Patent Description:
When operating electrical power grids, it is a well-known challenge to match the supply of electricity to rapidly-fluctuating demand. Conversely, the use of intermittent power sources such as solar, wind and other renewables results in short-term fluctuations in generating capacity.

Even a transient mismatch between the supply and demand of electricity can cause an unacceptable variation in supply frequency across the grid. Consequently, it is routine to employ a mixture of generating assets with different attributes. Those assets typically comprise continuously-operating base-load sources, such as power stations powered by gas, coal or nuclear energy, and faster-reacting short-term sources, such as generators powered by gas turbines or diesel engines.

In addition, it is common for electrical power grids to employ load-balancing measures that involve temporary storage of energy. Energy may be stored in various ways, for example as electrochemical energy in batteries or as potential energy in water reservoirs, such as are used in pumped-storage hydroelectricity schemes. Other, less mature, energy-storage solutions include the use of flywheels or of compressed air. In each case, the stored energy can be released almost instantly to supply or to generate electricity on demand.

Elegantly, excess electricity from periods of low demand can be converted into electrochemical or potential energy to be saved for periods of higher demand. Typically this involves using the excess electricity to charge batteries or to pump fluids to higher heads or pressures. The fact that such an arrangement must be a net consumer of electrical energy is outweighed by benefits to the overall grid system, including more efficient use of base-load sources and minimising overcapacity of very expensive generating assets.

Energy is stored and discharged cyclically, most typically on a daily cycle reflecting different levels of demand for electricity during daytime and night-time periods. However to maintain control of the power grid, storage and discharge actions may be planned and executed on timescales ranging from days to seconds.

There is an increasing need for electrical power grids to be supplemented by short-term, quick-reacting energy storage systems. That need is driven by both supply-side and demand-side challenges. The main supply-side challenge is the increased reliance upon renewable energy sources, which can only provide a discontinuous or intermittent supply. A major demand-side challenge is how to recharge the rapidly-growing number of electrically-powered vehicles.

As a result, there is a need to find additional ways of storing very large amounts of energy that can be accessed quickly enough to generate electricity on demand. However, provision of sufficiently large battery installations and pumped-storage schemes would be extremely complex and expensive and raises significant environmental and planning concerns. Also, battery installations are prone to degrade with repeated charge/discharge cycles over a period of time.

Various subsea energy-storage solutions have been proposed. Examples are disclosed in <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and the Applicant's International Patent Application No.<CIT>.

<CIT> discloses a hydroelectric generator that is driven by pressurised air. A tubular structure that houses a turbine is anchored to the seabed. The lower end of the structure comprises an air injection system that releases pressurised air into the structure. The resulting mixture of air and water rising within the structure spins the turbine.

More generally, various proposals have been made for subsea power stations using turbines that spin in a flow of water, examples being the turbine assemblies disclosed in <CIT> and <CIT>. However these and other prior art disclosures are not fully enabling; they merely illustrate the turbine assembly on a schematic or symbolic level and do not consider the practicalities of how to make such machines work efficiently in a subsea environment.

<CIT> describes an underwater power generation unit comprising turbines that are driven using a flow of water. <CIT> describes an underwater unit for a hydroelectric power plant. Additional examples of hydroelectric turbo-generator systems are described in <CIT>, <CIT>, <CIT> and <CIT>.

The present invention proposes practical turbo-generator arrangements for use in subsea energy storage systems, such as the systems described in <CIT>. Those systems comprise a pressure-resistant vessel defining a fluid storage volume, exemplified by a pipeline, and a pump to evacuate seawater from the storage volume. Consequently, fluid remaining within the storage volume is at a pressure lower than the ambient pressure defined by the hydrostatic pressure of the surrounding seawater. Inward flow of seawater in response to that pressure differential spins a turbine that drives a generator to produce electric power.

Pelton turbines are well known in the field of hydroelectric power. They are characterised by a circumferential array of dished vanes that are shaped like shallow cups or buckets. Water flowing along a penstock from an upstream reservoir arrives at the turbine with high velocity. The high-velocity water is distributed between a circumferential array of tangentially-oriented injection nozzles that direct respective jets of pressurised water at the buckets of the turbine. The buckets reverse the flow of the jets to maximise the momentum change and hence the reaction force applied to the turbine.

The turbine is connected by a shaft to a co-axial alternator or generator to form a turbo-generator assembly. As the generator is heavy and bulky, small or freestanding hydroelectric installations are typically oriented such that the turbine and generator spin about a horizontal axis. A vertical axis is usually only adopted in very large hydroelectric installations such as dams, where massive structures of reinforced concrete can be built to support the generator atop the turbine or vice versa.

