GB2514773A

GB2514773A - Underwater turbine installation apparatus and methods

Info

Publication number: GB2514773A
Application number: GB1309851.2A
Authority: GB
Inventors: Michael Betschart
Original assignee: Andritz Hydro Hammerfest UK Ltd
Current assignee: Andritz Hydro Hammerfest UK Ltd
Priority date: 2013-06-03
Filing date: 2013-06-03
Publication date: 2014-12-10
Anticipated expiration: 2033-06-03
Also published as: GB201309851D0; GB2514773B

Abstract

The invention relates to an installation apparatus 101 and method for the installation or removal of a tidal turbine nacelle151 on an underwater substructure 153. The apparatus comprises a buoyant platform 101 with may comprise two hull portions 103, 103b with a number of inflatable buoyancy units 107. The apparatus also comprises lifting equipment 109, 113, 123, 124, (125 figure 7) for lowering and/or raising the tidal turbine nacelle, and positioning means for positioning the installation apparatus relative to the underwater substructure. The apparatus enables the installation or removal of a turbine nacelle on a substructure without having to deploy a separate barge or heavy lift vessel. Instead, such an installation apparatus can be towed and positioned by a workboat, multicat or any other suitable marine vessel 161. Positioning can be achieved by a propulsion system comprised in the apparatus itself, or manoeuvred by another vessel.

Description

1 Underwater Turbine Installation Apparatus and Methods 3 The present invention relates to the field of underwater turbines such as tidal turbines 4 installed in areas of tidal flow. More specifically, the present invention concerns improvements in the installation, and removal, of underwater turbine nacelles. In a 6 particular embodiment of the invention there is provided an installation apparatus which 7 comprises a buoyant platform and lifting equipment for lowering and/or raising the tidal 8 turbine nacelle relative to the buoyant platform, and in another embodiment of the 9 invention there is provided a method of installing a turbine nacelle on a substructure using such apparatus.

12 Background to the invention

14 It is recognised that the long-term financial viability of marine energy conversion devices such as tidal turbines is contingent on reducing the costs of installation and access for 1 maintenance. Studies have shown that the cost of marine operations accounts for a 2 significant percentage of the lifecycle costs when standard installation and retrieval 3 processes are used. A key consideration in making these devices commercially viable is 4 therefore to minimise or at least reduce costs.

6 Presently, the installation of tidal turbines is realised using vessels such as dynamic 7 positioning (DP) vessels or moored barges which are provided with large cranes to lower 8 the turbine and/or substructure onto the seabed. The cost of operating such vessels is 9 significant. Furthermore, access to such vessels may be limited due to demands in the hydrocarbon exploration and production industry which also makes use of such vessels.

12 It has been proposed that the production of a bespoke DP barge for dedicated installation 13 and maintenance would meet requirements, but the cost is likely to be prohibitive.

Adopting a similar but more cost-effective approach, Open Hydro's "OpenHydro Installer" is 16 a heavy lift barge which has been custom built to enable full scale tidal turbine 17 deployment. However, the "OpenHydro Installer" is still a barge which requires other 18 vessels to manoeuvre and position it, and the structure of the barge is particularly geared 19 towards the enclosed blade type turbine preferred by OpenHydro.

21 WO 2009/127415 (in the name OpenHydro Group Limited), discloses a method of 22 installing a hydroelectric turbine onto the seabed by lowering it from such a barge using a 23 number of cables or support lines attached to the turbine base. However, it can be difficult 24 to ensure that the turbine is aligned with the base as it is lowered into position. In any case, the barge was built at significant cost which in turn increases the effective energy 26 cost. Furthermore this barge requires its own winches which if on long term charter is 27 expensive and on short term is time consuming and also expensive to mobilise and 28 commission.

Accordingly, the applicant recognises that there is an outstanding requirement for a 31 solution that permits cost effective and efficient tidal turbine installation.

33 Accordingly, it is an object of at least one aspect of the present invention to provide an 34 apparatus for the installation of a tidal turbine that is simpler to produce and/or easier to operate than current or proposed solutions.

2 Further aims and objects of the invention will become apparent from reading the following

3 description.

