Improved Underwater Turbine Installation and Removal Apparatus and Methods The present invention relates to the field of underwater turbines such as tidal turbines installed in areas of tidal flow. More specifically, the present invention concerns improvements in the installation, and removal, of underwater turbine nacelles. In a particular embodiment of the invention there is provided a frame which supports a turbine nacelle and guides the turbine nacelle onto a substructure, and in another embodiment of the invention there is provided a method of installing a turbine nacelle on a substructure using such a frame. Background to the invention It is known, for example from WO 2009/127415 (in the name OpenHydro Group Limited), to install a hydroelectric turbine onto the seabed by lowering the turbine onto the seabed using a number of cables or support lines attached to the turbine base. The turbine including the base, or the turbine in isolation from the base, may be so lowered. However, it can be difficult to ensure that the turbine is aligned with the base as it is lowered into position. One solution to this problem may be found in WO 2004/015264 (in the name Hammerfest Str0m AS) in which a guiding apparatus for guiding a turbine nacelle onto a turbine support structure is disclosed. Guide wires are disposed between the support structure and a surface based vessel, and the turbine nacelle is lowered into position on the support structure from the surface along the guide wires. An alternative but similar solution to this problem may be found in WO 2012/019673 (in the name Voith Patent GmbH) in which the turbine nacelle is coupled to a floating device, which makes the installation unit buoyant, and pulled downwards onto the support structure by a number of cables extending between the floating device and a winch on the support structure. However, the Applicant has deemed such installation methods to be too complex and technically demanding in practice and as such requires a yet further alternative installation methodology. Accordingly, it is an object of at least one aspect of the present invention to provide an apparatus for the installation of an underwater turbine that is simpler and easier to implement than existing solutions. Further aims and objects of the invention will become apparent from reading the following description.

Summary of the invention According to a first aspect of the invention, there is provided a frame for the installation or removal of an underwater turbine nacelle on a substructure, the frame comprising a support configured to support the weight of the nacelle during installation or removal and a guide arranged to align the frame with the substructure as the frame is moved towards the substructure. Preferably, the guide comprises at least one pair of guide members arranged on the frame to form a substantially V-shaped void which receives the substructure. Preferably, the at least one pair of guide members are arranged to form an obtuse angle there between. Optionally, the guide comprises at least two pairs of guide members. Preferably, the guide is disposed towards a lower end of the frame. Most preferably, the guide members are fixed. Preferably, the guide members are rigid. Alternatively, the guide members are deformable or flexible. Optionally, the guide members are provided with a protective layer. The protective layer may be a coating or a bumper for example. Optionally, the guide members comprise rollers. Preferably, the support comprises a generally U-shaped member arranged, configured or adapted to encircle at least a portion of the turbine nacelle. The U-shaped member may be disposed around, over, or under, the turbine nacelle. Preferably, the support comprises at least one attachment to attach the turbine nacelle to the support. The attachment may comprise a support cup adapted to receive

corresponding lugs or projections on the turbine nacelle, or vice versa. Alternatively, or additionally, the support comprises one or more support members which are arranged, configured or adapted to cradle the turbine nacelle in the frame. Alternatively, or additionally, the support attaches to the turbine nacelle using hydraulic locks. Alternatively, the support comprises a platform arranged, configured or adapted to support the turbine nacelle. Optionally, the support comprises a concave upper surface to receive the turbine nacelle. Preferably, the frame further comprises a plurality of cable connectors for connection to one or more winches via one or more cables for raising and lowering the frame with respect to the substructure. Optionally, the frame is arranged to accommodate a cable connected to the nacelle.

