DK2440708T3 - Foundation for offshore tower - Google Patents

Description

DESCRIPTION

Field of the invention [0001] The present invention relates to devices for installation on a seabed in a body of water. More specifically, the invention relates to a structure for seabed installation, for supporting offshore wind turbines.

[0002] The invention also relates to a mooring line arrangement with means for mooring lines, tensioning and aligning the supporting structure with vertical.

Background of the invention [0003] Complete guyed tower for supporting offshore wind turbines is described in Norwegian patents N0326937 and NO326904. In one operation, the entire structure is erected. However, due to their height, these structures cannot be upended by ballasting only. They need first to be landed with their lower end onto a foundation base that provides reaction to the upending accomplished by an external force exerted by e.g. tug/installation vessel. Further, these publications do not provide means for an important prerequisite for satisfactory dynamic behavior in water areas with waves i.e. the tensioning of the mooring lines.

[0004] Known technologies for tensioning mooring lines and permanently fixing them in a tensioned state are based on the use of chain segments and chain stoppers. The main disadvantage this technology shows for taut mooring systems, is the inability to facilitate adjustment in reasonably small load and length steps that are of paramount importance for tensioning taut moorings in particular at low elasticity of the lines. The smallest adjustment steps are equal to length of two chain links and yields unacceptable/unfeasible large steps in the tension. Another known technology applies a chain wheel with ratchet stoppers that allows finer adjustments than the use of the chain stoppers. However, the chain wheels for large dimension chains are heavy, space consuming and expensive. Further, both these techniques require to applied a tension load of a magnitude that is at least equal to the intended tensioning force in the line and the anchor to which the mooring line is connected can be exposed to a large lifting load up to nearly the double of the tensioning load and be decisive for the bearing capacity of the anchor. Finally, the use of chain as a part of the mooring line adds to the geometrical stiffness of the mooring lines due to increased catenary effects, which may not be desired for taut moorings.

[0005] Loads from the mooring system that need to be transferred into seabed are large, hence large capacity anchors are needed. It is anticipated that in many applications, anchors loaded with solid ballast to enhance their capacity will be the most economical solution. Dimensions of such anchors are so large that they make standard installation techniques expensive and most importantly, sensitive to dynamic effects from waves. In order to overcome the limitations standard installation techniques and to lower the installation costs, a novel design of an anchor suitable for a novel installation method is described herein. The anchor is designed for wet tow to the installation site, for transfer to a small height above seabed by a free descent and for final lowering onto the seabed by means of a buoyant and ballastable element.

Summary of the invention [0006] It is thus provided a support system for an offshore structure such as a wind turbine, comprising a lower column for supported via its first end by the seabed; at least one mooring arrangement connected at a first end to the lower column and at a second end to at least one respective anchor means, and wherein each mooring arrangement comprising at least a tubular portion attached via connection means in an end-to-end relationship to a flexible portion.

[0007] In a preferred embodiment, each mooring arrangement comprises at least two tubular portions attached to the lower column and arranged substantially in a side-by-side relationship and via respective connection means attached in an end-to-end relationship to two respective flexible portions, each flexible portion being connected to respective anchors.

[0008] The connection means are preferably interconnected via a tensioning device, whereby the distance between the connection means may be controlled and adjusted.

[0009] In one embodiment, the tubular portion comprises a rod element. In one embodiment, the tubular portion comprises a buoyant element. In one embodiment, the flexible portion comprises a wire, fibre rope, steel rope, or similar.

[0010] In one embodiment, each anchor means comprise a foundation plate element for placement on or in connection with the seabed and having an upwardly extending and peripheral wall element thereby defining an upper compartment above said foundation plate element and having an upwardly directed opening. The anchor means further comprises a dividing plate inside the upper compartment thereby defining a container between the foundation plate element and the upper compartment, said container comprising means for fluid communication with fluids outside the container. In one embodiment, the upper compartment comprises means for fluid communication with fluids outside the container.

