The method according to any one of [1] to [5], further comprising adding at least a third series of links between the mooring legs by connecting separate rigging lines between a mooring leg attached to one of the anchor points and two or more selected mooring legs from other mooring legs each attached to another of the anchor points, so that each mooring leg is connected to two or more of the remaining mooring legs by a respective rigging line. [7] The step of tensioning the down-line may be performed by a dedicated tensioner device installed on each anchor point; or A dedicated tensioner device installed on the floating structure The method according to any one of [1] to [6], which is performed for each pull-down line in any one of the following ways. [8] Connecting the downdraft lines between each buoyancy assembly and its associated anchor point includes: Connecting the ends of the tensioning lines to a dedicated tensioner device; positioning a free end of each tensioning line on the seabed; for each buoyancy assembly, connecting an end of an auxiliary line to said buoyancy assembly; allowing the free ends of the auxiliary lines to hang freely for each buoyancy assembly; connecting a free end of the auxiliary line from each of the buoyant assemblies with a free end of a tensioning line of the anchor point associated with the respective buoyant assembly to form the pull-down line between each buoyant assembly and its anchor point; The method according to any one of [1] to [7], comprising: [9] Connecting the downdraft lines between each buoyancy assembly and its associated anchor point includes: connecting an end of a tensioning line to said anchor point; positioning a free end of each tensioning line on the seabed; for each buoyancy assembly, connecting an end of an auxiliary line to a dedicated tensioner device on said floating body; allowing the free ends of the auxiliary lines to hang freely for each buoyancy assembly; connecting a free end of the auxiliary line from each of the buoyant assemblies with a free end of a tensioning line of the anchor point associated with the respective buoyant assembly to form the pull-down line between each buoyant assembly and its anchor point; The method according to any one of [1] to [7], comprising:

[10] The adding of at least the first series of links may be performed on the tensioning line as part of the downhaul line while the tensioning line is being laid on the seabed; or Or the method of claim 8 or 9, wherein adding the at least a first series of links is performed on an auxiliary line as part of the downhaul line while the auxiliary line is hanging freely from the floating body.

[11] Connecting the free end of the auxiliary line to the free end of the associated tensioning line comprises: connecting a free end of the auxiliary line with a first end of an impact stabilizer element; connecting a second end of the impact stabilizer element with a free end of the associated tensioning line; The method according to [8] or [9], comprising:

[12] Connecting the free end of the auxiliary line to the free end of the associated tensioning line comprises: Raising the free end of the tensioned line from the seabed by the installation vessel; and using an ROV to carry the free end to the free end of the auxiliary line and couple the free end; The method according to [8] or [9], comprising:

[13] The step of attaching the mooring legs to each anchor point comprises: Positioning the free end of the mooring leg on the seabed. and wherein connecting the mooring legs with the associated buoyancy assembly while the mooring legs are in a slack mode further comprises: Raising the free end of the mooring leg from the seabed by the installation vessel; - carrying said free end to said associated buoyant assembly; attaching a free end of the mooring leg onto the associated buoyancy assembly with a mooring line connector; The method according to any one of [1] to

[12] , comprising:

[14] A method according to any one of [1] to

[13] , wherein the mooring legs consist of two or more mooring lines between each anchor point and an associated buoyancy assembly.

[15] The method according to any one of [1] to

[14] , wherein the central body comprises a tower having at least one radial connection element to each buoyant assembly, the buoyant assemblies being interconnected by lateral connection elements, and the wind turbine is mounted on top of the tower.

Claims

1. 1. A method for installing a tension leg platform based floating body at multiple anchor points at sea using an installation vessel and a remotely operated underwater vehicle (ROV), comprising: the floating body comprises a central body and at least three buoyant assemblies positioned around the central body in a horizontal plane, each of the buoyant assemblies being at least connected to the central body and adapted to be connected to an associated one of the anchor points; The method comprises: - Attaching a mooring leg with at least one mooring line to each anchor point; - for each anchor point, connecting a pull-down line between each buoyancy assembly and its associated anchor point; - tensioning the lowering line so that the floating object is lowered relative to the water surface to an installation lifting level below an operational lifting level; - for each anchor point, connecting said mooring leg with said associated buoyancy assembly while said mooring leg is in slack mode; after connecting the mooring lines with the buoyancy assembly, paying out the haul lines to transfer a load to the mooring legs so that the floating body is raised upward from the installed lifting level to the operational lifting level where the mooring legs are in tension mode; A method comprising:

2. 2. The method of claim 1, further comprising adding at least a first series of links between the down lines by connecting separate rigging lines between a down line attached to one of the anchor points and two or more selected down lines from other down lines each attached to another of the anchor points, so that each down line is connected to a respective one of two or more of the other down lines by a respective separate rigging line.