1 Summary of the invention

3 According to a first aspect of the invention, there is provided an installation apparatus for 4 the installation or removal of a tidal turbine nacelle on an underwater substructure, the apparatus comprising a buoyant platform, lifting equipment for lowering and/or raising the 6 tidal turbine nacelle, and positioning means for positioning the installation apparatus 7 relative to the underwater substructure.

9 The apparatus is therefore a tool that can enable the installation or removal of a turbine nacelle on a substructure without having to deploy a separate barge or heavy lift vessel 11 with lifting capability such as a DP barge. A tool according to the first aspect can itself be 12 packaged and shipped to a location where it is required, and (for example) when towed by 13 and positioned by a workboat, multicat or any other suitable marine vessel can serve the 14 same purpose and hence replace a larger and more expensive to operate heavy lift vessel. This positioning could be either achieved via moorings or via a OP system on the 16 towing vessel. The installation apparatus may also be self-propelled and therefore 17 independently positionable.

19 Preferably, the lifting equipment lowers and/or raises the tidal turbine nacelle relative to the buoyant platform.

22 The positioning means may comprise attachment means for attaching the buoyant 23 platform to a marine vessel such as a work boat or multicat which can subsequently 24 position the installation apparatus. Alternatively, the positioning means may comprise a propulsion system. Further alternatively, or additionally, the positioning means may 26 comprise mooring apparatus to moor the installation apparatus to, or relative to! the 27 underwater substructure.

29 Optionally, the positioning means is configured, or further configured, to control the position of the turbine nacelle and/or a frame associated with the turbine nacelle during 31 installation or retrieval of the turbine nacelle. This may be achieved by propulsion means, 32 steering aids and/or fins comprised in the installation apparatus. Alternatively, this may be 33 achieved by controlling the position of a vessel to which the installation apparatus is 34 attached or tethered, or which can otherwise manoeuvre the installation apparatus (for example by pushing). The installation apparatus could be moored, for example to the 1 substructure. Further alternatively, or additionally, a frame which supports or is suspended 2 from the turbine nacelle during lowering or retrieval may comprise the positioning means - 3 that is, the lifting/lowering/guide frame itself may be provided with propulsion means, 4 steering aids and/or fins.

6 In a particular embodiment of the invention, the buoyant platform comprises one or more 7 hollow steel tanks. The buoyancy of the tanks can be controlled by flooding with seawater 8 as required.

Optionally, the buoyant platform comprises at least two hull portions disposed on either 11 side of a void which accommodates the turbine nacelle when in a raised position.

12 Preferably, each hull portion is provided with one or more buoyancy devices. Optionally, 13 the buoyancy devices are inflatable. Alternatively, the buoyancy devices are solid.

14 Alternatively, the buoyant platform comprises a single hull portion.

16 Optionally, the apparatus comprises a support configured to support the weight of the 17 nacelle during installation or removal, the lifting equipment attachable to the nacelle by the 18 support. Preferably, the support comprises a lifting frame. The support may comprise one 19 or more A-frames.

21 Alternatively, the lifting equipment is attachable directly to the nacelle.

23 Preferably, the lifting equipment comprises at least one sheave block configured to receive 24 at least one corresponding wire from a winch on board the marine vessel to which the apparatus is attached. Alternatively, the winch on board the marine vessel may be directly 26 attached to the nacelle or a corresponding support.

28 Alternatively, the lifting equipment comprises one or more winches.

Preferably, the apparatus comprises a winch frame which comprises the lifting equipment 31 and is removeably attachable to the buoyant platform. Optionally, the winch frame is 32 configured to span a void between two hull portions.

34 Optionally, the apparatus comprises a guide configured to align the turbine nacelle with the substructure as it is progressed towards the substructure. Preferably, the guide is 1 disposed towards the lower end of a support which supports the weight of the turbine 2 nacelle; the support may be a lifting frame.

4 Alternatively, the guide is configured to be suspended beneath the turbine nacelle.

6 In an embodiment of the invention, the guide is disposed on a frame for the installation or 7 removal of an underwater turbine nacelle on a substructure. The frame comprises a 8 support which is configured to support the weight of the nacelle during installation or 9 removal, and further comprises the guide is arranged to align the frame with the substructure as the frame is moved towards the substructure. Alternatively, the nacelle 11 can be suspended from or otherwise attached to, for example by hanging off hooks, the 12 frame without the provision of a weight-bearing support. In any case, the frame 13 particularly aids in installation by guiding the turbine nacelle onto the substructure in a 14 predetermined manner, and aids in removal as the guide allows the frame to be guided onto the turbine nacelle prior to lifting away from the substructure. The support or the 16 guide may be omitted in particular embodiments so as to benefit from each feature 17 separately; in a particular embodiment this allows the guide to be mounted (or formed) on 18 the substructure, and in another particular embodiment the frame can be suspended from 19 the turbine nacelle.

