Field of the invention

[0001] The present invention relates to wave energy converters, being apparatuses that convert energy provided by waves in a body of water into electrical energy.

Background of the invention

[0002] Wave power is a form of renewable energy that is a desirable alternative to non-renewable energy sources, such as oil and coal. Apparatuses that harness the energy of waves are known as wave energy converters. The current technology for wave energy conversion is in its infancy, and therefore, a wide variety of wave energy converters having vastly different designs have been proposed.

[0003] A wave energy converter (WEC) is usually a vessel designed to float in a body of water, typically the ocean, and convert the mechanical energy of ocean waves into useful electrical energy. There are obvious benefits to utilising wave motion for energy generation, for example, the abundance of ocean waves, the low (if any) emissions in energy generation, and low environmental impact.

[0004] In the applicant’s earlier PCT application (PCT/AU2017/051043), there is disclosed a WEC comprising a pendulum adapted to swing in response to wave action. The pendulum includes one or more permanent magnets arranged at a distal end thereof which, when the pendulum swings, induce an electromotive force (EMF) in one or more stationary stator coils located beneath the pendulum. Accordingly, the swinging of the pendulum in response to wave action generates electrical energy via electromagnetic induction.

[0005] However, ocean waves are not regular and their periodicity and amplitude is stochastic, which can cause low efficiency in power generation. The ocean is also a difficult environment, tending to damage any exposed moving parts.

[0006] It is an object of the present invention to provide a WEC design that may overcome or address some of the disadvantages or deficiencies of prior WEC designs, or at least a useful alternative WEC design. [0007] Reference to any prior art in the specification is not an acknowledgment or suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant, and/or combined with other pieces of prior art by a skilled person in the art.

Summary of the invention

[0008] In a first aspect of the present invention there is provided a wave energy conversion (WEC) device comprising a pair of hulls connected for mutual rotation about a pivot axis extending in a transverse direction of the WEC device, each hull supporting: an arcuate track extending in a plane defined by longitudinal and vertical axes of the WEC device such that the transverse pivot axis is substantially normal to said plane; a body configured to move along the track in response to pitching of the respective hull and surging or heaving of the WEC device due to wave action; and an energy converter configured to convert movement of the body relative to the track into electrical energy.

[0009] The arc described by each arcuate track may be a cycloidal arc. Each arcuate track may comprise at least two rails and the respective body may comprise at least one rail cart comprising a set of wheels mounted to said rails.

[0010] Each energy converter may comprise at least one rotary generator mounted to the at least one rail cart and driven by rotation of the wheels of said at least one rail cart. The at least one rotary generator may be driven directly by rotation of the wheels with no intervening gearbox. In particular, the at least one rotary generator may be a hollow shaft pancake motor mounted directly to an axle of the set of wheels.

[0011] Each hull may have a generally tubular form extending in the transverse direction of the WEC device. At least a portion of each hull may have a cam shaped cross-section. At least a portion of each hull may have a waisted-cam shaped crosssection in which a substantially circular, upper section tapers to a waist before flaring slightly into a lower, keel section.

[0012] Each hull may have a pair of mooring outriggers extending from port and starboard ends of the hull in a substantially longitudinal direction away from the other hull, each mooring outrigger terminating in a mooring hardpoint. Each hull may have a pair of pivot outriggers extending from port and starboard ends of the hull in a substantially longitudinal direction towards the other hull, each pivot outrigger terminating in a hinge arrangement at the transverse pivot axis. The mooring and pivot outriggers may be tubular frames held in compression by: port and starboard mooring stays extending from the port and starboard mooring hardpoints, respectively, back to the hull, port and starboard pivot stays extending from the port and starboard hinge arrangements, respectively, back to the hull, and port and starboard upper stays extending between the port and starboard mooring hardpoints and the corresponding port and starboard hinge arrangements.

[0013] The track, body and energy converter of each hull may be substantially enclosed by a tubular structure supported on the respective hull. The tubular structure may be hermetically sealed.

