GB2344843A

GB2344843A - Gravity securing system for offshore generating equipment

Info

Publication number: GB2344843A
Application number: GB9828078A
Authority: GB
Inventors: Neven Joseph Sidor
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-12-18
Filing date: 1998-12-18
Publication date: 2000-06-21
Anticipated expiration: 2018-12-18
Also published as: GB9828078D0; GB2344843B

Abstract

A gravity securing system for a piece of offshore generating equipment such as a wind turbine consists of gravity legs arranged in a star pattern on the seabed and connected to a mast 3 stabilised by tensioned cables. The latter are attached to the legs at points 3e along their length. It can be installed by utilising the legs as hulls of a transportation vessel, launching the system, with the legs arranged alongside one another with the mast, turbine and associated winching equipment on board, and then towing the complete system out to the installation site for sinking the legs thereat, after the legs have been arranged into the configuration required by the seabed contours. Addition of a further pair of buoyancy hulls 2 for temporarily supporting the mast 3 facilitate raising the mast and turbine as a result of sinking the legs, the temporary pair of buoyancy hulls providing a fulcrum.

Description

GRAVITY SECURING SYSTEM FOR OFFSHORE GENERATING EQUIPMENT.

This invention relates to a gravity securing system for offshore generating equipment, at sea or in a lake for example.

Designs for securing offshore generating equipment (such as wind turbines) have utilised piled foundations to date. This however imposes constraints on the choice of site as the geology of the seabed must not only be understood but also respected.

Moreover the unit cost of a single installation could be prohibitive in waters deeper than, for example, 15 meters where conventional derrick barges cannot operate. Also piling operations themselves are vulnerable to bad weather owing to the length of time they take.

It would therefore be desirable to provide a means for securing a piece of equipment to the seabed which does not require piling to be undertaken into the seabed.

According to the present invention there is disclosed a gravity securing system for offshore equipment, the system comprising a plurality of anchoring legs suitable for siting on a seabed, and, in situ, being of sufficient size to resist movement therefrom, means for securing the legs to one another, a mast, means for securing the mast to at least one of the legs, thereby forming a leg and mast assembly, and means for stabilising the leg and mast assembly. The legs may be sufficiently massive or long to resist movement from the seabed. Preferably, the legs are adapted to rest horizontally or flush on the seabed.

The equipment may be electrical generating equipment, for example a wind turbine.

In a preferred embodiment, there are three legs, secured together at an angle of approximately 120 degrees to one another. Three legs provide a naturally stable footing. The legs may comprise substantially hollow buoyancy chambers permitting them to be floated to an installation site and then to be sunk into position by filling with water.

The legs can be folded relative to one another into a towing configuration such that they lie substantially parallel to one another for towing to an installation site, and the legs can be unfolded from the towing configuration into a submergence configuration by swinging them about one another for sinking into position on the seabed.

The legs may be secured together using flexible bindings around protruding cleats, permitting them independence of movement vertically and they may be constricted horizontally by friction acting on a series of fenders.

Hawsers of controllable length attached to each leg and to a platform on the mast, may facilitate stabilising the leg and mast assembly. The length of each hawser can be controlled by a winch on the platform, for example. The length of the hawsers can be locked and the winches can be disconnected from the system; for example the length of the hawsers are locked by locking the drums of the winches, and the winches are then removed from the drums for reuse.

The platform may be connected to the mast via an axle, thereby allowing the platform to fold almost parallel to the mast when the mast it is in a lowered or transporting position. This platform may be free to slide up and down the mast, thereby permitting the platform to be positioned along the mast, for example to allow the platform to be winched up to the top of the mast using a further system of cables and pulleys, thereby enabling the platform to be used to raise and lower other equipment up and down the mast.

The plurality of legs, in a submergence configuration, can be arranged to radiate from an intersection point, thereby forming a star pattern. The mast may be secured to the legs at this intersection point and the intersection point could comprise an universal coupling, the coupling permitting rotational movement of the mast both from a lowered transportation configuration into a raised operational configuration and perhaps about the axis of the mast itself.

