GB2598616A

GB2598616A - Floating body and mooring system

Info

Publication number: GB2598616A
Application number: GB2013964.8A
Authority: GB
Inventors: Borgen Eystein
Original assignee: Riggmor As
Current assignee: Riggmor As
Priority date: 2020-09-04
Filing date: 2020-09-04
Publication date: 2022-03-09
Also published as: WO2022049263A1; GB202013964D0

Abstract

A mooring system 100 for a floating body 1 comprises at least one arm member 2, each arm member configured to be buoyant in water and to be rotatably attached to the floating body at or near a first end of the arm member via a rotatable joint 3. At least one mooring line 4 is configured to attach to the arm member at a longitudinal position spaced apart from said first end. At least one anchor member 5 is provided and the at least one mooring line is configured to connect the arm member to the at least one anchor member. A method of mooring a floating body using the mooring system is also disclosed. A floating body moored in a body of water by a taught mooring line which does not restrict the heave motion of the floating body is disclosed. A mooring system comprising a power export cable (27, Fig 7) and a mechanical swivel (26, Fig 7) is also disclosed.

Description

FLOATING BODY AND MOORING SYSTEM

Technical Field

The present invention relates to a mooring system for a floating body. In particular, the present invention may relate to a mooring system for a floating body that supports a wind turbine.

Backetround There are two main systems for mooring of a floating body, such as a boat: (i) a catenary system, with or without buoys, and 00 taut mooring line systems, normally using tension legs.

Most often a floating body that needs to be positioned at a geographical point at sea, with deviations within acceptable limits, is moored using a catenary mooring system. Through gravity, the catenary moorings take the shape of a free-hanging line between the floating body and the sea-bed, and hang horizontally at the seabed, such that the seabed anchors are subjected to horizontal drag forces. The catenary moorings provide a restoring force on the floating body predominantly through the weight of the catenary moorings. Methods are known for designing such catenary moorings to satisfy the acceptable limits. However, in deep waters the catenary moorings will contribute a very considerable weight. Additionally, the mooring footprint at the sea floor is very large, and increases with increasing depth.

A known system of tension leg mooring, comprising legs formed from multiple steel tubular steel members, called tendons, is used for a variety of applications. The legs are tensioned via buoyancy in the floating body. The tension in the legs enables the mooring system to position a floating body such that the body can resist horizontal forces, as well as enabling control of the yaw of the floating body. For example, a Tension Leg Platform (TLP) system is used in the oil production industry. When designed correctly, these systems function well. However, the vertical forces in the taut legs may be very large.

One way of designing top end connections of the taut lines is to build horizontal cantilevers stretching from a floating body of a spar type, or a floating body having a semi-submersible platform. These cantilevers are then passive members that have to be strong enough to withstand the large vertical forces from the taut lines.

Tension leg platforms with horizontal rigid mooring arms connected to vertical taut mooring lines have also been proposed as foundations for floating wind turbines, for example in US20150259044A1. Such tension leg platforms have to be designed to withstand the thrust force from the wind turbine and the resulting extremely large bending and overturning moments, which are transferred to the floating foundation.

Such a system limits the surge motion of the platform, and enables almost zero pitching, which is an advantage in reducing the motions and accelerations at the top of the wind turbine tower. In addition, the yaw restoring moment is well accommodated due to the high pre-tension in the vertical mooring lines. However, the large overturning moment has to be resolved by very large vertical forces in the taut mooring system in order to maintain the stability of the system. Therefore, such systems will require a mooring and anchor system 5-10 times the size and cost of a conventional catenary mooring system, as used for spar type or semi-submersible type floating wind turbines.

However, even if the catenary mooring systems are less costly than the tension leg platforms with horizontal rigid arms, the catenary type systems are still costly. In addition, a catenary type mooring system has a large footprint and is therefore often in potential conflict with fisheries or in-field pipelines and subsea structures in the same area.

Another type of tension leg system is proposed in W02004097217A1. This is a single leg tension leg platform with a downwind oriented wind turbine mounted on top of a stayed tower, where the large overturning moment from the wind turbine thrust force is resisted by the hydrostatic stability of a spar type floating foundation. The stays are introduced to resist the large bending moments and to reduce the tower and substructure fatigue loads. To avoid the rotor blades clashing into the up-wind mounted stays, the wind turbine is rigidly fixed to the tower which means there is no yaw bearing between the wind turbine and the tower. Instead, the entire floating tower weathervanes (i.e. heads into the direction of the wind) around a swivel at the bottom of the floating tower. However, with only one central tension leg this system has limited yaw stability, and the yaw angle relative to the wind direction needs to be controlled by active individual pitching of the rotor blades, or by using subsea yaw motors at the swivel.

Different single line weathervaning mooring systems have also been proposed for floating production storage and offloading vessels (FPS0s). One solution is to employ a horizontal stiff member in a mooring system by using a yoke. The yoke connects a floating body, for instance a FPSO, to a fixed point for offloading purposes. Alternatively, the other end of the yoke may be connected to another floating body, as for instance a buoy, also used for offloading purposes. Such a design is often more or less dependent on a triaxial connection at the fixed point. The same applies if the connection point is to a floating buoy. The three axes are: a vertical axis, a horizontal axis perpendicular to the line between the two bodies, and a horizontal axis in the line between the two said bodies. At the end of the yoke, where there is a ship in the example used here, there is a hinge with rotation about a horizontal axis. At this end, the yoke is typically completed with two members in a v-shape where the apex of the v-shape is at the triaxial point, such that there is a spread between the members at the ship end. The ship may be permanently connected at the bow end in the described manner, in which case there is offloading at the rear end. An equally often used application of the described system is where the ship is connected only during offloading, and is then disconnected for transporting the product (e.g. oil or gas product) to shore. In the disconnected state, the yoke may be supported by its own buoyancy, by a buoyant member connected to the yoke, or may be rested on the seafloor in shallow water cases.

