Patent Description:
To date, floating wind turbines have typically been configured in parallel straight-line arrays. The turbines of such arrays are typically connected electrically in series by dynamic umbilicals and are also connected electrically to a floating substation structure.

Conventionally, floating wind turbines are anchored offshore via mooring lines that are connected to respective anchors placed on, or embedded into, the seabed. Such an arrangement is practical where the water is relatively shallow, for example with a depth of up to around <NUM>. However, it is difficult to achieve sufficient stability with conventional mooring arrangements in significantly deeper water. Consequently, excessive movement of turbines relative to the seabed, and relative to each other, can generate unacceptable fatigue in the umbilicals or cables that are used to transfer power within a windfarm or to export power from the windfarm.

Another problem is that where multiple floating wind turbines are grouped in an offshore windfarm, the water column becomes congested with mooring lines. This presents a risk of clashing with or between mooring lines, especially as the horizontal spread of the moorings tends to increase with water depth.

<CIT> discloses an anchoring pattern for multiple wind turbines, in which at least some mooring or stabilising lines of each turbine share subsea foundations with mooring or stabilising lines of other turbines. The arrangement disclosed relies on a primary taut vertical cable and auxiliary catenary cables serving as legs. However, the use of a vertical cable in deep water is impractical and presents an unacceptable risk of failure.

In <CIT>, the wind turbines of an array are connected together by stabilising connecting lines. Intermediate weights on the connecting lines confer a degree of stability that may be sufficient for use in shallow water but are not sufficient for use in deep water. A larger stabilising system would be needed in that case.

<CIT> discloses further examples of anchoring arrangements for floating wind turbines.

In view of these problems, the inventors have identified a need for improved layouts and mooring arrangements for offshore power-generation facilities, notably for windfarms that comprise multiple floating wind turbines. The advent of deep-water installation of wind turbines and the desirability of connection with subsea energy storage systems presents new challenges, especially in remote locations.

The prior art includes numerous mooring proposals for floating oil and gas production facilities. However, that prior art does not teach a solution to the problems addressed by the present invention. For example, <CIT> discloses a mooring line pattern for a floating production vessel, in which catenary mooring lines double as flowlines for hydrocarbon production fluids.

Other prior art from the subsea oil and gas industry is too complex or otherwise wholly unsuitable for the purposes of the invention. For example, <CIT> teaches combining mooring lines and production risers through a disconnectable buoy between the seabed and the surface. In <CIT>, a platform is anchored in deep water using a combination of deep-water mooring lines supported by intermediate buoys and shallow-water moorings connected to the buoys. <CIT> discloses an even more complex mooring pattern in which various mooring lines are connected together.

Against this background, the invention provides a floating electric power-generation group that comprises: a floating hub such as a spar buoy or a buoyant platform that is anchored to a plurality of subsea foundations by a plurality of anchor lines; and at least two floating power producer units that are connected electrically and mechanically to the hub. The power producer units are distributed around a substantially circular array that is centred on the hub. The power producer units are each moored by a plurality of mooring lines, at least one of those mooring lines extending in a radially-inward direction toward the hub to effect mechanical connection to the hub and at least one other of those mooring lines extending in a radially-outward direction away from the hub toward one of the subsea foundations to effect mechanical connection to the subsea foundation.

The anchor lines and/or the mooring lines may, for example, be catenaries or taut legs that are held in tension by buoyant upthrust of the hub and/or the power producer units.

The outwardly-extending mooring lines may each be joined at a lower end to a lower chain section of a respective anchor line, or directly to a respective subsea foundation. In either case, each of the outwardly-extending mooring lines may conveniently share one of the subsea foundations with one of the anchor lines.

Similarly, the inwardly-extending mooring lines may each be joined at an upper end to an upper section of a respective anchor line, or directly to the hub.

Each power producer unit is suitably moored between an adjacent pair of the anchor lines radiating from the hub. In top plan view, the power producer units are preferably closer to the hub than the subsea foundations.

Each of the mooring lines extending from the power producer units may converge with or intersect one of the anchor lines. For example, the mooring lines may intersect the anchor lines at junctions between a major central wire or rope section of the anchor line and a minor upper or lower section of the anchor line. The upper and/or lower sections of the anchor line may, for example, be of chain.

The power producer units may be substantially equidistant from the hub. In any event, the power producer units and the anchor lines may alternate circumferentially around the hub.

Conveniently, the hub may house switchgear in a dry environment. The hub may similarly house a step-up transformer that is connected to a power export link. Similarly, dry splice connections may be made between power umbilicals and the power producer units and the hub. Such umbilicals thereby effect electrical connections between the power producer units and the hub and may hang as catenaries between the power producer units and the hub.

The invention also provides a floating electric power-generation group that comprises: a floating hub and at least two floating power producer units that are connected electrically to the hub and are distributed around a substantially circular array that is centred on the hub; wherein the floating hub is anchored to a plurality of subsea foundations that are located in plan view between the power producer units and the hub; wherein the power producer units are each moored by a plurality of mooring lines, at least one of those mooring lines extending in a radially-inward direction toward one of the subsea foundations of the floating hub that is located in plan view between the power producer unit and the hub to effect mechanical connection to the subsea foundation, and at least one other of those mooring lines extending in a radially-outward direction away from the hub toward a further (additional) subsea foundation located in a radially-outward direction of the power producer unit in plan view to effect mechanical connection to the further (additional) subsea foundation.

Each power producer unit may additionally be moored by a further mooring line that extends inwardly toward the hub to effect mechanical connection to the hub.

The power-generation group may generally include any of the features described above in connection with the first aspect of the present invention.

The inventive concept also embraces a set comprising a plurality of the electric power-generation groups of the invention, the hub of each of those groups being connected electrically to the hub of at least one other of those groups.

The power-generation groups of the set may be arranged in at least two rows. Alternatively, the power-generation groups of the set may include a central power-generation group and an array of power-generation groups that at least partially surround the central power-generation group, for example in a looped or circular array.

Elegantly, anchor lines extending from hubs of different groups of the set may be anchored to common subsea foundations.

The hub of at least one group of the set may be configured to serve as an electrical substation for the hub of at least one other group of the set.

