Floating structure for transporting and installing floating tension leg platform

Abstract

A floating structure (1) for transporting and installing a floating tension leg platform (TLP) (2) having a plurality of legs (3) radiating from a central point (4) is provided. The floating structure comprises: controllable ballast tanks (6, 6 ') for changing the buoyancy of the floating structure; and engagement means (9) engaged with the legs (3) of the TLP and maintaining the legs (3) at the desired draft during transportation and installation. The floating structure comprises a plurality of floating structure portions intended to be positioned horizontally near each leg of the TLP and provided with engagement means (9), in such a way as to engage mating engagement means provided on the legs of the TLP, such that in an operational position of the floating structure in which the floating structure portions are positioned horizontally alongside the legs of the TLP, the engagement means (9) engage mating engagement means provided on the legs of the TLP. The engagement means can be loaded on the mating engagement means with a downward force opposite the upward buoyancy of the TLP, and the engagement means are further arranged such that the engagement means can be engaged with the mating engagement means in the above-mentioned manner even when the legs of the TLP are at least partially located at a height above the sea level.

Description

Technical Field

[0001] The present invention relates to a floating structure for transporting and installing a floating tension leg platform (TLP) having a plurality of legs radiating from a central point. Background Art

[0002] Part of the energy transition is the development of offshore wind farms, where the use of seabed fixed structures is currently the latest technology. However, such fixed structures are limited to sites with limited water depth. The demand for renewable energy is growing rapidly, and therefore industrial, large-scale solutions for floating offshore wind farms are being studied, which can also take advantage of deeper sites. Therefore, there is great interest in the use of TLPs to support wind turbines, which can be moored to the seabed (directly or indirectly) using mooring cables (or tendons) attached to its legs. Due to its high power / weight ratio and small footprint, wind turbine foundations based on TLPs have great potential in the offshore floating wind market.

[0003] In the installed moored condition, the pre-tension in the tendons provides stability to the entire TLP system (TLP together with wind turbine components) due to the excessive buoyancy of the TLP. In this installation condition, the waterplane area of ​​the TLP is preferably small in order to limit the impact of wave loads, thereby reducing motions and accelerations.

[0004] However, the TLP system still lacks the tendon recovery and stabilization functions during the transportation and installation phases towards the desired offshore wind farm site. At free floating draft, the TLP system can remain stable, but its seaworthiness is limited and its dynamic stability range is insufficient for safe transportation. During the subsequent installation phase (or disassembly phase when the TLP system is decommissioned, for example), the TLP system is submerged and leaves a limited waterplane area, and the TLP system becomes unstable due to its high center of gravity (mainly caused by the high rise wind turbine on top of the wind turbine tower) and the large overturning moment caused by wind-induced drag loads.

[0005] Currently, TLPs are usually transported to their location on the deck of large crane vessels or by adding external (temporary) buoyancy at the TIF (transportation mounting frame) on their legs to keep them stable while being towed out by tugboats.

[0006] In addition, currently, TLPs are usually installed at their draft depth by placing temporary water ballast inside. However, this requires each individual TLP to be equipped with a ballast system, which greatly increases the structural complexity and cost of such TLPs, and also requires mitigation measures to prevent the corrosive environment inside the TLP. Summary of the invention

[0007] In view of the above, the object of the present invention is to provide a floating structure intended for transporting and installing a floating tension leg platform, with which such transport and installation can be carried out in a safe and reliable manner, without the need for (any or excessive) additional systems and / or construction measures regarding the TLP.

[0008] Therefore, according to the present invention, such a floating structure comprises: a controllable ballast tank for changing its buoyancy; and an engagement device for engaging with the legs of the TLP and maintaining the legs at a desired draft during transportation and installation, wherein the floating structure comprises a plurality of floating structure portions intended to be horizontally positioned adjacent each leg of the TLP, the floating structure portions being provided with engagement devices which engage with mating engagement devices provided on the legs of the TLP in such a manner that, in an operating position in which the floating structure portions of the floating structure are horizontally positioned alongside the legs of the TLP, the engagement devices are capable of loading the mating engagement devices with a downward force opposite to the upward buoyancy of the TLP, and wherein the engagement devices are further arranged so that they are capable of engaging with the mating engagement devices in such a manner even when the legs of the TLP are at least partially located at a height above sea level.