[0014] The present invention also contemplates a method for facilitating maintenance of a WEC device comprising a pair of hulls connected for mutual rotation about a pivot axis extending in a transverse direction, each hull supporting an arcuate track extending in a plane defined by longitudinal and vertical axes of the WEC device such that the transverse pivot axis is substantially normal to said plane, a body configured to move along the track in response to movement of the WEC device due to wave action and an energy converter configured to convert movement of the body relative to the track into electrical energy, the method comprising the steps of: bringing the WEC device into a maintenance configuration, in which the hulls are closer together than in an operating configuration; bringing a work boat into position relative to the WEC device such that there is substantial angular alignment between an outboard end of one of the tracks of the WEC device and a receiving arrangement provided on the work boat; and moving the body from said track onto said receiving arrangement in order to facilitate maintenance.

[0015] In this regard, in a further aspect of the present invention there is provided a method for facilitating maintenance of the WEC device of the first aspect, the method comprising the steps of: bringing the WEC device into a maintenance configuration, in which the hulls are closer together than in an operating configuration, such that there is substantial angular alignment between an outboard end of one of the tracks of the WEC device and a receiving arrangement provided on a deck of a work boat; manoeuvring the work boat into a position where its deck is located below the outboard end of said one of the tracks of the WEC device such that said track is substantially aligned in the longitudinal direction with the receiving arrangement provided on the deck; and lowering said track into substantial vertical alignment with the receiving arrangement such that the body can be moved from said track onto said receiving arrangement.

[0016] In a further aspect of the present invention there is provided a method for facilitating maintenance of the WEC device of the first aspect, the method comprising the steps of: manoeuvring a work boat into a position where its deck is located below an outboard end of one of the tracks of the WEC device such that said track is substantially aligned in the longitudinal direction with a receiving arrangement provided on the deck; and bringing the WEC device into a maintenance configuration, in which the hulls are closer together than in an operating configuration, such that there is substantial angular and vertical alignment between the outboard end of said track of the WEC device and the receiving arrangement provided on the deck and the body can be thereby moved from said track onto said receiving arrangement.

[0017] The method may further comprise the step of coupling the work boat to the WEC device to prevent or at least minimise any relative translational movement between said track and the receiving arrangement. The receiving arrangement may be a complemental track.

[0018] In a further aspect of the present invention there is provided a method of gaining access to an energy converter of the WEC device according to the first aspect, wherein each track comprises a pair of rails and the respective body comprises at least one rail cart comprising a set of wheels mounted to the rails, and wherein each energy converter comprises at least one rotary generator mounted to the at least one rail cart and driven by rotation of the wheels of said at least one rail cart, the method comprising the steps of: bringing the WEC device into a maintenance configuration, in which the hulls are closer together than in an operating configuration, such that there is substantial angular alignment between an outboard end of one of the sets of at least two rails of the WEC device and a set of complemental rails provided on a deck of a work boat; manoeuvring the work boat into a position where its deck is located below the outboard end of said one set of rails of the WEC device such that said one set of rails is substantially aligned in the longitudinal direction with the set of complemental rails provided on the deck; lowering said one set of rails into substantially vertical alignment with the set of complemental rails; coupling the work boat to the WEC device to prevent or at least minimise any relative translational movement between said one set of rails and the set of complemental rails; and rolling the at least one rail cart onto the set of complemental rails such that the at least one rotary generator mounted to said at least one rail cart becomes accessible.

[0019] The track, body and energy converter of each hull may be substantially enclosed by a tubular structure supported on the respective hull, and the deck of the work boat may be provided with a mating structure that supports the set of complemental rails, said mating structure having an open cross section that matches an external shape of outboard ends of the tubular structures thereby providing a selfalignment function in the transverse and vertical directions during the lowering step, and, after the coupling step, the method may comprise a step of opening the outboard end of the respective tubular structure before said at least one rail cart is rolled out.

[0020] The WEC device may be provided with water ballast and the lowering step may comprise increasing the amount of water ballast in the hulls to increase their displacement and thereby lower the outboard end of said one pair of rails.

[0021] As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps.

