GB2534874A

GB2534874A - Base for offshore structure

Info

Publication number: GB2534874A
Application number: GB1501670.2A
Authority: GB
Inventors: Abbattista Martino; Richards Edward
Original assignee: Marine Current Turbines Ltd
Current assignee: Marine Current Turbines Ltd
Priority date: 2015-02-02
Filing date: 2015-02-02
Publication date: 2016-08-10
Also published as: GB201501670D0

Abstract

A method of manufacturing a subsea foundation 6 comprising: carrying out a bathymetric survey of an area of seabed, determining a surface profile (8, fig.2) for that area, creating a mould (5, fig.2) based on that profile, and forming a subsea base 6 is within the mould. The produced base 6 has a flat upper surface and a lower surface which conforms to the seabed 4. The base 6 is lowered into place; a thin layer of concrete grout 10 may be placed beneath the base. A flexible seal 9 may be placed around the perimeter to contain the grout. Each base 6 may feature a location identifier, specifically an RFID tag to identify each base. Each base may be gravity base foundation or a landing platform for a gravity base foundation. The subsea base may be adapted for use with a tidal turbine.

Description

BASE FOR OFFSHORE STRUCTURE

This invention relates to a method of manufacturing a base for an offshore structure, in particular for a tidal turbine and a base so formed.

Typically, offshore wind turbines have been installed in locations with low current flow, where there is a generally flat, sandy seabed, allowing the seabed to be prepared by dredging and a base installed by sinking or lowering a gravity base foundation onto the prepared seabed. In some cases, sand and gravel have been provided on the dredged surface to even out the seabed for the wind turbine base.

However, tidal turbines are typically installed in areas of high current flow and as a result the seabed typically has a very uneven rocky surface. Adding sand and gravel is of little benefit, as these are quickly washed away, leaving just the rocky surface. In accordance with a first aspect of the present invention, a method of manufacturing a base for a subsea structure comprises carrying out a bathymetric survey of an area of seabed on which one or more subsea structures are to be installed; determining a surface profile of a site for a base of one of the subsea structures from the bathymetric survey of the area; manufacturing a mould having a surface profile corresponding to the surface profile of the seabed at the site; manufacturing a base having a surface corresponding to the mould surface profile and a flat surface; and separating the mould and the base.

The present invention addresses the problem of being able to install bases for subsea structures, such as tidal turbines with a flat upper surface to receive the turbine, or a gravity base foundation for the turbine, by surveying the seabed and manufacturing a base which follows the contours of the seabed, but has a flat top to receive the turbine, or gravity base foundation.

Preferably, the method further comprises manufacturing flotation chambers in the base.

Preferably, the method further comprises fitting a flexible seal around a perimeter of the base.

Preferably, the method further comprises manufacturing a plurality of moulds for a plurality of sites identified from the bathymetric survey and applying a location identifier to each mould and base.

Preferably, the base is one of a landing platform or a gravity base foundation.

Preferably, the landing platform is formed as an annular disc.

Preferably, the method further comprises providing a side wall projecting above the annular disc.

In accordance with a second aspect of the present invention, a method of installing a base for a subsea structure comprises manufacturing a base in accordance with the method of any preceding claim; manoeuvring the base into place at the site; and fixing the base to the seabed.

Preferably, the manoeuvring comprises floating the base into position and filling flotation chambers with sea water to sink the base in position.

Preferably, the method further comprises pumping concrete grouting into a cavity formed between a perimeter seal, the base and the seabed.

In accordance with a third aspect of the present invention, a base for a subsea structure comprises a first substantially regular surface to receive a subsea structure and a second, irregular surface, the irregular surface corresponding to a seabed surface profile at a specific subsea location.

Preferably, the base further comprises a flexible skirt mounted around a perimeter of the irregular surface.

Preferably, the base comprises one of a gravity base foundation or a landing platform for a gravity base foundation.

Preferably, the landing platform comprises an annular disc.

Preferably, the base further comprises a perimeter wall projecting above the regular surface and encompassing the annular disc.

Preferably, the subsea structure comprises a tidal turbine.

An example of a base for an offshore structure and a method of manufacturing such a base according to the present invention will now be described with reference to the accompanying drawings in which: Figure 1 illustrates a vessel carrying out an offshore survey of the seabed for use in the method of the present invention; Figure 2 illustrates construction of a mould reproducing the seabed surface profile using the data obtained in the survey of Fig.1; Figure 3 illustrates the manufacture of a base using the mould of Fig.2; Figure 4 shows the base constructed in Fig.3 having been installed on the seabed Figure 5 is a flow diagram of the method of the present invention; and, Figures 6a to 6d illustrate an example in which the base is a landing platform and a gravity base foundation is installed on the landing platform subsequently.

Tidal sites normally have a rocky seabed of uneven slope, with boulders of different shapes and sizes. Gravity based foundations may be up to 20m in diameter. Putting these onto the seabed means that the foundation is not horizontal due to the uneven nature of the seabed. By contrast, wind turbine sites tend to be in relatively flat water with gravel or sand bases and can be raked before the foundations are fitted, or have extra gravel supplied to fill any voids. In tidal sites, the rate of flow would immediately scour out any gravel and sand that was deposited for this purpose. Conventional gravity base foundations for use with offshore wind turbines are usually built onshore and transported offshore by vessels and installed by lifting and sinking procedures.