[0011] Preferably, the foundation plate element comprises downwardly extending skirt elements thereby defining a lower compartment underneath the plate element.

[0012] In a preferential embodiment, each anchor means comprises an attachment device for a respective mooring arrangement and having a pull-in means for a respective one of a flexible portion, and locking means which permits a pull-in operation but prevents the flexible portion from being pulled out of the attachment device.

[0013] The attachment device further comprises, in one embodiment, pins for connecting the flexible portion to the pull-in and locking means, said pins being arranged in a substantially perpendicular relationship, similar to a universal joint.

[0014] In one embodiment, the support system comprising three mooring arrangements.

[0015] Preferably, the lower column is supported by a foundation base resting on the seabed. In a preferred embodiment, the mooring arrangement first end is connected to the lower column in a region near a second end of the lower column. The lower column second end extends above sea level when installed on the seabed.

[0016] The inventive guyed supporting structure and mooring arrangement with integrated tensioning solves a set of typical issues associated with this type of structures and mooring of them, in particular: • The foundation can be upended by ballasting only, hence simplifying both the installation and the design of the interface of the structure with the foundation base • The mooring lines can be tensioned with small steps • The tensioning does not expose the anchor to add additional loads • The arrangement of the mooring lines and the tensioning system facilitate fine alignments of the support structure with vertical • Post tensioning and tension adjustments during the life are enabled [0017] In principle the embodiment of the supporting structure is similar to that described in the patents mentioned above. However, there is a main differentiator, i.e. in the present invention the entire supporting structure is divided into two separate parts i.e. the lower supporting structure hereafter named as the lower column, and the upper supporting structure often named as column, shaft, or tower. This split of the structure reflects the current philosophy of wind offshore farm developers who requires one contractor to deliver the lower column and another contractor to deliver the tower with the turbine and rotor. In the technical sense, splitting of the entire structure enables upending by ballast control only, i.e. without application of an external upending force.

[0018] The inventive mooring comprises at least 3 pairs of mooring members where the members are arranged in pairs. Further, the members are interconnected with a line or similar that is provided with a device that can shorten the distance between the mooring members in points where the interconnecting line is attached. In this manner the tension in the mooring members can be effectively increased or adjusted. By adjusting the length of the interconnecting line at the individual pairs of mooring members the upper connection points of the mooring members, hence the lower column at this elevation can be moved in the horizontal plane. Since the lower end of the lower column is prevented to move in the horizontal plane, the result of these adjustments is change of the angle of the lower column with the vertical and can be applied to achieve the desired angle, typically vertical. Further, the lower end of each mooring member is provided with a termination that locks itself in a fixed position at the anchor that is able to transmit loads into the anchor. The mooring members can be composed of cables (wire ropes, chain, fiber ropes) and rods (tubular, beams) and combination of these. Any of these mooring members can either be made buoyant or distributed or lump weights can be added in a conventional manner to achieve the required characteristics of the entire mooring system.

[0019] For installation purposes the mooring members are provided with a forerunner line attached to the lower end, which can pull the termination into the lock position. Method for installing the entire mooring system with minimizing the amount of subsea work is also provided.

In applications where several or many [0020] The structural arrangement of the lower column, composition of the mooring member and the method of installation is explained for an embodiment of the lower column and corresponding mooring system, with reference to the attached drawings.

Brief description of the drawings [0021] These and other characteristics of the invention will be clear from the following description of a preferential form of embodiments, given as a non-restrictive examples, with reference to the attached figures wherein:

Figure 1 is principle sketch of a side view of an embodiment of the inventive lower column attached to mooring,

Figure 2 is a top view of the embodiment in fig. 1,

Figure 3 is a top view of one pair of mooring members illustrating the individual components

Figure 4 and 5 are vertical section and top view of the anchor

Figure 6 and 7 are vertical section and top view of the foundation base

Figure 8 is a top view of the anchor rigged for transport and installation

Figures 9 though 12 illustrate the transport and installation of the anchors

Figure 13 shows the situation at the installation site when the foundation base and all anchors have been installed Figure 14 shows the tow of the lower foundation to the installation site.