Optionally, the frame comprises a gap to accommodate the cable. Alternatively, the frame is arranged to actively or passively position the cable during installation. Alternatively, or additionally, the frame comprises a cable support adapted to hold a cable clear of the substructure during installation. Optionally, the cable support is adapted to hold a corresponding cable support arm in a first position clear of the substructure during installation and release the cable and/or the cable support arm after installation. Alternatively, or additionally, the frame comprises a cable support adapted to lift a cable clear of the substructure during removal. Optionally, the cable support is adapted to hold a corresponding cable support arm in a first position proximal to the substructure before removal and lift the cable and/or the cable support arm during removal. According to a second aspect of the invention there is provided a method of installing an underwater turbine nacelle on a substructure, the method comprising supporting the nacelle on a frame according to the first aspect, lowering the frame and nacelle towards the substructure, and advancing the frame and nacelle towards the substructure such that the guide aligns the frame and nacelle with the substructure. Preferably, the method comprises lowering the frame and nacelle until the nacelle engages with the substructure. Preferably, the method comprises continuing to lower the frame to release the nacelle. Preferably, the method comprises withdrawing the frame from the substructure. Preferably, the method comprises retrieving the frame to the surface. Preferably, the method further comprises transporting the nacelle to the substructure on a buoyant structure. The buoyant structure may be a barge, another suitable type of vessel, or a buoyancy frame.. Preferably, the frame and nacelle are lowered from the buoyant structure using one or more winches attached to the frame. Preferably, the frame and nacelle are advanced towards the substructure by moving the buoyant structure.

Optionally, the buoyant structure is moved by a tug boat. Alternatively, or additionally, the buoyant structure is moved by a work vessel. The buoyant structure may be attached to the work vessel or integrally formed with the work vessel. Of course, the buoyant structure may be self-propelled. Optionally, the method further comprises connecting a cable to the nacelle, and holding the cable clear of the substructure during installation. Preferably, the cable is paid out from a winch. Alternatively, or additionally, the cable is held clear of the substructure using a cable support arm. Preferably, the cable support arm releases the cable after installation. Alternatively, the cable support arm is released from the frame after installation. Alternatively, the method further comprises automatically connecting a cable to the nacelle. Preferably, the cable is connected via a wet-mate connector. Embodiments of the second aspect of the invention may comprise features corresponding to the preferred or optional features of the first aspect of the invention or vice versa. 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 lowering a frame according to the first aspect to a location below the nacelle, advancing the frame towards the substructure such that the guide aligns the frame with the substructure, and raising the frame to support the nacelle. Preferably, the method comprises continuing to raise the frame to disengage the nacelle from the substructure. Preferably, the method comprises retrieving the frame and nacelle to the surface. Embodiments of the third aspect of the invention may comprise features corresponding to the preferred or optional features of the first or second aspects of the invention or vice versa. According to a fourth aspect of the invention there is provided a method of installing an array of turbines comprising installing at least one turbine nacelle on at least one substructure of the array in accordance with the method of the second aspect. Preferably, the method of the second aspect is repeated for each of the turbines of the array. Embodiments of the fourth aspect of the invention may comprise features corresponding to 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 buoyant structure for installing a turbine nacelle on a substructure in accordance with the method of the second aspect. Preferably, the buoyant structure comprises one or more winches arranged to lower and/or raise a frame according to the first aspect. Optionally, the one or more winches are arranged around a void through which the frame is lowered and/or raised. Preferably, the buoyant structure is a barge. Alternatively, the buoyant structure comprises a buoyancy frame. According to a sixth aspect of the invention, there is provided a fleet for installing a turbine nacelle on a substructure in accordance with the method of the second aspect, the fleet comprising a barge for transporting and installing the turbine nacelle, and a tug boat for manoeuvring the barge. Preferably, the fleet further comprises a work vessel for supporting and/or further manoeuvring the barge. Embodiments of the fifth to sixth aspects of the invention may comprise