US4475802 proposes a rigid, vertical anchor pipe hinged against a single anchor and articulately connected to the FPSO via a rocker arm above the surface. The rocker arm is hinged at each end and a sub-surface buoyancy tank is connected to the rocker arm via a truss structure. This system has a limited ability to accommodate large horizontal displacements of the moored body, due to the near 90 degree angle between the yoke and the mooring line/pipe. A separate buoyancy tank has to be provided near the surface and connected via an additional truss structure to the yoke above the surface, which complicates the fabrication of the system. This system is also not able to control the yawing of the floating body, for instance if the current has another direction to the wind direction, which is essential for a floating wind turbine. Having the buoyancy of the system close to the surface will also result in large wave forces on the buoyancy tank.

A similar solution is proposed in US4029039 with the same disadvantages as noted above.

Yet another similar single line weathervaning mooring system is disclosed in JPS58202185A, which also comprises a separate underwater buoyancy tank connected via a truss structure to a rocker arm above the surface, and a hinge connecting the rocker arm to the FPSO about a horizontal axis. In addition, the outer rocker arm connection point is to a floating buoy which is held in place by a catenary mooring system. Similar to the above mentioned prior art, the rocker arm is mounted in a substantially horizontal position, or inclined downwards from the connection at the FPSO towards the combined center of buoyancy of the rocker arm and buoyancy tank.

This system has a better ability to accommodate large horizontal displacements of the moored body due to the catenary type mooring to the seabed. However, a catenary mooring has its own disadvantages, with long legs, large footprint and high costs. As for the prior art mentioned above, a separate buoyancy tank has to be provided near the surface and connected via an additional truss structure to the yoke, which complicates the fabrication of the system. This system is also not able to control the yawing of the floating body. As above, having the buoyancy close to the surface will also result in large wave forces on the buoyancy tank.

US6983712B2 discloses a spread moored FPSO vessel with offloading by tandem connection to a shuttle tanker via a submerged yoke with a flotation buoy in one end.

The yoke is connected to the FPSO in a flexible manner at one end, and to the shuttle tanker via a buoy that provides flotation at the other end. The yoke is a truss like structure. A vertical mooring line connects the yoke to a seabed anchor. This system relies on the FPSO being moored with a full catenary mooring system controlling both the horizontal displacements as well as the yawing of the FPSO. This system is only suitable for providing a link for mooring a floating vessel to another vessel which has its own separate spread mooring system.

The present invention aims to provide an improved solution for mooring a floating body, and in particular a floating wind turbine, which controls both the horizontal position and the yaw orientation of the floating body and which overcomes the disadvantages of the aforementioned prior art.

Summary of Invention

In a first aspect there is provided a mooring system for a floating body, said system comprising: at least one arm member, each said arm member configured to be buoyant in water and to be rotatably attached to said floating body at or near a first end of the arm member via a rotatable joint; at least one mooring line configured to attach to said arm member at a longitudinal position spaced apart from said first end; and at least one anchor member, wherein the at least one mooring line is configured to connect the arm member to the at least one anchor member.

The rotatable joint may be configured to rotate about a horizontal axis perpendicular to a longitudinal axis of the arm member.

The rotatable joint may be a hinged joint.

Each mooring line may be configured to attach to a respective arm member at or substantially close to a centre of buoyancy of the arm member.

Each arm member may comprise at least one ballast compartment.

Each arm member may comprise a single pipe configured to be inherently buoyant.

The system may comprise two or more arm members attached to said floating body, each arm member may be connected to an anchor member via a respective mooring line.

One or more spreader bars may be located between two or more respective mooring lines.

The floating body may comprise a spar buoy.

The system may comprise two arm members spaced substantially 180 degrees apart around the floating body.

The floating body may comprise a semi-submersible platform, said platform comprising two or more horizontal beams and two or more downwardly extending columns at spaced apart positions.

The system may comprise two or more arm members configured to be attached at or close to a lower end of said respective columns.

The system may comprise at least three downwardly extending columns at spaced apart positions.

Each arm member may comprise drag damping fins.

Each arm member may be configured to rotate about the rotatable connection such that a longitudinal axis of the arm member varies between an angle of approximately 5 degrees to 180 degrees from the vertical.

The system may comprise a latch configured to secure the arm member to the floating body when the arm member is positioned at an angle of approximately 5 degrees to the vertical.

A fender may be arranged between the floating body and each rocker arm to prevent contact between a second end of the rocker arm and the floating platform.

In use, the one or more anchor may be located on a seabed and the one or more arm members may be connected to the floating body via the rotatable joint at a position below a surface of the water.

The rotatable connection may be located at a sub-surface position below a centre of buoyancy of the respective arm member.

The system may comprise a tower or support structure mounted on the floating body and a wind turbine mounted on said tower or support structure.

The wind turbine may be rotatably connected to the tower or support structure by a yaw bearing.

The system may comprise a subsea swivel, wherein the tower or support structure is arranged to rotate about said swivel.

The yaw bearing may be activated in response to a signal detecting a yaw misalignment relative to a wind direction in order to rotate said tower about said swivel to align itself with the wind direction.

The wind turbine may be configured to rotate a maximum of 179 degrees in each direction relative to the tower or support structure.

The tower may be asymmetrical.

The floating body may have a centre of buoyancy positioned above a total centre of gravity of the floating body, and the floating body may be moored in a body of water by a taut mooring line which does not restrict heave motion of said floating body.

The system may comprise a power export cable and a mechanical swivel, said power export cable configured to be routed through said mechanical swivel; the system optionally further comprising a power swivel assembly configured to be disengaged to unwind said power export cable and subsequently re-engaged.

The arm member may be configured to resist lateral forces relative to a longitudinal axis of the arm member, and the arm members are preferably configured to resist such lateral forces from a direction up to and including 90 degrees.

In another aspect there is provided a floating body having a centre of buoyancy positioned above a total centre of gravity of the floating body, wherein the floating body is moored in a body of water by a taut mooring line which does not restrict heave motion of said floating body, such that the heave restriction force will be within 50% of the forces in said taut mooring line.

In another aspect there is provided a method of mooring a floating body in a body of water, the method comprising the steps of: rotatably attaching one or more arm members configured to be buoyant in water to said floating body at or near a first end of the arm member via a rotatable joint, such that each arm member is substantially located below a surface of the water; positioning each arm member at an angle of between 10 degrees and 70 degrees from a horizontal axis of the floating body, such that the arm member extends upwardly from the rotatable joint; attaching a mooring line to each arm member at a longitudinal position spaced apart from said first end; attaching each arm member to a seabed anchor via the mooring line such that the mooring line is taut and substantially vertical; wherein downward rotation of the arm member in response to a horizontal force applied to the floating platform enables the mooring line to incline at an angle to the vertical.

