Field of the Invention

[01] The present disclosure relates to an offshore platform for, amongst other applications, a wind turbine. In particular, the present application is concerned with floating type offshore platforms (i.e., platforms with buoyancy) and a method for deploying the platform in the sea.

Background

[02] It has long been appreciated that offshore wind farming provides significant advantages over onshore alternatives: wind is generally much stronger off the coasts and more continuous throughout the day. Historically, most offshore wind farms employ fixed-foundation wind turbines in relatively shallow water, allowing their deployment not only in coastal areas but also lakes and fjords. More recently, floating wind turbines for use in deeper waters - e.g., over 100 m (metres) deep - have started to be developed and deployed.

[03] One technique for anchoring a floating wind turbine is a tension leg platform. Here a submersible platform is attached to the seabed by vertical moorings originating from each of the corners of the platform, with the moorings being undertension to maintain the platform at a certain depth so that the platform is stable.

[04] A typical technique for tensioning a mooring chain line (and achieving the required depth of the platform) is the linear chain tensioner, or linear chain jack, which is fixed to the platform to feed the mooring chain through itself until the chain is suitably tensioned. One tensioner is provided for each mooring chain. Such tensioners are incredibly large, not only in terms of the physical chain feeding mechanism, but also in the footprint required for e.g. storing the chain after its fed through the mechanism, and also heavy, which has implications for the buoyancy of the platform. They are also expensive, thereby putting at risk the commercial viability of installing the offshore wind platform.

[05] It is therefore highly desirable to develop alternative techniques, and accompanying apparatuses, for submerging and stabilising a wind turbine platform.

Summary

[06] The present invention is defined according to the independent claims. Additional features will be appreciated from the dependent claims and the description herein. Any embodiments which are described but which do not fall within the scope of the claims are to be interpreted merely as examples useful for a better understanding of the invention.

[07] The example embodiments have been provided with a view to addressing at least some of the difficulties that are encountered with current approaches to floating windfarm platforms, whetherthose difficulties have been specifically mentioned above orwill otherwise be appreciated from the discussion herein. In particular, it is an aim of the present disclosure to provide an improved tensioning technique for submerging a floating platform that does not require multiple chain tensioning systems.

[08] Accordingly, in one aspect of the invention there is provided a floating offshore platform for a wind turbine. The platform comprises an at least partly submersible superstructure, a ladder element provided on the superstructure, and a tensioner unit configured to engage with, and be moveable along, the ladder element, and also configured to couple to an installation line anchored to the seabed. Walking the tensioner unit along the ladder element initially tensions the installation line, and once the line is under tension, then causes the platform to submerge. The provided tensioner system is advantageously smaller than contemporary systems, having fewer mechanical components and therefore fewer points of failure.

[09] Moreover, it is particular envisaged that the tensioner unit is detachable from the ladder element, so that it can be reused on second, third, fourth, platforms, and so on, thereby significantly reducing costs associated with installing platforms as part of an offshore windfarm, as each platform does not have to be provided with a dedicated tensioning system. As such, the platform may suitably comprise means to secure one or more mooring lines to the superstructure.

[10] In some examples, the platform may comprise a plurality of ladder elements provided on its superstructure, with a corresponding plurality of tensioner units. The ladder elements are suitably located such that forces on the platform 100 applied by the plurality of tensioner units coupled to them substantially counterbalance, so that the platform is maintained substantially level in order to support a wind turbine. Put another way, a plurality of ladder elements may be suitably located so as to avoid any torques on the platform when the tensioner units are active (assuming the tensioner units are maintained substantially in equilibrium).

[11] Suitably, in another aspect of the invention, there is provided a method of deploying a floating offshore platform for a wind turbine. The method comprises coupling an installation line to a tensioner unit engaged with a ladder element provided on an at least partly submersible superstructure of the platform (the installation line being previously or subsequently anchored to the seabed), controlling the tensioner unit to walk along the ladder element, thereby submerging the platform, and controlling the tensioner unit to stop walking along the ladder element when the platform reaches a predetermined depth. After the predetermined depth has been reached, the method may comprise attaching at least one mooring line to the platform, decoupling the installation line from the tensioner unit, and disengaging the tensioner unit from the ladder element.

[12] In another aspect of the invention there is provided a tensioning system for use in deploying a floating offshore platform. The system comprises a ladder element configured to be detachably couplable to a superstructure of the platform, and a tensioner unit configured to engage with, and be moveable along, the ladder element, and configured to be couplable to an installation line anchored to the seabed.