The skilled reader will appreciate that hydropower solutions that work on land or near the surface will not necessarily work deep underwater. For example, parts affected by seawater corrosion or by the growth of marine organisms cannot be maintained easily in seabed installations. Also, it is impractical to build massive supporting structures deep underwater.

A subsea energy storage system must define a sufficiently large storage volume for the required energy capacity and must withstand the hydrostatic pressure of deep water. Yet, the system must also be practical to construct and to install on the seabed, and must continue to work efficiently and reliably when installed. There is a need for subsea turbo-generator arrangements that are compatible with these objectives.

Against this background, the present invention resides in a turbo-generator assembly for producing electrical power underwater. In one expression of the inventive concept, the assembly comprises: a pressure-resistant shell that defines a sealed internal chamber; at least one water inlet extending through the shell to effect fluid communication between the chamber and a body of water surrounding the shell; a turbine 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; at least one tubular penstock structure that is in fluid communication with the chamber via the or each water inlet; and a drainage receptacle that communicates with the chamber to receive water falling from the turbine. The drainage receptacle is attached to or integrated with an accessory module of a pipeline or with a towhead module of a pipeline bundle; and the shell is separably mountable and sealable to the drainage receptacle. In this case, the turbine could be reversible to expel water from the chamber into the body of water that surrounds the shell.

The or each penstock structure preferably extends outside the shell to be exposed externally to the surrounding water, where it is conveniently supported by the shell or by another part of the assembly. The or each penstock structure suitably extends upwardly from the or each water inlet on an upright, preferably substantially vertical axis. The or each penstock structure may comprise a tapering accelerator portion disposed between an intake portion and the or each water inlet.

The chamber of the assembly may further contain a duct that communicates with the or each water inlet and with a circumferential array of nozzles that surrounds the turbine.

The shell may comprise a domed portion around the turbine. A generator may be supported by the shell. A transformer may also be supported by the shell or by another part of the assembly. The spin axis, which is preferably upright, and more preferably substantially vertical, suitably intersects the transformer.

The drainage receptacle suitably has an outlet for fluid communication with a fluid storage volume. Conveniently, the pipeline or the bundle serves as a fluid storage volume communicating with the assembly. The turbine may be a Pelton turbine, and the turbo-generator assembly may be arranged to maintain a gas-filled space within the chamber, the Pelton turbine being arranged to turn on the spin axis in the gas-filled space.

The inventive concept extends to a power plant for producing electrical power underwater, the power plant comprising at least one turbo-generator assembly of the invention, whose internal chamber is in fluid communication with a fluid storage volume that is capable of holding fluid at a pressure substantially below hydrostatic pressure prevailing around the or each turbo-generator assembly. Preferably, the internal chamber of the or each turbo-generator assembly is positioned above the fluid storage volume and the shell of the or each turbo-generator assembly is exposed externally to surrounding seawater.

The power plant may further comprise at least one pump that is in fluid communication with the fluid storage volume and that is arranged to expel water from the fluid storage volume.

Where the fluid storage volume comprises a pipeline or a pipeline bundle, the or each turbo-generator assembly is suitably supported by the pipeline accessory module or by the bundle towhead module.

Embodiments of the invention provide a turbine structure for producing electrical power subsea. The structure comprises: a pressure-resistant body; a sealed chamber inside the body containing a gas such as air; at least one water inlet in fluid communication with seawater; and a Pelton turbine inside the chamber. The turbine preferably has a vertical spin axis to reduce fatigue and vibration due to asymmetry, and to ease installation and maintenance. The sealed chamber may be in fluid communication with a storage volume for gas or other fluid.

The fluid storage volume may be installed by installing a pipeline or pipeline bundle suspended from an installation vessel. Subsequently, the turbo-generator assembly may be attached to an accessory module of the pipeline or to a towhead module of the bundle.

The inventive concept also finds expression in methods of generating electrical power underwater. One such method comprises: drawing a flow of water under hydrostatic pressure into a sealed chamber at lower than hydrostatic pressure; forming one or more jets from the incoming flow of water; and impinging the or each jet of water against a Pelton turbine spinning in a gas in the chamber. Preferably, the incoming flow of water is accelerated along a penstock.

Preferably, the turbine is spun in the chamber about an upright axis while the gas is confined in the chamber, for example in an upper portion of the chamber around the turbine. Gas may be allowed to rise into the chamber from an underwater fluid storage volume that is in fluid communication with the chamber and is disposed at a level beneath the chamber.