The method may comprise positioning two or more arm members with a horizontal distance therebetween, such that the two or more arm members create a pair of horizontal force components in the mooring system further creating a yaw restoring moment in response to environmental forces yawing said floating body out of its initial yaw orientation.

The floating body, said one or more arm members, and said seabed anchor may form a mooring system according to the aspects above.

In a still further aspect there is provided a mooring system comprising a power export cable and a mechanical swivel, said power export cable configured to be routed through said mechanical swivel; the system further comprising a power swivel assembly configured to be disengaged to unwind said power export cable and subsequently re-engaged.

Brief Description of Drawings

Some preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a side elevation view of a mooring system for a floating body; Figure 2 is a side elevation view of a mooring system for a floating body; Figure 3 is a side elevation view of the system of Figure 2 in a single mooring configuration; Figure 4a is a top down view of the system of Figure 3 in a misaligned condition; Figure 4b is a top down view of the system of Figure 3 in a yawed condition; Figure 5a is a view of a semi-submersible platform moored with rocker arms; Figure 5b is a front view of the platform of Figure 5a; Figure 5c is a side view of the platform of Figure 5a; Figure 5d is a top view of the platform of Figure 5a; Figure 5e is a front view of the platform of Figure 5a with the rocker arms in a rotated position; Figure 6 is a top view of a semi-submersible platform similar to that of Figure 5 but with the rocker arm members in an alternative configuration; and Figure 7 is a side view of a power export cable for use in a mooring system.

Detailed Description

Described herein with reference to Figures 1 to 7 is a mooring system for a floating body. The system may comprise at least one arm member (e.g. a rigid mooring arm or yoke), each arm member configured to be buoyant in water and to be rotatably attached to the floating body at or near a first end of the arm member via a rotatable joint. At least one mooring line is configured to attach to the arm member at a longitudinal position spaced apart from said first end. The system may comprise at least one anchor member, and the at least one mooring line may be configured to connect the arm member to the at least one anchor member.

The floating body may have a centre of buoyancy positioned above a total centre of gravity of the floating body, and may be moored in a body of water by a taut mooring line which does not restrict heave motion of said floating body, such that the heave restriction force will be within 50% of the forces in said taut mooring line.

Also described herein is a method of mooring a floating body in a body of water, comprising the steps of: rotatably attaching one or more arm members configured to be buoyant in water to said floating body at or near a first end of the arm member via a rotatable joint, such that each arm member is substantially located below a surface of the water; positioning each arm member at an angle of between 10 degrees and 70 degrees from a horizontal On use) axis of the floating body, such that the arm member extends upwardly from the rotatable joint; attaching a mooring line to each arm member at a longitudinal position spaced apart from said first end; attaching each arm member to a seabed anchor via the mooring line such that the mooring line is taut and substantially vertical. Downward rotation of the arm member in response to a horizontal force applied to the floating platform enables the mooring line to incline at an angle to the vertical.

Positioning each arm member upwards initially provides a larger range of rotation of the arm member, and hence a larger flexible range for the mooring system, which can absorb larger motions of the floating body before the mooring becomes tight.

The inventive mooring systems described herein may be used for any type of structure that is to be positioned floating on water, for example, a vessel, an oil rig, a platform for a wind turbine or the like, and so on. The systems may be used as a main, long term mooring system of any stand-alone floating body. The inventive systems employ one or more buoyant rocker arm members in a new configuration, thereby ensuring yaw stability, as well as providing increased resistance to large waves acting on the floating structure, without large forces in the mooring lines, as compared to prior art systems.

The inventive mooring systems have reduced costs compared to a catenary mooring system and also have a far smaller footprint.

In use, a floating body (such as a spar buoy or semi-submersible floating platform for supporting a wind turbine, for example) that is to be moored is positioned on water.

One or more mooring arms or yokes, hereafter denoted as arm members or rocker arm members, are rotatably connected at, or close to, a first end to the floating body at a sub-surface (i.e. underwater) position, at one or more locations around the floating body. A substantially vertical mooring line is connected to each of the rocker arm member(s) at a position between first and second ends of the rocker arm member(s), or at the second end of the rocker arm member(s). Buoyancy is provided to the rocker arm member(s) in order to create a pre-tension in the substantially vertical mooring lines when installed.

To avoid the rigid rocker arm member(s) resolving the large overturning moments caused by wave motions, and the thrust force acting on the wind turbine (where present), the rigid rocker arm member(s) are rotatably connected to the floating body about a horizontal axis. When forces are acting on the mooring system, the rocker arm member(s) will rotate downwards (i.e. away from the water surface) about the rotatable connection (which may be a hinged connection) at the first end of the rocker arm member, and will allow the substantially vertical mooring lines to become inclined and therefore resist the horizontal loads.

The systems described herein are designed to resist lateral horizontal forces in all directions and accompanying moments acting on the buoyant rocker arm members from the mooring lines, which will add additional requirements for the strength of the hinge connections to resist the global bending moments in the hinge connection. For example, the arm members may be configured to resist lateral forces relative to a longitudinal axis of the arm member, preferably from a direction up to and including 90 degrees.

The floating mooring systems may comprise at least one floating body 1, one or more a buoyant rocker arm members 2, a hinge connection 3 between the rocker arm member(s) 2 and the floating body 1, at least one anchor 5, and one or more taut mooring lines 4 connected to the anchor(s) 5. Where more than one mooring line is present, a spreader beam 10 may be located between two mooring lines 4.

Figures 1 and 2 illustrate an exemplary mooring system 100 for a floating body 1, in elevation. In this example, a wind turbine 11 comprising a wind rotor 24 and tower 13, is mounted on the floating body 1. The floating body 1 may be of a spar buoy type and may be configured to float at or just below a surface 9 of the body of water, which may comprise a river, estuary, lake, sea, ocean and the like. A pair of anchors 5 is provided at a bottom 8 of the body of water (e.g. at the sea bed).