21 Alternatively, or additionally, the guide comprises one or more wires or cables for 22 connection to the substructure, the apparatus configured to lower the turbine nacelle onto 23 the substructure along said one or more wires or cables.

Alternatively, the guide comprises one or more wires or cables for connection to the 26 substructure, the apparatus configured to pull at least part of the buoyant platform and the 27 turbine nacelle along the one or more wires or cables down onto the substructure.

29 Optionally, the positioning means comprises a connection frame connected to the buoyant platform at one end and comprising a connector to attach the connection frame to the 31 marine vessel.

33 Optionally, the connector is articulated to allow relative movement between the apparatus 34 and the work vessel. The connector may comprise a hinge. Alternatively, the connector may comprise one or more chains.

2 Optionally, the buoyant platform comprises an arrangement of tubular members.

3 Optionally, the connection frame also comprises an arrangement of tubular members.

Optionally, the apparatus further comprises one or more imaging devices configured, 6 arranged or adapted to observe the turbine nacelle and/or components of the apparatus.

7 These may be disposed on a frame or directly on the turbine nacelle for example.

9 According to a second aspect of the invention there is provided a method of installing a tidal turbine nacelle on an underwater substructure, the method comprising transporting 11 the turbine nacelle to the substructure in or on an installation apparatus according to the 12 first aspect, and lowering the turbine nacelle to the substructure using the lifting 13 equipment.

Optionally, the method further comprises connecting a winch cable from a winch on board 16 the marine vessel to the nacelle via the lifting equipment of the apparatus.

18 Optionally, the method comprises connecting one or more wires or cables to the 19 substructure and lowering the turbine nacelle onto the substructure along said one or more wires or cables.

22 Alternatively, the method comprises connecting one or more wires or cables to the 23 substructure and pull at least a part of the buoyant platform and the turbine nacelle along 24 the one or more wires or cables down onto the substructure.

26 Embodiments of the second aspect of the invention may comprise features corresponding 27 to the preferred or optional features of the first aspect of the invention or vice versa.

29 According to a third aspect of the invention there is provided a method of removing an underwater turbine nacelle from a substructure, the method comprising transporting an 31 apparatus according to the first aspect to the substructure, and raising the turbine nacelle 32 from the substructure to the apparatus using the lifting equipment..

1 Embodiments of the third aspect of the invention may comprise features corresponding to 2 the preferred or optional features of the first or second aspects of the invention or vice 3 versa.

According to a fourth aspect of the invention there is provided a method of installing an 6 array of tidal turbines comprising installing at least one turbine nacelle on at least one 7 substructure of the array in accordance with the method of the second aspect.

9 Preferably, the method of the second aspect is repeated for each of the turbines of the array.

12 Embodiments of the fourth aspect of the invention may comprise features corresponding to 13 the preferred or optional features of the first to third aspects of the invention or vice versa.

According to a fifth aspect of the invention, there is provided a marine vessel and an 16 installation apparatus according to the first aspect positioned behind or attached to the 17 marine vessel.

19 Optionally, the marine vessel is a work boat. Alternatively, the marine vessel is a multicat.

Alternatively the marine vessel is a tug or a anchor handling vessel.

22 Embodiments of the fifth aspect of the invention may comprise features corresponding to 23 the preferred or optional features of the first to fourth aspects of the invention or vice 24 versa.

26 According to a sixth aspect of the invention, there is provided a tidal turbine comprising a 27 turbine nacelle and an underwater substructure, the turbine nacelle having been installed 28 on the substructure using one or more of: a frame according to the first aspect; a method 29 according to the second or fourth aspect, or a marine vessel according to the fifth aspect.

31 Embodiments of the sixth aspect of the invention may comprise features corresponding to 32 the preferred or optional features of the first to fifth aspects of the invention or vice versa.