In a transportation configuration, the mast may be carried on a boom spanning between two buoyancy hulls either side of a central one of the legs. This boom may be braced to maintain a 90 degree alignment to both the hulls and the leg. The boom acts as a fulcrum for the mast, such that by sinking the legs, the buoyancy of the two buoyancy hulls enable the mast to be lifted from a substantially horizontal transportation configuration to a substantially vertical operational configuration. The two buoyancy hulls can then be disconnected from the boom once the leg and mast assembly has been fully stabilised.

Glands and valves may be fitted to the legs to allow them to be emptied or refilled with air, so that the system can be relocated.

There is also disclosed a method of deploying a seabed securing system onto the seabed comprising the steps of floating a seabed securing system as described above to an installation site and sinking the legs onto the seabed whilst rotating the mast from a transportation configuration into a substantially vertical operating configuration.

The securing system of this invention operates due to gravity, not piling. It enables an offshore electrical generating device easily to be taken direct from a harbour or shipyard to an installation site at sea.

The present invention provides a system of components which both facilitates the towing of a turbine to an offshore location, and its installation through the actions of buoyancy and gravity alone. With no mechanical fixing or piling to the seabed, the entire system, including, for example, the turbine, can also periodically be removed back to dry land for maintenance.

The functions and structural loadcases under which these components must perform vary, depending on the point within the installation sequence that one is considering; accordingly, a specific embodiment of the present invention, and how it is assembled will now be described with reference to the attached annotated figures A-J, which describe the system applied to a wind turbine. Although this system can work with a range of turbine sizes and water depths, the present illustration assumes a turbine blade diameter of about 48m and a water depth of about 22m.

FIG A (isometric with inset detail (i)) LAUNCH CENTRAL GRAVITY LEG The central gravity leg (1) consists of a welded plate steel box girder filled with concrete (see detail (i)). Transportable segments are welded together on a suitable quayside before the required total length is craned into the water. The proportions of concrete and air inside the section are determined by balancing the design loadcase for the final installation with the need for buoyancy in the temporary case.

FIG B (isometric with inset detail (ii); all submerged parts not shown) LAUNCH BUOYANCY HULLS.

Two buoyancy hulls (2) are craned into the water either side of the central gravity leg. Their construction can be similar to that of the gravity legs but without the concrete infill. They are spaced apart using a bracing frame (2a). This consists of a central boom combined with a series of marine hawsers. The function of the bracing frame is to maintain the plan alignment of the central boom at 90 degrees to the hulls while permitting sufficient vertical twist to accommodate wave action. The function of the buoyancy hulls will become apparent in the subsequent diagrams. They should be surrounded with sufficient fenders (2b) to prevent the locking together of hulls and gravity legs.

FIG C (isometric with inset detail (iii) ; all submerged parts not shown) LAUNCH OUTER GRAVITY LEGS The outer gravity legs (la) are constructed in an identical manner to the central leg. The combined assembly of three legs interspersed with two hulls is then moored together using marine hawsers acting on a series of protruding cleats (lb). It can now be seen how a tapering shape of the gravity legs allows them to come into contact at one end while keeping sufficient clearance for the hulls along their length. Continuous fenders are fixed to the extremities of each leg where they touch. See also detail iii.

FIG D- (isometric with inset details (iv) and (v); all submerged parts not shown) CONNECT MAST AND TURBINE Next the mast and turbine (3) are craned into position and connected to one end of the central gravity leg (see detail (iv)) using a universal pin joint (3a) which allows the mast to rise and fall about both vertical planes. A triangular winching platform (3b) has already been attached to the mast along its length via an axle (3c) which allows it to swivel, as shown in detail (v). Mast and platform are allowed to rest on the centre of the buoyancy hull bracing boom (2a), where a temporary and flexible rope connection is made. It can now be appreciated that the buoyancy hulls (2) should be sized to counteract the combined downward force on the bracing boom created by the cantilevered mast and turbine. Only a small upward force is applied from the other mast end to the central gravity leg. One cable from each of the three cable winch drums (3d) should be connected to each of the three swivelling cable cleats (3e) on the gravity legs.