Another method of generating electrical power underwater in accordance with the invention comprises: drawing a flow of water under hydrostatic pressure into a sealed chamber at lower than hydrostatic pressure; accelerating the incoming flow of water along a penstock; and impinging the accelerated flow of water against a turbine spinning in the chamber. The incoming water may be accelerated along the penstock outside the chamber.

Water may be drained from the turbine into an underwater fluid storage volume that is in fluid communication with the chamber and is disposed at a level beneath the chamber.

The turbine structure may comprise two or more inlets for seawater. The or each inlet may comprise a vertical or upright tube. The tube may have at least one bore and/or at least one filtering device. The inlets may be fluidly connected to at least one injection manifold, such as a ring around the turbine, comprising or communicating with at least one injection nozzle.

During a power production phase, seawater may be admitted into the injection manifold by a pressure difference between the interior of the body and the surrounding seawater. Waste water falling from the turbine after transferring its energy to the turbine may also be drained or evacuated to the gas storage volume by a pressure difference.

The body may comprise a base and an upper cover that can be brought together to define a sealed volume between them. The base may define a drainage receptacle for water falling from the turbine and may comprise an outlet leading to the gas storage volume. Conversely, the upper cover may comprise a shell and the at least one inlet. The upper cover may also enclose the turbine and support an alternator or generator block.

The upper cover of the body may be sealed to the base by one or more of the following releasable connections: a threaded connection; a dogleg lock; a collet connector system; and/or a pinbox connector system.

In summary, the invention provides a subsea turbo-generator unit for producing electrical power. The unit comprises a pressure-resistant shell that defines a sealed internal chamber. At least one water inlet extends through the shell to effect fluid communication between the chamber and a body of water surrounding the shell. A turbine is supported within the chamber to spin 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 may turn about a vertical spin axis. The or each water inlet may communicate with at least one tubular penstock structure that may be supported by the unit outside the shell. The chamber communicates with, and drains water into, a fluid storage volume such as a pipeline that may be positioned at a level beneath the chamber.

Referring firstly to <FIG> of the drawings, a turbo-generator assembly <NUM> of the invention comprises a hollow, rigid, pressure-resistant and self-supporting domed shell or housing <NUM>. The housing <NUM> is rotationally symmetrical around a substantially vertical central axis <NUM> and so is circular in plan view.

The housing <NUM> contains a generally toroidal manifold or ring duct <NUM> for high-pressure water that encircles the central axis <NUM>. The housing <NUM> also encloses, and the duct <NUM> also surrounds, a Pelton turbine <NUM> that is supported to spin about the central axis <NUM>. Such a turbine <NUM> is characterised by an array of circumferentially-facing buckets <NUM> that are distributed angularly around the central axis <NUM>.

As best shown in <FIG>, the ring duct <NUM> supports, and is in fluid communication with, an array of nozzles <NUM> that face inwardly from the ring duct <NUM> and are spaced angularly from each other around the central axis <NUM>. The nozzles <NUM> are offset angularly from radial alignment with respect to the central axis <NUM>, all in the same circumferential direction. Thus, the nozzles <NUM> have tangential orientation to direct jets of high-pressure water from the ring duct <NUM> into the buckets <NUM> of the turbine <NUM> with substantial circumferential or tangential momentum.

The ring duct <NUM> is also in fluid communication with one or more elongate penstock structures <NUM>, through which the ring duct <NUM> receives high-pressure water, in use, from the surrounding sea. The or each penstock structure <NUM> is supported by the assembly <NUM>, in this example by the housing <NUM> of the assembly <NUM>, but is otherwise self-supporting so as to project from the housing <NUM> into the surrounding sea.

In this example, there are two penstock structures <NUM> in mutual and symmetrical opposition about the central axis <NUM>. The penstock structures <NUM> shown here are largely straight and on parallel, substantially vertical axes parallel to the central axis <NUM>, which is preferred for compactness and ease of installation. Upright orientation also creates a helpful gradient in hydrostatic pressure along the length of each penstock structure <NUM>. However, in principle, each penstock structure <NUM> could have any suitable shape or orientation.

Each penstock structure <NUM> is tubular and comprises an enlarged intake portion <NUM> upstream of a frusto-conical venturi or accelerator portion <NUM> that tapers in a downstream direction, in this case downwardly. An injector pipe <NUM> downstream of the accelerator portion <NUM> curves inwardly toward the central axis <NUM> to extend through the wall of the housing <NUM> into fluid communication with the ring duct <NUM> within the housing <NUM>.