[0022] Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings [0023] Figure 1 is a side view of a wave energy conversion (WEC) device according to an embodiment of the present disclosure as deployed in its operational configuration;

[0024] Figure 2 is an end view of the WEC device;

[0025] Figure 3 is a top view of the WEC device;

[0026] Figure 4 is a long section view of the WEC device taken along the line A - A depicted in Figure 3;

[0027] Figure 5 is an enlarged side view of the forward hull and tubular structure of the WEC device as depicted in Figure 1 ;

[0028] Figure 6 is an enlarged view of the forward hull and tubular structure of the WEC device as depicted in Figure 4;

[0029] Figure 7 is a side view of the WEC device in a towing configuration;

[0030] Figure 8 is a perspective view of the WEC device in the towing configuration towed by an Anchor Handling Tug (AHT) vessel;

[0031] Figure 9 is a side view of the WEC device in a maintenance configuration and the AHT vessel servicing the WEC device; and

[0032] Figure 10 is a perspective view of the WEC device and the AHT vessel as depicted in Figure 9.

Detailed description of the embodiments

[0033] Figures 1 , 2 and 3 depict side, end and top views, respectively, of a wave energy conversion (WEC) device (10) deployed in its operational configuration. Accordingly, Figure 1 depicts the view in the transverse direction of the WEC device, Figure 2 the view in the longitudinal direction, and Figure 3 the view in the vertical direction. The WEC device 10 is configured to convert the mechanical energy of ocean waves into useful electrical energy. The manner in which this achieved is described below. [0034] The WEC device 10 comprises a pair of hollow, watertight hulls, forward hull 12A and aft hull 12B, configured to float in water and connected such that they can mutually rotate about a pivot axis extending in a transverse direction of the WEC device 10. Accordingly, the WEC device 10 is configured such that each hull 12A, 12B can pitch independently around the transverse pivot axis or both hulls 12A, 12B can surge and/or heave in unison.

[0035] Each hull 12A, 12B is fabricated from a suitable material familiar to those skilled in the art, e.g. steel, aluminium, concrete, fibreglass, etc., and has a generally tubular or prismatic form extending in the transverse direction of the WEC device 10. In an alternative embodiment, each hull may be formed from syntactic foam, which provides the primary hull hydrostatic buoyancy, supported and held in place by a steel load carrying structure. The depicted hull form can assist in facilitating pitching about the transverse axis and reducing rolling about the longitudinal axis of the WEC device 10. In this embodiment, the generally tubular form has a length of approximately 30 metres in the transverse direction of the WEC device 10, a height of approximately 12.5 metres in the vertical direction, with approximately 8 metres below the water plane, and a width of approximately 6.5 metres at the water plane in the longitudinal direction of the WEC device 10.

[0036] As can be seen from Figures 1 , 2 and 4, the generally tubular form has a cross-section that varies along its length in the transverse direction, but which is symmetrical about the longitudinal axis of the WEC device 10. Specifically, short lengths of each hull 12A, 12B at its port and starboard ends have a cam shaped cross-section, which then transition to a central length having a waisted-cam shaped cross-section. As depicted in Figures 4 and 6, the waisted-cam shaped cross-section has a substantially circular, upper section (i.e. the base circle of the cam shape) tapering to a waist or pinch point before flaring slightly in a keel section (i.e. the lobe of the cam shape). It will be appreciated that alternative embodiments of the WEC device may utilise a different hull form and or different cross-sections, e.g. a substantially circular cross-section, capped with rounded end plates rather than the flat end plates of the depicted embodiment, etc., as long as the hull presents a bluff body in the direction of wave travel and has an overall shape that minimises drag during rotation. Further, in whatever form they take, each hull 12A, 12B may be provided with movable ballast, e.g. water ballast, to vary the trim and/or displacement of the hull, thereby altering the natural period of the hull and/or assisting with manoeuvring the WEC device 10 during towing, installation, maintenance, etc.

[0037] The forward hull 12A is provided with a pair of stayed, port and starboard mooring outriggers 16A, 18A that extend forward in the longitudinal direction of the WEC device 10 from the port and starboard ends of the forward hull 12A. Each mooring outrigger 16A, 18A is approximately 9 metres in length and terminates in a mooring hardpoint 17A, 19A approximately 8 metres above the design waterline of the forward hull 12A. In this embodiment, each mooring outrigger 16A, 18A comprises a flat, tubular frame extending away from the hull 12A and converging at the mooring hardpoint 17A, 19A. As best shown in Figure 5, each tube of the mooring outriggers 16A, 18A is provided at the hull 12A with a termination cone that couples with a mating socket arrangement of a structural sub-frame (not shown) within the hull 12A. As also best shown in Figure 5, each tube of the mooring outriggers 16A, 18A, is provided at its other end with a termination cone that couples with a similar mating socket arrangement of the mooring hardpoint. Each mooring hardpoint 17A, 19A comprises a yoke of a universal joint that is stayed back to the keel of the hull 12A with a pair of mooring stays, as well as to a corresponding hinge arrangement with a pair of upper stays. A spider is received in the mooring hardpoint yoke and carries the yoke of a chain stopper (see e.g. 21 A in Figure 5), providing a universal joint between each mooring outrigger 16A, 18A and its respective mooring chain 48.