3. 3. The method of claim 2, wherein in the tensioned mode of the down line, the rigging lines connected by adding the first series of links are horizontal at a first predetermined level between the anchor point and the floating body.

4. 4. The method of claim 2 or 3, wherein in a horizontal projection, the rigging lines in the first series of links form a simple polygon having a number of polygonal sides equal to a number of anchor points when the haul line is in the tensioned mode.

5. 4. The method of claim 3, further comprising adding at least a second series of links between the down line attached to one of the anchor points and the two or more down lines each attached to another of the anchor points, such that in the second series of links, each down line is connected to two or more of the other down lines by a respective separate rigging line, the second series of links being at a second predetermined level below or above the first predetermined level.

6. 6. The method of claim 5, further comprising adding at least a third series of links between the mooring legs by connecting separate rigging lines between two or more mooring legs selected from a mooring leg attached to one of the anchor points and other mooring legs each attached to another of the anchor points, so that each mooring leg is connected to two or more of the remaining mooring legs by a respective rigging line.

7. The step of tensioning the down line may be by a dedicated tensioner device installed on each anchor point, or A dedicated tensioner device installed on the floating structure The method according to any one of claims 2 to 6, wherein the method is carried out for each down-draw line in any of the following steps:

8. connecting the pull down lines between each buoyancy assembly and its associated anchor point; - connecting the ends of tensioning lines to the dedicated tensioner devices installed on the associated anchor points; - positioning a free end of each tensioning line on the seabed; - for each buoyant assembly, connecting an end of an auxiliary line to said buoyant assembly; - for each buoyant assembly, allowing a free end of said auxiliary line to hang freely; - connecting a free end of the auxiliary line from each of the buoyancy assemblies with a free end of a tensioning line of the anchor point associated with the respective buoyancy assembly to form the pull-down line between each buoyancy assembly and its anchor point.

9. connecting the pull down lines between each buoyancy assembly and its associated anchor point; - connecting an end of a tensioning line to said anchor point; - positioning a free end of each tensioning line on the seabed; - for each buoyancy assembly, connecting an end of an auxiliary line to a dedicated tensioner device on said floating body; - allowing the free ends of the auxiliary lines to hang freely for each buoyancy assembly; - connecting a free end of the auxiliary line from each of the buoyancy assemblies with a free end of a tensioning line of the anchor point associated with the respective buoyancy assembly to form the pull-down line between each buoyancy assembly and its anchor point.

10. adding at least a first series of links is performed on the tensioned line as part of the down line while the tensioned line is being laid on the seabed; or 10. The method of claim 8 or 9, wherein either the adding of the at least a first series of links is performed on an auxiliary line as part of the downhaul line while the auxiliary line is hanging freely from the floating body.

11. Connecting the free end of the auxiliary line with the free end of the associated tensioning line includes: connecting the free end of the auxiliary line with a first end of an impact stabilizer element; - connecting a second end of the impact stabilizer element with a free end of the associated tensioning line; The method according to claim 8 or 9, comprising:

12. Connecting the free end of the auxiliary line with the free end of the associated tensioning line comprises: raising the free end of the tensioning line from the seabed by the installation vessel; using an ROV to carry the free end to the free end of the auxiliary line and join the free ends; The method according to claim 8 or 9, comprising:

13. Attaching the mooring legs to each anchor point comprises: - positioning the free end of the mooring leg on the seabed; and wherein connecting the mooring legs with the associated buoyancy assembly while the mooring legs are in a slack mode further comprises: - Raising the free end of the mooring leg from the seabed by the installation vessel; - carrying said free end to said associated buoyancy assembly; - attaching the free end of the mooring leg onto the associated buoyancy assembly with a mooring line connector; The method according to any one of claims 1 to 12, comprising:

14. A method according to any one of claims 1 to 13, wherein between each anchor point and an associated buoyancy assembly, the mooring leg consists of two or more mooring lines.

15. 15. The method according to any one of claims 1 to 14, wherein the central body comprises a tower with at least one radial connection element to each buoyant assembly, said buoyant assemblies being interconnected by lateral connection elements, and the wind turbine is mounted on top of said tower.

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