The floating body 1 is configured to conned to two buoyant rocker arm members 2 via respective hinged connections 3. The rockers arm members 2 are located around a circumference of the spar (i.e. the floating body 1), preferably diametrically opposite to one another. Each rocker arm member 2 may be positioned at an angle 6 to the horizontal, such that in use the rocker arm members 2 point generally upwards towards a surface 9 of the water. Each rocker arm member 2 is configured to connect to a first end of a respective mooring line 4. A second, distal end of each mooring line 4 is configured to connect to a respective anchor 5 on the seabed 8. The longitudinal position of the connection point of the mooring line 4 along the rocker arm member 2 is pre-determined using an optimisation process, as discussed in more detail with respect to Figure 5 below.

Advantageously, the use of two rocker arm members 2 pointing at different angles to the vertical axis of the spar type floating body 1 may create a yaw stable mooring system.

In use, the angle that the taut mooring lines have to the vertical is reasonably free, but will preferably be close to zero. In an equilibrium situation for the system i.e. without environmental forces such as wind, wave action and tidal action acting on the system, the angle between each mooring line and the vertical is most often equal to zero. As shown in Figure 1, a subsea spreader beam 10 may be located between the two taut mooring lines 4 to prevent the mooring lines 4 from getting too close during large yaw angles of the floating body 1. The spreader beam 10 may serve to increase the yaw stiffness and yaw restoring moment of the floating body 1 at large yawing angles.

Preferably, this spreader beam 10 is located so that the lengths of the mooring lines 4 above and below the spreader beam or beams 10 are similar or substantially the same.

As illustrated in Figure 2, the floating body 1 may in use experience a heave motion indicated by arrow 7, caused by environmental forces. The rocker arm member(s) 2 may be configured to rotate about the hinge connections 3 such that the angle of each rocker arm member 2 from the horizontal may vary.

Figure 3 is an elevation view of the system 200 similar to that of Figures 1 and 2 but using a single mooring configuration i.e. having a single anchor 5 and a single rocker arm member 2. The floating body 1 to be moored may be of a spar type, but could also be a semisubmersible platform or any other floating body. In this example, the wind turbine 11 is mounted downwind of the weathervaning tower 13 and floating body 1 (wind direction is indicated by reference VV). The tower 13 and floating body 1 is configured to weathervane about a subsea (or underwater) swivel (not shown), which could be part of the anchoring system, i.e. integrated with the mooring line or in either end of it. In another embodiment, the wind turbine may be mounted upwind of the tower but downwind of said swivel connection.

An asymmetric stiffening or support structure, for example a stay 14 and spreader beam 12 system, may be provided on the upwind and/or downwind side of the tower.

Any other type of suitable support structure, such as a truss or the like, could also be used. In use, a taut anchor line member 4 connects the single rocker arm member 2 with a single seabed anchor 5.

The single rocker arm member 2 is installed at an inclined angle pointing upwards, towards the water surface 9. A taut mooring line 4 is connected between the single rocker arm member and the single anchor 5 on the seabed.

A conventional yaw bearing 15 is introduced at the top of the tower 13 between the wind turbine 11 and tower 13. It will be appreciated that instead of a conventional tower to support the wind turbine, any other suitable support structure could be used, either on top of the floating body or integrated with it. The bearing 15 enables the wind turbine 11 to be yawed a certain amount relative to the tower 13. In the case of an asymmetrical tower, the yaw angle of the wind turbine 11 relative to the tower 13 and floating body 1 is limited to up to a maximum of around 179 degrees in each direction, or normally up to a maximum of about 80-120 degrees, depending on the configuration of the wind rotor relative to the yaw axis of rotation and the configuration of said stiffening structure upwind / downwind of the tower. The wind turbine 11 in this case cannot yaw fully 360 degrees due to the stiffening structure (e.g. the upwind mounted stays 14 and spreader beam 12), which would then clash with the rotor blades.

Figure 4a is a top down view of the single mooring configuration system 200 shown in Figure 3. In this example, the wind direction W and the turbine wind rotor 24 thrust direction 16 is aligned with the axis of rotation of the wind rotor 24 and which always acts perpendicular to the rotor 24 plane, is misaligned by an angle 19.

In response to the misalignment 19 of the rotor thrust force vector 16 to the incoming wind W, effectively the wind turbine's yaw misalignment, the wind turbine 11 is turned a few degrees on the yaw bearing 15 on top of the tower 13. The rotor thrust force vector 16, which as discussed above always acts substantially perpendicular to the rotor 24 plane, will then have a horizontal offset 17 to the anchor 5. As described in more detail below, this offset 17 multiplied by the rotor thrust force 16 will create a yaw restoring moment 18 which will assist the system to align itself with the wind. In fig. 4a and 4b the anchor 5 is shown visible for clarity, but is in reality located below the rocker arm member in these top views and therefore should be partly hidden by said rocker arm member.

Figure 4b is also a top down view of the single mooring configuration 200 shown in Figure 3. In this example, the wind turbine 11 has been yawed approximately 20 degrees clockwise relative to the tower 13, floating body 1 and rocker arm member 2 by turning the yaw bearing 15 on top of the tower 13. The misalignment between the wind direction W and the turbine rotor thrust direction 16 is thereby decreased, and a clockwise yaw moment 18 is simultaneously created by the offset 17 and the vector of the rotor thrust force 16, which turns the entire floating system clockwise about the anchor connection point, reducing the wind misalignment still further.

The mooring system 200 of Figures 3, 4a and 4b combined with a partially yawable wind turbine (relative to the floating body and rocker arm member) has an additional advantage, due to its connection point considerably offset from the centre of the wind turbine's vertical axis, of stabilizing the wind turbine in the direction of the wind. As described above, when necessary, the wind turbine's normal yawing system at the top of the tower can be used to turn the wind turbine a few degrees to one side and in that way create an additional yawing moment due to the thrust force acting perpendicular on the rotor plane will then have a horizontal offset to the anchor point. The result will be that the wind turbine weathervaning tower and floating body can be actively assisted in yaw about a subsea swivel using a standard yaw tracking control system and small adjustments in the wind turbines own yaw bearing at the top of the tower. In addition, the large heave forces subjected to the prior art systems will be avoided due to the hinged rocker arm member. This is a superior method of controlling the yawing compared to prior art single moored wind turbines, which are tracking the wind passively and with large heave forces in the mooring system.