Figures 15-23 show various stages of the installation sequence

Detailed description of an embodiment [0022] Referring initially to figures 1 and 2, the illustrated embodiment of a lower column 1 is installed in water body W and resting on a foundation base 2 that is supported by seabed S. In place on the foundation, stability and verticality of the lower column 1 is provided with three pairs of mooring members 3a,b, 4a,b, and 5a,b, each attached to respective anchor pairs 6a,b, 7a,b and 8a,b. The pairs of mooring members are tensioned and the verticality of the lower column secured by interconnecting lines of adjustable length 9,10 and 11.

[0023] An example of composition of the mooring is illustrated in a top view in figure 3. Both mooring members 3aare at the upper end connected to the lower column 1 resting on the foundation base 2(only party shown in figure 3). At the lower end the mooring members 3a,b are attached to anchors 6a,b. At both upper and lower ends of the mooring members are connections allowing free rotation in plane perpendicular their longitudinal axis, although with some practical limitations of the angles that are larger that the angles expected to occur. This ensures that no bending moments will occur on the individual parts of the mooring members 3a,b. In this embodiment the upper section 12a,b of the mooring member 3a,b comprise a rod, preferably a tubular of required stiffness (i.e. the product of the cross section area and the elasticity of the material). In most applications the optimum design is so that the water buoyancy counterweights the weight of the section. The use of rods/tubulars in the upper section may have advantages in terms of resistance to accidental loads such as from drifting vessels and debris. The lower section 15a,b in the shown embodiment is made of a rope, i.e. steel wire or fiber rope. The sections12a,b and 15a,b are attached to each other by means of respective connecting plates 18a,b. To the first connection plate 18a is attached a tensioning line 9 whose opposite end is connected to a drum 24 with a ratchet wheel on the second connection plate 18b. The drum 24 can be revolved by a standard torque tool operated by a standard remotely operated vehicle. By revolving the drum 24, the tensioning line 9 is shortened and the splice points of the mooring members are moving towards each other and due to geometrical changes of the mooring members either a tensioning force of motion to top connection point or combination of both occurs. Hence the tensioning and vertical alignment is facilitated. It is advantageous to design all the mooring members 3a,b, 4a,b, and 5a,b identically.

[0024] Figures 4 and 5 illustrate an embodiment of the anchors 6,7,8 that is compatible with the innovative installation method explained in description of Figures 8 to 11.. Vertical section through the anchor 6,7,8 in figure 4 shows on seabed S resting a bottom slab 34 that supports vertical wall 35. The vertical wall 35 confines a from above open space 36 filled with solid ballast 39 and a closed space 37 separated by a roof element 41. Space 36 is open to air during towage to the installation site when the anchor is floating on the water surface. Seawater can flood this space through an orifice 42 controlled by valve 44. The closed space 37 is filled by air during transport and installation of the anchor, and upon completion of installation it is flooded by seawater through orifices 42' valves 44' and the air is expelled for the compartment through a vent orifice 43 controlled by valve 45. In order to enhance the soil reaction to loads from the anchor the bottom slab 34 can be provided by skirts 38 or other elements embedded into the seabed such as piles (not shown). Loads from the lower mooring member 15,16,17 are transmitted into the structure of the anchor 6,7,8 through an attachment device 40. The attachment devise 40 enables pull-in of the line 15,16,17 into a position in which becomes permanently secured in position and able of transmitting tension loads from the line. This may be achieved by using by using commercially available connectors. However, due to magnitude of loads typical for the foundation the availability of such connectors is limited and they are expensive. An optional embodiment of the attachment device is shown in Detail A where the lower mooring member 15,16,17 is an wire rope terminated by a socket 46 attached via a rotation free bolt 47 to a transition element 48 that is further attached via a rotation free bolt 49 to an end stopper 50. As the rotation bolts 47 and 49 are perpendicular, the wire rope may change its direction without exerting any moment in its termination and moments exerted into the end stopper are minimized. In lock position the end stopper 50 transfers to tension into the attachment device through a contact with a pair of locking arm 51a,b that can pivot about an axis 52. During pull-inn of the line and end stopper into the final and locked position the end stopper 50 is drawn by means of a pull-in line 53 into the attachment device. Indicated by dashed lines is a position in which the pair of locking arms has pivoted to an extreme position 51'a,b. In next position during the pull-inn the locking arms loose the support from the end stopper 50 and fall down on the opposite side of it. When tension in the pull-in line 53 is released, the end stopper 50 moves outwards the attachment devise until getting into contact with the locking arms 51. These are prevented by a load bearing member 54 from pivoting outwards, hence any outward motion of the end stopper 50 due to tension loads in the wire rope 15,16,17 is prevented and the connection is established.