[0038] The forward hull 12A is further provided with a pair of stayed, port and starboard pivot outriggers 20A, 22A that extend aft in the longitudinal direction of the WEC device 10 from the port and starboard ends of the forward hull 12A. Each pivot outrigger 20A, 22A is approximately 17.5 metres in length and terminates at a hinge arrangement located approximately 3 metres above the design waterline of the forward hull 12A. In this embodiment, each pivot outrigger 20A, 22A comprises a flat tubular frame extending away from the hull 12A and converging at the hinge arrangement. As best shown in Figure 5, each tube of the pivot outriggers 20A, 22A is provided at the hull 12A with a termination cone that couples with a mating socket arrangement of the structural sub-frame (not shown) within the hull 12A. As also best shown in Figure 5, each tube of the pivot outriggers 20A, 22A is provided at its other end with a termination cone that couples with a similar mating socket arrangement of the hinge arrangement. Each hinge arrangement is stayed back to the keel of the hull 12A with a pair or pivot stays, as well as to the corresponding mooring hardpoint with the upper stays as described above.

[0039] The combination of the flat, tubular mooring and pivot outrigger frames with the mooring and pivot stays back to the hull 12A at the keel and the upper stays across the hull 12A results in simplified load paths and enable the frames to be held under compression and the stays to be under tension in all sea states. During assembly of the WEC device 10, once alignment has been made, the void between the respective termination cone and mating socket is filled with a load bearing grout. This compression only joint greatly simplifies assembly and improves load transfer across the WEC device 10.

[0040] The aft hull 12B is identical to the forward hull 12A, though rotated 180° around the vertical (Z) axis of the WEC device 10. Accordingly, the port and starboard mooring outriggers 16B, 18B of the aft hull 12B extend in the aft direction, and the port and starboard pivot outriggers 20B, 22B extend in the forward direction.

[0041] In this embodiment, the pivot axis is defined by a port hinge axis formed through the mating of the port hinge arrangements of the port pivot outriggers 20A, 20B and a starboard hinge axis formed through the mating of the starboard hinge arrangements of the starboard pivot outriggers 22A, 22B. In this embodiment, the port and starboard hinge arrangements consist of synthetic polymer bushes, typical of that of rudder bearings and familiar to those skilled in the art. It will be appreciated that, in alternative embodiments, the pivot axis may be defined by a single, monolithic axle (see e.g. axle 14 in Figure 10) that extends from port pivot housings provided on the port pivot outriggers 20A, 20B to starboard pivot housings provided on the starboard pivot outriggers 22A, 22B.

[0042] Each hull 12A, 12B supports an arcuate tubular structure, forward tubular structure 24A supported on forward hull 12A and aft tubular structure 24B supported on aft hull 12B, extending in a plane defined by the WEC device’s 10 longitudinal and vertical axes (Y, Z) and arranged such that the ends of the tubular structure 24A, 24B curve upwards when the WEC device 10 is in the operational configuration as depicted in Figures 1 , 2 and 4 to 6. As will be appreciated, the arcuate tubular structures 24A, 24B can be supported on their respective hulls 12A, 12B in a variety of different manners depending on the form of the hull. In this embodiment, a support cradle 26A, 26B provided on the upper surface of the hull 12A, 12B receives a substantially central portion of the tubular structure 24A, 24B. As can be seen in Figures 1 and 4 to 6, the cradle is not formed diametrically opposite the keel of the hull, but rather slightly offset in a direction away from the pivot axis. Such a configuration can assist with trimming the keel of the hull away from the pivot axis.