Although, it might be possible to pump in grouting whilst supporting the gravity base foundation in a horizontal orientation from a ship, some sort of seal is needed between the base of the foundation and the seabed in order to stop the grouting just pouring out again before it can set, which is difficult to achieve and expensive to manufacture. A huge amount of grout would be required to fill in the void between the seabed and conventional flat gravity base foundations, which would also require a levelling and a reliable sealing procedure to be in place. This means that such a method would be limited to cases where the gaps between the seabed and the base are not too large, otherwise the sealing is not practical and the cost for the grout becomes excessive.

The present invention addresses the problem of installing tidal turbine bases on an inclined, uneven or rocky seabed by obtaining accurate contour information from a bathymetric survey of the site and then making a base on shore which follows the contours as surveyed. This means the contoured foundation can be lowered into position and only a small amount of grouting is required to fix the foundation in place.

The seals to achieve this are much smaller and so more practical and the amount of concrete grouting needed is also far less, saving cost.

Using high quality bathymetry data a mould is created, for example out of expanded polystyrene, or with other techniques, to build a gravity base foundation onshore which approximately fits on the seabed surface of the chosen location. This procedure reduces the amount of grout to be used to bond the foundation on the seabed and the overall installation procedure to be carried out offshore.

The advantages of the invention are that a base, such as a gravity base foundation or light concrete landing platform are provided onto which a tidal turbine can be positioned subsequently, may be installed on a rocky seabed without any seabed preparation, without a levelling procedure to hold the foundation at the correct orientation whilst grout is pumped in and without needing large amounts of grout to bond the foundation to the seabed.

Figs. 1 to 5 illustrate the steps involved in manufacturing and installing a base according to the present invention. Fig.1 illustrates a vessel 1 carrying out a bathymetric survey of the seabed 4 at a chosen location, using a transducer 3 towed along below the level 2 of the sea. In the example shown, the survey is carried out using Sonar, for example using a multibeam echosounder. The data that has been collected in the survey is analysed to determine the best locations at which to site a base and the surface profile 8 at each site is used to create a mould 5, as shown in Fig.2, which reproduces the profile of the seabed at the chosen location. The mould may be made of expanded polystyrene or be made using other suitable techniques, such as wooden blocks of different lengths, or a rubber surface manipulated by jacks.

Side walls, or a perimeter wall 7 may be added to the profiled part 8 of the mould 5, so that the base 6 can be formed, as shown in Fig.3. The base may be either a gravity base foundation, or a landing platform for the gravity base foundation. The base is typically constructed from concrete, the concrete being poured into the mould formed by the profiled part and perimeter wall. Examples are a light concrete of the order of 2.4te/m3, or a heavy concrete containing magnetite ore, with a density of the order of 3.7te/m3. Typically, the base is reinforced with steel bar. The manufactured foundation may be solid and carried by a barge before being lowered into place, or may be provided with floatation chambers which are opened to the sea when in position, to sink the foundation into place.

After the concrete has set, the base and the mould are separated and the base is transported to the location of site identified in the survey, where the base is to be installed on the seabed. As the profile of the surface of the base in contact with the seabed has been formed to follow the profile of the seabed as closely as possible, the base is oriented so that its flat upper surface is horizontal, ready for a subsea turbine to be installed on top.

A sealing skirt 9 is fitted around the perimeter of the base and grout 10 is pumped between the base and the seabed inside the skirt, as can be seen in Fig.4. This also takes care of minor changes in the contours of the seabed since the survey caused by movement of rocks, or inaccuracies in converting the survey to the mould.

Fig.5 is a flow diagram of a method of installing a base for a sub-sea turbine according to the present invention. The process starts by determining 20 the required location of a subsea turbine base and carrying out 21 a bathymetric survey of the seabed at that location. In practice, a larger area is likely to be surveyed for installation of multiple subsea turbines and a more specific location chosen for the site of each base from the larger survey. The detailed seabed data for each specific location is extracted from the survey data and given a location identifier associated with the surface profile at that location. Onshore, a mould is created 22 which has a surface profile that corresponds with the surface profile of the seabed at the chosen location. The mould is typically made from expanded polystyrene foam, although other materials may be used. Using the mould, a base is produced 23, which may be a gravity base foundation, or a landing platform, for the subsea turbine. The base has a profile on its lower edge which matches, very closely, the profile of the seabed at the chosen location and a flat surface opposite to the profiled surface to receive the subsea turbine on the gravity base foundation, or to receive a gravity base foundation, if the base is landing platform for a gravity base foundation The mould and the base may also include the location identifier, for example by embedding an RFID tag in each, or by embossing a reference number in the mould side wall.

The mould and base are separated and the base is transferred 24 to the location for installation. The base may be solid and carried on a barge, or it may have flotation chambers, allowing it to be towed into position and then sunk once on station. In order to seal the base to the seabed, a skirt may be provided around the perimeter of the base and grout is pumped 25 under the skirt into gaps formed between the seabed and the lower surface of the base. When the grout has set, the skirt may be removed for reuse.