In order to accommodate an exchange of energy, normally in the form of electricity but also by hydraulics or a gas line, and communication signals, a power export cable may be routed through the centre of a mechanical swivel arranged in any of the mooring systems described herein. A separate power swivel assembly is arranged, said power swivel assembly comprising an electrical slip ring or an electrically disconnectable circuit breaker which may be disengaged to unwind said cable and then be re-engaged, ideally by automated action, in response of a cable twist detection system or a yaw rotation counting system. By separating the mechanical swivel in the mooring system with the electrical and communication swivel it will be possible to locate the electrical swivel in air protected inside the tower or the nacelle and for easy access for maintenance. Further detail of the power export cable will be discussed with reference to Figure 7.

The combination of a single a rocker arm member, a single taut mooring line and a partially moveable yaw bearing at the top of the tower on a single point moored spar wind turbine therefore enables improved yaw control of the floating body.

Figures 5a to 5f illustrate another exemplary floating mooring system 300 in which the floating body takes the form of a semi-submersible (semisub) platform 1'. Figures 5b, Sc and 5d respectively illustrate front, side and top down views of the system of Figure 5a; Figure 5d also includes an enlarged view of the hinged connection 3. Figure 5e illustrates a front view of the system of Figure 5a in the case where environmental forces are acting on the floating body 1' and the wind turbine 11, and the rocker arm members 2 are rotated down towards the seabed due to the increased forces transmitted by the taut mooring lines 4. Figure 5f is a front view of the system of Figure 5a with the rocker arm members 2 configured substantially in the vertical position to enable towing and installation of the system.

The semi-submersible platform 1' illustrated in Figures 5a to 5f may be configured to carry a wind turbine 11. The platform 1' may be triangular in shape, i.e. may comprise three apexes, and may be formed of a plurality of horizontal (in use) beams. The triangular platform 1' is configured to be connected to a first respective end of first 21 and second 22 columns via two of the three apexes. It will be appreciated, however, that the semi-submersible platform 1' may take different forms (e.g. quadrilateral) or other forms suitable, depending upon the specific application. In this case, additional columns and/or rocker arm members may be provided).

In use, the first ends of the columns 21, 22 will be closer to the water surface than the second (i.e. lower) ends of the columns 21, 22. The lower ends of the columns 21, 22 may be located at a depth of around 15 to 30 metres from the water surface, depending upon the design draft of the platform 1'.

The wind turbine 11 and tower 13 is in this example are positioned on a third column 23. The third column 23 is configured to be connected to the third apex of the triangular floating platform 1'. The wind turbine 11 of this example is configured to yaw at least 360 degrees in both directions relative to the tower 13 by virtue of a conventional yaw bearing mounted at the top of said tower 13.

As illustrated in Figure 5a, two rocker arm members 2 are provided in this exemplary system, each configured to be connected at or near respective second ends of the first 21 and second 22 columns by hinged connections 3. In one example, the connections 3 are arranged as a hinge rotatable about a substantially horizontal pivot axis, substantially perpendicular to a longitudinal axis of the rocker arm member 2. This axis (not shown) is substantially perpendicular to the respective rocker arm member 2 extending from the connection point with the column 21 or 22.

In the example of Figure 5a, the rocker arm members 2 have a substantial length and a substantial cross-sectional area. Preferably, the rocker arm members 2 comprise pipes or tubulars, ideally substantially hollow pipes or tubulars. The longitudinal dimension (i.e. length) of the rocker arm members 2 will ideally be selected to ensure that the rocker arm members 2 do not under normal circumstances interfere with other floating objects (e.g. boats) at the water surface. Where the rocker arm members 2 are hollow, the cross sectional area and wall thickness of the rocker arm members 2 may be arranged to provide sufficient strength to resist the forces and bending moments acting on the rocker arm members 2, and to provide sufficient buoyancy for the necessary pre-tension in the mooring lines 4. The rocker arm members 2 may be divided into compartments, each compartment being substantially filled with ballast or substantially empty for contributing buoyancy. The above-mentioned features of the rocker arm members 2 may apply to any of the examples discussed with reference to Figures 1 to 6 herein.

Each of the rocker arms 2 illustrated in Figure 5a is configured to attach or otherwise connect to a respective mooring line 4 at a longitudinal position spaced away from the hinge connection 3. The longitudinal distance between the mooring line 4 attachment point (not shown) and the outer end of the rocker arm member 2 may be denoted as overhang. This overhang (as well as the cross-sectional area of the rocker arm members 2) may be pre-determined using an optimisation process. This process aims to provide a safe and ample restriction of horizontal drift and dynamic motions in operational as well as survival conditions for the floating body 1,1'. A larger overhang will increase the percentage of the buoyancy forces in the rocker arm members 2 which can be transferred to the mooring lines 4 as pretension in the vertical mooring lines 4.

However, a smaller overhang i.e. a larger distance from the hinge connection 3 to the mooring line 4 attachment point on the rocker arm member 2 will result in less flexible motion of the floating platform 1,1' until the mooring lines 4 become fight. Therefore, an attachment position for the mooring line 4 substantially in the longitudinal centre of each rocker arm member 2 (which may also be the centre of buoyancy of the rocker arm member 2) may be preferable. It will be appreciated that afore-mentioned description of the overhang may apply equally to the examples illustrated in Figures 1 to 7.

Advantageously, each separate rocker arm member 2 may be one single pipe with inherent buoyancy without the use of an external buoy. The systems therefore comprise fewer moving parts and have substantially no buoyancy at the surface where wave forces are larger. Additionally, each rocker arm member 2 is mounted at or close to the bottom of the floating body 1,1' (i.e. either directly to the lower end of a spar type floating body 1, or close to its centre of gravity, or to the lower ends of the columns 21, 22 of the semi-submersible floating body 1'), with the rocker arm member 2 in use pointing at an upwards angle and the hinge connection 3 located below the centre of buoyancy of the rocker arm member 2. This arrangement provides the mooring systems 100,200,300,400,500 described herein with more inherent flexibility, as the rocker arm member 2 can rotate further (downwards) until the mooring system becomes fight and loses its articulating flexibility.

