WO2015139942A1

WO2015139942A1 - Water current turbine

Info

Publication number: WO2015139942A1
Application number: PCT/EP2015/054315
Authority: WO
Inventors: Matthew HARRISON; Robert COLCLOUGH; Frank Ibach; Matthew Reed
Original assignee: Marine Current Turbines Limited
Priority date: 2014-03-17
Filing date: 2015-03-02
Publication date: 2015-09-24
Also published as: GB201404747D0; GB2524252A

Abstract

A modular water current turbine array comprises a plurality of submersible turbine assemblies (2), a conversion module (13, 7, 36) for power and/or frequency conversion and a conversion module mounting (12, 38, 6, 38), At least one conversion module (13) is common to two or more turbine assemblies (2).

Description

WATER CURRENT TURBINE

This invention relates to a modular water current turbine array, in particular using a submersible turbine assembly in tidal flows.

Conventional turbine assemblies for tidal waters include surface piercing tower mounted turbines, as well as seabed mounted, or floating, submersible turbines. A particular problem with submerged turbines, such as the seabed mounted type, is that if any part of the system fails, it is necessary to raise the complete turbine and powertrain unit to carry out a repair, or replacement, which due to its size and weight requires the use of a heavy lift vessel. Such vessels are not typically owned by the turbine operator, but must be chartered on a daily basis at significant expense and may not be

immediately available. For surface piercing tower mounted turbine assemblies, the lift is more straightforward, as the turbine is mounted on a cross member fitted to the lifting gear on the tower, but it still results in extended periods when the turbine is out of commission whilst the faulty part is worked on.

In accordance with a first aspect of the present invention, a modular water current turbine array, the array comprising a plurality of submersible turbine assemblies; one or more conversion modules for power and/or frequency conversion and one or more conversion module mountings; wherein at least one conversion module is common to two or more turbine assemblies.

The power conversion module or frequency conversion module may be mounted on extensions to the power train housing, but preferably, at least one of the module mountings is remote from the turbine assembly.

The modules and module mountings may be installed in a float or vessel, but preferably, the module mountings are installed on the seabed.

Preferably, the modules further comprise controllable buoyancy, whereby the modules are retrieved from and returned to their mountings.

In accordance with a second aspect of the present invention, an array according to the first aspect, wherein the plurality of submersible turbine assemblies are coupled to the seabed; wherein each turbine assembly comprises a turbine mount and a powertrain; wherein the array further comprises one or more electrical modules removably installed in one or more mountings and connected to the plurality of turbine assemblies, each electrical module being retrievable from its mounting independently of retrieval of the powertrain and the turbine unit; wherein at least one electrical module comprises a conversion module for power and/or frequency conversion, common to two or more turbine assemblies.

The large and heavy powertrains can be left in place in their submerged locations whilst the electrical module is retrieved. As the electrical module is smaller and lighter, as well as containing components with a shorter mean time to failure, then the turbine assembly, repairs and maintenance on the electrical module can be carried out more easily and at a lower cost than with a conventional turbine assembly where the electrical components are an integral part of the powertrain.

The array may comprise separate electrical modules for frequency conversion, power conversion and transforming from low voltage to high voltage.

Preferably, a separate electrical module for frequency conversion is provided for each turbine assembly.

Preferably, a common electrical module for transforming and switching is provided for the array of turbine assemblies.

Preferably, the turbine assembly mounting comprises one of a submerged pile and a float anchored to the seabed.

Preferably, the electrical module is located on a surface piercing tower, or a float.

Preferably, the electrical module is located in its mount on the seabed, or on the turbine assembly.

Preferably, the electrical module further comprises a variable buoyancy housing by which the module is installed in its mount.

Preferably, the mount further comprises connectors to receive electrical inputs from each powertrain and a connector to connect an output to a cable at export voltage.

Preferably, the electrical module connects to its mount via a stab connector.