Advantageously, the set may further comprise at least one subsea energy storage unit that is electrically connected to at least one of the hubs. Such an energy storage unit may, for example, comprise a storage volume in fluid communication with pumping machinery that is arranged to expel water from the storage volume and with generating machinery that is arranged to generate electricity from a flow of water entering the storage volume. The pumping machinery may be positioned at a greater water depth than the generating machinery.

The storage volume of the energy storage unit may be elongate, extending between groups of the set. Terminal structures at each end of the storage volume may conveniently house the pumping and generating machinery. In particular, the pumping machinery may be housed in a terminal structure at one end of the storage volume and the generating machinery may be housed in a terminal structure at an opposite end of the storage volume. In top plan view, each terminal structure is preferably located closer to a hub than the power producer units of a group comprising that hub. The energy storage unit suitably comprises at least one cable that effects electrical connection between the hubs of different groups of the set.

Wet-mate connections may be made between the pumping machinery, the generating machinery and umbilicals suspended from the hubs.

The hub of at least one group of the set may be configured to switch power generated by the power producer units of that group to drive the pumping machinery of the energy storage unit. Conversely, the hub of at least one group of the set may be configured to combine power generated by the power producer units of that group with power generated by the generating machinery of the energy storage unit.

The inventive concept also provides a set of floating electric power-generation groups each comprising a floating hub and a plurality of floating power producer units that are connected electrically to the hub.

The hub of at least one of the power-generation groups may be connected electrically to and configured to serve as an electrical substation for the hub of at least one other of the power-generation groups.

The power-generation groups of the set may be arranged in at least two rows including a first row and a second row. In this case the hub of at least one of the power-generation groups in the first row may be connected electrically to and configured to serve as an electrical substation for the hub of at least one of the power-generation groups in the second row.

Alternatively, the power-generation groups of the set may include a central power-generation group and an array of power-generation groups that at least partially surround the central power-generation group. In this case the hub of the central power-generation group may be connected electrically to and configured to serve as an electrical substation for the hub of at least one of the surrounding power-generation groups.

The hubs and/or power producer units of the power-generation groups may be connected to subsea foundations on the seabed. At least one of the power-generation groups may share at least one subsea foundation with at least one other of the power-generation groups.

Where the power-generation groups of the set are arranged in at least two rows, each of the power-generation groups may share at least one subsea foundation with at least one other power-generation group in its own row and at least one subsea foundation with at least one other power-generation group in another row.

Alternatively, where the power-generation groups of the set include an array of power-generation groups that at least partially surround a central power-generation group, the central power-generation group may share at least one subsea foundation with at least one of the surrounding power-generation groups, and preferably with each of the surrounding power-generation groups. The surrounding power-generation groups may each also share at least one subsea foundation with at least one other of the surrounding power-generation groups.

The inventive concept also extends to an offshore electric power-generation arrangement, comprising: a plurality of electric power-generation groups of the invention (each group comprising a hub that is connected electrically to a plurality of power producer units); and at least one subsea energy storage unit that electrically connects the hub of one of the groups to the hub of another of the groups.

Correspondingly, the inventive concept may be expressed as a method of generating electrical power offshore, which method comprises: generating electrical power using a plurality of electric power-generation groups of the invention (each group comprising a hub that is connected electrically to a plurality of power producer units); and conveying electrical power from the hub of one group to the hub of another group via at least one subsea energy storage unit. For example, electrical power may be converted to potential energy by pumping water from a storage volume of the subsea energy storage unit. Thereafter, the stored potential energy may be converted to recovered electrical power by admitting a flow of water back into the storage volume. Water may conveniently be pumped out of the storage volume at a greater depth than the depth at which water is admitted into the storage volume.

Embodiments of the invention provide a floating windfarm, comprising: a central buoyant hub anchored to the seabed by a plurality of mooring lines; and at least two floating wind turbines electrically connected to the central hub, and mechanically connected to the central hub by at least one radial mooring line; wherein the floating wind turbines are also anchored to the seabed by mooring lines, at least one of the wind turbine mooring lines being connected to a bottom section of a mooring line of the central hub. The mooring lines may, for example, comprise a combination of chains and/or spiral strand wire and/or synthetic wire.

The central hub may be a spar buoy or a platform, which may be anchored by catenary mooring legs or by taut tendons. For example, <CIT> shows a spar buoy and <CIT> shows a tensioned leg platform.

The floating wind turbines may be at the same distance as each other from the central hub, for example distributed around the circumference of a circle centred on the hub.

The or each radial mooring line may be a catenary line between the wind turbine and the central hub or the upper third of a mooring line of the central hub.

The central hub is apt to carry a power substation or may export power to a subsea substation.

Thus, the inventive concept may be exemplified by arranging wind turbines in a circular configuration around a central tower or spar structure that serves as an interface with subsea infrastructure.

The invention is apt to accommodate very large-diameter offshore wind turbines as they become available. By employing proven spar or tower solutions, the invention can accommodate water depths of up to <NUM>. The invention also enables deep-water storage of electrical energy generated offshore, noting that high-voltage dynamic umbilicals are not practical with existing technology.

In summary, the invention provides a floating power-generation group that comprises a floating hub, such as a spar buoy, which is anchored to subsea foundations by anchor lines. Floating power producer units such as wind turbines are connected electrically and mechanically to the hub. The power producer units are each moored by mooring lines. At least one mooring line extends inwardly toward the hub to effect mechanical connection to the hub and at least one other mooring line extends outwardly toward a subsea foundation.

The groups may be combined as a set whose hubs are connected electrically to each other, for example via subsea energy storage units. Anchor lines of different groups can share subsea foundations. The storage units suitably comprise pumping machinery to expel water from an elongate storage volume such as a pipeline and generating machinery to generate electricity from a flow of water entering the storage volume. The pumping machinery may be in deeper water than the generating machinery.

Referring firstly to the offshore floating wind turbine unit <NUM> shown in <FIG>, a rotor of a wind turbine <NUM> comprises a set of blades <NUM> extending from a hub <NUM>. The hub <NUM> turns relative to a nacelle <NUM> that contains power-generation machinery to be driven by the turning hub <NUM>, including a gearbox and an electrical generator. As is conventional, the nacelle <NUM> is mounted atop a tower <NUM> and can turn relative to the tower <NUM> about an upright axis to align the blades <NUM> in accordance with the prevailing wind direction.