[0025] Figure 5 is a top view of the anchor 6,7,8 where the vertical wall 35 is shaped in form of an vertical cylinder resting on a square bottom slab 34. The wire rope 15,16,17 is attached to the anchor by means of the attachment device 40.

[0026] Fig. 6 shows a vertical section of the foundation base 2 resting with it bottom section 55 on seabed S and skirts 38 penetrated into the seabed S. The lower column indicated by dashed lines vull be deployed in the center of the foundation base fitting the conical recess 56. The contact face 57 between the recess and the lower column is clad with material with properties ensuring that the loads from the column can be transferred into the foundation. Particular design concern is to obtain desired torque capacity between these two structures. As the base can be transported and installed in the same manner as the anchor, the outfitting for these operations is identical to that for the anchor. I.e. the base is provided with a buoyancy compartment confined by the bottom section 55, vertical walls 35 and 35' and roof section 41; orifices 42 and 43; and valves 44 and 45. For guiding of the lower column during the final stage of its lowering onto the base a guide 58 is provided.

[0027] Figure 7 is a top view of the foundation base 2 wfnere the bottom section 55 with skirts 38 indicated by dashed lines are shaped as a squares that provide resistance various loads, in particular to the torque.

[0028] Figure 8 illustrates the anchor 6 outfitted for transport and installation wth lower part 15 of the mooring devise On its upper end the line 15 is terminated with the connecting plate 18 wfnile on the opposite end the lime terminates with the end stopper 50 to which the pull-in line 53 is connected. To the upper end of the pull-in line 53 is attached the buoy 31 with messenger line 53' provided with the end float 32. All the components are seafastened to the base 34 of the anchor.

[0029] Figures 9 to 12 shows the preferred transport and installation method for the anchors and the foundation base, here illustrated for anchor. The same applies for the foundation base.

[0030] The anchor 5 floating on surface of a water body W shown in figure 9 is connected to towing vessels 59 and 60 and is being transported to the installation site.

[0031] Figure 10 illustrates the situation-upon arrival of the towing spread to the installation site where the foundation base 2 has already been installed. An installation vessel 61 has attached a distance line 63 from the foundation base 2 to the anchor 5 and is now being engaged with connecting an installation buoyancy tank 62 to a line 64 from the anchor 5. The trailing towing vessel indicated by numeral 60 in figure 8 has been detached from the anchor.

[0032] Figure 11 shown the descent of the anchor 5 to a position close to seabed S. The descent was initiated by opening the orifice 42 by means of valve 44 (see figure 4) enabling flooding of the ballast compartment 36. The descent progresses because the anchor has buoyancy that is somewhat smaller that it's submerged weight. Shown is the stage where the anchor descents through water while tensioning the line 64 thus gradually engaging the installation buoyancy tank 62. Towing vessel keeps the distance line 63 tensioned, thus maintaining positional control of the anchor 5.