[0043] A long section view of the WEC device 10 taken along the line A - A of Figure 3 is depicted in Figure 4, with an enlarged view of the forward tubular structure 24A supported on forward hull 12A depicted in Figure 6. The aft tubular structure 24B supported on the aft hull 12B is identical to the tubular structure 24A supported on the forward hull 12A. Accordingly, it will be appreciated that the description below of the forward tubular structure 24A applies equally to the aft tubular structure 24B.

[0044] In this embodiment, the tubular structure 24A describes a cycloidal arc derived from a circle of radius 8 metres, extending through an arc angle of approximately 90°, with an arc length of approximately 30 metres and a tubular diameter of approximately 5 metres. It is formed from a skin 32A laid over a tubular lattice of annular ribs 34A and arcuate stringers 36A. This tubular structure 24A encloses an arcuate track that is substantially co-extensive with the structure 24A. In this embodiment, the track comprises a pair of rails 38A that are laid on the annular ribs 34A and extend from one end of the tubular structure 24A to the other such that the tubular structure 24A and pair of rails 38A describe the same arc. In another, non-illustrated embodiment, the track may take a segmented form, such that its arc profile can be adjusted by hydraulic, pneumatic or mechanical means operating on individual parts of the track. It will be appreciated that, as a section view, only one rail 38A of the pair of rails 38A is depicted in Figures 4 and 6.

[0045] A body 40A is enclosed by the tubular structure 24A and is configured to move along the rails 38A in response to pitching of the forward hull 12A around the transverse axis and surging or heaving of the WEC device 10 due to wave action. In this embodiment, the body 40A takes the form of a pair of coupled rail carts 40A mounted to the rails 38A via a set of wheels 42A. The wheels 42A may be standard conical wheels for centralising the rail carts 40A between the rails 38A. Advantageously, the rail carts 40A may be commercial off the shelf (COTS) products, such as rail carts used in mining environments, which may be provided with extra corrosion protection to increase their lifespan in the marine environment. It will be appreciated that, in other, non-illustrated embodiments, the body may take an alternative form, such as a single rail cart, a body that interfaces with the track through a rack-and-pinion system, a cable-tethered body with a single, continuous cable (or two, discrete cables) wrapping around a drum and motor at each end of the track, and/or a body with an adjustable natural frequency that can be varied by raising and lowering the centre of gravity of the body, e.g. through water ballast or mechanical means on the body itself.

[0046] The tubular structure 24A also encloses an energy converter (not shown) configured to convert movement of the body 40A relative to the track 38A into electrical energy. In this embodiment, the energy converter comprises a plurality of rotary generators mounted to each of the rail carts 40A and driven by rotation of the wheels 42A. Specifically, a hollow shaft pancake motor (not shown) is mounted directly to each axle of each wheel set, thereby avoiding the need for a gearbox. The diameter of the wheels 42A may then be selected based on the expected linear velocity ranges of the body 40A to achieve angular velocities appropriate for direct drive power take off. Advantageously, the rotary generator may be a COTS product, such as hollow shaft pancake motors of the type currently used in small wind turbines.

[0047] Accordingly, it can be seen that the tubular structure 24A provides an enclosed environment in which all the relevant moving parts of the WEC device 10 are protected from the marine elements. In some embodiments, the tubular structure 24A may be hermetically sealed. The internal climate (temperature, humidity, etc.) of the tubular structure 24A may be controlled through the use of fans and/or heat exchangers, which may be powered internally (i.e. by power generated by the energy converter) or externally from an outside power source. It will be appreciated that such a level protection may not always be necessary and, depending on how well protected the individual components are for the marine environment, it may be possible for the tubular structure 24A to be omitted, completely open or merely provide a minor level of spray and/or rain protection.

[0048] The electrical energy generated by the energy converter can be extracted from the WEC device 10 in a variety of ways. For example, it may be exported directly to a power substation located remote to the WEC device, e.g. on shore, via an external power cable extending from one or both of the hulls 12A, 12B. Such an external power cable may be a power cable as described in the applicant’s earlier PCT application (PCT/AU2017/051043). Alternatively, it may be stored on board, e.g. in a (removable) battery or as a compressed hydrogen derivative produced from an on board electrolyser, for later extraction. The generated electrical energy can be transferred from the rail carts 40A to the respective extraction means by a flexible internal cable. Alternatively, the track may further comprise a third, power take off rail electrically connected to the rotary generators of the rail carts 40A by a sliding or brushed connection.