It should be noted that the three cable winch motors (3f) shown in detail (v) are mounted separately to the winch drums as they constitute temporary plant within this system.

FIG E- (side elevation; above and below the water line; with inset detail (vi)).

TOW OUT The entire assembly can now be towed out to the installation Site. This diagram shows clearly how the cantilevered load of the mast and turbine (3) is transferred to the two buoyancy hulls (2) via their bracing boom (2a), helped by restraints at the mast base pin (3a), the winching platform axle (3c), and the winching platform frame (3b).

FIG F- (isometric with inset detail (vii) ; all submerged parts not shown) CREATE TRIPOD BASE When the assembly is directly over the installation site, the hawsers attached to the cleats (lb) at the towing end of the gravity legs are released (ld). The action of towing alone then swings the outer two legs round until a tripod shape is created in plan.

This can be secured with a new hawser connection (le) as illustrated in detail (vii). The cable winches connected to the two outer legs (3d) should be released to track this manoeuvre. It can now be seen why the cable connection cleats on each leg (3e) should be able to swivel.

FIG G- (side elevation with inset detail (viii); above and below water line) SINK GRAVITY TRIPOD BASE Water is now allowed to fill the hollow chambers of the three gravity legs. As they sink, their mass pulls one mast end down with them, while the fulcrum created by the boom (2a) on the buoyancy hulls forces the other end (together with the turbine) to rise vertically.

FIG H- (side elevation; above and below water line) WINCH MAST VERTICAL With the gravity legs resting on the seabed, the mast is held in its near vertical position by the action of the boom (2a) on the buoyancy hulls (2) alone, with the winching platform still in contact with the cross boom. The slack is now taken in on the three cable winches to finally level the mast. This manoeuvre also serves to lift the winching platform (3b) clear of the hulls, thus releasing them.

FIG J- (isometric, with inset detail (ix); all parts shown whether submerged or not) TOW AWAY BUOYANCY HULLS The system is now in its final working configuration. It should be noted how the restraint offered by the tripod of gravity legs is maximised by locating the swivelling cable cleats (3e) in positions that create a series of lever arms. The longer these levers, the greater the restraint. It should also be noted that the system would work equally well on a sloping or uneven seabed, taking into account the flexible hawser connections between the three gravity legs and the universal coupling at the base of the mast. All that now remains is for the cable winch drums (3d) to be locked rigid, for the winch motors (3f) to be disconnected and stowed away on the buoyancy hulls, and for the latter to be towed away after the bracing frame assembly (2a) has been dismantled.

These should. all be thought of as temporary plant, capable of reuse on the next installation.

Two options exist when it comes to sizing the gravity legs. If the most important factor is the curbing of their length, then step G above should be followed by the pumping of cement or grout into their buoyancy chambers. However, as stated previously, it is also possible for the entire system to be brought back to dry dock for periodic maintenance. In this case, the concrete within the legs alone must provide the required mass, and when the time comes, steps E to J above must be repeated in reverse, the only difference being that air must be pumped into the gravity legs, thus displacing water and recreating the buoyancy to lift them. Periodic maintenance of the turbine could also be facilitated by designing the axle connecting winching platform and mast (3c) so that it can be released from the mast to slide vertically. Such a feature would permit the lowering of the mast in situ using two of the cable winches.

Application of this system to submerged tidal generators would follow the same sequence, only the turbine would be attached below the winching platform. An isometric view of such an arrangement is shown in FIG K. Here the section of mast above the water line (4) is used for lifting the turbine (5) out of the water for inspection and maintenance using the winching platform.

The present invention has been described above purely by way of example. It should be noted that modifications in detail may be made within the scope of the invention as defined by the claims appended hereto.