In general, each mooring line 4 is stretched from the connection point on the respective rocker arm member 2 to an anchor 5 at the seafloor (not shown here) in a taut manner in a vertical or substantially vertical direction when no environmental forces are acting on the system. Thus, the anchor 5 must be designed to accept vertical loads. The tension in the taut mooring line 4 is determined by the buoyancy of the rocker arm member 2 and the overhang. In one example, not shown here, additional buoyancy may be provided by a separate buoy moored to the rocker arm member, in addition to the length of the overhang.

As illustrated in Figures 5a to Sc, the rocker arm members 2 of the exemplary system 300 are initially configured to point outwards from the first and second columns 21, 22 and towards the third column 23 in a substantially upwards direction (in use) and at an angle that is larger than zero but less than 90 degrees relative to the horizontal, when no environmental forces are acting upon the platform 1. Ideally, the angle between the rocker arm members 2 and the horizontal should be between 10 and 70 degrees. In one example, the angle between the rocker arm members 2 and the horizontal may be approximately 30 degrees from the horizontal. When environmental forces, such as wind, current and waves, are gradually increasing from a theoretical zero force situation, the rocker arm members will gradually start to rotate downwards.

The angular position of the rocker arm members 2 may improve the range of flexible motion of the moored floating platform 1,1' until the mooring lines 4 of the mooring system become tight. High forces resulting in shock loads to the system will occur when the longitudinal axis of any of the rocker arm members 2 becomes near parallel to the taut mooring lines 4 (i.e. when the rocker arm members 2 approach the vertical position). Should this occur, there would be insufficient geometrical stiffness within the system and unacceptable loads may result. Hence, when horizontal environmental forces from waves, wind or current act on the semi-submersible platform 1,1', the forces in the mooring lines 4 will force the rocker arm members 2 to rotate downwards via the respective hinged connections 3 and thus allow the mooring lines 4 to become inclined away from the vertical and hence more able to resist horizontal forces.

Figure 5e illustrates the reaction of the rocker arm members 2 when wave, current or wind forces are pushing the floating platform 1' backwards, i.e. in-plane or in the direction of the view. The wind turbine 11 may or may not be operating in this case, depending on whether the maximum operational wind speed, typically 25 m/s, is exceeded. The mooring lines 4 are forced backwards at the top. Due to the resulting horizontal force components acting on each rocker arm member 2 at the attachment point to the taut mooring lines 4, a considerable out-of-plane bending moment will be transferred to the inner section of the rocker arm members 2, as well as to the hinge connections 3. These bending moments 3a act about a substantially vertical axis. The rocker arm members 2 are configured to resist these large bending moments, by the structural strength inherent in the hollow pipe section (where present) of the rocker arm members 2 themselves. The overall system strength may also be increased by locating the contact points for the hinge connections 3 spread apart horizontally at each side of each the first (inner) ends of each rocker arm member 2.

In response to the horizontal forces acting on the floating body 1,1', the rocker arm member(s) 2 rotate downwards towards the seabed, allowing the substantially vertical mooring lines to become inclined and therefore resist the horizontal loads. When said horizontal forces are reduced, the rocker arm member(s) 2 will again rotate upwards towards the water surface, back to their original position, and the mooring lines will become substantially vertical again, when no environmental forces are acting. When a wave passes by the same will happen, i.e. the rocker arm member(s) 2 will rotate up and down to accommodate the cyclic horizontal and vertical movements of the floating body 1,1'.

By carefully selecting the horizontal distance between two rocker arm members 2 attachment points to their respective mooring lines, a pair of horizontal force components, depending on the distance between the arms, will make a moment arm that creates a moment to resist any yaw rotation of the floating system. As discussed above, In Figure 5a two parallel rocker arm members 2 are respectively connected to first 21 and second 22 columns and point towards the side of the platform 1' where the third column 23 is located. In this way the mooring system's geometrical centre is close to the centre of forces acting on the semi-submersible platform 1' from waves and currents in an extreme environmental event, hence reducing the yawing moments acting on the system.

Figure 6 is a top down view of a system 400, similar that of the system 300 of Figure 5, comprising a semi-submersible floating platform 1', but with the rocker arm members 2 instead connected to the first column 21 and the third column 23 (on which the wind turbine 11 is mounted), and pointing away from the second column 22.

An advantage of the configuration of Figure 6 is that service boats can moor up alongside the first 21 and second 22 columns and have direct access to the wind turbine 11 without running the risk of colliding with one of the rocker arm members 2 in the event that the rocker arm members 2 rotate too far up towards the water surface in heavy seas or weather conditions.

In general, during transportation to the field (i.e. to the sea, estuary, river etc. where the floating body is to be moored, and in the case of anchor line failure, the rocker arm members may be stored substantially in a vertical position along a hull of the floating body (e.g. a spar, a semi-submersible, or other type of floating body) to which the members are connected. In this vertical position, the rocker arm members may be locked by a device (e.g. a latch) that can actively be unlocked during installation.

In another embodiment of the invention, a power export cable 27 is routed through the centre of a mechanical swivel 26 in the mooring system 500 illustrated in Figure 7. A separate power swivel assembly 36 is arranged, comprising an electrical slip ring or an electrically disconnectable circuit breaker, known to a person skilled in the art, which may be disengaged to unwind said cable 27 and then be re-engaged, ideally by automated action in response to a cable twist detection system or a yaw rotation counting system. The cable twist detection could, for example, be based on measuring the rotational position of two points on the cable 27 at a known distance above each other, or by a strain gauge or other systems known to the skilled person in the field.

Figure 7 illustrate a side view of an anchor 5 fixed to the seabed 8, a substantially torsionally resistant connection 34 which connects the anchor 5 to the mechanical swivel 26. The torsionally resistant connection 34 may be the lower part of a taut mooring line 4. The swivel 26 may comprise a lower swivel member 29 connected to the torsionally resistant connection 34, the lower swivel member 29 connected via a bearing 35 to an upper swivel member 28 arranged to freely rotate on the bearing 35 about the swivel's 26 longitudinal and substantially vertical axis. The cable 27, which may comprise an electrical cable and a signal cable, including a fibre optic cable, is arranged to enter the swivel 26 via an opening 30 in the lower swivel member 29; to pass through a central opening 31 in the bearing 35; to continue into the upper swivel member 28; and to exit through an opening 32 arranged in the upper swivel member 28. A torsion resistant tension member 33, which may coincide with the mooring line 4 illustrated in Figures 1 to 6, and may consist of a single line or several lines in parallel, is connected to the upper swivel member 28 in order to connect the swivel 26 in a substantially torsion resistant manner to a rocker arm member 2, as illustrated in Figures 1 to 6 above.