features corresponding to the preferred or optional features of the first to fourth aspects of the invention or vice versa. According to a seventh aspect of the invention, there is provided an underwater turbine comprising a turbine nacelle and a substructure, the turbine nacelle having been installed on the substructure using one or more of: a frame according to the first aspect; a method according to the second or fourth aspect; a barge according to the fifth aspect; or a fleet according to the sixth aspect. Embodiments of the seventh aspect of the invention may comprise features corresponding to the preferred or optional features of the first to sixth aspects of the invention or vice versa. Of course, it will be understood that the invention may be used to support the weight of the nacelle during installation or removal, or to align the frame with the substructure as the frame is moved towards the substructure, as separate functions or features. Therefore in accordance with an eighth aspect of the invention there is provided a frame for the installation or removal of an underwater turbine nacelle on a substructure, the frame comprising a support configured to support the weight of the nacelle during installation or removal. Such a frame may be aligned with the substructure using alternative means, such as guide means on the substructure and/or movement of a vessel from which the frame is lowered. Furthermore in accordance with a ninth aspect of the invention there is provided a frame for the installation or removal of an underwater turbine nacelle on a substructure, the frame comprising a guide arranged to align the frame with the substructure as the frame is moved towards the substructure. Such a frame may be configured, arranged or adapted to be suspended from the turbine nacelle rather than supporting it, or may be aligned with the substructure prior to the turbine nacelle being connected thereto. The guide may comprise at least one pair of guide members arranged on the frame to form a substantially V-shaped void which receives the substructure. Embodiments of the eighth and ninth aspects of the invention may comprise features corresponding to the preferred or optional features of the first to seventh aspects of the invention or vice versa, and equivalent methods of use corresponding to the second, third and fourth aspects form further aspects of the invention. In such methods, lowering cables may be attached directly to the nacelle. According to a yet further aspect of the invention, there is provided a frame, a method of installing a turbine nacelle or removing a turbine nacelle from a substructure, or a method of installing a turbine array, substantially as herein described with reference to the appended drawings.

Brief description of the drawings Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings (like reference numerals referring to like features) in which: Figure 1 is a schematic representation of a frame according to at least one aspect of the invention and a corresponding turbine nacelle and substructure, in three stages of the installation of the turbine nacelle on the substructure, in accordance with another at least one aspect of the invention; Figures 2 to 13 are schematic representations of a tug, barge, and work vessel being used to install a turbine nacelle on a substructure in accordance in accordance with at least one aspect of the invention; Figure 14 is a schematic representation of an alternative frame according to at least one aspect of the invention and a corresponding turbine nacelle and substructure, at an intermediate stage of the removal of the turbine nacelle from the substructure; and Figures 15 to 17 are schematic representations of the alternative frame being used to install a turbine nacelle on a substructure in accordance with at least one aspect of the invention.

Detailed description of preferred embodiments As discussed in the background to the invention above, known installation methods are too complex and technically demanding in practice and as such an alternative installation methodology is desired. An embodiment of the present invention is illustrated in Figure 1 and overcomes this problem with the prior art. To demonstrate the features of the invention and the benefits of the invention in use, Figure 1 shows three stages in the installation of a turbine nacelle 151 on a corresponding substructure 171 using a frame 101 according to an embodiment of the present invention. The frame 101 comprises a generally U-shaped member 103 which, in use, encircles and supports a significant portion of the turbine nacelle 151. The frame also comprises four support cups 1 13 which cooperate with corresponding lugs 153 on the turbine nacelle 151. The cooperating cups 113 and lugs 153 permit alignment of the frame 101 with the turbine nacelle 151 , as well as serving a load-bearing purpose. The funnel shape of the particular cups 113 employed in this embodiment can assist in guiding the frame 101 (for example in a lifting operation) in the event that there is misalignment between the frame 101 and the nacelle 151. The frame 101 also comprises two fixed guide members 105 disposed towards a lower end of the frame 101 and, by