[0009] The floating structure according to the present invention provides an opportunity to transport and install a TLP in a safe and reliable (stable) manner. When a TLP (or a TLP system, such as a TLP provided with a wind turbine) is in a floating position and its legs are at least partially located at a height above sea level, a floating structure having its corresponding floating structure portion (the floating structure portion is provided with a coupling device) can be arranged side by side with the legs of the TLP, so that when the floating structure is ballasted, the coupling between the coupling device on the floating structure and the matching coupling device on the TLP is activated. Therefore, there is no need to provide a system that can position the floating structure above the TLP (for example, requiring the TLP to be below sea level). According to the present invention, while the TLP is in a normal floating position (which means a position generated by buoyancy and weight without the use of tendons), the floating structure can be engaged with the TLP in a desired manner.

[0010] During transport and installation, a floating structure with a large span and limited waterplane area holds the TLP in a stable position with good seakeeping properties (e.g. legs fully submerged during transport). The floating structure also allows the TLP to be lowered to the desired depth at the installation site (offshore wind farm) to allow hook-up of the mooring lines (tendons), after which the floating structure can be deballasted and the TLP will be moored in a stable manner by the tensioned mooring lines due to its buoyancy.

[0011] The mating engagement means on the leg of the TLP may comprise an identifiable device and therefore be a separate device provided on said leg, but it is also conceivable that a portion of the leg which is part of its normal structure (e.g. its top surface) is used to define or function as the mating engagement means (thus, in this case, the structure of the TLP is not altered).

[0012] For safety reasons, the engagement device can be provided with a member for improving the adhesion between the engagement device and the counterengagement device, or a member made of a soft material. Such a member for improving adhesion can, for example, include a vacuum pad or a magnet.

[0013] Alternatively or additionally, the floating structure may be provided with a locking member (eg a locking pin) for locking the engagement device to the mating engagement device. Such a locking member may cooperate with a dedicated member or part of the TLP.

[0014] To ensure optimal fit between the engagement means and the mating engagement means, the engagement means may have a shape that is complementary to the mating engagement means at a predetermined interface where it engages with the mating engagement means. For example, if the legs of the TLP are formed as circular tubes (whose upper surface defines the mating engagement means), the engagement means may have a similar circular shape.

[0015] To further help avoid local stresses between the engagement means and the counter-engagement means, the underside of the engagement means may be provided with a softer material than the rest of the engagement means, such as wood, plastic or rubber.

[0016] In a preferred embodiment, the engagement means comprises a protruding cam arranged and embodied to engage with a counter engagement means.

[0017] When the cam has a stationary position relative to the floating structure, the complexity of the construction of the floating structure can be minimized.

[0018] However, in alternative embodiments of the floating structure, the cam may be movable relative to the floating structure, such as linearly in the vertical direction or rotatably in the horizontal or vertical direction. Preferably, the cam may be movable in a motion-controlled or motion-restrained manner. Such movement of the cam may be used to control the manner in which the cam and the mating engagement device cooperate, particularly (but not limited to) in the process of connecting the floating structure to the TLP or separating it from it. The motion-controlled movement may be particularly useful for compensating for relative movement between the floating structure (e.g. caused by waves) and the TLP (e.g. which may have a stationary position in its moored state), such as to prevent severe collisions between them. The movement of the cam may also be used to avoid collisions between the floating structure and the TLP in situations to be discussed later (particularly during their disconnection).

[0019] In special cases, the vertical movement of the cam may also be used to (at least partially) lift the TLP out of the water, for example to reduce drag during transportation.