An example of a modular water current turbine system will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates an embodiment of the present invention showing a module being retrieved from its operational location;

Figure 2 illustrates the module of the embodiment of Fig.1 in more detail;

Figure 3 is a schematic diagram of the energy generating system of Fig.1 ; Figure 4 illustrates a first embodiment of a sub-sea turbine array according to the present invention;

Figure 5 illustrates a second embodiment of a sub-sea turbine array according to the present invention;

Figure 6 illustrates a second embodiment of a sub-sea turbine array according to the present invention;

Conventional tower mounted sub-sea turbine systems, such as described in EP2613046, arrange for electrical power cables, control and instrumentation cables, plus any hydraulic or pneumatic hoses to be threaded though the surface piercing tower, into the housing at the top of the support column and thence, as required, to the shore. However, typically, the power electronics are within the powertrain and hence to access power conversion equipment and carry out any maintenance on the electrical components in the turbine system, it is necessary to raise and lower the complete turbine and powertrain assembly. For a turbine system mounted on the sea bed, the complete turbine and powertrain assembly must be raised, using a heavy lift ship. Even with a turbine and powertrain mounted beneath a floating pontoon, there is still a requirement to raise the complete assembly out of the water to access electrical systems for maintenance. Some float mounted systems are designed to pivot out of the water, such as EP 1467091, but others would need to be lifted up onto a barge, or ship. In all these cases, the turbine is out of commission throughout.

An example of a water current turbine system for generating energy from tidal or other currents is illustrated in Fig.1. Fig.1 shows a turbine assembly mounted to the seabed on a pile 3 by means of a mechanical connection. A powertrain module 2 of the turbine assembly converts flow into torque and electrical power and comprises a turbine hub and turbine blades, as well as a gearbox, generator and pitch systems. The turbine typically has two or three blades. A low/medium voltage cable 4 from the electrical power generation part of the powertrain is led to the seabed and along the sea bed to a separate power unit 5 which comprises a mounting 6 and a removable electrical frequency conditioning module 7. Low/medium voltage is typically in the range of 690V to 6.6kV, preferably of the order of 4kV, for example 4.16kV, the limiting factor being the voltage at which wet-mate connections are possible. Thus up to 1 lkV is currently feasible. The frequency conditioning module 7 of the power unit 5 typically comprises frequency conversion equipment and switch gear for the turbine enabling the electrical power from the powertrain to be harmonised with grid power. As the frequency conversion module is the smallest and lightest part, alternatively, this may be mounted as a removable module of the powertrain (not shown) in a mounting allowing easy connection and release to remove the module. The frequency conditioning unit 5 is coupled via another seabed cable 15 to a substation and transmission unit 14 comprising a mounting 12 and a removable power conditioning module 13. In an alternative embodiment (as shown in Fig.3), the functions of the frequency

conditioning module and power conditioning module may be combined in a single removable module. A transformer part of the substation to transform up to a suitable voltage for exporting power to the grid may be provided. This part may either be provided separately, or as part of the power conditioning module 13. An example export voltage is 33kV. Transforming up from the low to medium voltage 690V to 1 lkV is required to minimise transmission losses to shore and the grid.

Fig.3 is an electrical schematic showing combined conditioned and

unconditioned power inputs. Frequency conditioning unit 36 of the module 13 may perform the required frequency conversion of the unconditioned generator output received from the powertrain 2 and also provide auxiliary power for the turbine, for example for use by the pitch system and for pitch system transformation. The frequency conversion is independent of the powertrain, but may only be a limited distance away from the powertrain due to a LV/MV connection to the turbine generator and the inductance of the cable 4 over the distance between the connection and the frequency conversion unit.

The arrangement illustrated requires independent switching of each turbine powertrain. This may be done at the substation 27, in the case where each powertrain has its own power conditioning unit 37, but where the power conditioning unit is shared between a number of powertrains 2, each of which have their own frequency conditioning unit 36, then the switching is carried out at the power conditioning unit 37. In Fig.3, the transformer 26 is separate from the frequency and power conditioning modules 36, 37. The transformer transforms the electrical power at low/medium voltage up to high voltage, typically 33kV or more, in order to minimise transmission losses to the shore and grid. As any connections at high voltage are within the dry unit, no wet mates are required once at export voltage. The transformer also allows for switching of the power units 5 e.g. for maintenance.

Generally, the frequency conditioning module 7, 36 has the lowest mean time to failure of the components of a turbine system. When a fault is detected, a service vessel 10 connects a lifting cable 8 from a crane 11 to a lifting point 9 on the frequency conditioning module 7 and lifts it onto the vessel 10. A replacement frequency conditioning unit may then be attached and lowered into position in the mounting 6, completing the circuit and allowing power generation from its turbine assembly to continue. If servicing, or maintenance, of the power conditioning module 13 is required, the same lift and replace process may be carried out with a replacement power conditioning module.