virtue of the angle formed there between, define a generally v-shaped void 107. In use, as described below, the guide members 105 guide the frame 101 with respect to the substructure 171 and as a result align the frame 101 with the substructure 171. The angle there between is preferably obtuse, although it will be understood that any angle less than 180 degrees may be employed. The guide members 105 are rigid; however they may be provided with a protective layer such as a coating, a bumper, or the like, to prevent damage to the substructure 171 when they are brought into contact. As the frame 101 is moved down the substructure it may be preferable to dispose rollers on the guide members 105 to prevent damage or at least to reduce friction between the frame 101 and the substructure 171. The frame 101 also comprises a number of cable connectors 109 for connection to corresponding cables 1 11 which are used to lower, raise and manoeuvre the frame 101 (for example from an installation or maintenance barge, not shown in Figure 1). In this embodiment the cable connectors 109 are disposed at three separate locations on the frame 101 selected to provide as much horizontal separation there between as possible to provide the most stability and control when lowering, raising and/or manoeuvring the frame 101. A gap 114 is also provided in the rear of the frame 101 to accommodate a cable 115 (for example, an electrical or pigtail cable) attached to the turbine nacelle 151 which trails behind the frame 101. By maintaining tension on the cable 1 15 it can be kept clear of the substructure 171 to prevent snagging; the gap 1 14 allowing for significant overlap of the frame 101 and the cable 1 15 without any contact there between. In Figures 1 (a) to (c) the frame 101 is being used to lower the turbine nacelle 151 onto the substructure 171. Note that in this example the substructure 171 comprises a female socket 173 which receives a corresponding male projection 155 on the underside of the turbine nacelle 151. In Figure 1 (a) the frame 101 is approaching the substructure 171 in a horizontal direction. The turbine nacelle 151 is displaced vertically from the substructure 171 sufficiently to allow clearance between the male projection 155 and the female socket 173. Note that there is a vertical clearance between the guide members 105 of the support frame 101 and the bottom of the male projection 155. As the frame 101 progresses towards the substructure 171 , the substructure 171 is received in the v-shaped void 107; continued progress of the frame 101 - in the event of lateral misalignment - brings the substructure 171 into physical contact with the or a guide member 105 and further progress of the frame 101 towards the substructure results in lateral centralisation of the support member 101 with respect to the substructure 171. In Figure 1 (b), the frame 101 is laterally centralised with the substructure 171 and the male projection 155 is vertically aligned with the female socket 173; by virtue of the clearance between the guide members 105 and the bottom of the male projection 155 the guide members 105 are in contact with the substructure 171 without any contact between the male projection 155 and the substructure 171. The frame 101 can then be lowered vertically such that the male projection 155 is received in the female socket 173 as shown in Figure 1 (c). Further lowering of the frame 101 disengages the support cups 113 from the lugs and thereby detaches the frame 101 from the turbine nacelle 151 , and vice versa, after which the frame 101 can be manoeuvred away from the substructure 171 leaving the turbine nacelle 151 in place on the

substructure 171. To remove the turbine nacelle 151 from the substructure 171 the operation is performed in reverse, although in that case the frame 101 is positioned under the turbine nacelle 151 ; the guide members 105 first contacting the substructure 171 lower down than during installation. Note that the placement of the lugs and support cups described above may of course be reversed such that the support cups are inverted and located on the turbine nacelle and the lugs located on the frame. Of course any equivalent engagement, coupling or attachment means may be employed. The frame 101 is shown as being constructed from a number of elongate members which are welded together in the configuration shown, although the frame may be formed of or in a single piece. The elongate members are shaped and preferably hollow, but may comprise I-beams, or a combination of both, which lend structural strength as well as comparative lightness to the frame overall. As may be observed in Figure 1 (c), this construction methodology produces low drag by effecting a number of apertures and through paths for fluid flow. The addition of fins to the frame may help to stabilise the frame in a fluid flow and thus assist in alignment with the substructure. Other modifications to the frame are foreseen. For example, while described as being rigid the guide members may instead or to a degree be deformable or flexible, for example to conform to the substructure when contact is made. This may also serve to dampen the impact and/or provide a basic mating mechanism