34 According to a yet further aspect of the invention, there is provided an installation apparatus, a method of installing a turbine nacelle or removing a turbine nacelle from a 1 substructure, a method of installing a turbine array, or a marine vessel incorporating an 2 installation apparatus or having an installation apparatus attached thereto, substantially as 3 herein described with reference to the appended drawings.

1 Brief descrirtion of the drawings 3 Aspects and advantages of the present invention will become apparent upon reading the 4 following detailed description and upon reference to the following drawings (like reference numerals referring to like features) in which: 7 Figure 1 is a schematic representation of an installation apparatus according to at least 8 one aspect of the invention, a corresponding turbine nacelle suspended from the 9 installation apparatus, and an anchor handling tug supply vessel towing the installation apparatus; 12 Figures 2 to 4 are further schematic representations of the installation apparatus shown in 13 Figure 1, and in conjunction with Figure 1 illustrating the subsea deployment of the turbine 14 nacelle; 16 Figure 5 is a schematic representation of an alternative installation apparatus according to 17 at least one aspect of the invention and a corresponding turbine nacelle; 19 Figures 6 to 8 are further schematic representations of the installation apparatus shown in Figure 5, in various views and with some features omitted; 22 Figure 9 is a schematic representation of the installation apparatus shown in Figures 5 to 8 23 attached to a work vessel; Figure 10 is a schematic representation of the installation apparatus being used to install a 26 turbine nacelle on a substructure in accordance with at least one aspect of the invention; 27 and 29 Figure 11 is a schematic representation of a number of stages in the installation of a turbine nacelle on a substructure in accordance with at least one aspect of the invention.

1 Detailed description of preferred embodiments

3 As discussed in the background to the invention above, known installation vessels are 4 expensive to operate in practice and the lack of accessible and cost effective alternatives is currently preventing large scale device deployment. As such, an alternative installation 6 methodology is desired.

8 An installation apparatus 1 according to an embodiment of the present invention is 9 illustrated in Figure 1 and overcomes a number of problems with the prior art. To demonstrate the features of the invention, Figures 1 to 4 illustrate the subsea deployment 11 of turbine nacelle 51. The installation apparatus 1 comprises a number of buoyancy units 12 7 which surround and define a void through which lifting equipment 9 is connected to the 13 turbine nacelle 51. The buoyancy units 7 in this embodiment comprise a number of steel 14 tanks or drums.

16 The installation apparatus 1 is tethered to a vessel 61, which in this embodiment is an 17 anchor handling tug supply (AHTS) vessel, which tows the installation apparatus 1 and 18 turbine nacelle 51 (which is suspended from the installation apparatus 1) to an installation 19 site. Once at site, or when approaching site, the installation apparatus 1 is pulled towards the stern of the AHTS vessel 61 at which point any seafastenings rigidly connecting the 21 turbine nacelle 51 to the installation apparatus 1 are removed so that it can be raised and 22 lowered. Winches on board the AHTS vessel 61 are then operatively connected to the 23 turbine nacelle 51 via sheave blocks 23 mounted on the installation apparatus by winch 24 lines 24.

26 The winch lines 24 can connect directly to the turbine nacelle as in the current 27 embodiment, but it is also foreseen that the winch lines 24 could connect to a lifting frame 28 aftached to, supporting or suspending, the turbine nacelle 51 (as in the later described 29 embodiment). Furthermore, the winch lines could be connected to winch lead lines already so connected.

32 As shown in Figure 3, the turbine nacelle 51 is then lowered from the installation apparatus 33 1, via sheave blocks 23, by winch lines 24 connected to winches on board the AHTS 34 vessel 61. A pigtail cable 26, connected to the turbine nacelle 51 and hanging in a lazy loop over a stern roller (not shown) on the AHTS vessel 61, is paid out by a tugger winch 1 mounted on the deck of the AHTS vessel 61. As noted below, the turbine nacelle 51 can 2 be guided onto a substructure by using cameras mounted on the turbine nacelle 51 (or an 3 associated lifting frame) and subsequently disconnected or disengaged from the winch 4 lines 24 (or lifting frame) using hydraulics.

6 The winch lines 24 are then retracted and, as shown in Figure 4, another vessel (in this 7 example a multicat 63) brought in to tow the installation apparatus 1 away from the AHTS 8 61 -from which the installation apparatus 1 is now detached. The AHIS 61 can then be 9 used to pay out cable 26 to the seabed.