Whilst the wind turbine <NUM> shown in <FIG> is conventional in layout, the invention places no limits on the size of the turbine <NUM> and indeed enables the use of exceptionally large turbines <NUM> as they become available. Each turbine <NUM> could, for example, have a generating capacity of <NUM> MW. This implies that each blade <NUM> could have a length of about <NUM> from the hub <NUM> to the tip, hence defining a rotor with a diameter of <NUM> or more.

The turbine <NUM> is supported by a buoyant raft <NUM> that, in this example, comprises three parallel upright tubular legs <NUM> connected by a triangular lattice frame <NUM>. The tower <NUM> of the turbine <NUM> extends upwardly and coaxially from one of the legs <NUM>. The other two legs <NUM> contain ballast tanks to counter tilt forces arising from wind acting on the turbine <NUM>. One of the three legs <NUM>, which may be the leg <NUM> that supports the tower <NUM>, houses switchgear controls and ballast pumps.

The turbine <NUM> and the raft <NUM> are apt to be fabricated, assembled and tested on shore or near shore, for example in a dry dock. The turbine <NUM> and the raft <NUM> may then be towed out together to, or reassembled at, an offshore windfarm site.

<FIG> shows a spar buoy <NUM> that exemplifies a floating hub for use in the invention. The spar buoy <NUM> comprises an upright cylindrical hull <NUM> that is characterised by its tall and narrow profile. Mooring lines <NUM> radiate from the hull <NUM> equi-angularly as shown; there may be more or fewer such mooring lines <NUM>. Again, conveniently, the components of the spar buoy <NUM> could be fabricated and assembled in a dry dock.

As will be explained, the floating hub of the invention could be defined by a buoyancy-supported structure other than a spar buoy <NUM>, such as a tower or a tension leg platform (TLP).

Advantageously, the spar buoy <NUM> or other floating hub houses high-voltage equipment in a dry environment, thereby avoiding the need for such equipment to be positioned subsea and especially in deep water. An example of high-voltage equipment is a boost transformer and its associated switchgear. As will be explained, a boost transformer may be used to raise generation voltage from, say, <NUM> kV to a transmission voltage of, say, <NUM> kV for export of electrical power to an onshore grid.

The use of a spar buoy <NUM> or other floating hub employs existing, proven technology to minimise vertical and horizontal movement under the action of wind, waves and currents when installed offshore in deep water. This enables the use of static riser elements to convey electrical power and also avoids the limitations of high-voltage dynamic umbilicals, for which the current maximum depth limit is regarded as being around <NUM>. In particular, the invention mitigates the problem of designing deep-water high-voltage dynamic umbilicals by reducing dynamic motions and associated stresses.

A spar buoy <NUM> or other floating hub has various other advantages. For example, it enables various dynamic umbilicals of the system to have standardised cross-sections, which reduces engineering costs and minimises the need for spares. In this respect, the spar buoy <NUM> or other floating hub may be configured for conventional pull-in and dry termination of umbilicals and risers using a central pull-in arrangement. Tie-in of umbilicals and risers during installation can therefore be achieved with a low-specification vessel that is less expensive and that has greater availability than a higher-specification vessel. Widening the range of available installation vessels is a particular benefit for installation in remote areas.

Turning next to <FIG>, this plan view shows a generating group <NUM> in which a spar buoy <NUM>, such as that shown in <FIG>, serves as a central hub for a circular array of wind turbine units <NUM> that are centred on that hub. In each wind turbine unit <NUM>, a turbine <NUM> floats on a buoyant raft <NUM> as shown in <FIG>. In this example, the generating group <NUM> comprises eight wind turbine units <NUM> that are equi-angularly spaced on a pitch circle <NUM> with the spar buoy <NUM> at its centre.

The optimal mutual spacing between the wind turbine units <NUM> is based upon the rotor diameters of their turbines <NUM> and is determined by the effects of turbulence between the rotating blades <NUM> of adjacent turbines <NUM>. If the turbines <NUM> are too close to each other, this could result a reduction of between <NUM>% and <NUM>% in the overall power generated by the installation.

Assuming eight <NUM> MW turbines <NUM> each with a rotor diameter of <NUM>, the pitch circle <NUM> has a diameter of <NUM> around the central hub defined by the spar buoy <NUM>. This reflects optimal spacing between the turbines <NUM>, regarded as a minimum of seven rotor diameters in line with the principal expected wind direction and four rotor diameters between adjacent turbines <NUM> in a direction transverse to that wind direction.

In the anchor pattern arrangement of the generating group <NUM> shown in <FIG>, the spar buoy <NUM> is moored to the seabed <NUM> by hub anchor lines <NUM> that hang from the spar buoy <NUM> as taut legs or catenaries and that extend radially from the spar buoy <NUM> in plan view. A subsea foundation <NUM> placed on or embedded in the seabed <NUM>, such as a suction pile, serves as an anchor at the outer end of each hub anchor line <NUM>. In this example, each subsea foundation <NUM> may be positioned at a radius of about <NUM> from the vertical centre line of the spar buoy <NUM>. Thus, the hub anchor lines <NUM> extend radially beyond the pitch circle <NUM> on which the wind turbine units <NUM> are positioned.

In this example, the hub anchor lines <NUM> are equal in number to the wind turbine units <NUM>. Consequently, there are eight hub anchor lines <NUM>. In plan view, each wind turbine unit <NUM> is interposed between an adjacent pair of hub anchor lines <NUM> such that the wind turbine units <NUM> and the hub anchor lines <NUM> alternate with each other, moving circumferentially around the spar buoy <NUM>.

Most of the length of each hub anchor line <NUM> comprises an intermediate central section 40C that may be made of steel wire or, preferably, a synthetic rope of a polymer such as polyester. Each hub anchor line <NUM> further comprises upper and lower sections 40U, <NUM> that are suitably made of steel chain for wear resistance. The upper section 40U extends from the central section 40C to the spar buoy <NUM>. The lower section <NUM> extends from the central section 40C to the associated subsea foundation <NUM>. Where the foundation <NUM> is embedded in the seabed <NUM>, part of the lower section of the hub anchor line <NUM> may also be embedded in the seabed <NUM>.