[0020] As an alternative to using a protruding cam, the engagement means may also comprise a recess in the floating structure (which may receive the mating engagement means of the TLP).

[0021] In a very particular embodiment, the floating structure according to the invention is provided with a release mechanism comprising a releasable locking member which releases the floating structure from the TLP in a controlled manner.

[0022] For example, the releasable locking member may include a rope, cable, or piston-cylinder assembly, a first end of which is releasably attached to one of the floating structure and the TLP, a second end of which is attached to the other of the floating structure and the TLP, and a distance between the first and second ends of which can be controlled.

[0023] A controlled release of the floating structure from the TLP can be used to avoid dangerous collisions between them. Without such releasable locking members, after the release of the floating structure from the TLP (e.g. when the latter has been installed and held at the desired position and depth by tendons) and during deballasting, the floating structure will start to move (upwards) gradually away from the TLP by a gradual increase in its buoyancy. In the initial stages of such movement, when the distance between the fixed TLP and the floating structure moving upwards is still quite moderate and the buoyancy of the floating structure is still quite limited, an uncontrolled movement of the floating structure (e.g. due to waves or other external factors) may result in a collision between the TLP and the floating structure, and a great risk of damage or a dangerous situation.

[0024] However, when using the controlled release mechanism according to the present invention, the floating structure can already be (partially) deballasted while the releasable locking member keeps the TLP connected (engaged) to the floating structure. Only when the floating structure has been sufficiently deballasted (generating a large buoyancy), the locking member can be separated or controlled in a way that increases the distance between its first end and the second end, which causes the floating structure to move upwards and away from the TLP. Because at this point, when this movement begins, the floating structure already has a large buoyancy, it is very unlikely (or even impossible) for the floating structure to collide with the TLP due to waves or other external influences.

[0025] Increasing the distance between the two ends of the locking member may include unwinding a cable or rope or extending a piston-cylinder assembly.

[0026] When the distance between the floating structure and the TLP has increased sufficiently, the locking member can be completely disconnected from at least one of the TLP or the floating structure (e.g., disconnected from the TLP so that the locking member remains connected to the floating structure for reuse with another TLP).

[0027] Other measures may be provided for avoiding collisions between the TLP and the floating structure. For example, the floating structure may be provided with means for ensuring rapid release between the floating structure and the TLP, such as one or more dump valves for rapid discharge of ballast water. Such rapid discharge of ballast water results in a sudden and rapid increase in the buoyancy of the floating structure, which also helps ensure that the floating structure moves only away from (and not towards) the TLP.

[0028] In construction, the floating structure according to the present invention may include a plurality of spaced-apart cylindrical ballast tanks connected by connecting beams, wherein the coupling means are provided on the ballast tanks.

[0029] This produces a very stable floating structure, both during transport and installation of the TLP and when floating alone. The stability comes mainly from the distance between the ballast tanks, each of which may have a fairly limited waterplane area, and which can be designed to obtain good seakeeping performance.

[0030] In one embodiment, the bottom of each ballast tank is provided with feet, the horizontal cross-section of which is larger than the horizontal cross-section of the rest of the ballast tank. This allows the floating structure to rest in a stable manner on the seabed (e.g. in a shallow harbor) when fully ballasted, and also when connected to a TLP (so that the TLP can be assembled with e.g. a wind turbine while remaining in a very stable position). Furthermore, the use of feet allows the floating structure to operate in shallow water.

[0031] The footings may or may not have ballast chambers. In addition, such footings may provide additional dynamic damping for the floating structure.

[0032] In a particular embodiment, in which the floating structure is intended for use with a three-legged TLP, the TLP includes three ballast tanks arranged in a triangular structure (viewed from above), with a first ballast tank at the top of the triangular structure and two other ballast tanks at adjacent corners of the triangular structure, each connected to the first ballast tank by a connecting beam.

