GB2534851A

GB2534851A - Turbine support structure

Info

Publication number: GB2534851A
Application number: GB1501537.3A
Authority: GB
Inventors: Harflett Luke; Rust David; Smith Jonathan
Original assignee: Marine Current Turbines Ltd
Current assignee: Marine Current Turbines Ltd
Priority date: 2015-01-30
Filing date: 2015-01-30
Publication date: 2016-08-10
Also published as: GB201501537D0

Abstract

A subsea turbine 8 support structure 1 comprises a plurality of support legs 5 joined to one another by a lattice 17; and a platform 3 mounted to the support legs e.g. above the surface of the water 4. A lifting mechanism e.g. winch 10 can be coupled to a turbine base 9 or the turbine 8 to lift the turbine which slides along a turbine base mounting e.g. slide rails 12a, 12b. The slide rails 12a, 12b may be fitted to the support legs or to the lattice framework and the turbine may be lowered or raised on the slide rails by the winch 10.

Description

TURBINE SUPPORT STRUCTURE

This invention relates to a subsea turbine support structure, in particular for marine water current turbines.

Tidal structures may be surface piercing or fully submerged. An example of a surface piercing structure is SeaGen, which has a large diameter steel tube with a cross beam mounted on the tube, so that the turbines fitted at each end of the crossbeam are away from the steel tube. However, this design is expensive to construct due to the loading of the turbines. There is a large moment on the cross beam and a large surface area of the cross beam must pass through the water to air interface. The turbines may also apply a twisting force to the structure if they are caught by waves as they exit the water. The design requires both turbines to be retrieved together, stopping power generation entirely, even if only one turbine requires maintenance.

In accordance with the present invention, a subsea turbine support structure comprises a plurality of support legs, a lattice joining the support legs to one another; and a platform mounted to the support legs; the support structure further comprising a lifting mechanism for coupling to one of a turbine base and the turbine; and a turbine base mounting; wherein the turbine base mounting is fitted to one of the support legs or to the lattice framework, and wherein the turbine may be lowered or raised on the turbine base mounting by the lifting mechanism.

This structure has the advantage that the lattice frame, being wide based, provides support directly behind the powertrain, rather than via a crossbeam which is subject to higher bending moments The turbine base mounting may comprise a slide rail fitted to the leg, or the turbine base mounting may comprise a slide rail fitted to the lattice, or both.

Preferably, a turbine base is provided on at least two of the support legs. Preferably, the lifting mechanism comprises a winch and associated rigging. Preferably, a single winch is provided on the platform, adapted for connection to each of the turbine bases separately.

Preferably, the support structure comprises four support legs.

Preferably, the platform is mounted to each of two adjacent ones of the support legs arranged such that a plane formed between the two adjacent support legs is substantially perpendicular to a plane of the platform; and the platform is mounted to the other two support legs such that a plane formed between the other two support legs is angled at an acute angle relative to the plane of the platform.

Preferably, the platform is mounted to the support legs such that a longitudinal axis of each of the four support legs is angled at an acute angle to the platform.

Preferably, the support legs are connected to a ground anchor by a pile inserted in each support leg.

Preferably, the support legs are connected to a ground anchor through a framework having a wider footprint than a footprint formed by the support legs.An example of a subsea turbine support structure and a method of operation will now be described with reference to the accompanying drawings in which: Figures 1 a and lb illustrate a first example of a support structure according to the present invention; Figure 2a and 2b illustrate a second example of a support structure according to the present invention; Figure 3 illustrates a third example of a support structure according to the present invention; Figure 4a illustrates a turbine base for mounting a turbine to a support structure according to the present invention; Figure 4b illustrates a turbine mounted to a support structure according to the present invention, using the turbine base of Fig.4a; Figure 5 is a flow diagram illustrating installation of a turbine on a support structure according to the present invention; and, Figure 6 is a flow diagram illustrating retrieval of a turbine on a support structure according to the present invention, for maintenance.