Brief Description of the Drawings

[13] For a better understanding of the present disclosure reference will now be made by way of example only to the accompanying drawings, in which:

[14] Fig. 1 shows an example floating platform with improved tensioning system;

[15] Fig. 2 shows another, preferred, example floating platform with improved tensioning system

[16] Fig. 3 shows an example tensioner unit for the tensioning system in more detail;

[17] Fig. 4 shows an example tensioner unit in operation moving along a ladder element;

[18] Fig. 5 summarises a method of submerging a floating platform comprising an example tensioning system.

Detailed Description

[19] At least some of the following example embodiments provide an improved technique for submerging a floating platform, particularly for wind turbines, which is stabilised by a tension leg system. In particular the example embodiments provide an alternative approach to tensioning which is simpler and more cost effective than current practices. Many other advantages and improvements will be appreciated from the discussed herein.

[20] Figure 1 shows an example floating offshore platform 100 for a wind turbine 10. The platform 100 is at least partly submerged to a set depth 102 below a surface 104 of a body of water (e.g., of a sea/ocean). As will be appreciated by those in the art, floating type platforms are buoyant, so that an upward force is generated on the (partly submerged) platform which attempts to the raise the platform 100 to the water surface 104.

[21 ] The platform 100 comprises a tensioning system 106, which comprises a tensioner unit 108 and a ladder element 110 (may also be termed a climbing ladder). The ladder element 110 is provided on a superstructure 112 of the platform 100, and is preferably oriented such that the ladder element 110 is at least partly oriented toward the seabed 116 (in a direction of travel along the ladder 110).

[22] In the present example, a single ladder element 110 is provided on a centrally aligned appendage 114 (i.e., on a central axis of the platform 100) which extends (vertically) into the ocean towards the seabed 116. In this way travel along the ladder element is provided in an advantageously simple up/down direction to/from the seabed 116 / sea surface 104.

[23] In other arrangements, a plurality of ladder elements 110 may be provided to correspond to a shape of the superstructure 112 in the plane ofthe water surface 104, with at least one ladder element 110 being provided to correspond to each of the edges or vertices on the platform (as viewed from above or below looking into/out of the water). For example, where the shape of the platform 100 with respect to the surface 104 is substantially triangular, at least three ladder elements may be provided, where the shape is substantially square, at least four ladder elements may be provided, and so on. Preferably the plurality of ladder elements 110 are equispaced across the platform 100; e.g., provided at each vertex of the platform, or centrally with respect to each edge of the platform.

[24] In some example arrangements the ladder element 110 forms part of the platform 100. For example, the ladder element(s) 110 may be manufactured as an integral part of the superstructure 112, or may be manufactured separately and fixedly attached (e.g., by welding) to the superstructure 112. In other examples, the ladder element 110 does not form a fixed component of the platform 100, instead being detachably coupled to the superstructure 112 using suitable coupling means. Although the ladder element 110 so arranged will have less mechanical strength than if it were fixed to the platform 100, such an arrangement has the advantage that the ladder element 1 10 may be reused on another platform, thereby reducing manufacturing costs and installation costs when the present techniques are implemented for platform installation across an entire windfarm.

[25] The tensioner unit 108 is configured to engage with the ladder element 110 and move along the ladder element 110 so as to be able to climb up and down the ladder. More specifically, the tensioner unit 108 comprises means to releasably engage one or more rungs or slots 120 of the ladder element 110, and means to move the tensioner unit 108 along the ladder element 110 (between disengaging and (re)engaging the rungs/slots 120).

[26] The tensioning system 106 also comprises an installation line 122 which is anchorable to the seabed 116. Suitably, the tensioner unit 108 is configured to couple (by suitable means) to the installation line 122; preferably, the installation line 122 is coupled at a lower part of the tensioner unit 108 closest to the seabed 116. Thus, as the tensioner unit 108 ascends the ladder element 1 10 - i.e., moves away from the seabed 116 - attempting to pull the installation line 122 behind it, the installation line initially tensions (i.e., takes out any slack in the line 122) and, while the installation line 122 is under tension, the climbing action of the tensioner unit 108 causes the platform 100 to submerge. It will be appreciated that the installation line 122 could be coupled to a top end of the tensioner unit 108 such that the tensioner unit 108 is configured to descend the ladder element 110, but this is not preferred due to the additional pulley mechanisms that would be required adding complexity to the system.