Each wind turbine unit <NUM> is moored, in turn, by a set of turbine mooring lines <NUM> in the form of taut legs or catenaries. Again, all or most of the turbine mooring lines <NUM> may be made of steel wire or of a synthetic rope of a polymer such as polyester.

In the example shown in <FIG>, a set of three turbine mooring lines <NUM> extends from the raft <NUM> of each wind turbine unit <NUM> to adjacent hub anchor lines <NUM> disposed on both sides of that wind turbine unit <NUM>. In most cases, two of the turbine mooring lines <NUM> extend from the wind turbine unit <NUM> in a radially-outward direction and one of the turbine mooring lines <NUM> extends from the wind turbine unit <NUM> in a radially-inward direction with respect to the spar buoy <NUM>. However, <FIG> also shows the option of one of the turbine mooring lines <NUM> extending from the wind turbine unit <NUM> in a radially-outward direction and two of the turbine mooring lines <NUM> extending from the wind turbine unit <NUM> in a radially-inward direction with respect to the spar buoy <NUM>.

In the variant shown in <FIG>, a set of four turbine mooring lines <NUM> extends from the raft <NUM> of each wind turbine unit <NUM> to adjacent hub anchor lines <NUM> on both sides of that wind turbine unit <NUM>. A first pair of those turbine mooring lines <NUM> extends from the wind turbine unit <NUM> in a radially-outward direction and a second pair of those turbine mooring lines <NUM> extends from the wind turbine unit <NUM> in a radially-inward direction with respect to the spar buoy <NUM>.

In the examples of <FIG> and <FIG>, each of the turbine mooring lines <NUM> intersects the respective hub anchor line <NUM> at a junction between the central section 40C and the lower section <NUM> of the hub anchor line <NUM> or at a junction between the central section 40C and the upper section 40U of the hub anchor line <NUM>. Thus, each turbine mooring line <NUM> may be joined to a respective hub anchor line <NUM> by a coupling at the top of the lower section <NUM> or at the bottom of the upper section 40U of the hub anchor line <NUM>. In this way, elegantly, the turbine mooring line <NUM> and the hub anchor line <NUM> share a common subsea foundation <NUM> and the upper and/or lower sections 40U, <NUM> may be shared by the turbine mooring line <NUM> and the hub anchor line <NUM>. <FIG> also shows that each turbine mooring line <NUM> may have an upper section 44U of chain adjoining the associated wind turbine unit <NUM>.

Moving on to <FIG>, this shows a set <NUM> of generating groups <NUM> like that shown in <FIG>, each generating group <NUM> comprising a circular array of wind turbine units <NUM> centred on a respective spar buoy <NUM> serving as a central hub for the array. In this example, the set <NUM> comprises five such generating groups <NUM> in two mutually-staggered rows <NUM>, a first row 48A comprising three generating groups <NUM> denoted 34A, 34B and 34D and a second parallel row 48B comprising two generating groups <NUM> denoted 34C and 34E. Elegantly, this arrangement allows generating groups <NUM> of one row <NUM> to share subsea foundations <NUM> of neighbouring generating groups <NUM> of the other row <NUM>, which foundations <NUM> serve as anchors for the spar buoys <NUM> of the respective generating groups <NUM>. Those shared foundations <NUM> are disposed between the rows <NUM> as shown.

As the turbine mooring lines <NUM> share the hub anchor lines <NUM> via link interfaces, and as at least some of the foundations <NUM> are also shared between hub anchor lines <NUM>, the installation requires fewer deep-water anchors. This beneficially declutters the seabed <NUM> and helps to protect the installation from fishing or other marine activities. A further benefit of this arrangement is to make space for terminal modules such as towheads <NUM> of energy storage bundles <NUM> to be located on the seabed <NUM> within the anchor pattern, as shown in <FIG> and <FIG>.

In this respect, <FIG> and <FIG> show elongate energy storage bundles <NUM> that terminate in towheads <NUM> within the anchor pattern of each generating group <NUM>. Such energy storage bundles <NUM> may employ the principles described in <CIT>, which discloses a pumped-storage system having pumping and hydropower generation components for, selectively, converting electricity into potential energy by expelling water from within a tank into the surrounding sea and for generating electricity from an incoming flow of water re-entering the tank under hydrostatic pressure. The tank comprises at least one elongate rigid pipeline that may be lowered to the seabed <NUM> as part of a towable unit or laid on the seabed <NUM> as a pipe string launched from a surface vessel.

Conveniently, in the example shown in <FIG> and <FIG>, each energy storage bundle <NUM> comprises: a pumping towhead 50P that supports a set of pumps for expelling water; a generating towhead <NUM> that supports a set of turbo-generator units comprising water turbines for generating electricity from an incoming flow of water, and an intermediate bundle of pipelines that together define a tank or power storage volume and establish fluid communication between the pumping towhead 50P and the generating towhead <NUM>. The pumping towhead 50P of each energy storage bundle <NUM> is electrically connected to and configured to be powered by the spar buoy <NUM> of a respective one of the generating groups <NUM> when water is to be expelled from the energy storage bundle <NUM> via the pumping towhead 50P. The generating towhead <NUM> of each energy storage bundle <NUM> is electrically connected to and configured to transmit power generated by its turbo-generator units to the spar buoy <NUM> of another one of the generating groups <NUM> when water is allowed to flow back into the energy storage bundle <NUM> via the generating towhead <NUM>.

For example, electricity may be generated in a subsea pumped-storage system by employing the principles described in <CIT>, which discloses an underwater turbo-generator unit that comprises a pressure-resistant shell defining a sealed internal chamber. At least one water inlet extends through the shell to effect fluid communication between the chamber and the sea surrounding the shell. A turbine is supported within the chamber to turn on a spin axis in response to admission of a flow of water into the chamber via the or each water inlet. The shell may be arranged to maintain a gas-filled space within the chamber, facilitating the use of a Pelton turbine that turns about a vertical spin axis. The chamber communicates with, and drains water into, a fluid storage volume such as a pipeline or bundle positioned at a level beneath the chamber.

