GB2503104A

GB2503104A - Submersible platform for attaching a plurality of energy producing devices

Info

Publication number: GB2503104A
Application number: GB201309397A
Authority: GB
Inventors: David Kerr
Original assignee: LUNAR ENERGY POWER Ltd
Current assignee: LUNAR ENERGY POWER Ltd
Priority date: 2012-05-29
Filing date: 2013-05-24
Publication date: 2013-12-18
Anticipated expiration: 2033-05-24
Also published as: WO2013179003A2; WO2013179003A3; GB201309397D0; GB2503104B; GB201209522D0

Abstract

Apparatus comprising a platform (2 figure 1) having an upper surface level 4 and a lower surface level 6. The upper surface has attachment means 13 for attaching one or more energy-producing devices. Between the upper and lower surface levels there is at least one void space i.e. open cells 10 which may be flooded to submerse the platform. In this way, it is possible to transport and position a platform at an off-shore location in a cost effective manner without the use of specialized vessels. Preferably, the platform is a raft and a plurality of tidal turbines are attached to the upper surface level of the platform to enhance the energy output of the turbines. Later embodiment relates to a method of installing a platform at an offshore location.

Description

APPARATUS AND METHOD

This invention relates to a raft foundation for an energy-producing device and in particular to a raft foundation for a tidal turbine device.

Typically, to transfer one or more turbine devices to an off-shore location requires the employment of suitable specialised large ships, which is extremely expensive and may require further specialised heavy-lift vessels at the final location to lift and/or place the turbine(s) in place.

According to a first aspect of the present invention, there is provided apparatus comprising a submersible platform structure having attachment means for the attachment of a plurality of energy-producing devices to said platform structure.

Owing to this aspect of the invention, it is possible to provide a platform having a plurality of energy-producing devices attached thereto in order to maximise the amount of energy obtained from the surrounding environment in which the platform structure is submersed.

According to a second aspect of the present invention, there is provided apparatus comprising a platform having an upper surface level and a lower surface level, the upper surface level including attachment means for attaching one or more energy-producing devices and wherein between the upper and lower surface levels there is at least one void space.

According to a third aspect of the present invention, there is provided a method of installing a platform at an offshore location comprising transporting the platform to the location, anchoring the platform at the location, said anchoring including filling at least one void space in said platform and attaching one or more energy-producing devices to the platform.

Owing to the present invention, it is possible to transport and position a platform at an off-shore location in a cost effective manner without the use of specialised vessels.

Advantageously, the platform is a raft and a plurality of tidal turbines is attached to the upper surface level of the platform to enhance the energy output of the turbines.

Advantageously, the energy-producing devices can subsequently be removed for the purposes of repair and maintenance.

The key considerations behind the raft foundation concept are:- * Energy capture can be enhanced by locating the tidal devices close together.

* The raft can be used to support several tidal devices positioned close together.

* The single raft foundation should be an economic means of supporting the devices.

The raft has sufficient buoyancy to give near neutral weight, so that it can be lowered to the sea bed using only small cranes or winches mounted on one or more standard tug boats. This will avoid the need for heavy lift cranes and specialised vessels and their associated high cost.

In order that the present invention can be clearly and completely disclosed, reference will now be made, by way of example, to the accompanying drawing, in which:-Figure 1 is a top plan view of a first embodiment of a platform for supporting a plurality of tidal turbine devices, Figure 2 is a cross-sectional view along the line Il-Il of Figure 1, Figure 3 is a partial top plan view of one end of the platform of Figure 1, Figure 4 is a cross-sectional view taken along the line IV-IV of Figure 3, Figures Sa to 5d show different stages of installation of the platform of Figures 1 to 4 Figure 6 shows a plan view of a skirted pad area, Figure 7 is a cross-sectional view taken along the line VIl-VIl of Figure 6, Figure 8 is a top plan view of a second embodiment of a platform for supporting a plurality of tidal turbine devices, Figure 9 is a cross-sectional view along the line IX-IX of Figure 8, Figure 10 is a partial top plan view of one end of the platform of Figure 8, Figure 11 is a cross-sectional view taken along the line Xl-Xl of Figure 10, and Figures 1 2a to 1 2d show different stages of installation of the platform of Figures 8 to 11.