Modularising the subsystems of the energy conversion chain allows low cost, frequent access to the items of the system with the lowest Mean Time Before Failure (MTBF), meaning that the availability of the system can be maintained at a high level, whilst minimizing the exposure to operation cost escalation due to costly operations with large high day rate offshore oil and gas ships. The retrieval of the subsystems is easier because they have a relatively low weight compared to the weight of the turbine and powertrain, so a smaller, more readily available and lower daily rate vessel may be used.

The mountings 6, 12 comprise fittings which allow simple, quick decoupling of the modules 7, 13 from the seabed mount to further minimize risk due to the short timeframe available at low current speeds at tidal sites. These relatively light weight modules may be retrieved in higher wave states than when retrieving the turbine and powertrain, as heave compensation can be used to reduce impact sensitivity when docking on a subsea frame. The low voltage connections between submerged powertrain and the frequency and power conversion modules are less costly and more reliable than high voltage connections.

Fig.2 illustrates an example of a mounting and a module for power conversion, when installed on the seabed, although as mentioned above the modules may be mounted in a releasable mounting near the power train. The precise shape of the module is not limited to the cylinder shown, but any suitable shape may be used. The module 13 connects to the mounting 12 with a guide cable 18 and a connector 19. The module may be retrieved or replaced using a cable connected to the lifting point 9, as described above, or it may have in addition, controllable buoyancy to assist with retrieval. The buoyancy can be adjusted by filling or emptying tanks of water or air. The mounting comprises a housing for the connectors from the frequency conversion unit 5 or powertrain 4, as well as support arms 16 and weighted anchors 17 at each end of the support arms. The same basic structure of releasable module may be used for the frequency conditioning modules, but the example described in more detail with reference to Fig.3 is for a power conditioning unit 14.

Although, the modular nature of the system would be useful even with a single turbine, it is more particularly suited to an array of turbines, as illustrated in Figs. 4 to 6 The power conditioning module 13 provides an enclosure for power electronics 32 which is chosen according to the hydrostatic pressure and cooling requirements of the specific implementation. In this schematic, an ancillary power transformer 20 is incorporated into the power conditioning unit. The ancillary power supply derived from a 690V connection to the transformer 20 is fed to the powertrain 2 via cable 21. Power generated in the powertrain is transmitted to the power conditioning electronics 32 through cable 4.

The unit has a mechanical mounting to a concrete base unit. The power conditioning module 13 has mechanical and electrical connections to the power conditioning unit base, for example using double wetmate connections, or by means of a stab connector. The module may be connected via a single stabbing plate to a transformer unit incorporating apparatus for exporting the power to the shore and the grid. The unit may also act as a subsea hub, including a transformer to transform to export voltage, typically 33 KV. Where the unit acts as a subsea hub, there are multiple high voltage leads 25 feeding to the transformer 26, the inputs 24 received from several other power conditioning units (not shown). Combining the substation and power conditioning equipment for transmitting high voltage to shore has the advantage of allowing turbines to be located long distances from shore, as not every turbine assembly requires cabling to the shore, or a transformer.

The subsea foundation 12 of the power conditioning unit 14 relies to a large extent on gravity to hold the power conditioning module 13 in place, with mating features, such as the wetmate connection, laid into the foundation, or stab connector mentioned above, between the module 13 and the wet-mate frame 12. The foundation is typically made of reinforced concrete, shaped to reduce load from snagging of any cables or mooring lines.

The power conversion module 13 may also include turbine switch gear, as well as the power conditioning equipment and ancillary power for turbine subsystems. The module has a mechanism to enable it to lock to the base, though it is preferable that actuators for locking the module rigidly to the foundation frame and actuators for moving the wet-mate electrical connection in and out of contact with its housing, are installed in the bases of the power conditioning unit, rather than in the retrievable module 13. However, the mechanism could be arranged with the actuators on the module. The module also has a built in lifting point, if a lift frame is also to be used, the module has mating features for mating with the lift frame. Unlike conventional systems, there is no requirement to shut down all connected systems in the array when removing or servicing a power conditioning unit, with the consequent effect on the power production of the array.