The housing <NUM> is surmounted by, and supports the weight of, a generator <NUM>, such as an alternator, and a transformer <NUM>. The generator <NUM> closes an open top of the housing <NUM> and is coupled to the turbine <NUM> by a drive shaft <NUM> that also spins on the central axis <NUM>. The transformer <NUM> is conveniently mounted on top of the generator <NUM> as shown in this example. However, the transformer <NUM> could instead be positioned elsewhere and connected to the generator <NUM> by cables or other conductors.

As the internal features of the generator <NUM> and the transformer <NUM> are conventional, internal details of them have been omitted from the sectional views of <FIG> and <FIG>. Those sectional views are taken on a plane that extends along the central axis <NUM>.

The housing <NUM> also has an open bottom that cooperates with and closes the open top of a drainage receptacle <NUM> that serves as a base or mount for the assembly <NUM>. The drainage receptacle <NUM> is hollow to define a drainage chamber within a tubular peripheral wall. The bottom of the housing <NUM> seals against the peripheral wall of the drainage receptacle <NUM>, for example by being seated into an upwardly-facing groove in the peripheral wall to compress a gasket or O-ring placed in the base of the groove.

The drainage receptacle <NUM> is in fluid communication with a submerged storage volume for holding fluid at a pressure lower than the ambient pressure defined by the hydrostatic pressure of the surrounding seawater. As will be explained, pressure within the storage volume is lowered by pumping out seawater, thus enlarging a gas pocket in a headspace above the reduced volume of seawater that remains in the storage volume. Consequently, a pump is also in fluid communication with the storage volume to create the pressure differential that stores potential energy in the seawater around the storage volume.

In this example, the storage volume is a pipeline <NUM>, meaning that the drainage receptacle <NUM> may conveniently be attached to, or integrated with, an accessory structure or module <NUM> of the pipeline <NUM>, atop the module <NUM> as shown in <FIG>. Buttresses extend radially from the peripheral wall of the drainage receptacle <NUM> to the top of the module <NUM> to brace the assembly <NUM>, which is supported by the tubular wall of the drainage receptacle <NUM>.

<FIG> shows the drainage receptacle <NUM> atop an in-line module <NUM> at an intermediate location along the length of the pipeline <NUM> whereas <FIG> shows the drainage receptacle <NUM> atop a terminal module <NUM> at an end of the pipeline <NUM>. In each case, the module <NUM> has one or more internal channels <NUM> that effect fluid communication between the pipeline <NUM> and the assembly <NUM> via the drainage receptacle <NUM>. In this example, the or each channel <NUM> incorporates a valve <NUM> such as a ball valve that can be closed to close the channel <NUM> and hence to close and seal the storage volume defined by the pipeline <NUM>.

When the valve <NUM> is closed as shown in <FIG> and a pump in fluid communication with the pipeline <NUM> is activated to expel seawater from within the pipeline <NUM>, the pipeline <NUM> holds fluid in the form of gas and water at low pressure. The system is therefore charged with potential energy due to the pressure differential with the surrounding seawater, which remains at high hydrostatic pressure. It will be noted from <FIG> that the water level <NUM> in the pipeline <NUM> is low and that a headspace <NUM> of gas such as air and water vapour above the water level <NUM> in the pipeline <NUM> is correspondingly large in volume.

When the valve <NUM> is opened as shown in <FIG>, seawater surrounding the assembly <NUM> is drawn into the penstock structure <NUM> through a perforated wall of the intake portion <NUM>. The perforated wall serves as a filter that blocks entry into the system of potentially damaging debris that could be entrained in the inrushing seawater. Other, or additional, filtering provisions are of course possible.

The accelerator portion <NUM> accelerates the incoming flow from the intake portion <NUM>, which therefore enters the injector pipe <NUM> with high velocity. At the interface between the injector pipe <NUM> and the ring duct <NUM>, the high-velocity water is deflected to follow the duct <NUM> in a circumferential direction corresponding to that of the jets projected by the nozzles <NUM>. The effect is that a high-pressure, high-velocity water flow impinges against the buckets <NUM> of the turbine <NUM> and so drives the turbine <NUM> efficiently.

A Pelton turbine <NUM> operates most efficiently when spinning in a gas such as air or water vapour. Consequently, after impinging on the buckets <NUM> of the turbine <NUM>, water drains or is evacuated from the turbine <NUM> into the drainage receptacle <NUM> and from there into the storage volume that is defined by the pipeline <NUM> in this example.

It will also be apparent that gas trapped in the pipeline <NUM> or other storage volume will tend to rise into the housing <NUM> around the turbine <NUM> and will be trapped there by the domed shape of the housing <NUM>. Gas remains trapped in the system to allow the volume of water in the pipeline <NUM> to change as pressure in the pipeline <NUM> is varied. The opposed dotted arrows within the module <NUM> of <FIG> illustrate the upward migration of gas into the housing <NUM> in exchange for downward flow of water into the pipeline <NUM>.