[0049] In an alternative, non-illustrated embodiment, the energy converter may comprise an alternative form of generator to provide the power take-off system, such as a linear generator of the type described in the applicant’s earlier PCT patent application (PCT/AU2017/051043). In this embodiment, one or more permanent magnets may be arranged on the body and configured to induce an electromotive force (EMF) in one or more stator coils located along the track as the body moves relative to the track. Such a linear generator may be used in conjunction with, or as a replacement to, the rotary generators of the previous embodiment.

[0050] As will be appreciated by a person skilled in the art, there are a myriad of different variables that affect the functioning of the WEC device 10, with the precise configuration and dimensions of the various features depending on the site specific environmental conditions. However, the inventors of the present invention have identified four key parameters that make the greatest contribution to governing the response of the WEC device 10:

1 . One key driver of response is the natural period associated with the arc through which the body 40A moves. The power extraction from the energy converter provides a damping effect that can modify this natural period. Matching this damped natural period of response most advantageously to the range of prevailing wave periods on site enables the pitch response of the WEC device 10 to be tuned. One advantage of using a cycloidal arc is that a cycloidal pendulum remains a tautochrone even when damped. Another key driver of response is that of the peak pitch period of the hulls 12A, 12B. The dimensions of the hulls 12A, 12B, including that of mass and inertia, are selected to provide the hulls 12A, 12B with a response that is sympathetic to that of the arc through which the body 40A moves. A further key driver of response is the length and incident angle of the mooring outriggers 16A, 18A, 16B, 18B and pivot outriggers 20A, 22A, 20B, 22B with respect to the hulls 12A, 12B. The mooring outriggers 16A, 18A, 16B, 18B determine the level of pitch-surge coupling in the WEC device 10, this coupling being an important contributor to long period wave response. However, the incident angle of the mooring outriggers 16A, 18A, 16B, 18B also affects the rate of consumption of fatigue life in the mooring system: a larger incident angle with respect to the water plane creates, during the natural motions of the WEC device 10, a larger range of tensions in the mooring system as a result of pitch. The incident angle of the pivot outriggers 20A, 22A, 20B, 22B must be so selected to balance out the pitching effect of the mooring outriggers 16A, 18A, 16B, 18B upon the hulls 12A, 12B whilst still enabling a productive range of motion of the hulls 12A, 12B with respect to each other. The final key driver of response is the configuration of the mooring system: its physical properties and pretension. The horizontal catenary stiffness of the mooring - the combination of material extensibility and/or suspended weight - determines the natural period of response for the WEC device 10 in surge. It will be appreciated that the selection of this surge natural period, in association with the surge-pitch coupling derived from selection of the mooring outrigger configuration as described above, creates an important contribution to the productivity of the WEC device 10. However, the proximity of the surge natural period of the WEC device 10 with respect to the prevailing natural period of the waves at the selected site strongly affects both the rate of consumption of fatigue life in the mooring system and the overall strength of mooring components required.

[0051] As will be understood by a person skilled in the art, the coupled interplay of these four key drivers of response, in conjunction with the damping provided by the power extraction to exploit them, and balance of power production against mooring fatigue consumption represents a complex optimisation challenge. Selection of the most preferred configuration is dependent on site specific environmental conditions and requires iterative and adaptive selection to establish that arrangement with the best overall WEC device 10 whole life deployment economics.

[0052] When not in operation, the WEC device 10 can be manually rotated about the pivot axis, bringing the hulls 12A, 12B closer together into a towing configuration as depicted in Figures 7 and 8, and into a maintenance configuration as depicted in Figures 9 and 10. Such towing and maintenance configurations can be achieved in a variety of ways, e.g. by winching or towing the hulls 12A, 12B together. The WEC device 10 can then be held in either the towing or maintenance configuration by making the hulls 12A, 12B fast to each other, or locking the hinge arrangements to prevent rotation with respect to the axle 14.