11 In this embodiment, the installation apparatus 1 is independently stable and the connection 12 to the AHTS vessel 61 can be a flexible attachment such as may be provided by chains. A 13 bumper disposed between the installation apparatus 1 and the AHTS vessel 61 (on the 14 installation apparatus 1 as illustrated and/or the AHTS vessel 61). A more rigid connection between a vessel and the installation apparatus may also be provided, such as the hinged 16 connection described in the alternative embodiment below.

18 Notwithstanding the nature of the connection, the apparatus enables the installation or 19 removal of a turbine nacelle on a substructure without having to deploy a separate barge or heavy lift vessel. Such an installation apparatus can be easily packaged and/or shipped 21 to a location where it is required, and when attached (flexibly, rigidly or otherwise) to a 22 suitable marine vessel, the resulting assembly provides a direct replacement for a larger, 23 slower and more expensive to operate heavy lift vessel which may also need to be 24 modified for the specific purpose of lowering and raising a turbine nacelle -the present invention is specifically made for this purpose. Of course, the installation apparatus could 26 be self-propelled and itself serve as a direct replacement for such vessels.

28 An alternative installation apparatus 101 according to an embodiment of the present 29 invention is illustrated in Figure 5 and overcomes a number of problems with the prior art.

To demonstrate the features of the invention, Figures 6 to 8 show alternative views of the 31 installation apparatus 101 with some features omitted to allow a clearer view of some of 32 the otherwise obscured features. Figure 9 shows the installation apparatus 101 attached 33 to a work vessel 161 and en route to an installation site carrying a turbine nacelle 151, and 34 Figure 10 shows the installation apparatus at the installation site and the turbine nacelle 151 being lowered towards an underwater substructure 153.

2 The installation apparatus 101 comprises two hull portions 103a,103b held in a spaced 3 apart arrangement by a connection frame 105. The spaced apart arrangement 4 accommodates the turbine nacelle 151 between the hull portions 103a,103b when it is in a raised or retracted position. The connection frame 105 provides for attachment of the 6 installation apparatus 101 to the work vessel 161 via a hinged connector 115, as shown in 7 Figure 9. The hinged connector 115 permits relative movement or rotation between the 8 apparatus 101 and the work vessel 161.

Each hull portion 103a,103b is provided with a number of buoyancy units 107 which lend 11 buoyancy to the apparatus 101 and thus provide a buoyant platform from (and/or to) which 12 the nacelle can be lowered (and/or raised). In this embodiment, the buoyancy units 107

13 are inflatable.

The turbine nacelle 151 is held in a lifting frame 109 which is attached to the nacelle 151 at 16 the front and back ends. The attachment is via a number of hydraulic connectors 111 17 which can be remotely actuated. The lifting frame 109 also comprises two sheave block 18 connectors 113 which cooperate with sheave blocks 123 to receive winch cables from the 19 work vessel to which the apparatus 101 is attached. It will be understood that many work vessels do not have winches of sufficient capacity to handle the raising and lowering of a 21 turbine nacelle otherwise; the sheave block arrangement provides mechanical advantage 22 to allow smaller vessels to deploy relatively large turbine nacelles.

24 As shown in Figure 10, the lifting frame 109 is lowered (and/or raised) by winches on board the work vessel 161 by cables 124 connected via sheave blocks 123 on the 26 apparatus 101 and sheave block connectors 113 on the lifting frame 109.

28 In this embodiment, the sheave blocks 123 themselves are located on a winch frame 125 29 which spans the space between the hull portions 103a,103b. In FigureS, the winch frame 125 is connected to the hull portions 103a,103b as would be the case during transport to 31 an installation site and during lowering (or raising) of the turbine nacelle 151. Figure 7 32 shows the installation apparatus 101 with the turbine nacelle and lifting frame omitted, and 33 the winch frame 125 raised up from the hull portions 103a,103b.

1 This permits the winch frame 125 to be attached to the turbine nacelle 151 or lifting frame 2 109 by one or more hydraulic connectors, while on a quayside for example, and the 3 turbine nacelle 151 then lowered onto the apparatus 101 (into the space between the hull 4 portions 103a,103b) using cranes or similar until the winch frame 125 engages with the hull portions 103a,103b. The winch frame 125 can then be fixed to the hull portions 6 103a,103b and thus the turbine nacelle 151 is held with the apparatus 101 from above.