[0033] The open portion of the triangular structure allows the floating structure to sail around the center point of the TLP (e.g., the tower of a wind turbine) and float towards and contact the three legs of the TLP with its ballast tanks. Each leg then contacts the floating structure (e.g., by a tip) (e.g., at the side wall of a cylindrical ballast tank) while the engagement means (e.g., cams) on the ballast tank are above the mating engagement means (e.g., upper surface) of the leg.

[0034] It is conceivable that the ballast tanks and connecting beams may be disconnected from each other, thereby enabling efficient transport and storage.

[0035] The floating structure according to the invention may include other features and may provide other advantages, which will be briefly mentioned below in a non-limiting manner.

[0036] The floating structure essentially adds a temporary tool to the TLP in order to provide the TLP with semi-submersible characteristics associated with good seakeeping, stability and increased maneuverability. The floating structure and its engaging means can sail over the legs of the TLP in order to then push the TLP downwards.

[0037] Similar to a semi-submersible, widely spaced columnar ballast tanks (if provided) provide significant stability to the entire system. This means that the waterplane area is minimized by concentrating the buoyancy in the ballast tanks. Minimized waterplane area helps reduce wave response. In addition, such columnar ballast tanks provide excellent space for ballast water and associated pumps / equipment.

[0038] Based on the positive buoyancy of the TLP and by ballasting and lowering the floating structure, the use of engagement means (e.g. cams) allows connection with the TLP (e.g. TIF of the legs). Here, the ballasting system of the floating structure is used to bring / lower the entire system (floating structure and TLP) to the installation depth, after which the floating structure is disconnected from the TLP, essentially by de-ballasing the floating structure. The floating structure can be safely disconnected from the TLP by translation, rotation and / or vertical clearance.

[0039] The floating structure may also be used to transport or store materials and equipment. For example, if tendons need to be installed from the floating structure, the tendons and installation equipment may be fitted to the floating structure. However, the floating structure may also provide a deck to accommodate other installation aids, such as power packs, cabins, small components, small cranes, walkways or tendons (carried by drums). Additional equipment may be equipped to the floating structure to allow controlled disconnection after hook-on as described above.

[0040] For larger TLPs, the design of the floating structure may be scalable, for example by replacing connecting beams with beams of other sizes, updating ballast volume and pump capacity, changing column diameter and height.

[0041] Finally, while typically the transportation and station keeping of floating structures is usually accomplished with the aid of tugboats (in line with normal offshore installation practice and in order to keep costs low), floating structures may also be equipped with DP (dynamic positioning) equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In the following, the present invention will be described with reference to the following drawings, in which:

[0043] Figure 1 is a perspective view of an embodiment of a floating structure connected to a TLP;

[0044] Figure 2 Also a perspective view of an embodiment of a floating structure connected to a TLP provided with a wind turbine;

[0045] Figure 3 Schematically illustrating the use of auxiliary components between the floating structure and the TLP;

[0046] Figure 4 The use of a locking member is shown;

[0047] Figure 5a-5c Three different embodiments of the cam are shown;

[0048] Figure 6 A vertically movable cam is schematically shown;

[0049] Figure 7 Three different embodiments with a rotatable cam are schematically shown;

[0050] Figure 8 An embodiment having a releasable locking member is shown; and

[0051] Fig. 9 The use of a drain valve is shown. DETAILED DESCRIPTION

[0052] exist Figure 1 and Figure 2 In the figure, two different views of an embodiment of a floating structure 1 according to the invention (note: in the following referred to as TIF - transport and installation frame) are shown. The TIF is used to transport and install a floating tension leg platform (TLP) 2 having a plurality of legs 3 radiating from a central point, such as a tower (or central column) 4 of a wind turbine 5 (the wind turbine is only Figure 2 ).

[0053] TIF1 includes a plurality of (here three) spaced apart controllable columnar ballast tanks 6, 6' to change its buoyancy. The ballast tanks 6, 6' are arranged in a triangular structure (seen from above), with the first ballast tank 6' located at the top of the triangular structure and the two ballast tanks 6 located at adjacent corners of the triangular structure. Each ballast tank 6 is connected only to the first ballast tank 6' by its own horizontal connecting beam 7, but not to each other. This leaves an open space between the ballast tanks 6 and between the connecting beams 7 for receiving TLP2.