[0053] The ability to bring the WEC device 10 into the towing and maintenance configurations can assist in the deployment and maintenance of the WEC device 10. For example, when towed in its transverse direction, the WEC device 10 in the towing configuration has improved handling characteristics compared with the WEC device 10 in its operating configuration when towed in either the transverse or longitudinal direction. Accordingly, the WEC device 10 can be assembled onshore and then towed more easily in its towing configuration to its deployment site by a work boat, such as an Anchor Handling Tug (AHT) vessel 46, using a towing bridle 47, as depicted in Figure 8. The WEC device 10 can then be moored at its deployment site using fixed chain stoppers on the forward mooring outriggers 16A, 18A and adjustable chain stoppers on the aft mooring outriggers 16B, 18B. [0054] Bringing the WEC device 10 into the maintenance configuration can also assist with in situ maintenance of the WEC device 10. More specifically, the maintenance configuration can improve access to the energy converter disposed in the tubular structure while the WEC device 10 remains moored at its deployment site, as discussed below with respect to Figure 10.

[0055] The WEC device 10 is designed such that drawing the hulls 12A, 12B together into the maintenance configuration brings the forward end of the rails 38A of the forward tubular structure 24A (and the aft end of the rails of the aft tubular structure 24B) into alignment with the horizontal plane. As depicted in Figure 10, the work deck 50 of the AHT vessel 46 is provided with a mating structure 51 and a set of rails 52 that correspond to the pair of rails 38A enclosed in the forward tubular structure 24A (and the pair of rails enclosed in the aft tubular structure 24B). In this embodiment, the mating structure 51 takes the form of a grillage supporting the set of rails 52 and has an open cross section that matches the external shape of outboard ends of the tubular structures 24A, 24B. Accordingly, in the embodiment of the WEC device 10 provided with water ballast, maintenance access to the rotary generator(s) mounted to the rail carts 40A in the forward tubular structure 24A can be achieved by:

1. bringing the WEC device 10 into the maintenance configuration by drawing the hulls 12A, 12B together such that the forward end of the rails 38A of aligned with the horizontal plane,

2. manoeuvring the AHT vessel 46 into a position such that the aft end of its work deck 50 is located between the port and starboard mooring outriggers 16A, 18A and the mating structure 51 is positioned below the forward end of the forward tubular structure 24A,

3. increasing the amount of water ballast in the hulls 12A, 12B to increase their displacement and thereby lower the forward end of the forward tubular structure 24A to press upon the mating structure 51 ,

4. coupling the AHT vessel 46 to the WEC device 10 to prevent/minimise any relative translational movement between the forward tubular structure 24A and the mating structure 51 , 5. opening the forward end of the forward tubular structure 24A, and

6. rolling the rail carts 40A out of the forward tubular structure 24A and onto the rails 50 provided on the work deck 52.

[0056] Any necessary servicing or maintenance work can be then carried out while the rail carts 40A are on the work deck 52, or the rail carts 40A can be stowed on the AHT vessel 46 (e.g. lashed to the work deck 52) for removal off site. The rail carts 40A can then be rolled back into the forward tubular structure 24A once any servicing/maintenance work has been completed, or a new rail cart (not shown) can be rolled into the forward tubular structure 24A from the work deck 52 if the removed rail carts 40A are to be taken off site. The forward tubular structure 24A can then be closed/resealed and the WEC device 10 uncoupled from the AHT vessel 46 before the amount of water ballast is reduced, thereby lifting the forward end of the forward tubular structure 24A off the mating structure 51 . The AHT vessel 46 can then be manoeuvred away from the forward hull 12A, before the WEC device 10 is lowered back into its operational configuration or steps 2 to 6 are repeated for the aft hull 12B.

[0057] At some deployment sites, it may be necessary to reduce the tension in the mooring lines 48 before the WEC device 10 can be folded into the maintenance configuration. However, it will be appreciated that this reduction in mooring tension is not always necessary and that at other deployment sites the WEC device 10 can be folded into the maintenance configuration while operating tension is maintained in the mooring lines 48.

[0058] It will be appreciated that the mating structure 51 having an open cross section that matches the external shape of the forward end of the forward tubular structure 24A (and the external shape of the aft end of the aft tubular structure 24B) assists with aligning AHT vessel 46 with the WEC device 10 in the transverse direction as the respective tubular structure 24A, 24B presses down upon the mating structure 51 . A mating detail (not shown), such as a cone or peg beneath the tubular structure 24A, 24B can then be used to make the final coupling between the AHT vessel 46 and the WEC device 10. [0059] It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.