8 Note that in an alternative embodiment, not illustrated here, the winch frame is integral 9 with the apparatus and the apparatus is instead floated over the turbine nacelle and the turbine nacelle or lifting frame subsequently attached to the winch frame or apparatus by a 11 suitable fastener. In a further alternative embodiment, the nacelle can be trailered onto the 12 buoyant body into an A-frame structure which lowers the nacelle between the hull portions.

14 Once the turbine nacelle 151 is located within the apparatus 101, the work vessel 161 tows the apparatus 101 (and attached nacelle 151) to the deployment site. It is then 16 moored to two or more pre-installed moorings by mooring lines 163.

18 In an alternative embodiment, to avoid the need for mooring the work vessel a work vessel 19 such as a multicat, tug or anchor handler with dynamic positioning could be employed.

21 Once in place over the substructure 153, the connection between the turbine nacelle 151 22 or lift trame 109 and the winch frame 125 is released and the nacelle 151 lowered using 23 the winches onboard the work vessel 161 via the sheave blocks 123 as described above 24 and as shown in Figure 10. Once the turbine nacelle 151 is in place, the connectors attaching it to the lift frame 109 can be released and the lift frame 109 recovered to the 26 surface.

28 In a removal or servicing operation, the apparatus 101 can be towed out to the turbine site, 29 positioned over the substructure and nacelle, and the lift frame lowered until it can be connected to the nacelle. Once connected, the nacelle can be lifted away from the 31 substructure and up towards the apparatus. Once retrieved, the work vessel can tow the 32 apparatus plus nacelle to a quayside or to another servicing location or vessel.

34 Figure 11 illustrates an alternative embodiment of the apparatus 201 in various stages during deployment of a turbine nacelle 251 on a substructure 253 in accordance with 1 another embodiment of the invention. In Figure 11(a) the apparatus 201 has been towed 2 to the installation location and positioned above the substructure 253, although as will 3 become apparent the apparatus 201 does not need to be directly above or aligned with the 4 substructure 253. Guide wires extend from the apparatus 201 to the substructure 253, where they are connected to a guide frame 210 which is clamped onto the substructure.

7 The guide frame 210 may be lowered from the apparatus 201 with the guide wires already 8 attached and the guide frame 210 subsequently attached to the substructure, or the guide 9 frame 210 may be attached to the substructure 253 and the guide wires subsequently attached to the guide frame 210. In an alternative embodiment the guide wires may be 11 connected directly to the substructure. In a further alternative embodiment the guide 12 frame is not connected to the substructure but has sufficient mass to adequately tension 13 the guide wires.

The turbine nacelle 251 is cradled by a lifting frame 209 which is lowered, as shown in 16 Figure 11(b), along the guide wires to guide the turbine nacelle 251 into place on the 17 substructure 253, as shown in Figure 11(c). The lifting frame 209 is then disengaged from 18 the nacelle 251 and the guide frame 210 disengaged from the substructure, leaving the 19 turbine nacelle 251 mounted on the substructure 253. The apparatus can then be towed away, although it is envisaged that the apparatus in this embodiment could instead be self- 21 propelled.

23 Note that it is foreseen that instead of using a lowering frame to lower the turbine nacelle 24 onto the substructure, the guide wires may be used instead as winch cables, and winches on the apparatus (or the guide frame or even the substructure) used to pull the apparatus 26 and the turbine nacelle down onto the substructure. Once attached to the substructure the 27 apparatus can release the turbine nacelle and the winch cables paid out to return the 28 apparatus to the surface.

The positive buoyancy provided by the apparatus makes the descent of the turbine nacelle 31 onto the substructure easier to control, and in the event of damage to the guide wires or 32 winch cables the turbine nacelle will be returned to surface rather than sinking to the 33 seabed and risking damage to the turbine nacelle and/or the substructure.

1 Alternatively, as shown in Figure 12, the buoyancy tanks of the apparatus 301 are flooded 2 with water to reduce the total buoyancy of the apparatus 301 plus turbine nacelle 351.