<FIG>, which is not to scale, shows a generating towhead <NUM> at an end of an energy storage bundle <NUM> lying on the seabed <NUM>. The towhead <NUM> is conveniently close to the vertical central longitudinal axis of the spar buoy <NUM>, and indeed in this example is directly beneath the spar buoy <NUM>. A power umbilical <NUM> hangs from the spar buoy <NUM> to make electrical connections between electrical equipment on the spar buoy <NUM> and a set of turbo-generator units <NUM> within the towhead <NUM>. The towhead <NUM> further includes a drainage void or chamber <NUM> beneath the turbo-generator units <NUM> that communicates with the bundle of pipelines 52B extending between the towheads 50P, <NUM>. A dynamic breather pipe <NUM> for allowing air to enter the energy storage bundle <NUM> as water is being expelled by the pumping towhead 50P and to leave the energy storage bundle <NUM> as water enters the energy storage bundle <NUM> via the generating towhead <NUM> extends from the chamber <NUM> to the atmosphere above the surface <NUM> and is conveniently supported by the spar buoy <NUM>.

A further dynamic power umbilical <NUM> hangs as a catenary between the raft <NUM> of the wind turbine unit <NUM> and the spar buoy <NUM>. <FIG> also shows the hub anchor line <NUM>, comprising a central section 40C of synthetic rope and upper and lower sections of chain 40U, <NUM>, hanging as a taut leg or catenary between the spar buoy <NUM> and a subsea foundation <NUM> on the seabed <NUM>, and two of the turbine mooring lines <NUM> that hang as taut legs or catenaries from the raft <NUM> of the wind turbine unit <NUM>. Each turbine mooring line <NUM> comprises an upper section 44U of chain fixed to the wind turbine unit <NUM>.

One of the turbine mooring lines <NUM> extends from the wind turbine unit <NUM> radially inwardly, with respect to the spar buoy <NUM>, to the junction between the central section 40C and the upper section 40U of the hub anchor line <NUM>. The other of the turbine mooring lines <NUM> extends from the wind turbine unit <NUM> radially outwardly, with respect to the spar buoy <NUM>, to the junction between the central section 40C and the lower section <NUM> of the hub anchor line <NUM>.

By way of example, the top of the hub anchor line <NUM> may be at a depth of about <NUM> beneath the surface <NUM> and the tops of the turbine mooring lines <NUM> may be at a depth of about <NUM> beneath the surface <NUM>. Conversely, the seabed <NUM> may be at a depth of several hundred metres, for example <NUM> to <NUM> or more.

<FIG> corresponds to <FIG> but shows four energy storage bundles <NUM> that extend between, and connect respective pairs of, the five generating groups <NUM>. The pumping towheads 50P of the energy storage bundles <NUM> are adjacent to the spar buoys <NUM> of the first row 48A of generating groups 34A, 34B and 34D. Conversely, the generating towheads <NUM> of the energy storage bundles <NUM> are adjacent to the spar buoys <NUM> of the second row 48B of generating groups 34C and 34E.

It will be noted that the towheads <NUM> of the energy storage bundles <NUM> are disposed close to the vertical longitudinal axes of the spar buoys <NUM> at the centres of the generating groups <NUM> and indeed may, as shown in <FIG>, lie directly beneath the spar buoys <NUM>.

Conveniently, the energy storage bundles <NUM> are apt to be used not only for energy storage but also to effect subsea electrical connections between the spar buoys <NUM> of the rows <NUM> of generating groups <NUM>. For example, the spar buoys <NUM> of the first row 48A of generating groups <NUM> may conveniently serve as substations for the spar buoys <NUM> of the second row 48B of generating groups <NUM>, with the first energy storage bundle 52A providing an electrical connection between the spar buoy <NUM> of generating group 34A and the spar buoy <NUM> of generating group 34C, the second energy storage bundle 52B providing an electrical connection between the spar buoy <NUM> of generating group 34B and the spar buoy <NUM> of generating group 34C and so on.

It will be apparent that the three generating groups 34A, 34B and 34D of the first row 48A provide for generation of power via their wind turbine units <NUM> and also for consumption of power via the pumps in the pumping towheads 50P of the associated energy storage bundles <NUM>. In other words, the three generating groups 34A, 34B and 34D together provide a focal point for generation of electricity, effected by conveying their electrical output to the substations provided by the two generating groups 34C and 34E of the second row 48B, or for energy storage, effected by switching their output to the pumps that consume that electrical output and convert it to potential energy.

Conversely, the two generating groups 34C and 34E of the second row 48B provide for generation of power via their wind turbine units <NUM> and also via the turbo-generator units <NUM> in the generating towheads <NUM> of the associated energy storage bundles <NUM>. The generating groups 34C and 34E also provide for switching their output either to an onshore power grid or to the pumps of the pumping towheads 50P that are located among the generating groups 34A, 34B and 34D.

<FIG> shows the set <NUM> of generating groups <NUM> situated offshore as part of a floating windfarm <NUM>, in which the rows <NUM> of generating groups <NUM> within the set extend generally parallel to a coastline <NUM>. A sloping seabed <NUM> shelving steeply away from the coastline is represented here by contour lines <NUM>. One contour line 70A representing a water depth of, for example, <NUM> extends through the three generating groups 34A,34B and 34D of the first row 48A further from the coastline <NUM>. The other contour line 70B representing a water depth of, for example, <NUM> extends through the two generating groups 34C and 34E of the second row 48B closer to the coastline <NUM>. Thus, efficiently, the pumping towheads 50P of the energy storage bundles <NUM> are at a greater water depth than the generating towheads <NUM> of the energy storage bundles <NUM>.

In this example, the spar buoys <NUM> of the two generating groups 34C and 34E of the second row 48B serve as substations for the three generating groups 34A, 34B and 34D of the first row. The two spar buoys <NUM> that serve as substations are connected to an onshore power grid <NUM> by respective high-voltage links <NUM> that cross the coastline <NUM>. The high-voltage links <NUM> are suitably located underwater and may, on land, be located underground.

Turning next to <FIG>, these drawings exemplify the electrical distribution system of the windfarm <NUM> shown in <FIG>.