for the frame. Furthermore, the support may be disposed around, over and/or under the turbine nacelle, and may be attached to the turbine nacelle in a variety of ways. Instead of support cups and lugs, which may of course be disposed on either or both of the frame and the turbine nacelle, or other attachment or locking mechanisms such as hydraulic locks, the turbine nacelle may simply be cradled or supported by one or more members on which the turbine nacelle rests or is received. However, an embodiment of the invention is foreseen in which the frame does not support the weight of the turbine nacelle but is instead suspended from the turbine nacelle. Similar attachment mechanisms and arrangements to those described above, which may be applied in a reverse sense if appropriate or required, may be employed. In such an embodiment, lowering lines or cables would be attached to the turbine nacelle directly, and the load bearing capacity of the frame can be vastly reduced as a result which may simplify design. Corresponding guide members or functionally equivalent arrangements allow such a frame to provide the alignment function provided by the exemplary embodiment described above. Similarly, an embodiment of the invention is foreseen in which the frame does not include a guide and simply supports the turbine nacelle during installation; in which case a corresponding guide may be provided on the substructure which receives and aligns the frame as it is manoeuvred towards the substructure. An installation procedure employing a tug 191 , a barge 193 and a work vessel 195 will now be described with further reference to Figures 2 to 13. Figure 2 shows the

substructure 171 preinstalled on the sea bed. The turbine nacelle 151 is held in a frame 101 as described above. The cables 11 1 (not visible in Figure 2) are attached to corresponding winches 194 mounted on the barge. Note that three winches 194 are positioned around a void through which the frame 101 and turbine nacelle 151 shall be lowered. Initially, the tug 191 tows the barge 193 into position, generally above the substructure 171. The barge 193 is then connected to the work vessel 195 which is already in place, itself tethered to subsea moorings 196. The work vessel 195 is able to control the longitudinal position of the barge 193, for example by using on-deck winches, with additional control provided by thrust of the tug 191. Figure 3 shows the turbine nacelle 151 as it is lowered from the barge 193 on the frame 101 , the vertical position controlled by the winches 194 which feed out the cables 1 11. At the same time, the pigtail cable 115 is lowered from the opposite end of the barge 193, using a constant tension winch for example. In Figure 4, the turbine nacelle 151 has been lowered to a depth at which the guide members 105 vertically overlap the substructure 171. The barge 193 is then moved forwards which in turn moves the frame 101 forwards as shown in Figure 5, for example by tug thrust, to ensure that the frame 101 , and specifically the guide members 105, contact the substructure 171. As described above, this results in vertical alignment of the male projection 155 and the female socket 173 as shown in Figure 6. The nacelle 151 is then lowered onto the substructure 171 , again using the winches 194 to pay out the cables 1 11 , and the male projection 155 is received in the female socket 173 as shown in Figure 7. The conical shape of the male projection 155 compensates for any remaining misalignment, which should only be minor. Continued lowering of the frame 101 , as shown in Figure 8, releases the turbine nacelle 151 from the frame 101 , and the barge is urged backwards as shown in Figure 9, for example by way of on-deck winches on the work vessel 195, to clear the frame 101 away from the turbine nacelle 151. The pigtail cable 1 15 continues to be paid out while this takes place to keep it clear of the frame 101 as it moves away from the turbine nacelle 151 and substructure 171. The winches 194 on board the barge 193 then lift the frame 101 upwards as shown in Figure 10, until it is fully recovered back to the barge 193 as shown in Figure 11. It is of course foreseen that the cable could be attached to the turbine nacelle and/or to the substructure as part of the installation procedure, for example via a wet-mate connection. The work vessel 195 can then winch in the barge 193, and the pigtail cable 1 15 can be passed from the barge 193 to the work vessel 195 as shown in Figure 12. Thereafter, the barge 193 can be towed away by the tug 191 , leaving behind an installed turbine nacelle 151 and an associated pigtail cable 1 15 held by the work vessel 195, as shown in