For each of the different locations of the modular frequency and power conditioning units, there are different benefits. In the example of Fig.4, an array of powertrains 2 each mounted by mechanical connections to a subsea pile 3 are shown. For each pile, a separate, subsea frequency conditioning unit 5 is provided, in close proximity with the powertrain, connected by cable 4 and low voltage connections. The output of the unit 5 is fed by cable 15 to the power conditioning unit 12, again using low voltage connections. The power conditioning unit services all six of the powertrains shown in the example. The number of powertrains per power conditioning unit is typically in the range of 6 to 30, although higher numbers are possible. The constraints are the cable length and array layout and optimisation of the costs of installing another power unit against the cost of using more cable for longer runs to the unit. Removing one or two frequency conditioning units for maintenance has no impact on the power generation of the remainder of the array and as the unit can be easily replaced with another, the time for which even this powertrain is out of commission is low. Even if large numbers of units are taken out of use, the system continues to operate with those remaining, although with some loss of efficiency. The units which do need to be removed regularly are light, keeping down the cost of retrieval. The transformer for transforming to export voltage is not required in every power conditioning unit. Where the power conditioning unit does not include an export transformer, that unit will be connected to a substation which acts as a common export transformer. As many as 30 power conditioning units may be connected to a single export transformer sub-station and thence to the shore and typically at least 6. As with the power conditioning units, the cable length and layout influence the precise number of units per substation, with the costs of installing another substation, or using more, smaller substations, against the cost of using more cable for longer runs to the substations being taken into account.

Fig. 5 shows an alternative, in which the array of powertrains 2 on their piles 3 are connected 4 to an off-shore surface piercing tower 29 which houses the frequency conditioning 30, power conditioning 31, control and substation equipment 33 for one or more tidal turbines deployed in the locality of the tower. This means that the tower can be deployed further offshore, but maintains the benefit that the electrical modules are in a less hostile environment than if permanently submerged and hence avoids the cost and complexity of critical high-voltage wet-mateable connectors and submerged equipment, as well as providing easy access for servicing and repair of less reliable components, such as the frequency conditioning module, and a safer working environment. In order to use the tower mounted frequency conditioning unit 30, the powertrains need to be relatively close, typically no more than 500 metres from the tower, although up to 5km may be possible. Beyond this distance, it is more efficient to install another platform. A hybrid option, if this is not possible, is to use retrievable subsea frequency conditioning units 5 for those of the powertrains 2 outside the range of the tower unit 30, and use the tower mounted power conditioning unit 31 and substation 33.

Fig.6 shows an array based on surface float mounted turbine assemblies. The turbine assembly is mounted beneath a pontoon 35, or has some other means of providing the correct amount of buoyancy so that the turbine 1 and powertrain 2 are submerged. Each assembly is connected via a chain 34 to an anchor 28 on the sea bed and a cable 4 from the powertrain connects to an individual seabed mounted frequency conditioning unit 5 with low voltage connections on to the submerged power conditioning unit 12. The submerged power conditioning unit service a plurality of powertrains and either includes or connects to a transformer to transform up to a suitable voltage for export to the shore. If so desired, the anchored floating turbine assemblies may be provided in the vicinity of a surface piercing tower and at least the power conditioning unit and transformer provided on the tower. The connections from the powertrains to the tower still go via the seabed and where the array is large and not all powertrains are close enough to the tower, the frequency conditioning units may be provide on the seabed, or as removable units on the powertrain, or on the pontoon.

Fig.7 shows an alternative embodiment, in which the powertrains 2 are mounted on piles on the seabed, in a similar fashion to the examples of Figs.4 and 5 and the electrical subsystems 13, 43, 27, 42 are mounted on a float or vessel 38 close to the powertrains. An anchor cable 41 connects the vessel 38 to the seabed and according to the design, may act as a guide or support for the cables from each powertrain. Within the floating vessel 38 are provided the frequency and power conversion modules 36, 37 in the power conditioning module 13, a pitch and control system 43, the transformer 27 and necessary computer systems 42 to operate the turbine assembly. The powertrain 2 is connected to the power module 13 via cable 4 and control of the pitch mechanism, generator torque demand and receipt of sensor data from the powertrain is by means of cable 39 to and from the pitch and control system 43. Export of power to the shore and grid is via cable 40. Access to the electrical subsystems for repair and maintenance is straightforward, yet the common vessel for multiple powertrains, close to the powertrains allows the array to be sited offshore. This arrangement has the advantage that all connections other than to the powertrain are dry. In another embodiment (not shown) only the frequency conversion part of the power unit 13 is housed in the floating vessel, on the basis that that is the most likely part to need to be accessed for maintenance. The array and units are otherwise as shown in Fig.4.