In the electrical block diagram of <FIG>, the generating groups 34A, 34B, 34C, 34D and 34E each comprise an array of eight wind turbine units <NUM> connected to associated switchgear <NUM> housed in the respective spar buoys <NUM>. Dry splice connections are made between the wind turbine units <NUM> and the switchgear <NUM>, which are all topside above the surface <NUM> or at least are housed in a dry environment.

The switchgear <NUM> of generating groups 34A and 34B is connected, via the first and second energy storage bundles 52A and 52B, to the switchgear <NUM> of generating group 34C, which serves as a substation for generating groups 34A and 34B. Conversely, the switchgear <NUM> of generating group 34D is connected, via the fourth energy storage bundle 52D, to the switchgear <NUM> of generating group 34E, which serves as a substation for generating group 34D.

The switchgear <NUM> of generating groups 34A, 34B and 34D is also connected to respective sets of pumps <NUM>, serving as subsea power consumers, located in the pumping towheads 50P of the associated energy storage bundles <NUM>. Specifically, the switchgear <NUM> of generating group 34A is connected to the pumping towhead 50P of the first energy storage bundle 52A. The switchgear <NUM> of generating group 34B is connected to the pumping towheads 50P of the second and third energy storage bundles 52B and 52C. The switchgear <NUM> of generating group 34D is connected to the pumping towhead 50P of the fourth energy storage bundle 52D. Whilst dry splice connections are made to the switchgear <NUM>, wet-mate connections <NUM> are made to the pumps <NUM> of the pumping towheads 50P.

The switchgear <NUM> of generating groups 34C and 34E is also connected to respective sets of turbo-generator units <NUM>, serving as subsea power producers, located in the generating towheads <NUM> of the associated energy storage bundles <NUM>. Specifically, the switchgear <NUM> of generating group 34C is connected to the generating towheads <NUM> of the first and second energy storage bundles 52A and 52B, whereas the switchgear <NUM> of generating group 34E is connected to the generating towheads <NUM> of the third and fourth energy storage bundles 52C and 52D. Again, whilst dry splice connections are made to the switchgear <NUM>, wet-mate connections <NUM> are made to the turbo-generator units <NUM> of the generating towheads <NUM>.

The wind turbine units <NUM> and turbo-generator units <NUM> of generating groups 34C and 34E are connected via their switchgear <NUM> to respective busbars and to respective step-up transformers that raise the voltage from <NUM>. kV to <NUM> kV for export of power along the high-voltage links <NUM> leading to shore.

<FIG> and <FIG> are electrical diagrams that represent generating groups 34B and 34C respectively. Only six wind turbine units <NUM> are shown in each of these examples, illustrating that there could be any number of wind turbine units <NUM> in the arrays of the generating groups <NUM>.

Each wind turbine unit <NUM> comprises a generator <NUM>, a transformer <NUM> and switchgear <NUM> for controlling a ballast pump <NUM> to keep the wind turbine unit <NUM> level under wind loading as explained previously. <FIG> and <FIG> also show the respective spar buoys <NUM> of generating groups 34B and 34C. Each spar buoy <NUM> is also equipped with a ballast pump <NUM> to control the draft <NUM> of the buoy <NUM>.

In generating group 34B shown in <FIG>, wet-mate connections <NUM> are made subsea between switchgear <NUM> aboard the spar buoy <NUM> and the pumping towheads 50P of energy storage bundles 52B and 52C for powering the pumping towheads 50P in order to expel water from energy storage bundles 52B and 52C. The sets of pumps <NUM> of each pumping towhead 50P are shown here with associated transformers <NUM> and switchgear <NUM>.

<FIG> also shows how the energy storage bundle 52B electrically connects the spar buoy <NUM> of generating group 34B to the spar buoy <NUM> of generating group 34C via wet-mate connections <NUM> and power cables <NUM> extending along the energy storage bundle 52B. In particular, parallel cables <NUM> are used to transmit power generated by the turbine units <NUM> of generating group 34B to the spar buoy <NUM> of generating group 34C for export, and to transmit power from generating group 34C to the spar buoy <NUM> of generating group 34B to support operation of the pumping towheads 50P.

In generating group 34C shown in <FIG>, wet-mate connections <NUM> are made subsea between switchgear <NUM> aboard the spar buoy <NUM> and the generating towheads <NUM> of energy storage bundles 52A and 52B for transmitting power generated by the generating towheads <NUM> to the spar buoy <NUM>. The sets of turbo-generator units <NUM> of each generating towhead <NUM> are shown here with associated transformers <NUM> and switchgear <NUM>.

<FIG> also shows the aforementioned cable connections <NUM> along energy storage bundle 52B for conveying parallel but opposite flows of power generated by the wind turbine units <NUM> and consumed by the pumps <NUM> associated with generating group 34B. Additionally, <FIG> shows how energy storage bundle 52A connects the spar buoy <NUM> of generating group 34C to the spar buoy <NUM> of generating group 34A via wet-mate connections <NUM>. A cable <NUM> extending along energy storage bundle 52A thereby conveys power generated by the wind turbine units <NUM> of generating group 34A to the spar buoy <NUM> of generating group 34C.

The spar buoy <NUM> of generating group 34C houses a busbar <NUM> and a boost transformer <NUM> in a dry environment. The generating outputs of the wind turbine units <NUM> of generating groups 34A and 34B and the turbo-generator units <NUM> of energy storage bundles 52A and 52B are aggregated at the busbar <NUM>. The boost transformer <NUM> steps up the voltage of those aggregated generating outputs to <NUM> kV for export to shore along one of the high-voltage links <NUM>.

The design philosophy of the example shown in <FIG> is to minimise the amount of equipment that has to be placed subsea and the number of wet-mate connectors <NUM> in the system. This involves trade-offs; for example, individual cables to each pump <NUM> could eliminate subsea switching at the cost of each pumping towhead 50P requiring multiple individual cables extending from the surface <NUM>. Consequently, the example shown in <FIG> employs readily-available subsea switching and synchronises supplies on the spar buoys <NUM> and towheads <NUM>.