[27] When the platform 100 has been submerged to the desired depth 102, mooring lines 124 (e.g., suitable strength steel chains) anchored to the seabed 116 may be secured to the platform 100 by suitable means to secure the platform 1 10 in place relative to the seabed 1 16. Analogous to prior systems, in some examples the installation line 122 may also be a mooring line, however this is not preferred. Instead, it is preferred that the installation line 122 be transferable, with the tensioner unit 108, for use in installation of further wind turbine platforms. The depth of the platform may be determined by a suitable sensor 118 provided on the platform 100 or as part of the tensioning system 106 (e.g., comprised as part of the ladder element 110 or the tensioner unit 108).

[28] Figure 2 shows a preferred example for the floating offshore platform 100. Here the superstructure 112 of the platform 100 is tetrahedral in shape, being formed from a plurality of hull segments 126. A first set 126a of the hull segments are arranged to be substantially parallel to the water surface 104; in other words, the three hull segments forming the first set 126a form a hollow base forthe platform 100. A second set 126b of the hull segments are arranged to extend upward, at an angle, from the first set 126a to join at a plinth 124 to which the wind turbine 10 is mounted; put anotherway, the three hull segments forming the second set 126b form a triangular pyramid arising from the triangular base formed by the first three hull segments 126a to complete the tetrahedral structure. Preferably at least the first set of hull segments 126a are each buoyant, and further preferably the second set of hull segments 126b are also buoyant.

[29] In this example the platform 100 comprises three ladder elements 110, each one being provided on an outer surface of each of the second set of hull segments 126b. Each ladder element 110 is arranged to extend along substantially an entire length of a hull segment 126b. Accordingly, three tension units 108, one per ladder element 110, are provided, each tension unit 108 being arranged to climb its respective ladder element 110 while coupled to a respective installation line 122. The vertices 128 between each of the first set of hull segments 126a may be provided with a guide for the installation line 122 (i.e., to prevent the installation line rubbing on the superstructure 1 12).

[30] As described above, mooring lines 124 (not shown in Fig. 2) are preferably attached to the superstructure when it has been submerged to the desired depth. In this example the platform 100 may be provided with suitable attachment means at each of the vertices 128.

[31] Figure 3 shows an example tensioner unit 108 and ladder element 110 in more detail. The tensioner unit 108 comprises an upper locking head 128, a lower locking head 130, and a hydraulic tension cylinder 132 therebetween.

[32] The tensioning cylinder 132 is designed to generate the force required to tension the platform 100. By way of example, the tension cylinder 132 may be arranged to generate a force equivalent to between 250 Te (tonnes) and 1000 Te, and further preferably equivalent to from 500 Te to 750 Te (inclusive). Suitably, the cylinder 132 may be manufactured from a high yield carbon steel body and stainless-steel rod (material 17/4 PH). This may be hard chrome plated to provide protection for offshore operation.

[33] The cylinder 132 comprises a suitable connector at each of the annulus (/rod) end and fullbore end for cooperatively coupling to the lower and upper locking heads 128, 130. For example, the cylinder may comprise male clevis arrangements 138, 140 which connect to the corresponding female clevis arrangements 142, 144 on the locking heads. The cylinder body and clevis arrangements may be coated in a protective paint to reduce rust.

[34] Preferably it is the annulus side of the cylinder 132 which is load bearing. Suitably, the annulus end of the cylinder is arranged to be coupled to the upper locking head 128 (with the fullbore end coupled to the lower locking head 130). In this way, the cylinder 132 acts in tension which removes the likelihood of a buckling effect on the cylinder rod.

[35] In some examples, the sensor 118 for determining depth is incorporated into the body of the cylinder 132. For example, the cylinder 132 may comprise a linear transducer, signals from which may indicate a number of strokes taken by the cylinder 132 (i.e., measure a stroke count) and provide a positional indication of where the cylinder is along the climbing ladder from which depth may be determined given a known starting depth of the platform 100 I tensioner unit 108. Moreover, the linear transducer may enable an operator to determine whether the cylinder 132 is operating correctly along its stroke, and to determine when the cylinder 132 is fully extended and retracted.

[36] In a preferred arrangement, the tension cylinder 132 is arranged to move the tensioner unit 108 within a channel 134 formed by the ladder element 110. Put another way, the tensioner unit 108 is situated within the channel 134 so that its motion is suitably guided/restricted, thereby helping prevent slippage of the tensioner unit off the ladder element 110.