As discussed above, current surface piercing offshore marine turbine mounting structures, such as a cylinder tower with a crossbeam incorporating an integrated powertrain recovery capability, are expensive to build because of the need to make them strong enough to cope with the loading due to the turbines being mounted on the ends of the crossbeam and the crossbeam having to pass through the air/water interface during the turbine retrieval operation. An alternative is a surface-piercing cylinder tower with a monopile foundation and the turbine powertrain mounted directly onto the tower.

The present invention provides a surface-piercing lattice tower structure, with independently recoverable turbine powertrains mounted on one or more legs of the structure. The turbine powertrains are mounted on bases, typically sliding on rails fitted to the leg and outside of the lattice close to and parallel with the leg. The base may be winched up the tower leg to allow for maintenance of the turbine powertrain which is mounted on it. Although examples of the lattice tower, or jacket structure are described with respect to a surface piercing wind turbine system, the same design of tower may be applicable in fully submerged installations of the order of 40m to 60m, giving the benefits of stability and individually recoverable turbines on a single support structure. A lifting cable, from a winch mounted at the top of the tower, o may be connected to a lifting point on the turbine powertrain when requried, with the powertrain housing and turbine base coupling designed to connect and disconnect easily when the turbine base is at the top of the support structure.

A first example is illustrated in Fig la. In this example, a four legged lattice structure I, anchored to the seabed 2, with a platform 3 above the air/water interface 4, is used. The lattice structure is essentially a truncated A-frame. Each of the four legs 5 of the structure 1 is inclined so that a footprint formed by the four legs at the seabed end 6 is larger than a footprint formed by the four legs at a platform end 7, as can be seen in Fig. 1 b. Planes 16 formed between each two adjacent legs are not perpendicular to a plane of the platform 3. For simplicity, the example shows a single turbine 8 mounted on a turbine base 9 and lowered into position on one of the legs 5, although each leg may support a turbine. Similarly, only one winch 10 is visible, but in practice, each turbine 8 on a leg 5 may have its own winch on the platform 3, or could share a winch with one or more other turbines. The platform may include a housing for control equipment, for monitoring and controlling transmission of power generated by the turbine. The platform is also provided with a crane 11 for general maintenance. Lifting and movement of a turbine 8 onto a turbine base 9 mounted on a leg of the support structure I is done using the winch 10 and its associated rigging.

The turbine base 9 is mounted to the leg on slide rails 12 with a lifting cable 13, for example a chain hoist, to the winch on the platform 3. The turbine powertrain is fitted to the turbine base 9 both for lifting the turbine up and down the structure and also when the turbine is in place and generating electricity. The turbine is mounted such that the blades 14 of the turbine do not come into contact with a lower part 5a of the leg, below the powertrain, as they rotate. Electrical cables 15 from the powertrain are fed up to the platform, either on the outside of the lattice tower structure, or in the centre of the lattice tower and protected by the framework. Each of the legs 5 of the tower are coupled together by a lattice framework 17 made up of horizontal and diagonal cross members.

Installation and retrieval of the turbine on the turbine base may be simplified by using the design of Fig.2a. In this case, the example illustrates a support structure for two turbines 8, with the legs 5 which do not receive a turbine forming a plane 8 between them which is at an angle with respect to the plane of the platform, as before, but with support legs 5b which receive a turbine being mounted so that a plane formed between those two support legs is perpendicular to the plane of the platform 3. This means that the turbine can be mounted to the turbine base 9 and slide rails 12 without having to adjust the position of the turbine to keep the blades clear of the bottom part 5a of the leg. Instead the rotating turbines are in a plane parallel with the plane of legs, so the clearance for the blades from the legs is the same at all points of the rotation cycle and the effect on the flow of any blocking by the structure will be similar.