Claims

CLAIMS 1. A gravity securing system for offshore equipment, the system comprising a plurality of anchoring legs suitable for siting on a seabed, and, in situ, being of sufficient size to resist movement therefrom, means for securing the legs to one another, a mast, means for securing the mast to at least one of the legs, thereby forming a leg and mast assembly, and means for stabilising the leg and mast assembly.
2. The system of claim 1, wherein the legs, in situ on a seabed, rest flush thereon.
3. The system according to claim 1 or 2, further comprising equipment in the form of electrical generating equipment.
4. The system according to claim 3, wherein the equipment is a wind turbine.
5. The system according to any preceding claim, wherein there are three legs.
6. The system of claim 5, wherein the legs, in situ, are secured together at an angle of approximately 120 degrees to one another.
7. The system according to any preceding claim, wherein the legs comprise buoyancy chambers suitable for filling either with air or water to enable flotation or submersion thereof.
8. The system according to any preceding claim, wherein the legs are substantially hollow, thereby permitting them to be floated to an installation site and then to be sunk into position.
9. The system according to any preceding claim, wherein the legs can be folded relative to one another into a towing configuration such that they lie substantially parallel to one another for towing to an installation site, and the legs can be unfolded from the towing configuration into a submergence configuration by swinging them about one another for sinking into position on the seabed.
10. The system as claimed in any preceding claim, wherein the legs are secured together using flexible bindings around protruding cleats, permitting them independence of movement vertically.
11. The system of claim 10, wherein the legs are constricted horizontally by friction acting on a series of fenders.
12. The system of any preceding claim, further comprising a hawser attached to each leg and a platform on the mast, the length of each hawser being controllable, wherein the hawsers facilitate stabilising the leg and mast assembly.
13. The system of claim 12, wherein the length of each hawser is controlled by a winch.
14. The system of claim 13, wherein the length of the hawsers can be locked and the winches can be disconnected from the system.
15. The system of claim 14, wherein the length of the hawsers are locked by locking the drums of the winches, and the winches are removed from the drums and can be reused.
16. The system according to any one of claims 12 to 15, wherein the platform is connected to the mast via an axle, thereby allowing the platform to fold almost parallel to the mast when the mast it is in a lowered position.
17. The system according to any one of claims 12 to 16, wherein the platform can slide up and down along the mast, permitting the platform to be positioned along the mast.
18. The system of claim 17, wherein the platform can be winched up to the top of the mast using a further system of cables and pulleys, and wherein this higher location allows it to raise and lower other equipment up and down the mast.
19. The system according to any preceding claim, wherein the plurality of legs, in a submergence configuration, radiate from an intersection point, thereby forming a star pattern.
20. The system according to claim 19, wherein the mast is secured to the legs at the intersection point.
21. The system of claim 20, wherein the intersection point is defined by an universal coupling, the coupling permitting rotational movement of the mast both from a lowered transportation configuration into a raised operational configuration and about the axis of the mast itself.
22. The system as claimed in any preceding claim, wherein, in a transportation configuration, the mast is carried on a boom spanning between two buoyancy hulls either side of one of the legs.
23. The system of claim 22, wherein the boom is braced to maintain a 90 degree alignment to both the hulls and the leg.
24. The system as claimed in claim 22 or 23, wherein the boom acts as a fulcrum for the mast, such that by sinking the legs, the buoyancy of the two buoyancy hulls enable the mast to be lifted from a substantially horizontal transportation configuration to a substantially vertical operational configuration.
25. The system as claimed in claim 22,23 or 24, wherein the two buoyancy hulls can be disconnected from the boom once the leg and mast assembly is vertical and has been fully stabilised.
26. The system as claimed in any preceding claim, wherein glands and valves are fitted to the legs.
27. A method of deploying a gravity securing system onto a seabed comprising the steps of floating a gravity securing system according to any preceding claim out to an installation site and sinking the legs onto the seabed whilst rotating the mast from a transportation configuration into a substantially vertical operating configuration.
28. A gravity securing system substantially as hereinbefore described with reference to the accompanying drawings.
29. An offshore electrical generating device substantially as hereinbefore described with reference to the accompanying drawings.
30. A method of deploying a gravity securing system substantially as hereinbefore described with reference to the accompanying drawings.