[0054] The bottom of each ballast tank 6, 6' is provided with a footing 8, the horizontal cross-section of which is larger than the horizontal cross-section of the rest of the ballast tank 6, 6'.

[0055] In the embodiment shown, the TIF 1 is intended for use with a three-legged TLP 2 and therefore comprises three ballast tanks 6, 6'. For use with a TLP having a different number of legs 3, the TIF may comprise a different corresponding number of ballast tanks. The number of connecting beams 7 may also vary.

[0056] Each ballast tank 6, 6' is provided with an engagement means, which in the embodiment shown is implemented as a cam 9 for engaging with an upper surface at the TIF region of a respective one of the legs 3 of the TLP 2, which upper surface acts as a mating engagement means.

[0057] In other embodiments not shown, the cam may engage with other parts of the leg 3 of the TLP 2. As an alternative to the cam 9, a recess (in the Figure 1 The recess accommodates (part of) the leg 3 (schematically shown by dashed line 10).

[0058] Cam 9 will be used to maintain the TLP2 at the required draft during transport and installation, which will be explained at a later stage.

[0059] As shown in the figure, TIF1 therefore includes parts (in particular ballast tanks 6, 6') intended to be positioned horizontally near each leg 3 of TLP2, and these parts are provided with cams 9, which engage with the legs 3 of TLP2 in such a way that in the operating position where the said parts of TIF1 are positioned horizontally side by side with the legs 3 of TLP2, the cams 9 are able to load the legs 3 with a downward force opposite to the upward buoyancy of TLP2.

[0060] The cam 9 may also be arranged so that it can engage with the leg 3 in the manner described even when the leg 3 of the TLP 2 is at least partially located at a height above sea level.

[0061] Figure 2 It is also shown that the TIF 1 may include a structure 11 for carrying objects 12 (such as materials or equipment). The ballast tanks 6, 6' may also support additional equipment, such as a crane 13 (only in Figure 2 ) or other objects.

[0062] like Figure 3 As shown, the cam 9 can be provided with a member 14, which is used, for example, to improve the adhesion between the cam 9 and the leg 3, in which case such a member 14 can, for example, include a vacuum pad or a magnet. Alternatively, the member 14 can also be made of a soft material (such as wood, plastic or rubber) to avoid high local stresses.

[0063] Reference Figure 4 A locking member 15 (eg a locking pin) may also be provided for locking the engagement means (cam 9) to the mating engagement means (leg 3).

[0064] like Figure 5a-5c As schematically shown in the three different embodiments, the engagement means (cam 9) will preferably have a complementary shape relative to the mating engagement means (leg 3) at the predetermined interface for engagement with the mating engagement means to ensure an optimal force transmission between them. Figure 5a In the embodiment, the upper surface of the leg 3 is flat and the lower edge of the cam 9 is also flat. Figure 5b In , both are rounded in a similar manner, while in Figure 5c In the embodiment, there are two spaced apart cams 9 whose lower edges are located at different heights to match the inclined upper surface of the leg 3.

[0065] In the embodiment shown above, the cam 9 has a stationary position relative to the TIF 1 (here relative to the ballast tanks 6, 6'). However, as an alternative, the cam 9 can be moved relative to the TIF, for example in the vertical direction V ( Figure 6 ) is moved linearly to a raised position 9' disengaged from the leg 3, or by rotating R( Figure 7), or inwardly, for example, rotated into a receiving opening (not shown) of a ballast tank 6, 6' ( Figure 7 b), or by rotating R in the horizontal direction ( Figure 7 Each mode moves to a new position 9' disengaged from the leg 3 (as shown in c).

[0066] The mobility of the cam 9 in particular allows sufficient clearance to be quickly established between TIF1 and TLP2, in particular during disconnection.