The power conditioning module as used in the examples of Figs.4 and 6 allows for a fully submerged tide energy system. This has the advantage over the surface piercing structure of Fig.5 that there is less material required in construction and installation in deep water, saving cost, but has the common benefit of separating the power generation part of the turbine system from ongoing maintenance obligations. Tidal turbines require the use of power conditioning hardware, control equipment and substation equipment to allow generated electricity to be exported into the electricity grid. Housing this equipment onshore limits the distance that the turbines can be positioned from the shore when each turbine requires a separate cable to shore, at significant cost, but the modular design of the present invention overcomes this issue, as many powertrains can make use of the same cable to shore by virtue of the common power conditioning unit.

The present invention overcomes the long downtime and reduces the lifting costs by the turbine system being of a modular design whereby components with a relatively short mean time to failure are in different modules from more reliable components allowing easy retrieval and replacement of the less reliable, or more regularly serviced, components. Once a module has been retrieved, it can be replaced immediately with another one of the same type, minimising the time during which the turbine and powertrain are not operable. This is particularly useful for permanently submerged energy generation systems, where it would have been necessary to conduct subsea operations to inspect, maintain or repair any system, or subsystem on the turbine and powertrain assembly. Instead of low reliability technology forcing frequent lifting and lowering of the turbine and powertrain and associated electrical component to and from the subsea mounting structures in strong tidal currents, which can be challenging, the invention means that only smaller, lighter modules need to be lifted frequently. Various methods have been proposed to make the turbine and powertrain lifting operation more efficient and less risky, but these do not address the problem of extended downtime.

Claims

1. A modular water current turbine array, the array comprising a plurality of submersible turbine assemblies; one or more conversion modules for power and/or frequency conversion and one or more conversion module mountings; wherein at least one conversion module is common to two or more turbine assemblies.

2. An array according to claim 1, wherein at least one of the module mountings is remote from the turbine assembly.

3. An array according to claim 2, wherein the at least one module mounting is installed on the seabed.

4. An array according to any preceding claim, wherein the at least one conversion module further comprises controllable buoyancy, whereby the module is retrieved from and returned to the mounting.

5. An array according to any preceding claim, wherein the plurality of submersible turbine assemblies are coupled to the seabed; wherein each turbine assembly comprises a turbine mount and a powertrain; wherein the array further comprises one or more electrical modules removably installed in one or more mountings and connected to the plurality of turbine assemblies, each electrical module being retrievable from its mounting independently of retrieval of the powertrain and the turbine unit; wherein at least one electrical module comprises a conversion module for power and/or frequency conversion, common to two or more turbine assemblies.

6. An array according to claim 5, wherein the array comprises separate electrical modules for frequency conversion, power conversion and transforming from low voltage to high voltage.

7. An array according to claim 5 or claim 6, wherein a separate electrical module for frequency conversion is provided for each turbine assembly.

8. An array according to any of claims 5 to 7, wherein a common electrical module for transforming and switching is provided for the array of turbine assemblies.

9. An array according to any of claims 5 to 8, wherein the turbine assembly mounting comprises one of a submerged pile and a float anchored to the seabed.

10. An array according to any of claims 5 to 9, wherein the electrical module is located on a surface piercing tower, or a float.

11. An array according to any of claims 5 to 9, wherein the electrical module is located in its mount on the seabed, or on the turbine assembly.

12. An array according to claim 11, wherein the electrical module further comprises a variable buoyancy housing by which the module is installed in its mount.

13. An array according to any of claims 5 to 12, wherein the mount further comprises connectors to receive electrical inputs from each powertrain and a connector to connect an output to a cable at export voltage.

14. An array according to any of claims 5 to 13, wherein the electrical module connects to its mount via a stab connector.