[0033] Figure 12 shows the situation when the anchor 5 has descended to a position in which is determined by the buoyancy of the installation buoyancy tank 62 and the length of the line 64. The vessel 59 keeps tension in the distance line 63 enabling to install the anchor in accurate distance from the foundation base 2. Attaining desired azimuth orientation the vessel 59 ensures also the desired orientation of the anchor. Upon retrieval of a filling hose 65 to its deck the installation vessel starts pumping water into the installation ballast tank 62. The increasing weight causes further descent and enables deployment of the anchor onto the seabed S. Water filling into the installation buoyancy terminates when the line 64 becomes slack, hence can be disconnected. Finally orifices 42' and 43 are opened e.g. by a remotely operated vehicle operating the valves 44' and 45 shown in figure 4.

[0034] Figure 13 shows the situation at the installation site when the foundation base 2 and all anchors 6,7,8 have been installed and the lower parts 15,16,17 of mooring lines have been buoyed off to water surface where they are supported by buoyancy elements 25,26,27. The pull-in lines 28,29,30 have also been buoyed off to water surface where they are supported by buoyancy elements 31,32,33 and provided with messenger lines 28',29',30' and floats.

[0035] The lower column 1 floating on surface of a water body W shown in figure 14 is connected to towing vessels 59 and 60 and is being transported to the installation site.

[0036] In figure 15 is shown the towing spread, i.e. the towing vessels 49 and 50 and the lower column 1 upon arrival to the installation site and upon upending of the lower column 1 by ballasting as described in NO 326904 and N0326937. Further, the upper part 12a of the mooring line has been released for transport position and is being connected by the installation vessel 61 to the lower part 15a of the mooring line. The towing vessel 60 (not shown due to scale) is still connected and assists the vessel 1 in keeping suitable position of the lower column 1.

[0037] Figure 16 shows in a top view the situation upon completion of connection of the mooring line 3a and during connecting the upper part 12b to the lower part 15b. Once the connection is completed the towing vessel 60 (not shown due to scale) can be released and demobilized.

[0038] In figure 17 the lines 3a,b and 4a,b have been connected and the installation vessel 61 is establishing the connection between the upper part 14b and the lower part 17b while the towing vessel 59 is keeping suitable position of the lower column 1.

[0039] In figure 18 all lines have been, the lower column 2 has been positioned to the required position above the foundation base 2 connected and a remotely operated vehicle 68 has open flooding valve in the lower column for ballasting with sea water and the lower column 1 is descending towards the guide 58 on the foundation base.

[0040] In figure 19 the lower column 1 has landed onto the foundation base 2 and the vessels 59 and 60 are in progress of pull-in of the mooring lines 3a and 4b into the lock position as explained in description of figure 4.

[0041] Figure 20 illustrates the final step of the installation, i.e. tensioning of the mooring lines and alignment with the vertical. Remotely operated vehicles 68 and 68' are winching the tensioning lines 9 and 10 while recording the tension and the verticality.

[0042] Fig. 21 illustrates optional type of anchors, mooring line segment and connector for the mooring legs. In this embodiment the anchoring of mooring legs is by means of piles 70 and a connector 71 is located at a suitable point along the leg. The pile is normally more economic solution than the anchor explained in figures 5 and 6 unless the driving of piles is difficult or impossible, which is the case for sediments with large boulders or shallow depth to bedrock. The connector is provided with a pull-in wire and facility for performing the initial tensioning of the mooring leg and to achieve similar application as the connecter 40 described in figure 4. Locating the connecter at an intermediate point along the mooring leg opens for an alternative installation method explained in figures that follow.

[0043] Fig. 22 is the situation upon installation of pile anchors 70 with the lower section of the mooring line 12 attached to it. At the other is attached the connection plate 9 to which is connected a tubular section of the upper segment 12, 13 14. These are buoyed off to surface by line 72 suspended from buoys 73.