In use, the power swivel assembly 36 is arranged in air above the mechanical swivel 26. The power swivel assembly 36 may comprise an electrical slip ring or an electrically disconnectable circuit breaker, which may be disengaged to unwind the cable 27 and then be re-engaged.

For any of the above-described systems, the filling of rocker arm member compartments for buoyancy may be carried out according to a plan for obtaining the desired properties (e.g. buoyancy). The systems may be installed by ballasting the rocker arm member(s) down, connecting the mooring line and then de-ballasting.

Alternatively or additionally, the rocker arm member(s) may be pulled down with a winch via a sheave on the pre-installed anchor. The mooring line(s) of any of the systems may be connected with the aid of working remotely operated underwater vehicle(s) (ROV).

The rocker arm member(s) of any of the described systems may be equipped with damping fins to increase drag, or other means for damping their motion in the case of anchor line failure. In case of a mooring line failure, the motion of the rocker arm members towards vertical may be stopped by a fender-like damping device on the hull of the floating body, so that the impact forces will be acceptable. The rocker arm member(s) may be locked to the hull, so that any uncontrolled banging against the hull is prevented.

By configuring the mooring system as described in the exemplary systems 100,200,300,400,500 of Figures 1 to 7, the mooring will substantially not restrict heaving motion, and will otherwise act almost as a catenary anchor line system. The main force in the mooring lines will be determined by the pre-tension provided by the designed buoyancy. The superimposed dynamic loads due to environmental loads will be acceptably low.

By playing on the variables within any of the above-described mooring systems 100,200,300,400,500, a versatile system for use at a range of depths may be provided. As an example only, for use with a 12-15MW wind turbine and at a depth of around 250 meters, the rocker arm members 2 may ideally have a length of approximately between 30 and 60 metres and a diameter of the rocker arm member 2 cross section may ideally be approximately between 3 and 7 metres. To create sufficient yaw resistance of the floating body the overhang is ideally approximately half the overall length of the rocker arm member.

As discussed above, to increase the geometrical flexibility in the mooring systems to absorb large first order wave motion, the length of the vertical mooring lines may be configured so that the rocker arm members are initially pointing upwards at an angle to the horizontal when no external environmental forces (wind, waves and current) are acting on the system. For the same reason, the center of buoyancy of the rocker arm members may be positioned higher than the hinged connection of the rocker arm members to the floating body when no environmental forces (wind, waves and current) are acting on the system. With this configuration, the mooring systems can accommodate a large mechanical rotation of the rocker arm member about the hinge connection at the inner end of the rocker arm member before the point at which the mooring lines become substantially parallel with the rocker arm members, at which point the stiffness of the mooring system and the forces in it will increase rapidly.

Ideally, the rocker arm member may be a pipe with sufficient diameter to resist the bending moments and at the same time accommodate substantially all, or the majority of, the required buoyancy. Since the center of buoyancy of the rocker arm member is in use initially positioned higher than the hinge connection, the buoyancy will contribute a larger moment about the hinge connection, compared to prior art systems with separate buoyancy tanks, and will resist the mooring line forces and accommodate a larger mechanical rotation of the rocker arm member about the hinge connection before the restoring moment from the buoyancy about the hinge connection is lost.

Another advantage over prior art is that the subsea location of the hinge connection, far below the water surface, is in the order of the length of the rocker arm member itself. This ensures that the buoyancy in the rocker arm member is less exposed to larger wave forces at or close to the surface.

The yaw resistance of the disclosed mooring systems will be a function of tension in the mooring lines, the smallest distance between the mooring lines, and the water depth. For very large yaw angles of the floating body, relative to the anchors on the seabed, the distance between the mooring lines at the mid-point will start to decrease gradually until the two or more mooring lines come into contact with each other and the moment arm between the mooring lines becomes virtually zero. At this point, the yaw restoring moment capacity is lost. In order to improve the situation, as discussed above a subsea spreader beam may be installed between the two or more mooring lines. For very large water depths more than one such spreader beam may be installed. With one spreader beam the yaw rotation and the yawing moment transferred through the anchor system can be doubled until the yaw restoring moment is lost.

Since the vertical rocker arm members in this way do not substantially restrict the overturning moments of the floating body, the size of the seabed anchor and the vertical mooring lines can be made much smaller than for a conventional tension leg platform. Since the vertical mooring lines typically will be several times shorter than a catenary type mooring, the systems described herein are envisaged to be less costly than both the traditional tension leg platform and the catenary moored spar systems. It has also the advantage of taking up much less space on the seafloor than a conventional catenary mooring system.

Since the mooring lines for the inventive systems do not substantially contribute to the stability of the tension leg foundation, but are rigidly fixed to the floating body in 5 out of 6 degrees of freedom, it can be sufficient to only use two tension legs and two anchors to ensure sufficient restoring forces to resist both the horizontal forces and the yaw moments acting on the system. This is in contrast to both the traditional tension leg platform and the spar with catenary type mooring, which need at least 3 mooring lines and anchors to properly stabilize the moored object both horizontally and in yaw.

It will be appreciated that various modifications and alterations may be made to the embodiments described herein, and that elements of the described embodiments may be combined with or substituted for one another. For example, any of the features of the exemplary mooring systems described above with reference to Figures 1 to 7 may be substituted or combined with one another. In addition, the above discussion of specific mooring systems may apply equally to other systems herein described.

The floating bodies described herein may be located in a variety of environment e.g. ocean, sea, lake, river estuary and the like. References herein to the seabed may there also be assumed to apply to the river bed, estuary bed and so on.