[0067] The cam 9 may be movable in a motion-controlled or motion-restrained manner (particularly in the vertical direction), for example to compensate for relative movements between TLP2 and TIF1 .

[0068] TLP2 is usually manufactured in a shipyard and then transported to a marshalling yard where it can be unloaded and moored in shallow waters in or near the port. TIF1 and its cam 9 can float above the moored TLP and can move it to the quayside or prepared seabed, and then bring it to the seabed or artificial support structure through ballast.

[0069] The unique design of the footing 8 (enlarged horizontal portion) allows the TIF to operate in shallow water, thereby optimizing the use of the port area or near-port space.

[0070] The triangular or V-shaped open space of the TIF 1 defined by the ballast tanks 6, 6' and the connecting beams 7 allows it to sail around the central column of the TLP (e.g. tower 4) and then float alongside the legs 3 and then float the cam 9 over the legs 3. Maneuvering the TIF 1 can be achieved by dedicated propulsion means or using tugboats.

[0071] The tip of each leg 3 will contact the corresponding side wall of the cylindrical ballast tank 6, 6', thereby ensuring the correct relative position between TIF1 and TLP2.

[0072] Next, by filling the ballast tanks 6, 6' with water to ballast the TIF1, the TIF1 will move downward until the cam 9 engages the legs 3 of the TLP2. By controlling the ballasting of the TIF1 (and thus the negative buoyancy), the force of the cam 9 loading the legs 3 of the TLP2 downward (opposite to the buoyancy of the TLP) and the draft of the entire TIF / TLP assembly can be controlled. Additional connections between the TIF1 and the TLP2 (such as cables or mechanical connections) can be used for additional safety during towing / transportation.

[0073] During transportation, TLP2 and TIF1 are connected together and thus are considered as a single floating unit. The main function of TIF1 is to provide additional stability to TLP2 (optionally equipped with wind turbines 5) for optimal seakeeping performance.

[0074] Typically, during such transport, the TLP legs 3 will be completely submerged. The central column of the TLP (tower 4) and the three columnar ballast tanks 6, 6' of the TIF1 are typically partially submerged, and all ballast tanks 6, 6' will provide waterplane area. The waterplane area and its distribution are designed so that the natural cycles of heave and pitch / roll are outside the prevailing wind and swell cycles. As a result, the entire assembly (TIF1, TLP2, optional wind turbine 5) has sufficient stability.

[0075] As mentioned previously, propulsion for the TIF1 can be provided externally via a tugboat or via a TIF with a propeller mounted. Both speed and heading control may require multiple tugboats.

[0076] The purpose of propulsion is to transport the entire assembly to the installation site in moderate sea conditions. Important design criteria are the maximum acceleration and towing speed experienced by the wind turbine 5. The latter is related to the total drag. The drag is reduced by rounded vertical cylindrical ballast tanks 6, 6' and smoothly shaped feet 8. The seaworthiness will also be designed to be able to "survive" any unforeseen severe weather conditions and remain within the movement limits of the wind turbine, thus providing robustness to the TIF1 in the operational scenarios.

[0077] A possible configuration of TIF1 may include a lifting mechanism (see Figure 2 The crane 13 in the figure can transport the TLP 2 when it is (fully or partially) lifted out of the water. This will reduce the drag of the entire system.

[0078] When arriving at the installation site, the ballast water in the ballast tanks 6, 6' will be used to increase the draft of the entire system. Ballasting can be done after arriving at the site, or in certain weather conditions it can also be started during transportation. The ballasted depth of TIF1 will be slightly deeper than the target installation depth of TLP2. This will allow the installation and assembly of the tendons; the length of the tendons varies depending on the site and location. An extension line or installation line will be connected to the upper part of the tendon to guide the upper part of the tendon into the tendon connector at the leg TIF of TLP2.