The example illustrates the lattice framework only in a region of the support structure below the turbine's installed location, but it is preferred that the lattice framework extends all the way up the structure, as in Fig. 1. In the examples shown in Figs. l a and 2a, the legs go directly from the platform to the seabed, without an intermediate framework, allowing the process of installing foundations for the lattice tower to be simplified by drilling through the legs 5 before the turbines are installed on the tower.

An alternative framework lattice tower is illustrated in Fig.3. The structure has four legs 5 connected by a lattice 17, with the legs arranged so as to form a substantially rectangular footprint 20, rather than the square footprint of the examples of Figs. l a and 2a. An end stop 22 may be provided at an end of the structure, remote from the platform, to engage with the turbine base 9 when it has been lowered into position. The legs 5b on which the turbines are mounted have the feature of Fig.2a that the turbines are able to be mounted in a plane perpendicular to the plane of the platform 3, but the overall bulk of the tower is reduced by bringing the opposite plane closer with the rectangular footprint 19. The framework between the legs 5 on the other side forms a plane which is angled at an acute angle to the plane of the platform. The tower is then mounted to a wider base framework 20 on the seabed, for stability. By reducing the overall material usage costs are reduced.

The turbine base 9 and its mounting arrangement 12a, 12b are illustrated in more detail in Figs.4a and 4k In this example, two slide rails are provided, one slide rail 12a is fixed to the leg 5 and the other slide rail 12b is fixed to the lattice framework close by. The turbine base 9 takes the form of a mounting frame with a coupling 23 by which the turbine may be fixed to the frame. The base engages with the slide rails and may include a fixing point to accept a lifting cable from a winch on the platform, or from a remote crane, for example mounted to a barge on the surface if the tower is a fully submerged one. Larger diameter tubing is used in a lower part 21 of the turbine base 9, where the loading is greatest when the turbine 8 is in place. Part of this tubing may come into contact with an end stop, for example as illustrated in the Fig.3 example, so that the turbine does not go too far down the structure and allow the blades to come into contact with the seabed.

Using a lattice framework helps to spread the load on the structure. The lattice has less overall surface area than a cylinder tower and by mounting each turbine on a leg of the structure, there is less twisting movement, so the structure has improved resistance to shear and torsion loads. In addition, the use of an open lattice gives rise to less flow turbulence than solid pile and crossbeam type support structures. This means that the turbines can be mounted closer to the main support structure, which is more efficient in terms of material use, as well as reducing being moments applied to the support due to the effect of the flow on the turbine. The structure is more stable in torsion, lighter and uses less material. A single turbine mounted on one leg of the lattice tower can be lifted more easily than a crossbeam with two turbines, so a smaller, lighter, lifting mechanism can be used. Although each turbine could have its own lifting mechanism installed on the platform above the surface of the water, in practice, a single central winch is preferred. The winch may be used to control lifting of each turbine in turn by simply disconnecting it on the platform and moving the rigging to the next turbine base. With one turbine on each leg, each turbine may be retrieved separately while the remainder continue generating, so limiting the losses in power generation during repair and maintenance. The other turbines remain in their installed position, so there is an increase in overall stability of the structure from the weight low down of the other turbines not being maintained at that time and a reduction in resonance due to weight high up on the structure.

By comparison with crossbeam mounted turbines on a cylindrical tower, a lattice tower simplifies installation and operation and has the advantage that splash-zone loads on the tower and torque loads on the foundation are reduced and the complex construction involving splines, a set of tapers that engage when the crossbeam is in the operating, or generating, position, with one half of the set at the centre of the crossbeam, the other half on the tower at the lowered crossbeam height, forming a good structural connection between the two. The lattice tower avoids the use of splines on the connection of the crossbeam to the tower is avoided, so reducing costs. The flexibility of being able to mount one powertrain on each leg and retrieve any powertrain independently of the others is also an advantage. The lattice structure is inherently more stable than a single cylindrical structure, particularly a lattice with four legs. The number of turbines does not need to match the number of legs. Surface piercing towers may be up to 60m deep. This depth allows for two turbines to be mounted to the same side of the tower. The turbines may be offset in height on different legs of the lattice tower. The flow pattern through the lattice is complex, but has a lesser effect in terms of shadowing than a pile.