[37] Suitably, the ladder element 110 is engaged by the locking heads 128, 130, with one locking head being engaged and the other disengaged with alternate extensions/retractions of the tensioning cylinder 132. Suitably, each locking head 128, 130 may be arranged to engage the ladder element 110 via at least one coupling to the ladder element 1 10: for example, each locking head 128, 130 may engage the ladder element via a slot in the base of the channel 134 (not shown). Preferably, however, each locking head 128, 130 is arranged to couple to the ladder element 110 by at least two couplings, which are further preferably provided on opposite sides of the locking heads 128, 130, so that each locking head 128, 130 engages the ladder element 110 on opposite sides of the tensioning unit 108: for example, the locking heads 128, 130 may engage with slots 120 provided in side walls 136 of the channel 134, as shown. It will also be appreciated that, in another example, one of the locking heads (e.g., an upper locking head 128) may engage slots 120 in the side wall 136 of the channel 134, while the other locking head (e.g., the lower locking head 130) may engage a slot in the base of the channel 134 (not shown).

[38] In more detail, in a preferred example the upper locking head 128 may comprise a pair of locking pawls 146 on opposite sides of the locking head 128 in an axis orthogonal to a major (z) axis of the tensioning cylinder 132. The locking pawls 146 each comprise a retaining lip 148 which engages an edge of a slot 120 provided in a side wall 136. The pawls 146 are actuated (i.e., to engage and disengage the slots 120) by a double acting hydraulic cylinder 150, and may be provided with a linear transducer by which it may be determined that the pawls 146 are engaged based on whether the cylinder 150 is extended or retracted. The pawls 146 may also be fitted with indicator flags to provide a visual indication of whether they are extended or retracted.

[39] The pawls 146 may be manufactured from high strength alloy steel (EN24T or similar) to provide the required characteristics for shear, bending and bearing. The upper locking head 128 may be suitably manufactured from a single billet of high strength carbon steel (min yield 355) suitably machined to provide the space/interfaces for the pawls 146 and cylinder 150. The present design beneficially reduces the number of (moving) parts and therefore possible failure points in the locking head 128.

[40] As shown, the lower locking head 130 may be formed similarly to the upper locking head 128, and so repeat description is omitted. In addition, however, the lower locking head 130 comprises a second female clevis arrangement 152 by which the tensioner unit 108 may be coupled to the installation line 122 (e.g., by the line 122 being provided with a suitably looped end). Suitably, the installation line 122 may be secured in place with a load monitoring pin arrangement, which may provide feedback on the tension load on the installation line 122.

[41] Figure 4 shows the example tensioner unit 108 of Figure 3 in operation climbing the ladder element 110. At step 401 the tensioner unit 108 is mounted to the platform 100 by engagement with the ladder element 110; specifically, the locking pawls 146 of the upper and lower locking heads 128, 130 are all engaged with the ladder element 1 10. The installation line 122 may be suitably attached to the lower locking head 130 either before or after the tensioner unit is coupled to the ladder element 110. Here the tensioner unit 108 is shown initialised with tensioning cylinder 132 in a retracted, or closed, configuration. It should be appreciated however that the tensioner unit 108 may alternatively be initialised in an extended, or open, configuration.

[42] At step 402, the pawls 146 of the upper locking head 128 are released. The fact that the pawls 146 are released may be confirmed by one or both of a hydraulic sensor (e.g., linear transducer) on the cylinder 150 and visual confirmation (e.g., using indicator flags). The cylinder 132 is then controlled to extend so that the upper locking head 128 slides within the channel 134 to rise up the ladder element 110. Once the tensioning cylinder 132 has been maximally extended (confirmed by e.g., a linear transducer as discussed above), the pawls 142 of the upper locking head 128 are extended to engage and lock into an appropriate slot 120 of the ladder element 110.

[43] At step 403, the locking pawls 146 of the lower locking head 130 are released (again this may be suitably confirmed before the procedures continue). The tensioning cylinder 132 retracts, pulling the lower locking head 130, and thereby the installation line 122, up the channel 134. The pawls 146 of the lower locking head 130 then engage the ladder element 110 to lock the lower locking head 130 in position. For the sake of expedience, it may now be taken that the installation line 122 is under tension, however it will be appreciated that steps 402 and 403 may be repeated until the movement of the tensioner unit 108 has climbed sufficiently far up the ladder element 110 to tension the installation line 122.