As the pumps <NUM> and turbo-generator units <NUM> of the energy storage bundles <NUM> preferably have a modular design for ease of maintenance, wet-mate connectors <NUM> are particularly apt for the power cables or umbilicals <NUM> that lead to and from those components. Qualified wet-mate connectors are limited to <NUM> kV and <NUM> A, which informs the choice of a voltage of <NUM> kV within the windfarm <NUM> with step-up to <NUM> KV for onward transmission via the high-voltage links <NUM> to shore.

Thus, the spar buoys <NUM> of the generating groups 34C and 34E serve as substations for all electrical power generated in the windfarm <NUM>, including stepping-up the voltage from <NUM> kV within the windfarm <NUM> to <NUM> kV for onward transmission to shore. As noted above, the spar buoys <NUM> of the generating groups 34C and 34E provide a dry environment for the requisite boost transformers <NUM> and the associated high-voltage switchgear <NUM> and splice connections.

Electrical power generated by the windfarm <NUM> can be routed either directly to shore or to storage in the energy storage bundles <NUM>. In principle, all electrical power generated by the windfarm may be stored by activating the pumps <NUM> with <NUM> kV <NUM> supplies. Alternatively, the energy storage bundles <NUM> could be replaced by one or several export cables eventually bundled together to convey electrical power to successive hubs, then to a main export cable extending towards an onshore substation.

If there is a base load onshore, the amount of subsea equipment could be reduced by directing the outputs of individual wind turbine units <NUM> directly to shore to support that base load.

The windfarm <NUM> may be controlled from onshore, for example via a fibre-optic communication system that can control power distribution, start-up routines, synchronisation of the individual wind turbines <NUM>, energy storage and generation systems. Additionally, monitored data from the pump s <NUM> and turbines <NUM> may be transmitted onshore and a digital twin may be produced from that data. This facilitates life-of-installation monitoring, predictive maintenance and condition monitoring.

When installing the windfarm <NUM>, especially in a remote location, the use of costly installation vessels must be optimised. Preferably, the main infrastructure may be installed by a specialised vessel in one campaign so that subsequent tow-out and tie-in operations can be performed by a lower-specification anchor-handling vessel with ROV capability.

An initial campaign using a specialised vessel may, for example, involve: installing substation spar buoys <NUM> and other structures; installing anchors and wet-parking the main mooring system for the spar buoys <NUM>; pulling-in and laying high-voltage links <NUM> to shore from spar buoys <NUM> serving as substations; and installing the first wind turbine units <NUM>. Subsequent installation of spar buoys <NUM> and umbilicals <NUM> could be achieved by a lower-specification vessel to connect lines and umbilicals as wind turbine units <NUM> become available.

Installation of the energy storage bundles <NUM> is optional and in any event could be delayed because, for example, towing such bundles <NUM> to the installation site would not require a high-specification vessel. Indeed, initially at least, power generation would be possible without adding energy storage to the windfarm. This would enable the windfarm to generate revenue at an early stage, potentially contributing to the cost of manufacturing and installing energy storage bundles <NUM> in the future.

<FIG> illustrates an alternative layout for a windfarm <NUM> including a set <NUM> of generating groups <NUM> of wind turbine units <NUM> in accordance with another possible embodiment of the present invention. The windfarm <NUM> illustrated in <FIG> includes nine generating groups <NUM>, each generating group <NUM> including eight wind turbine units <NUM> surrounding a central a spar buoy <NUM>.

The generating groups <NUM> of the windfarm <NUM> illustrated in <FIG> are generally similar to the generating groups <NUM> described above in connection with the windfarm <NUM> illustrated in <FIG>, <FIG> and <FIG>. For example, the spar buoy <NUM> of each group <NUM> is connected to the seabed <NUM> by a plurality of hub anchor lines <NUM> extending to subsea foundations <NUM>, and the wind turbine units <NUM> of each group <NUM> are moored by mooring lines <NUM> that are each connected to one of the hub anchor lines <NUM>. However, each of the wind turbine units <NUM> in the windfarm <NUM> is provided with a set of four turbine mooring lines <NUM> (instead of three turbine mooring lines <NUM> as shown in <FIG>, <FIG> and <FIG>), including two inwardly-extending mooring lines <NUM> and two outwardly-extending mooring lines <NUM>. Each of the inwardly-extending turbine mooring lines <NUM> is connected to the central spar buoy <NUM> via the upper section 40U of one of the spar buoy's anchor lines <NUM>, and each of the outwardly-extending turbine mooring lines <NUM> is connected to a subsea foundation <NUM> via the lower section <NUM> of one of the spar buoy's anchor lines <NUM>, as illustrated in <FIG>.

As in the windfarm <NUM> of <FIG>, <FIG> and <FIG>, within each generating group, each of the wind turbine units <NUM> is electrically connected to the central spar buoy <NUM> via a dynamic umbilical <NUM> (three of which are shown in <FIG>), with the central spar buoy <NUM> being configured to act as a central hub for the wind turbine units <NUM> of its respective generating group <NUM>. However, in the windfarm <NUM> of <FIG> the wind turbine units <NUM> are each additionally electrically connected to other wind turbine units <NUM> of their respective generating group <NUM> by additional hanging cables <NUM> that extend between adjacent pairs of wind turbine units <NUM> in order to form a network.

As shown in <FIG>, the set <NUM> of generating groups <NUM> includes a central group <NUM>' that is surrounded by a looped array of eight other groups <NUM>" that together form a circular arrangement. This is in contrast to the layout of the windfarm of <FIG>, <FIG> and <FIG>, in which the generating groups <NUM> are instead arranged in parallel rows.

Advantageously, the subsea foundations <NUM> to which the hub anchor lines <NUM> of the central group <NUM>' are attached are each shared by a hub anchor line <NUM> of one of the surrounding groups <NUM>", and the surrounding groups <NUM>" also each share subsea foundations <NUM> with the adjacent groups <NUM>" in the outer ring, thereby further reducing the total number of subsea foundations <NUM> required by the windfarm <NUM> and decluttering the seabed <NUM>. In the embodiment illustrated in <FIG> the subsea foundations <NUM> to which the hub anchor lines <NUM> of the central group <NUM>' are connected are more widely spaced and arranged on a wider pitch circle than the subsea foundations <NUM> to which the hub anchor lines <NUM> of the surrounding groups <NUM>" are connected in order to facilitate the sharing of a subsea foundation <NUM> with each one of the surrounding groups <NUM>". However, this need not be the case in all embodiments. For example, the spacing of the subsea foundations <NUM> of the central group <NUM>' could be the same as the spacing of the subsea foundations <NUM> of the surrounding groups <NUM>" in alternative embodiments in which the number of surrounding groups <NUM>" is reduced or in which the central group <NUM>' shares subsea foundations <NUM> with only some of the surrounding groups <NUM>".