[0044] Fig. illustrates the situation where all legs have been connected, the column is ready for deployment onto the base 2 and finally for tensioning of the legs. In this case the mooring legs the upper sections of mooring legs are made up of two elements provided with a hinged connection 74. The lower end of the upper part of the mooring leg is connected to the lower segment via a pull-in line 28 running over a sheave (not shown) in the connector 71 and to surface buoy 31. The pull-inn line is provided with a stopper transferring the tension in the preassembled mooring leg in this temporary phase. When the pull-in line is tensioned, the connector 71 and the termination at the lower end of the upper mooring leg segment are pulled together and into a locked position. This concludes the pretension. The final tensioning of the system and verticality alignments are performed as explained in fig. 20.

[0045] The invented support structure is particularly useful in water depths more than approximately 35 metres.

REFERENCES CITED IN THE DESCRIPTION

This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

Patent documents cited in the description • NQ328937 (00031 [00361 • ΝΟ32β904 [0003] [00301

Claims (15)

A support system for an offshore structure, such as a wind turbine, characterized by a lower column (1) for support, via its first end, to the seabed (S); at least one anchoring device (3a, b, 4a, b, 5a, b) connected at a first end to the lower column (1) and at a second end to at least one respective anchor means (6a, b, 7a, b, 8a, b), and wherein each anchoring device comprises at least one tubular section (12a, b) which is secured to a flexible section via connecting means (18a, b) in an end-to-end arrangement. A support system 136 according to claim 1, wherein each anchoring arrangement comprises at least two tubular sections (12a, b) which are attached to the lower column (1) and which are substantially arranged in a side-by-side relationship. and via respective connecting means (18a, b) are fixed in an end-to-end relationship with two respective flexible sections (15a, b), each flexible section being connected to respective anchors (6a, b). Support system according to claim 2, wherein the connecting means (18a, b) are connected to each other via a clamping device (9, 10, 11, 24), whereby the distance between the connecting means (18a, b) can be controlled and adjusted. Support system according to any one of claims 1 to 3, wherein the tubular section (12a, b) comprises a rod element. A support system according to any one of claims 1 to 4, wherein the tubular section (12a, b) comprises a flowable element. Support system according to any one of claims 1 to 5, wherein the flexible section (15a, b) comprises a wire, a fiber rope, a steel rope or the like. A support system according to any one of the preceding claims, wherein each anchor means (6a, b, 7a, b, 8a, b) comprises a foundation plate element (34) for placement on or in connection with the seabed, and comprising a wall element ( 35) which extends upwardly and is circumferentially, thereby defining an upper space (36) above the foundation plate element and having an upwardly facing opening. A support system according to claim 7, further comprising a dividing plate (41) within the upper space (36), defining a container (37) between the foundation plate element (34) and the upper space (36), wherein the container comprises means (43). , 45, 42 ', 44') for fluid connection with liquids outside the container. A support system according to claim 7 or claim 8, wherein the upper compartment (36) comprises means (42, 44) for fluid communication with liquids outside the container. A support system according to any one of claims 7 to 9, wherein the foundation plate element (34) comprises skirting elements (38) extending downwardly, thereby defining a lower space below the plate element. A support system according to any one of the preceding claims, wherein each anchor means (6a, b, 7a, b, 8a, b) comprises a fastener (40) for a respective anchoring device and a retracting means (53) with a respective flexible section (15a, b) as well as locking means (51a, b, 52.54) which allow for retractability but prevent the flexible section from being pulled out from the fastener. A support system according to claim 11, wherein the fastening device (40) further comprises pins (47, 49) for connecting the flexible section to the retracting and locking means, which pins are arranged substantially perpendicular to each other in a similar manner as in a universal. A support system according to any one of the preceding claims, comprising three anchoring devices. A support system according to any one of the preceding claims, wherein the lower column is supported on a foundation (2) which rests on the seabed. A support system according to any one of the preceding claims, wherein the first end of the anchoring device (3a, b, 4a, b, 5a, b) is connected to the lower column in a region near the other end of the lower column. A support system according to any one of the preceding claims, wherein the other end of the lower column extends above the seawater level when the support system is installed on the seabed.