[44] At step 404, an analogous process to step 402 is conducted. The locking pawls 146 of the upper locking head 128 release the ladder element 1 10, and then the tensioning cylinder 132 extends to push the upper locking head 128 up the ladder element 110. The locking pawls 146 of the upper locking head 128 are then controlled to re-engage the ladder element 110 higher up than where they started.

[45] At step 405, an analogous process to step 403 is conducted. Once again, the locking pawls 146 of the lower locking head 130 release the ladder element 110, and the tensioning cylinder 132 is controlled to retract. In contrast to step 403, however, as the installation line 122 is now under tension (due to being anchored to the seabed 116 and fully extended), the contraction of the cylinder 132 exerts a downward force on the platform 100 thereby pulling it deeper into the water.

[46] Steps 404 and 405 are repeated until the platform 100 is (at least partly) submerged to a desired depth (the depth may be determined by a sensor 118 as discussed above). At this point the tensioner unit 108 may be controlled to stop climbing the ladder element 110, and mooring lines secured to the platform 100 at suitable locations (and anchored to the seabed 116). The tensioner unit 108 may then be walked back down the ladder element 1 10 to release tension on the installation line 122, the installation line 122 decoupled from the tensioner unit 108, and the tensioner unit 108 decoupled from the ladder element 110 (not necessarily in that order). In this way the tensioner unit 108 may be used in the deployment of another platform, and does not need to be made an integral component of the platform 100. In addition, the ladder element 110 may also be removed from the platform 100 for reuse, if it has not been manufactured as an integral component of course.

[47] It will be appreciated that the steps 402 to 405 may be controlled by a suitable operator individually, the operator checking/confirming the expected operation of the tensioning cylinder 132 and the release/engagement of the locking heads 128, 130 at each stage. For example, an operator may be located on a nearby support vessel provided with suitable radio control of a hydraulic supply/return controlling actuation of each of the cylinders 132, 150 (including radio receipt of any data from any relevant sensors). Alternatively, the climbing operation may be entirely automated, being instead controlled by a suitable computer system which suitably monitors the relevant sensor data to check the operation of the cylinder 132 and locking heads 128, 130 at each stage before initiating the next.

[48] In the case of the platform 100 being provided with two or more tension units 108, for example three tensioner units 108 as in the preferred example of Fig. 2, then each tensioner unit 108 may be controlled in sequence to perform one climbing step. Put another way, a first tensioner unit performs step 402/404 followed by 403/405, followed by a second tensioner unit performing step 402/404 followed by 403/405, followed by a third tensioner unit performing step 402/404 followed by 403/405 (and so on if there are more tensioner units), before returning to the first tensioner to repeat step 402/404 followed by 403/405 if necessary, and so on. The process may continue until each of the tensioner units have performed the same number of steps (e.g., climbed to the same height on their respective ladder element) and/or until a substantially same tension is measured on each of the respective installation lines.

[49] Figure 5 summarises a method of deploying an offshore platform for a wind turbine including the above tensioning system.

[50] At step 501 , a tensioner unit is coupled to a ladder element provided on an at least partly submersible superstructure of an offshore platform via suitably cooperating engagement means.

[51] At step 502, an installation line is coupled to a tensioner unit (the installation line being previously or subsequently anchored to the seabed). Steps 501 and 502 may be performed in the opposite sequence, if desired, or even substantially simultaneously.

[52] At step 503, the tensioner unit is controlled to walk along the ladder element (preferably upward, so as to climb that ladder element), in doing so submerging the platform as a result of tension on the installation line.

[53] At step 504, the tensioner unit is controlled to cease its movement along the ladder element when the platform has been submerged to a predetermined depth.

[54] At step 505, mooring lines (anchored to the seabed) are secured to platform. Optionally (and indeed preferably), the installation line may then be decoupled from the tensioner unit and the tensioner unit subsequently detached from the platform.

[55] Thus, in summary, exemplary embodiments of an improved system/technique for deploying a floating offshore platform have been described. Components of the exemplary embodiments may be manufactured industrially. An industrial application of the example embodiments will be clear from the discussion herein.

[56] Although preferred embodiment(s) of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes may be made without departing from the scope of the invention as defined in the claims.

[57] Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

[58] All of the features disclosed in this specification, and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. [59] Each feature disclosed in this specification may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

[60] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification, orto any novel one, or any novel combination, of the steps of any method or process so disclosed.