Referring to Figures 1 to 5, a raft foundation or platform 2 constructed from concrete and having dimensions of substantially 60m x substantially 21 m on plan x substantially Sm high comprises an upper surface level 4, side wall portions 5 and a lower surface level 6. Between the levels 4 and 6 there is at least one void space and preferably there are a plurality of void spaces in the form of open cells 10 (which have no roof section at the upper surface level) and a plurality of closed cells 12 (which do have a roof section at the upper surface level). Inside of the outer perimeter of the raft 2 defined by the side wall portions 5, the plurality of cells 10 and 12 are defined by inner wall portions 5' upstanding from the bottom wall portion 6.

The main load case determining thickness of the raft 2 is the water pressure on the closed cells 12 during installation. The rectangular cell design, as shown in Figures 1 to 5, is limited to about 40m water depth. The upper surface level 4 includes attachment means 13 for mounting turbine units 14 upon the raft 2. Advantageously, the raft 2 includes attachment means 13 for a plurality of turbine units 14. Preferably, the tidal turbine units are the commercially available ROTECH" tidal turbine devices which comprise a tidal turbine and duct contained within a tubular steel frame 16 which has four substantially 500mm diameter tubular steel columns 18 protruding from the base of the frame 16. The attachment means 13 connect to the steel frame 16 and includes four substantially 600mm diameter columns 19 with flared stabbing guides 19a at the column heads. The columns 19 are located directly above the inner wall portions 5' for transferring the weight of the turbine units 14 directly through the raft 2. The columns 18 and 19 need a wall thickness of substantially 30mm to withstand the bending moment from operation of the turbine. This wall thickness will allow a reasonable tolerance for installation when the columns 18 stab into the columns 19. A latching device (not shown) may also be provided to secure the frame 16 to the columns 19.

Advantageously the turbine units 14 can be installed and subsequently retrieved as required for maintenance purposes by use of a surface vessel.

The columns 19 are connected to the inner wall portions 5' by a conventional base-plate and holding down bolts. Alternatively, they could be cast into the raft inner wall portions 5' which could be locally thickened at each column location for that purpose (see Figure 4).

An installation site for the raft 2 is usually assumed to be around 40m water depth (mean water level) with a peak current of substantially around 4.2m/s (appropriate for stability calculations, but the normal peak spring tide operating current for the tidal turbine would be less than this).

The raft 2 is supported on a plurality of pads 20, preferably six, distributed evenly on the underneath surface of the bottom wall 6 of the raft 2; one in each of the four corners of the raft 2 and two at the edges of the middle region of the length of the raft 2. Skirts 22, preferably of metallic material such as steel, are arranged to surround each pad 20 and/or the whole footprint of the raft and to initially contact a sea bed SB followed by injection of grout 24 pumped via a conduit from a surface support vessel and contained within the skirted pad area.

The raft foundation 2 is preferably constructed on-land adjacent to a quay and possibly transferred onto a submersible barge by a multi-wheel transporter and towed to the installation site on the submersible barge. Alternatively, the raft foundation could be directly towed to the site without the use of the submersible barge.

The raft 2 will advantageously be constructed above ground on a temporary platform so that the multi-wheel transporter can be driven under the structure to pick it up.

Tower cranes are likely to be needed for construction purposes, but heavy lift cranes are not required.

A typical medium sized submersible barge could accommodate two of the rafts 2.

The raft 2 will be moved onto the barge by the multi-wheel transporter and set down on steel stools to prevent premature damage to the skirts 22. The barge will be towed to site by tug boats 23, timed to arrive at the installation site in a period of neap tides (when the tidal range is at its minimum). On arrival at the site, and subject to suitable wind and wave conditions, the barge will be submerged and the raft 2 floated off.

Installation will commence as slack tide (the moment that the tidal current ceases) approaches according to the following sequence:- * Flooding of the the open cells 10.

* Adding water to the end closed cells 12 to trim the raft level in the water and to sink the raft 2 with a small net downward weight (for example 50t), supported by small cranes or winches on Ihe tug boals 23.

* Lower to the seabed SB, at a rate of about 2.Om per minute (2Ominutes for 40m water depth), during the slack tide period.

* After landing on the seabed SB, flooding the closed cells 12.

* Grouting of the pads 20 to be contained by the skirts will then proceed during subsequent slack tide periods.

Removal of the raft 2 and its attached turbine units 14 is achievable by reversal of this installation procedure with the initial stage requiring pumping air into the cells in order to recover the raft 2.

Sea bed conditions at potential installation sites typically consist of bare rock, coarse gravel or possibly stiff clay. For any site with potential for tidal power, the tidal velocities will be high enough to remove any tine sediments or soft clays.