As described herein, yaw refers to a turning rotation of a floating body, or a wind turbine or the like relative to a floating body in the case of a yaw bearing on top of a tower, about a vertical axis running through the floating body or tower; pitch refers to an up/down rotation of a floating body about a transverse axis running horizontally across the floating body; roll refers to a tilting rotation of a floating body about a longitudinal axis running horizontally through the length of the floating body. As described herein, heave refers to a translational vertical (up/down) motion; sway refers to a translational transverse (side-to-side) motion; surge refers to a translational front/back motion of a floating body.

As described herein, horizontal may refer to an axis substantially parallel to a surface of the body of water.

Claims

CLAIMS: 1. A mooring system for a floating body, said system comprising: at least one arm member, each said arm member configured to be buoyant in water and to be rotatably attached to said floating body at or near a first end of the arm member via a rotatable joint; at least one mooring line configured to attach to said arm member at a longitudinal position spaced apart from said first end; and at least one anchor member, wherein the at least one mooring line is configured to connect the arm member to the at least one anchor member.
2. The mooring system of claim 1, wherein said rotatable joint is configured to rotate about a horizontal axis perpendicular to a longitudinal axis of the arm member.
3. The mooring system of claim 1 or 2, wherein said rotatable joint is a hinged joint.
4. The mooring system of any preceding claim, wherein each mooring line is configured to attach to a respective arm member at or substantially close to a centre of buoyancy of the arm member.
5. The mooring system of any preceding claim, wherein each arm member comprises at least one ballast compartment.
6. The mooring system of any preceding claim, wherein each arm member comprises a single pipe configured to be inherently buoyant.
7. The mooring system of any preceding claim, comprising two or more arm members attached to said floating body, each arm member connected to an anchor member via a respective mooring line.
8. The mooring system of claim 7, wherein one or more spreader bars is located between two or more respective mooring lines.
9. The mooring system of any preceding claim, wherein the floating body comprising a spar buoy.
10. The mooring system of claim 9, said system comprising two arm members spaced substantially 180 degrees apart around the floating body.
11. The mooring system of any one of claims 1 to 8, wherein the floating body comprises a semi-submersible platform, said platform comprising two or more horizontal beams and two or more downwardly extending columns at spaced apart positions.
12. The mooring system of claim 11, said system comprising two or more arm members configured to be attached at or close to a lower end of said respective columns.
13. The mooring system of claim 11 or 12, comprising at least three downwardly extending columns at spaced apart positions.
14. The mooring system according to any preceding claim, wherein each arm member comprises drag damping fins.
15. The mooring system according to any preceding claim, wherein each arm member is configured to rotate about the rotatable connection such that a longitudinal axis of the arm member varies between an angle of approximately 5 degrees to 180 degrees from the vertical
16. The mooring system of claim 15, comprising a latch configured to secure the arm member to the floating body when the arm member is positioned at an angle of approximately 5 degrees to the vertical.
17. The mooring system of any preceding claim, wherein a fender is arranged between the floating body and each rocker arm to prevent contact between a second end of the rocker arm and the floating platform.
18. The mooring system of any preceding claim, wherein in use, the one or more anchor are located on a seabed and the one or more arm members are connected to the floating body via the rotatable joint at a position below a surface of the water.
19. The mooring system of claim 17, wherein in use the rotatable connection is located at a sub-surface position below a centre of buoyancy of the respective arm member.
20. The mooring system of any preceding claim, comprising a tower or support structure mounted on the floating body and a wind turbine mounted on said tower or support structure.
21. The mooring system of claim 20, wherein said wind turbine is rotatably connected to the tower or support structure by a yaw bearing.
22. The mooring system of claim 21, further comprising a subsea swivel, wherein the tower or support structure is arranged to rotate about said swivel.
23. The mooring system of claim 22, wherein said yaw bearing is activated in response to a signal detecting a yaw misalignment relative to a wind direction in order to rotate said tower about said swivel to align itself with the wind direction.
24. The mooring system of claim 21, 22 or 23, wherein the wind turbine is configured to rotate a maximum of 179 degrees in each direction relative to the tower or support structure
25. The mooring system of claim 24, wherein the tower is asymmetrical.
26. The mooring system as claimed in any preceding claim, wherein said floating body has a centre of buoyancy positioned above a total centre of gravity of the floating body, and wherein the floating body is moored in a body of water by a taut mooring line which does not restrict heave motion of said floating body.
27. The mooring system as claimed in any preceding claim, comprising a power export cable and a mechanical swivel, said power export cable configured to be routed through said mechanical swivel; the system further comprising a power swivel assembly configured to be disengaged to unwind said power export cable and subsequently re-engaged.
28. The mooring system of any preceding claim, wherein the arm member is configured to resist lateral forces relative to a longitudinal axis of the arm member, and wherein the arm members are preferably configured to resist such lateral forces from a direction up to and including 90 degrees.
29. A floating body having a centre of buoyancy positioned above a total centre of gravity of the floating body, wherein the floating body is moored in a body of water by a taut mooring line which does not restrict heave motion of said floating body, such that the heave restriction force will be within 50% of the forces in said taut mooring line.
30. A method of mooring a floating body in a body of water, the method comprising the steps of: rotatably attaching one or more arm members configured to be buoyant in water to said floating body at or near a first end of the arm member via a rotatable joint, such that each arm member is substantially located below a surface of the water; positioning each arm member at an angle of between 10 degrees and 70 degrees from a horizontal axis of the floating body, such that the arm member extends upwardly from the rotatable joint; attaching a mooring line to each arm member at a longitudinal position spaced apart from said first end; attaching each arm member to a seabed anchor via the mooring line such that the mooring line is taut and substantially vertical; wherein downward rotation of the arm member in response to a horizontal force applied to the floating platform enables the mooring line to incline at an angle to the vertical.
31. A method as claimed in claim 30, comprising positioning two or more arm members with a horizontal distance therebetween, such that the two or more arm members create a pair of horizontal force components in the mooring system further creating a yaw restoring moment in response to environmental forces yawing said floating body out of its initial yaw orientation.
32. A method as claimed in claim 31, wherein said floating body, said one or more arm members, and said seabed anchor form a mooring system as claimed in any of claims 1 to 28.
33. A mooring system comprising a power export cable and a mechanical swivel, said power export cable configured to be routed through said mechanical swivel; the system further comprising a power swivel assembly configured to be disengaged to unwind said power export cable and subsequently re-engaged.