Turning next to <FIG>, these drawings show the turbo-generator assembly <NUM> in the wider context of a subsea power plant that has a pump arranged to expel seawater from an elongate storage volume. They also exemplify ways in which such a power plant may be installed on the seabed.

<FIG> and <FIG> show a subsea power plant of the invention embodied as a towable bundle unit <NUM>. The unit <NUM> comprises a pipeline bundle <NUM> connecting a leading towhead <NUM> and a trailing towhead <NUM>. The bundle <NUM> comprises two or more substantially parallel pipes that extend substantially the full length of the bundle <NUM> between the towheads <NUM>, <NUM>.

The leading towhead <NUM> contains a pump so that after the unit <NUM> has been installed, water can be pumped from within the pipes of the bundle <NUM> into the surrounding sea. The trailing towhead comprises a module <NUM> and a drainage receptacle <NUM> onto which the turbo-generator assembly <NUM> can be docked, for example after the unit <NUM> has been installed as shown in <FIG>. Water admitted through the turbine <NUM> of the turbo-generator assembly <NUM> under hydrostatic pressure drives the generator <NUM> of the turbo-generator assembly <NUM> to produce electricity on demand.

Pipes of the bundle <NUM> serve as one or more energy storage tanks that can be of any reasonable length, and therefore of any internal capacity that may reasonably be required. Such a bundle unit <NUM> has proven resistance to hydrostatic pressure and can be fabricated and installed in a single operation using well-known and reliable methods.

As is well known in the art, pipes of the bundle <NUM> may be surrounded by an external carrier pipe. A carrier pipe and/or the pipes within any carrier pipe may be configured to resist the hydrostatic pressure at the operational depth. Alternatively, exposed pressure-resistant pipes of the bundle <NUM> may be clustered around a central core pipe or spine. A central core pipe may itself be pressure-resistant to add energy-storage capacity to the bundle <NUM> or it may remain flooded to act solely as a structural element.

The various pipes of the bundle <NUM> are typically of steel but any of them could be largely of polymers or of composite materials. Additional layers or components can be added to the pipes, such as an internal liner or an outer coating. Such additional layers or components may comprise polymer, metal or composite materials. Also, pipes can be single-walled or of double-walled pipe-in-pipe (PiP) construction.

Other elongate elements such as auxiliary pipes and cables may be included in the bundle <NUM>, extending in parallel with the other pipes of the bundle <NUM> in well-known fashion to carry fluids, power and data signals between the towheads <NUM>, <NUM>. As is also conventional, longitudinally-distributed transverse spacers may hold the various pipes and other elongate elements of the bundle <NUM> relative to each other.

A typical pipeline bundle <NUM> is a few kilometres in length, for example about <NUM> long. Its maximum length may be constrained by the availability of land at onshore fabrication facilities such as spoolbases or yards, However, a pipeline bundle <NUM> can be made longer by fabricating it from multiple bundle sections coupled end-to-end. In principle, therefore, a bundle <NUM> assembled from two or more such bundle sections could be of any reasonable length.

Thus, the bundle unit <NUM> is shown in <FIG> and <FIG> both interrupted and greatly shortened. Also, the depth of the water between the surface <NUM> and the seabed <NUM> will usually be much greater than these schematic views would suggest.

Integrating the bundle <NUM> and the towheads <NUM>, <NUM> to form the towable unit <NUM> allows the unit <NUM> to be prefabricated, assembled and tested onshore or in sheltered water before it is towed offshore for installation. Conveniently, therefore, multiple elongate elements can be towed together to an installation site as a single integral unit and installed on the seabed simultaneously in one operation. Reducing the number of subsea-connected interfaces simplifies the installation process and improves the reliability of the system, as compared with connecting units at a subsea location and performing tests there instead.

The towheads <NUM>, <NUM> incorporate buoyancy, or provide for buoyancy to be attached, to offset their weight during towing. For example, buoyancy may be added directly to the towheads <NUM>, <NUM> by attaching buoys or buoyancy modules to them.

The bundle <NUM> may also contribute buoyancy to the unit <NUM> by virtue of air or other gas contained within a sealed carrier pipe. However, as noted above, an external carrier pipe is optional; pipes of the bundle <NUM> may instead be clustered around a central core pipe or spine. Additional external buoyancy may also be provided on, or attached to, a carrier pipe, a core pipe or other pipes of the bundle <NUM>.