The calculated bearing pressures are relatively high. These pressures should be acceptable on most un-weathered rock surfaces, subject to geological assessment for any likely planes of weakness resulting from rock jointing and bedding planes at each installation site. If there is significant weathering or sediment cover on the seabed, it may be necessary to increase the number and/or size of the skirted area pads 20 on the bottom of the raft 2, which can be done within the raft footprint. If detailed hydraulic modelling were to show forces significantly higher than estimated for a particular site, then solid ballast could be added to the open cells 10 to increase the "on bottom" weight.

The raft 2 provides a stiff and robust structure. The longitudinal and transverse closed cells 12 provide relatively strong box beams which have good bending strength and torsional resistance. This is important not only for the on seabed condition, in which there may be some uneven contact pressures owing to an undulating seabed surface, but also for transportation and installation.

The main load cases to be considered for the structural design of the raft 2 are as follows:- * "On sea bed" with the turbine devices operating: The loads from the turbine devices and the raft 2 itself will be transferred to the foundation pads 20 by the raft 2 being subject to bending forces.

* Transfer from the construction site onto the submersible barge.

* Floating of the raft 2 prior to lowering to the seabed SB. This load case determines the thickness and reinforcement for the lower surface 6 to the open cells 10.

* During installation; the deep immersion condition determines the thicknesses of the walls of the closed cells 12.

The thicknesses of the lower surface or bottom wall 6, the side walls 5 and roof sections of the upper surface level 4 for the closed cells 12 are determined by the water pressures in the final stage of installation. The water pressure at touch-down on the seabed is substantially 40t/m2 at the underside of the raft 2 and 35t1m2 at the top of the raft 2. Suitable thicknesses required for these conditions are:- * 400mm for the bottom wall 6 for the open and closed cells 10 and 12. The reinforcement required is determined by the initial floating condition prior to installation when this slab could be subjected to the water pressure at up to 5m depth. The thickness of bottom wall of the open cells 10 could be adjusted to balance weight versus buoyancy at detailed design stage for individual installations.

* 400mm for all the side walls 5 and inner wall portions 5'.

* 375mm for the roof sections of the upper surface level 4.

These thicknesses have been selected to limit reinforcement required to resist bending stresses to about 2% (by cross sectional area of concrete). This is relatively heavy reinforcement but this is not unusual for marine structures.

The overall bending of the raft 2 when in place determines the reinforcement required in the longitudinal directions of the box beams, i.e. the closed cells 12. It may be beneficial to include some pre-stressing strand (not shown) which could allow a reduction in reinforcement and would assist in preventing cracks developing during construction from shrinkage.

The contact area with the seabed SB needs to satisfy the following requirements:- * To be able to transmit the loads to the seabed.

* To accommodate reasonable variations in seabed level (up to around 0.25m in variation).

* If possible, to accommodate a range of sea bed materials without major change to the raft design.

As a result, the proposed foundation arrangement is provided with the skirts 22, surrounding the pads 20, for contact with the seabed SB. The skirts 22 serve to penetrate or punch into the seabed surface and crushing locally to follow the seabed profile. A honeycomb structure of the skirts 22 could be used to achieve this purpose. The raft 2 will tend to rest initially on three of the six pads 20 and thus the skirts 22 need to be able to penetrate and/or crush sufficiently to achieve contact at each of the six pads 20. The grout 24 is to be placed after installation of the raft 2 on the seabed SB by pumping it from a support vessel through flexible hoses 25 connected to pipework 27 cast into the raft 2. The grout 24 will be contained within a fabric formwork, i.e. a mould into which the grout 24 is pumped, which will be attached to the underside of the raft 2 during construction. The formwork is needed to prevent excessive leakage of grout 24 past the skirts 22, which are likely to form an imperfect seal with the seabed SB. If the sea bed material is such that the skirts 22 will penetrate around the full perimeter, then it may be possible to omit the formwork.

Specialist cement-based grouts designed to be placed underwater are commercially available and the use of a fabric formwork is a well-established method for containing grout placed underwater. It will be important to place the grout 24 rapidly during slack tide periods of around half-an-hour. Typical placing rates are up to around 25m3 per hour. The volume of each pad 20 is substantially 8m3 and therefore it is possible to complete the operation for two pads 20 per tide, completing the whole operation over three tides.