[0079] The tendons will be pre-installed (pre-laid mooring system) or installed from TIF1. In this case, the tendon material and the required installation equipment will be installed on the deck of the TIF (see Figure 2 The large "deck" space is an important feature of this TIF1, allowing for safe walkways, equipment, and storage of equipment 12 and small components.

[0080] TIF1 provides significant stability to the entire system, which is crucial during the hook-up of the tendon. This will minimize the relative movement between the pre-installed tendon and the tendon connector at leg 3 of TLP2. Logically, this will increase the weather window for hook-up and safety during operation. In the case where the tendon will be installed starting from TIF1, the relative movement between the lower part of the tendon and the anchor point is minimized.

[0081] After hooking is made, TIF1 will begin to de-ballast and slowly move upwards. As a result, the tendon is tensioned. This will continue until the tendon is loaded to the target pre-tension level. At this point, TIF1 and its cam 9 will begin to separate from the leg 3 of TLP2. TIF1 will continue to de-ballast at a carefully designed rate so that the separation between TIF1 and TLP2 remains safe. Or more precisely, the gap will usually increase quickly enough so that the probability of re-contact (retouch) is low.

[0082] During the first stage of separation between TIF1 and TLP2, the buoyancy of TIF1 is still moderate and there is a risk of TIF1 colliding with TLP2 (held stationary by its tendons) due to external influences (e.g. waves).

[0083] Therefore, from the perspective of reliability, robustness and safety, additional equipment can be added to the TIF1 for releasing the TLP2 from the TIF1 in a controlled manner. It can be some kind of release mechanism with a releasable locking member. Such a mechanism can also be used to create a horizontal gap. It is also an option to apply a soft contact material on the TIF1.

[0084] Figure 8 An embodiment of such a release mechanism is schematically shown, in which a releasable locking member 16 (e.g. a rope, cable or piston-cylinder assembly) is provided, the first end 16' of which is releasably attached to the TIF1 (ballast tanks 6, 6') and the second end 16" of which is attached to the TLP2 (leg 3) (moored by tendons 17). The locking member 16 initially holds the TIF1 in firm contact with the TLP2 until the TIF1 is sufficiently deballasted to obtain a large residual buoyancy. The locking member 16 can then be released (e.g. at the second end 16") and the TIF1 will move upwards at a faster speed without the risk of colliding with the TLP2. It is also possible that the locking member 16 temporarily remains connected to both the TIF1 and the TLP and is released, for example, using a winch (at Figure 8 18) increases the length (between ends 16' and 16") so that TIF1 will steadily move upward and away from TLP2. Alternatively, part 18 may be a piston housing of a piston-cylinder assembly, in which case 16 represents the piston rod.

[0085] When the gap between TIF1 and TLP2 in the vertical and horizontal directions is large enough, TIF1 can be moved away from TLP2. In addition, cam 9 can be moved to a safe position. If TIF1 is outside the installation site, the installation of TLP2 is completed.

[0086] To further assist in the safe and rapid release of the TIF1 from the TLP2, additional means (eg, one or more discharge valves 19) may be provided for rapidly discharging ballast water from the ballast tanks 6, 6' of the TIF1.

[0087] With TLP2 installed, TIF1 can be towed back to port. Prior to this voyage, the draft of TIF1 can be optimized (by ballasting or deballasting) for the expected sea (wave) and wind conditions. As a result, TIF1 has enough stability to be towed back in moderate and moderately rough conditions.

[0088] On reaching the final destination the draft can be reduced, allowing the TIF 1 to be moved to shallow water for further operations or storage. The dedicated footings 8 will again allow a shallow draft and allow the TIF 1 to rest on the (flat) sea bed.

[0089] The invention is not limited to the described embodiments which may vary significantly within the scope of the invention defined by the appending claims.