Various towing methods may be used to transport the unit <NUM> to an offshore installation site. In particular, the unit <NUM> may be towed at various depths in the water. The choice of towing depth involves a trade-off between various factors. For example, the unit <NUM> may be surface-towed at or near to the surface <NUM>, which is easiest to manage. However, surface water dynamics may generate fatigue in the pipeline bundle <NUM>, which is a factor that limits the allowable tow distance. Conversely, towing near the seabed <NUM> protects the bundle <NUM> from the influence of surface water dynamics and limits risks during subsequent lowering to the seabed <NUM> at the installation site. However, controlling the unit <NUM> is more challenging at depth and is only feasible if the contours of the seabed <NUM> permit.

<FIG> shows the preferred option of a mid-water towing method in which the unit <NUM> is towed at an intermediate depth in the water column between the surface <NUM> and the seabed <NUM>. Here, the unit <NUM> is safely clear of the contours of the seabed <NUM> and is beneath significant influence from wave action near the surface <NUM>. Specifically, <FIG> shows a favoured mid-water towing method known in the art as the 'controlled-depth towing method' or CDTM, as described in <CIT>.

Mid-water towing is a good compromise that ensures low-stress installation without the use of large crane vessels that depend on low sea states. This makes installation less weather-sensitive and reduces the cost of installation vessels significantly. However, mid-water towing requires precise management of buoyancy.

In all towing methods, the unit <NUM> is held in tension by chains or lines <NUM> extending fore and aft from the respective towheads <NUM>, <NUM> to respective installation vessels such as tugs <NUM>. The bundle <NUM> acts in tension between the towheads <NUM>, <NUM> during towing, with tensile loads being borne principally by a carrier pipe or core pipe of the bundle <NUM>.

The speeds of, and spacing between, the tugs <NUM> are adjusted to keep the unit <NUM> at the required depth having regard to the effect of drag forces and tension in the lines <NUM>. Optionally, a third patrol/survey vessel <NUM> ahead of the leading tug <NUM> surveys the route and monitors the towing operation.

In the CDTM, the bundle <NUM> is made neutrally or slightly negatively buoyant at the required depth by the addition of buoyancy and/or ballast chains spaced along its length. In the example shown, ballast chains <NUM> spaced along the bundle <NUM> add weight that offsets any positive buoyancy of the bundle <NUM>. As a result of the added ballast weight, the bundle <NUM> hangs between the towheads <NUM>, <NUM> as a catenary.

When the unit <NUM> reaches an installation site, the unit <NUM> is lowered toward the seabed <NUM> while the lines <NUM> are paid out from the tugs <NUM>. The unit <NUM> can be lowered to the seabed <NUM> by removing external buoyancy from the unit <NUM> or by adding ballast to the unit <NUM>. The unit <NUM> then settles on the seabed <NUM> as shown in <FIG>, with the bundle <NUM> resting on and supported by the seabed <NUM> between the towheads <NUM>, <NUM>.

<FIG> shows the towheads <NUM>, <NUM> landed on and supported by pre-installed foundations <NUM>. The foundations <NUM> may, for example, be embedded structures such as suction piles or pin piles. Alternatively, all or part of the foundations <NUM> could be integrated with the towheads <NUM>, <NUM> or be installed after the towheads <NUM>, <NUM> have been landed on the seabed <NUM>.

<FIG> also shows, in dashed lines, other features that are apt to be installed after the unit <NUM> has been installed. Specifically, anchors <NUM> such as staples or pins are spaced along the bundle <NUM> to fix the bundle <NUM> to the seabed <NUM>. Also, a power cable <NUM> connects the unit <NUM> to an electrical power grid <NUM> via a control system <NUM>, both of which may be situated wholly or partially above the surface <NUM> or on land. In principle, it may instead be possible to connect a power cable <NUM> to the unit <NUM> before towing or installing the unit <NUM>.

Like numerals are used for like features in <FIG>, which exemplifies how a subsea energy storage tank could instead be defined by a pipeline <NUM> that is launched from an installation vessel <NUM> on the surface <NUM>. During installation, the pipeline <NUM> hangs as a catenary from the installation vessel <NUM> toward the seabed <NUM>. In principle, depending upon its materials and dimensions and the depth of water, the pipeline <NUM> could be installed by any method for installing subsea pipelines as known in the art, such as reel-lay, S-lay or J-lay. A J-lay operation is shown here, by way of example.