Referring to Figures 6 to 12, an alternative embodiment of the raft is shown in which the only major difference from the embodiment shown in Figures 1 to 5 is the shape of the closed cells 12. Instead of the rectangular cross-sectional shape of the closed cells 12 of Figures ito 5, the closed cells 12' of the alternative embodiment are of a substantially circular cross-sectional shape. This alternative shape is structurally stronger than a rectangular cross-sectional shape and thus, for similar wall thicknesses, allows installation at greater water depths. In addition, at a given depth, the weight of concrete in this alternative design for the raft 2 is less than for the first embodiment.

The closed cells 12' have a circular internal cross section and an octagonal external cross section and have a minimum wall thickness of 300mm. The octagonal external shape has been selected for ease of construction, particularly at the junctions between closed cells 12', but could be other shapes, such as circular, if preferred.

The substantially circular cylindrical form of the closed cells 12' with the aforementioned wall thickness can withstand the water pressure at touch-down on the sea bed SB in up to about 80m water depth. Circumferential reinforcement may further be required to withstand the bending forces due to the differential water pressure between the top and bottom of each closed cell 12'.

The bottom wall 6 has a thickness of around 300mm and any reinforcement required is determined by the floating condition prior to installation when this slab could be subjected to a water pressure at up to Sm depth. Additional stiffening walls around 250mm thick may be included in order to limit the bottom wall thickness and reinforcement quantities. The overall bending of the structure when in place will determine the amount of reinforcement required in the longitudinal directions of the closed cells 12'. It may be beneficial to include some pre-stressing strand, as mentioned before in relation to the first embodiment. Such a pre-stressing strand allows a reduction in reinforcement and would assist in preventing cracks developing during construction from shrinkage.

A grade 50 concrete is proposed both embodiments the raft 2. This is a typical concrete grade for marine structures and will satisfy both structural and durability requirements.

Claims

CLAIMS1. Apparatus comprising a submersible platform structure having attachment means for the attachment of a plurality of energy-producing devices to said platform structure.
2. Apparatus according to claim 1, wherein the plurality of energy-producing devices are a plurality of tidal turbines attached to an upper surface of the platform.
3. Apparatus according to claim 1 or 2, wherein the energy-producing devices are removable.
4. Apparatus according to claim 2 or claim 3 as appended to claim 2, wherein the tidal turbines are positioned close together.
5. Apparatus according to any one of claims 2 to 4, wherein the upper surface includes attachment means for mounting the tidal turbines.
6. Apparatus according to any one of claims 2 to 5, the plafform further comprising a lower surface and wherein between the upper and lower surface levels there is at least one void space.
7. Apparatus comprising a platform having an upper surface level and a lower surface level, the upper surface level including attachment means for attaching one or more energy-producing devices and wherein between the upper and lower surface levels there is at least one void space.
8. Apparatus according to claim 7, wherein a plurality of the energy-producing devices are a plurality of tidal turbines attached to the upper surface of the platform.
9. Apparatus according to claim 7 or 8, wherein the energy-producing devices are removable.
10. Apparatus according to any one of claims 7 to 9, wherein there are a plurality of void spaces in the form of a plurality of open cells and closed cells.
11. Apparatus according to claim 10, wherein the plurality of cells are defined by inner wall portions upstanding from a bottom wall portion.
12. Apparatus according to claim 10 or 11, wherein the closed cells have a rectangular cross-section.
13. Apparatus according to claim 10 or 11, wherein the closed cells have a substantially circular cross-section.
14. Apparatus according to any one of claims 7 to 13, wherein a skirt surrounds the whole footprint of the platform.
15. Apparatus according to any one of claims 7 to 14, wherein the platform is supported on a plurality of pads distributed evenly on the underneath surface.
16. Apparatus according to claim 15, wherein the pads include respective skirts arranged to surround each pad.
17. Apparatus according to claim 14 or 16, wherein the or each skirt has a honeycomb structure.
18. A method of installing a platform at an offshore location comprising transporting the platform to the location, anchoring the platform at the location, said anchoring including filling at least one void space in said platform and attaching one or more energy-producing devices to the platform.
19. A method according to claim 18, wherein there are a plurality of void spaces in the form of a plurality of open cells and closed cells and said anchoring includes flooding the open cells.
20. A method according to claim 19, wherein said anchoring includes adding water to the closed cells at end regions of the platform to trim the raft level in the water and to sink the raft with a small net downward weight, supported by small cranes or winches.
21. A method according to claim 20, and further comprising, after landing the platform on the seabed, flooding the closed cells.
22. A method according to any one of claims 18 to 21, wherein the platform is supported on a plurality of pads distributed evenly on the underneath surface, the method further comprising grouting the pads, grout material being contained by skirts surrounding the pads.