The spar buoy <NUM> of each of the surrounding groups <NUM>" is electrically connected to the spar buoy <NUM> of the central group <NUM>', which is configured to act as a substation for the spar buoys <NUM> of the surrounding groups <NUM>". The spar buoys <NUM> of the surrounding groups <NUM>" may each be connected to the spar buoy <NUM> of the central group <NUM>' using direct connections <NUM> as shown in <FIG>, in which the wind turbine units <NUM>, anchor lines <NUM> and mooring lines <NUM> have been omitted for clarity. The connections may, for example, be provided by dynamic umbilicals, by cables running along the seabed <NUM> and/or by energy storage bundles of the type described above in connection with <FIG> and <FIG>. However, in other cases one or more of the surrounding groups <NUM>" may be connected to the spar buoy <NUM> of the central group <NUM>' indirectly via one or more of the other surrounding groups <NUM>". The spar buoy <NUM> of the central group <NUM>' is further connected to a high-voltage link <NUM> which is configured to export power from the windfarm <NUM>, for example to an onshore grid (not shown).

Turning finally to <FIG>, these drawings show various alternative arrangements within the inventive concept for mooring and connecting central hubs and wind turbine units <NUM> of generating groups <NUM>. For example, whilst the arrangements shown in <FIG> and <FIG> have a spar buoy <NUM> serving as a central hub, <FIG> shows that the central hub could instead be a tension-leg platform <NUM>.

In <FIG>, a spar buoy <NUM> floating at the surface <NUM> is moored by three hub anchor lines <NUM> that extend to respective subsea foundations <NUM> embedded in the seabed <NUM>. In this example, upper and lower sections 40U, <NUM> of chain are optional. A wind turbine unit <NUM> is moored by three turbine mooring lines <NUM>. The hub anchor lines <NUM> and the turbine mooring lines <NUM> could be substantially or entirely of polymer rope. Two of the turbine mooring lines <NUM> extend to, and share, the subsea foundations <NUM> of two of the hub anchor lines <NUM>. The other turbine mooring line <NUM> is connected directly to the wind turbine unit <NUM> and the spar buoy <NUM> as a radial line.

<FIG> also shows a dynamic umbilical <NUM> extending between the wind turbine unit <NUM> and the spar buoy <NUM> and an export cable <NUM> extending from the spar buoy <NUM> to export power, for example to another generating group <NUM> or directly to the shore.

In <FIG>, the spar buoy <NUM> floating at the surface <NUM> is moored by four hub anchor lines <NUM> that extend to respective subsea foundations <NUM> embedded in the seabed <NUM>. In this example, each hub anchor line <NUM> has a central section 40C of polymer rope and upper and lower sections 40U, <NUM> of chain. Again, the wind turbine unit <NUM> is moored by three turbine mooring lines <NUM>. Two of the turbine mooring lines <NUM> extend to, and intersect, respective ones of the hub anchor lines <NUM> at the junction between the central section 40C and the lower section <NUM>, hence sharing their lower sections <NUM> of chain. The other turbine mooring line <NUM> intersects one of the hub anchor lines <NUM> at the junction between its central section 40C and its upper section 40U, hence sharing its upper section 40U of chain adjoining the spar buoy <NUM>.

<FIG> also shows a dynamic umbilical <NUM> extending between the wind turbine unit <NUM> and the spar buoy <NUM> and an export umbilical <NUM> that hangs from the spar buoy <NUM> to export power along an energy storage bundle <NUM> that lies on the seabed <NUM>.

The tension-leg platform <NUM> shown in <FIG> comprises four taut legs <NUM> in this example, each extending to a respective subsea foundation <NUM> embedded in the seabed <NUM>. Here, the wind turbine unit <NUM> is moored by four turbine mooring lines <NUM>.

Two of the turbine mooring lines <NUM> extend to, and share, the subsea foundations <NUM> of two of the legs <NUM>. A third turbine mooring line <NUM> extends to an additional subsea foundation <NUM>. The fourth turbine mooring line <NUM> is connected directly to the wind turbine unit <NUM> and to the platform <NUM> as a radial line.

Again, <FIG> also shows a dynamic umbilical <NUM> extending between the wind turbine unit <NUM> and the platform <NUM> and an export umbilical <NUM> that hangs from the platform <NUM> to export power along an energy storage bundle <NUM> that lies on the seabed <NUM>.

Many other variations are possible without departing from the inventive concept. For example, it would be possible for a turbine mooring line <NUM> to extend to a subsea foundation <NUM> and to be fixed to the subsea foundation <NUM> in parallel to a hub anchor line <NUM>, or to be fixed to a separate subsea foundation <NUM>. Similarly, it would be possible for a turbine mooring line <NUM> to extend to a central hub such as a spar buoy <NUM> and to be fixed to that hub in parallel to a hub anchor line <NUM>. In those cases, each turbine mooring line <NUM> may comprise a central section 40C of steel wire or synthetic rope and upper and/or lower sections 40U, <NUM> of steel chain.

Claim 1:
A floating electric power-generation group, comprising:
a floating hub (<NUM>) that is anchored to a plurality of subsea foundations (<NUM>) by a plurality of anchor lines (<NUM>); and
at least two floating power producer units (<NUM>) that are distributed around a substantially circular array that is centred on the hub (<NUM>), and are connected electrically and mechanically to the hub (<NUM>);
wherein the power producer units (<NUM>) are each moored by a plurality of mooring lines (<NUM>), at least one of those mooring lines (<NUM>) extending in a radially-inward direction toward the hub (<NUM>) to effect mechanical connection to the hub (<NUM>) and at least one other of those mooring lines (<NUM>) extending in a radially-outward direction away from the hub (<NUM>) toward one of the subsea foundations (<NUM>) to effect mechanical connection to the subsea foundation (<NUM>).