Claims

1. A floating structure (1) for transporting and installing a floating tension leg platform (2), the tension leg platform having a plurality of legs (3) radiating from a central point (4), the floating structure comprising: Controllable ballast tanks (6, 6') for changing the buoyancy of the floating structure; and an engagement device (9) for engaging with the legs (3) of the tension leg platform and maintaining the legs (3) at a desired draft during transport and installation, wherein the floating structure comprises a plurality of floating structure portions, the floating structure portions being intended to be positioned horizontally adjacent each leg (3) of the tension leg platform, the floating structure portions being provided with engagement devices (9) for engaging with mating engagement devices provided on the legs (3) of the tension leg platform (2) in such a manner that in an operating position in which the floating structure portions of the floating structure are positioned horizontally alongside the legs of the tension leg platform, the engagement devices are capable of loading the mating engagement devices with a downward force opposing the upward buoyancy of the tension leg platform, and wherein the engagement devices (9) are further arranged to engage with the mating engagement devices in such a manner even when the legs (3) of the tension leg platform are at least partially located at a height above sea level.

2. The floating structure according to claim 1, wherein the engagement device (9) is provided with a member (14) for improving the adhesion between the engagement device and the mating engagement device, or a member made of a soft material.

3. The floating structure according to claim 2, wherein the member (14) comprises a vacuum pad or a magnet.

4. A floating structure according to any of the preceding claims, provided with locking means (15), such as locking pins, for locking the engagement means (9) to the counter-engagement means.

5. A floating structure according to any one of the preceding claims, wherein the engagement means (9) has a shape complementary to the counter engagement means at a predetermined interface for engagement with the counter engagement means.

6. A floating structure according to any one of the preceding claims, wherein the engagement means (9) is arranged to engage with a top surface of the leg (3) of the tension leg platform (2).

7. A floating structure according to any one of the preceding claims, wherein the engagement means (9) comprises a protruding cam arranged and implemented to engage with the counter engagement means.

8. A floating structure according to claim 7, wherein the cam (9) has a rest position relative to the floating structure (1).

9. The floating structure according to claim 7, wherein the cam (9) is movable relative to the floating structure (1), such as linearly in a vertical direction (V) or rotatably in a horizontal or vertical direction (R).

10. The floating structure according to claim 9, wherein the cam (9) is movable in a motion-controlled or motion-restrained manner.

11. A floating structure according to any one of claims 1 to 6, wherein the engagement means (9) comprises a recess (10) in the floating structure part.

12. A floating structure according to any one of the preceding claims, provided with a release mechanism comprising a releasable locking member (16) for releasing the floating structure (1) from the tension leg platform (2) in a controlled manner.

13. A floating structure according to claim 12, wherein the releasable locking member (16) comprises a rope, a cable or a piston-cylinder assembly, a first end (16') of which is releasably attached to one of the floating structure and the tension leg platform, a second end (16") of which is attached to the other of the floating structure and the tension leg platform, and the distance between the first end and the second end of which can be controlled.

14. The floating structure according to any of the preceding claims, provided with means for ensuring a quick release between the floating structure (1) and the tension leg platform (2), such as one or more discharge valves (19) for quick discharge of ballast water.

15. A floating structure according to any one of the preceding claims, wherein the floating structure comprises a plurality of spaced-apart cylindrical ballast tanks (6, 6'), the ballast tanks being connected by connecting beams (7), and wherein the joining means (9) are arranged on the ballast tanks.

16. The floating structure according to claim 15, wherein the bottom of each ballast tank (6, 6') is provided with a footing (8), the horizontal cross-section of the footing being larger than the horizontal cross-section of the rest of the ballast tank.

17. A floating structure (1) according to claim 15 or 16, which is intended for use with a three-legged tension leg platform (2), comprising three ballast tanks (6, 6') arranged in a triangular structure, wherein a first ballast tank (6') is located at the top of the triangular structure, and the other two ballast tanks (6) are located at adjacent corners of the triangular structure and are both connected to the first ballast tank by a connecting beam (7).

18. The floating structure according to any one of claims 15 to 17, wherein the ballast tanks (6, 6') and the connecting beams (7) can be disconnected from each other.

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