Lattice structures are well known for oil and gas installations, providing the support for a platform on which equipment is mounted. Lattice structures have also been used as mountings for wind turbines, but only by mounting a single turbine to a central monotube and then mounting the central monotube to the top of the underwater lattice.

Figs.5 and 6 illustrates the steps involved in deploying a turbine on the structure and retrieving it for maintenance. In order to install a turbine, the turbine base 9 must be moved 30 to its upper position on the leg 5 of the lattice structure, above the water level 4 for a surface piercing structure, or as high as possible for a fully submerged structure. The turbine is transferred 31 from a barge onto the turbine base, using a crane, on the barge itself, or the winch on the top of the lattice tower, the winch rigging having been suitably adjusted. For a fully submerged turbine and structure, the turbine may not be disconnected from the crane until it has reached its installed position on the structure. For a surface piercing structure, electrical cables are connected 32 to the turbine for supplying power and for receiving generator output for transmission to the shore before lowering 33 the turbine 8 on the turbine base 9 down the slide rails 12 on the structure. Lowering continues until the turbine base reaches 34 the correct depth. This may be determined, for example, by an end stop on the structure preventing further downward movement, or by reaching a set mark on the winch cable. Once installed, an operator on the platform initiates a signal to switch 35 the turbine to power generating mode.

If a turbine reaches a scheduled maintenance date, or if a fault is identified that requires remedial action, the operator sends a signal to switch off 40 power generating mode and the winch is engaged 41 to lift the turbine on its base up the slide rails to its upper position above the water level 4. The base may be fixed 42 at this position to give access to the turbine for maintenance from the platform, or lifted off the base onto a barge for removal for shore based maintenance. During such an event all other turbines on the structure can continue in power generation mode, undisturbed. If the fault is known to require maintenance ashore, a replacement turbine may be brought on the barge and substituted for the one being repaired, thereby minimising the downtime.

Claims

CLAIMS1. A subsea turbine support structure comprising a plurality of support legs, a lattice joining the support legs to one another; and a platform mounted to the support legs; the support structure further comprising a lifting mechanism for coupling to one of a turbine base and the turbine; and a turbine base mounting; wherein the turbine base mounting is fitted to one of the support legs or to the lattice framework, and wherein the turbine may be lowered or raised on the turbine base mounting by the lifting mechanism.
2. A support structure according to claim 1, wherein the turbine base mounting comprises a slide rail fitted to the leg.
3. A support structure according to claim 1 or claim 2, wherein the turbine base mounting comprises a slide rail fitted to the lattice.
4. A support structure according to any preceding claim, wherein a turbine base is provided on at least two of the support legs.
5. A support structure according to any preceding claim, wherein the lifting mechanism comprises a winch and associated rigging.
6. A support structure according to claim 5, wherein a single winch is provided on the platform, adapted for connection to each of the turbine bases separately.
7. A support structure according to any preceding claim, comprising four support legs.
8. A support structure according to claim 7, wherein the platform is mounted to each of two adjacent ones of the support legs arranged such that a plane formed between the two adjacent support legs is substantially perpendicular to a plane of the platform; and the platform is mounted to the other two support legs such that a plane formed between the other two support legs is angled at an acute angle relative to the plane of the platform.
9. A support structure according to claim 8, wherein the platform is mounted to the support legs such that a longitudinal axis of each of the four support legs is angled at an acute angle to the platform.
10. A support structure according to any preceding claim, wherein the support legs are connected to a ground anchor by a pile inserted in each support leg.
11. A support structure according to any of claims 1 to 9, wherein the support legs are connected to a ground anchor through a framework having a wider footprint than a footprint formed by the support legs.