Conveniently, as shown in <FIG>, the pipeline <NUM> may include modules <NUM>, any or all of which may comprise or support pumping and power-generation facilities like those of the towheads <NUM>, <NUM> in <FIG> and <FIG>. As noted above, such modules <NUM> may be disposed at an end of the pipeline <NUM> or may be inserted within the length of the pipeline <NUM>. The modules <NUM> are therefore analogous to well-known pipeline accessories such as in-line tee assemblies (ILTs or ITAs), pipeline end manifolds (PLEMs) and pipeline end terminations (PLETs). Thus, using well-known techniques, the modules <NUM> may be incorporated into the pipeline <NUM> as it is launched into the sea.

The modules <NUM> are exemplified here by a terminal or end module 42A welded to an end of the pipeline <NUM> and an in-line module 42B welded between neighbouring sections of the pipeline <NUM> disposed end-to-end. Another terminal or end module 42A will be welded in due course to the other end of the pipeline <NUM>, to close and seal that end of the pipeline <NUM> on completion. As is conventional, the modules <NUM> could have mudmat foundations <NUM> as shown but other foundations such as the aforementioned piles shown in <FIG> could be used instead.

The pipeline <NUM> may be of single-walled construction or could instead be of twin-walled pipe-in-pipe (PiP) construction. Again, the pipeline <NUM> may be of steel, polymer or composite material and may comprise additional layers or components such as an internal liner or an outer coating. For example, some installation techniques such as S-lay will allow the pipeline <NUM> to have an outer weight coating of concrete to stabilise it on the seabed <NUM>.

In J-lay operations as shown in <FIG>, the pipeline <NUM> is assembled from pipe joints in an upright J-lay tower <NUM> on an installation vessel <NUM> offshore. The pipeline <NUM> hangs near-vertically to a sagbend approaching the seabed <NUM>, thus assuming a J-shape.

Pipe joints are lifted into the tower <NUM> to be welded to the top of a suspended pipe string. The tower <NUM> is shown here as being vertical for simplicity but in practice it could be pivoted or gimballed to depart from the vertical. Welding operations are performed at a welding station <NUM> near the base of the tower <NUM>.

A fixed lower bushing <NUM> beneath the welding station <NUM> and a travelling upper bushing or clamp <NUM> on the tower <NUM> support the pipe string in alternation. The lower bushing <NUM> and the travelling clamp <NUM> cooperate in a 'hand-over-hand' arrangement to lower the pipe string as successive pipe joints are added.

<FIG> shows a turbo-generator assembly <NUM> being docked with the module 42B after that module 42B has been landed on the seabed <NUM>. The assembly <NUM> is suspended from a lifting wire <NUM> that hangs from a winch or crane of a vessel, not shown, on the surface <NUM>.

After the pipeline <NUM> has been installed, a power cable <NUM> extends from the modules 42A, 42B, for example to connect them to an electrical power grid via a control system as shown in <FIG> and <FIG>. Again, anchors such as staples or pins could be spaced along the pipeline <NUM> to fix the pipeline <NUM> to the seabed <NUM>, but such anchors are not shown in <FIG>.

Stacking major components of the assembly <NUM> along the vertical central axis <NUM> simplifies installation and maintenance, allowing the assembly <NUM> as a whole, or any of its major components, to be lowered from or raised to the surface together or separately. Subsea-releasable, ROV-operable fastenings may be provided between the stacked components for this purpose. In this respect, reference is made to <FIG>.

<FIG> shows the assembly <NUM> mounted atop an in-line module <NUM> of a pipeline <NUM> via the drainage receptacle <NUM>. A lifting wire <NUM> is attached centrally to the top of the assembly <NUM>. <FIG> shows the assembly <NUM> now suspended from the lifting wire <NUM> and being lifted off, or lowered onto, the drainage receptacle <NUM>, which remains attached to the module <NUM>.

The assembly <NUM> may also be assembled or disassembled subsea. For example, <FIG> shows the generator <NUM> and transformer <NUM> of the assembly <NUM> being lifted off, or lowered onto, the housing <NUM> of the assembly <NUM>, which remains attached to the module <NUM> via the drainage receptacle <NUM>. Conversely, <FIG> shows the transformer <NUM> being lifted off, or lowered onto, the generator <NUM>, which remains attached to the module <NUM> via the housing <NUM> and the drainage receptacle <NUM>.

Finally, <FIG> show another embodiment of the invention in which multiple turbo-generator assemblies <NUM> are grouped together on a towhead <NUM>.

The towhead <NUM> has integral drainage receptacles <NUM> on its upper horizontal face, onto which the turbo-generator assemblies <NUM> can be mounted. The towhead <NUM> is at an end of an elongate storage volume, which is defined by a parallel pair of pipeline bundles <NUM> in this example. Valves to control incoming fluid flow and hence power generation are not shown in these simplified drawings but could be incorporated at any suitable location in the flowpath, upstream and/or downstream of the turbines in the turbo-generator assemblies <NUM>.