3 When negative buoyancy is achieved, the apparatus 301 and turbine nacelle 351 begin to 4 sink towards the seabed, and the vessel 361 is then manoeuvred over the apparatus 301 as shown in Figure 12(b) where on-board winches are then used to control the descent of 6 the apparatus 301 and nacelle 351, and the positioning of the nacelle 351 relative to the 7 substructure 353. Similarly to Figure 11, guide wires can extend between the vessel 361 8 and a guide frame 310 which guide the turbine nacelle 351 onto the substructure 353 as it 9 is lowered as shown in Figure 12(c). This may assist or be an alternative to positioning using the vessel 361.

12 Once in place on the substructure 353, the apparatus 301 can be retrieved and towed 13 away by the vessel 361 as shown in Figure 12(d). Prior to detaching the apparatus 301 14 from the turbine nacelle 351 it may be preferable to further flood the buoyancy tanks to compensate for the change in buoyancy resulting from the nacelle 351 being removed 16 from the assembly, and pull the apparatus 301 to the surface against negative buoyancy.

17 Otherwise, the apparatus 301 might be allowed to float to the surface.

19 It is envisaged that the attachment of the lift frame to the nacelle, or indeed the attachment of the nacelle to the substructure in an installation operation, can be assisted by the use of 21 ROVs. It is also envisaged that the use of subsea cameras mounted on the lift frame, 22 nacelle, substructure and/or any other part of the apparatus can be used to aid docking of 23 the nacelle on the substructure as well as monitoring the behaviour of attached cables and 24 any dry-mate or wet-mate connections.

26 The apparatus described in at least one embodiment above comprises an arrangement of 27 steel tubulars which effectively form two cages which receive and support the buoyancy 28 units. However, it is ancitipated that the hull portions could comprise solid or sheet steel 29 hulls. Furthermore, the buoyancy units can be solid instead of inflatable. For example, the apparatus described with reference to Figures 1 to 4 comprises large steel tanks which 31 provide buoyancy.

33 In an alternative embodiment of the invention, the lifting frame can cradle or otherwise 34 extend below the nacelle, and comprise a guide disposed towards a lower end of the frame to guide the frame with respect to the substructure as it is lowered into place. The 1 frame can be moved towards the substructure by appropriate thrusting or other position 2 control of the work vessel to which the apparatus is attached. It will be understood that 3 alignment of the frame with the substructure during installation serves to align the nacelle 4 with the substructure, and during removal serves to align the frame with the nacelle.

6 In a further alternative embodiment of the invention the frame does not support the weight 7 of the turbine nacelle but is instead suspended from the turbine nacelle and the winch 8 cables connected directly to the turbine nacelle. However, the frame is provided with a 9 corresponding guide or functionally equivalent arrangement to allow the alignment function provided by the embodiment described immediately above.

12 The invention provides an installation apparatus and corresponding methods for the 13 installation or removal of a tidal turbine nacelle on an underwater substructure. A buoyant 14 platform, lifting equipment for lowering and/or raising the tidal turbine nacelle, and positioning means for positioning the installation apparatus relative to the underwater 16 substructure, enables the installation or removal of a turbine nacelle on a substructure 17 without having to deploy a separate barge or heavy lift vessel. Instead, such an 18 installation apparatus can be towed by and positioned by a workboat, multicat or any other 19 suitable marine vessel and hence serve the same purpose. Positioning can be achieved by a propulsion system comprised in the apparatus itself, or manoeuvring by another 21 vessel.

23 Throughout the specification, unless the context demands otherwise, the terms comprise' 24 or include', or variations such as comprises' or comprising', includes' or including' will be understood to imply the inclusion of a stated integer or group of integers, but not the 26 exclusion of any other integer or group of integers.

28 The foregoing description of the invention has been presented for the purposes of 29 illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in 31 order to best explain the principles of the invention and its practical application to thereby 32 enable others skilled in the art to best utilise the invention in various embodiments and 33 with various modifications as are suited to the particular use contemplated. Therefore, 34 further modifications or improvements may be incorporated without departing from the scope of the invention as defined by the appended claims. In particular, it will be 1 appreciated that features of the embodiments described herein may be interchanged with 2 or added to one another in alternative embodiments of the invention, and that 3 combinations of features other than those expressly claimed are within the scope of the 4 invention.