If the base is a gravity base foundation, then the tidal turbine is installed directly onto this. If the base is a landing platform, then, as illustrated in Figs.6a to 6d, a further step of installing the gravity base foundation is required. Concrete gravity base foundations may be used as foundations for offshore structures of any sort. They ensure stability of the structure by their mass and transfer lateral loading to the seabed, for example by friction, by an adhesive bond between the grout and seabed, or by compression on inclined surfaces.

The landing platform in the example of Figs.6a to 6d, shown in Fig.6a in plan view, comprises a substantially annular disc 11, typically made of light concrete, with a side wall 16 along its outer perimeter projecting a little way above the flat top surface of the annular disc. The sidewall may vary in profile about the perimeter as illustrated in the sections A-A in Fig.6b and B-B in Fig.6c. In Fig.6b, the sidewall shows a basic thickness 13, whereas in Fig.6c, the side wall shows reinforcing sections 12. The base has a profiled lower surface 14 which is manufactured in the same way as the profiled surface of the gravity base foundation is formed, as described above with respect to Fig.5, by taking bathymetric survey data for the site on which it is to be installed. Data is extracted for an annular base and an annular mould is formed, which has a surface profiled in the same way as the seabed surface profile at that site. Concrete is poured into the mould so that the lower surface 14 of the landing platform has a profile which matches the profile of the seabed at the chosen site and the upper surface of the annular disc is flat, apart from the projecting sidewall. Alternatively, the profiled part 14 of the base may be formed separately and then fixed to the bottom surface of a flat annular disc with sidewalls.

The use of an annular landing platform has the advantage that it is a much lighter structure to transport, lift and install, than the gravity base foundation and the landing platform can be built with low strength concrete. It requires lighter offshore cranes for lifting and can be levelled on the seabed with more available standard devices. The opening in the centre helps when lowering the landing platform through the water column, as well as reducing weight. The landing platform is put in place on the seabed with the skirt 9 fitted around the perimeter and grout is applied to seal the profiled surface 14 of the landing platform to the seabed 4. Once the landing platform is securely fixed on the seabed 4 and sealed in place as described above, a conventional gravity base foundation 15, with flat upper and lower surfaces, is simply lowered onto the landing platform. Friction between the two concrete masses provides sliding resistance between the landing platform and the gravity base foundation 15 and the landing platform wall 16 confines the gravity base foundation along the perimeter.

The gravity base foundation may be of any type, such as floating, solid or modular foundations. The profiled lower surface 14 of the base allows installation in otherwise angled, irregular surfaces on a rocky seabed. Once the landing platform has been sealed in place, the gravity base installation is relatively straightforward, as the perimeter wall 16 of the landing platform acts as a guide during installation. The grouting operation is performed on a lighter structure which is easier to manoeuvre than the gravity base foundation 15, itself.

Claims

CLAIMS1. A method of manufacturing a base for a subsea structure, the method comprising carrying out a bathymetric survey of an area of seabed on which one or more subsea structures are to be installed; determining a surface profile of a site for a base of one of the subsea structures from the bathymetric survey of the area; manufacturing a mould having a surface profile corresponding to the surface profile of the seabed at the site; manufacturing a base having a surface corresponding to the mould surface profile and a flat surface; and separating the mould and the base.
2. A method according to claim 1, wherein the method further comprises manufacturing flotation chambers in the base.
3. A method according to claim 1 or claim 2, wherein the method further comprises fitting a flexible seal around a perimeter of the base.
4. A method according to any preceding claim, wherein the method further comprises manufacturing a plurality of moulds for a plurality of sites identified from the bathymetric survey and applying a location identifier to each mould and base.
5. A method according to any preceding claim, wherein the base is one of a landing platform or a gravity base foundation.
6. A method according to claim 5, wherein the landing platform is formed as an annular disc.
7. A method according claim 6, further comprising providing a side wall projecting above the annular disc.
8. A method of installing a base for a subsea structure, the method comprising manufacturing a base in accordance with the method of any preceding claim; manoeuvring the base into place at the site; and fixing the base to the seabed.
9. A method according to claim 8, wherein the manoeuvring comprises floating the base into position and filling flotation chambers with sea water to sink the base in position.
10. A method according to claim 5 or claim 6, wherein the method further comprises pumping concrete grouting into a cavity formed between a perimeter seal, the base and the seabed.
11. A base for a subsea structure comprising a first substantially regular surface to receive a subsea structure and a second, irregular surface, the irregular surface corresponding to a seabed surface profile at a specific subsea location.
12. A base according to claim 11, further comprising a flexible skirt mounted around a perimeter of the irregular surface.
13. A base according to claim 11 or claim 12, wherein the base comprises one of a gravity base foundation or a landing platform.
14. A base according to claim 13, wherein the landing platform comprises an annular disc.
15. A base according to claim 14, wherein the base further comprises a perimeter wall projecting above the regular surface and encompassing the annular disc.
16. A base according to any of claims 11 to 16, wherein the subsea structure comprises a tidal turbine.