As can be appreciated in the sectional views of <FIG>, the pipeline bundles <NUM> are in fluid communication with the turbo-generator assemblies <NUM> through branched manifold channels <NUM> in the towhead <NUM>. Specifically, the turbo-generator assemblies <NUM> and their associated drainage receptacles <NUM> are in parallel longitudinal rows on the towhead <NUM>. Each pipeline bundle <NUM> is in fluid communication with a respective row of turbo-generator assemblies <NUM> through a respective manifold channel <NUM>.

It would of course be possible for the pipeline bundles <NUM> to communicate with each other and with all of the turbo-generator assemblies <NUM>. Valves may be provided to segregate the pipeline bundles <NUM> and the turbo-generator assemblies <NUM> from each other to isolate failures and to facilitate maintenance or replacement of components.

<FIG> shows five turbo-generator assemblies <NUM> already installed on respective drainage receptacles <NUM> of the towhead <NUM> pre-installed on the seabed <NUM>. A sixth turbo-generator assembly <NUM> is shown being lowered onto the open top of a sixth drainage receptacle <NUM> of the towhead <NUM>.

<FIG> shows all of the turbo-generator assemblies <NUM> in place on top of the towhead <NUM>.

<FIG> shows the system in a wholly or partially discharged state. Consequently, the water level <NUM> in the towhead <NUM> and the pipeline bundles <NUM> is high and the headspace <NUM> of gas above the water level <NUM> is correspondingly small in volume. The headspace <NUM> is divided into multiple gas pockets, one for each of the turbo-generator assemblies <NUM>, corresponding to the branches of the manifold channels <NUM>.

<FIG> shows the system charged with potential energy due to a pressure differential with the surrounding seawater. The water level <NUM> in the towhead <NUM> and the pipeline bundles <NUM> is therefore low and the headspace <NUM> is correspondingly large in volume. The headspace <NUM> now extends between all of the turbo-generator assemblies <NUM>.

Many other variations are possible within the inventive concept. For example, the drainage receptacle <NUM> could be integrated with or recessed into the storage volume or with or into any structure, such as a pipeline accessory module <NUM>, that communicates fluidly with a storage volume such as the pipeline <NUM>. A drainage receptacle <NUM>, as a distinct structure, could therefore be omitted.

The or each penstock structure <NUM> could be provided with one or more valves that are capable of controlling or blocking fluid flow. For example, one-way valves may admit inrushing water but block the egress of gas. Valves in the or each penstock structure <NUM> may be provided instead of, or in addition to, any valve between the assembly <NUM> and the storage volume, such as the valve <NUM> described above.

Whilst it is preferred for the storage volume to comprise a pipeline or pipe bundle, the storage volume need not necessarily be an elongate structure. The storage volume could instead take other suitable pressure-resistant shapes such as spherical, part-spherical, ellipsoid or dome-shaped. Also, the storage volume need not be a wholly manufactured structure but could instead include a natural formation such as a subterranean chamber or a subsea well that has been depleted of hydrocarbons or is otherwise no longer economic to exploit.

A Pelton turbine <NUM> is preferred for its compactness and efficiency. However, in a broad sense, the turbine could be a reversible turbine such as a Francis turbine. In that case, the generator <NUM> could serve as a motor to spin the turbine in reverse, thereby to expel water from the storage volume along the penstock structures <NUM> and out into the surrounding sea. This may make it unnecessary to provide a separate pump to evacuate the storage volume.

Claim 1:
A turbo-generator assembly (<NUM>) for producing electrical power underwater, the assembly comprising:
a pressure-resistant shell (<NUM>) that defines a sealed internal chamber;
at least one water inlet (<NUM>) extending through the shell (<NUM>) to effect fluid communication between the chamber and a body of water surrounding the shell;
a turbine (<NUM>) supported within the chamber to turn on a spin axis (<NUM>) in response to admission of a flow of water into the chamber via the or each water inlet (<NUM>);
at least one tubular penstock structure (<NUM>) that is in fluid communication with the chamber via the or each water inlet (<NUM>); and
a drainage receptacle (<NUM>) that communicates with the chamber to receive water falling from the turbine (<NUM>), wherein the drainage receptacle (<NUM>) is attached to or integrated with an accessory module (<NUM>) of a pipeline (<NUM>) or with a towhead module (<NUM>, <NUM>) of a pipeline bundle (<NUM>),
characterised in that the shell (<NUM>) is separably mountable and sealable to the drainage receptacle (<NUM>).