Figure 13. This process can of course be repeated to install an array of turbines, or indeed performed in reverse, mutatis mutandis, to remove the turbine nacelle 151 from the substructure 171. It will also be readily appreciated that the installation method may equally be performed with fewer (or indeed more) vessels; for example the barge could be self-propelled in which case the tug and/or the work vessel may be dispensed with. The described installation method provides a good fit with existing operating practices. It is also foreseen that instead of a barge, any suitable type of vessel or buoyancy frame may be employed for the transport and installation of a turbine nacelle using the frame of the present invention. It may be self-propelled or moved by a tug, work boat or the like. Alternatively, a suitable buoyant structure may be integrally formed, fixed, or removeably attached to a work vessel. As noted above, alternative embodiments of the invention may omit a support for the turbine nacelle or omit a guide to align the frame (and hence the turbine nacelle in an installation process) with the substructure. Corresponding modifications to the installation and/or removal methods mat therefore be required. For example, in an embodiment in which the frame is suspended from the turbine nacelle, lowering wires may be connected directly to the turbine nacelle. In another embodiment in which the frame omits a guide a corresponding guide may be disposed on the substructure which subsequently guides the frame and hence nacelle onto the substructure during installation, and guides the frame onto the nacelle during removal. An alternative frame 201 , which for the avoidance of doubt supports and guides the turbine nacelle (but may of course also be modified as above) is now described with reference to Figure 14. The frame can be seen to comprise a generally U-shaped member 203 which, like corresponding member 103 above, encircles and supports turbine nacelle 251. Also shown in Figure 14 are two support members 204 which extend between opposite sides of the U-shaped member 203, underneath the turbine nacelle 251 to support it. While in both embodiments of the invention described herein the frame comprises a generally U-shaped member, it will be understood that any suitably shaped member or members arranged, configured or adapted to support the turbine nacelle may be employed. For example, the turbine nacelle may be carried on a platform having a concave upper surface which receives and supports the turbine nacelle - the concave upper surface may be shaped to correspond to the external surface of the turbine and/or comprise a deformable material able to conform to said external surface. The frame 201 can also be seen to comprise two pairs of guide members 205 which in use, and as described above, guide the frame 201 with respect to the substructure 271 as it is moved toward the substructure 271. Having two (or more) pairs of guide members may assist in ensuring vertical alignment with the substructure 271 , particularly in the event that the substructure 271 is inclined or in some other way non-vertical. In this embodiment, the cable 215 is supported by a cable support arm 214 mounted on the turbine nacelle 251 which is hinged towards a top end and comprises a hook which cooperates with a retaining arm 217 of the frame 201. When the frame 201 is supporting the turbine nacelle 251 , as shown, the retaining arm 217 holds the cable support arm 214 in a substantially horizontal position which keeps the cable 215 clear of the substructure 271 during installation - as described more fully below. Figure 15 shows the turbine nacelle 251 immediately following installation on the substructure 271. In this position, the retaining arm 217 still holds the cable support arm 214, and hence the cable 215, away from the substructure 271. In Figure 16, the support 201 has been lowered away from the turbine nacelle 251 and the retaining arm 217, which is hingedly attached to the frame 201 , has swung downwards and disengaged from the hook on the cable support arm 214, allowing the cable support arm 214 to hang vertically. The frame 201 can then be manoeuvred away from the substructure 271 as shown in Figure 17 and as described above. The invention provides a frame for the installation or removal of an underwater turbine nacelle on a substructure. The frame comprises a support which is configured to support the weight of the nacelle during installation or removal, and further comprises a guide arranged to align the frame with the substructure as the frame is moved towards the substructure. The frame particularly aids in installation by guiding the turbine nacelle onto the substructure in a 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 guide may be omitted in particular embodiments so as to benefit from each feature separately; in a particular embodiment this allows the guide to be mounted (or formed) on the substructure, and in another particular embodiment the frame can be suspended from the turbine nacelle. Throughout the specification, unless the context demands otherwise, the terms 'comprise' 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 exclusion of any other integer or group of integers. The foregoing description of the invention has been presented for the purposes of 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 order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, 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 appreciated that features of the embodiments described herein may be interchanged with or added to one another in alternative embodiments of the invention, and that

combinations of features other than those expressly claimed are within the scope of the invention.