This invention relates to a base structure for supporting structures on a sea bed or river bed, and in particular for supporting structures such as wind turbines or water flow turbines on a sea bed or river bed.

Background of the invention

A tidal stream generator is disclosed in WO 2013/054085. The generator is mounted on a support column which is supported by a support structure on the sea bed. Such support structures can be complex and expensive to install. They may require the use of divers or submersible equipment to secure the support structure to the sea bed. There is a requirement to enable a support structure to be towed to an offshore location and quickly and easily installed on the sea bed. There is also a requirement that a support structure can be installed from the sea surface, for example from a support vessel, without requiring the use of divers or submersible equipment. WO 2010/143966 discloses a foundation base which uses rigid walls or skirts which penetrate the seabed to transfer vertical loads to the seabed. The depth of the skirts must be designed specially for each location. The skirts extend permanently from below the base slab of the foundation base and are thus prone to damage during transport of the foundation base.

It is an object of the present invention to overcome one or more of the above problems. Although the invention is described in relation to use at sea, any reference to the sea is to be understood to include a river, a lake or an estuary or any other body of water in which a fixed structure is to be installed, and any reference to a sea bed is to be understood to include a river bed, a lake bed or an estuarial bed or other bed beneath a body of water.

Disclosure of the invention

According to a first aspect of the invention there is provided a base structure for supporting a structure on a sea bed, the base structure comprising:

a base unit having a perimeter and a downwardly facing bearing surface, a skirt depending from the perimeter of the base unit,

ballast material adapted to cause the skirt to engage with the sea bed, and at least one infill grout conduit communicating with the bearing surface of the base structure.

The base unit may be of reinforced or prestressed concrete, GRP, steel or other metal, or a combination thereof.

The base structure may further comprise a superstructure fixed to the base unit. The superstructure may be of reinforced or prestressed concrete, GRP, steel or other metal, or a combination thereof.

In use the base structure is lowered onto the sea bed, so that the downwardly facing bearing surface of the base unit is in contact with one or more high points on the sea bed, such as protruding rocks. Water is still present in the void between the bearing surface and the sea bed except at locations where the bearing surface is in contact with the high points. When the skirt engages with the sea bed, under the weight of the ballast therein, the skirt effectively seals off the void, to form a closed volume bounded by the bearing surface, the sea bed and the skirt.

Preferably the ballast material is introduced into the skirt while the base structure is in situ on the sea bed. Alternatively the ballast material may be a permanent component of the skirt; for example the ballast may be the self weight of at least some of the material from which the skirt is constructed. Preferably the skirt extends continuously around the perimeter of the base unit. Alternatively the skirt may comprise a plurality of skirt sections which seal against each other in use. The skirt is preferably flexible. The skirt may comprise a flexible, substantially impervious material.

The skirt may include a pocket portion comprising a substantially closed chamber and extending along the perimeter length of the skirt. The pocket portion may be substantially tubular. The pocket portion may be at the lower edge of the skirt.

Alternatively the pocket portion may extend substantially over the whole depth of the skirt. The pocket portion may comprise an inner layer and an outer layer joined along the upper and lower edges thereof to form a closed pocket. The base structure may include at least one ballast conduit communicating with the pocket portion. The ballast conduit may be a grout conduit. This enables the pocket portion of the skirt to be filled with pumped ballast material, which by its weight causes the skirt to engage with the sea bed under gravity.

The skirt may be secured at its upper perimeter edge to the perimeter of the base unit. The base structure may include an attachment system which attaches the lower perimeter edge of the skirt to the base unit. The attachment system may include a release mechanism which causes the lower perimeter edge of the skirt to become unattached to the base unit. The skirt may therefore be attached to the base unit by its upper and lower perimeter edges in a stowed mode and may be attached to the base unit by its upper perimeter edge only in a deployed mode, whereby the lower perimeter edge is free to conform to and engage with the sea bed in the deployed mode. In use the skirt can be deployed while the base structure is in situ on the sea bed. The skirt is preferably of sufficient flexibility and/or sufficient depth such that the lower perimeter edge remains in engagement with the sea bed when the base unit is lifted to a level position. The lifting to a level position may be achieved by pumping pressurised grout or other cementitious material into the void between the bearing surface and the sea bed. Ballast material, for example a cementitious material such as grout, may be fed into the closed pocket portion of the skirt to increase the weight of the skirt and to urge the lower perimeter edge to conform to and engage with the sea bed under gravity. The skirt may include one or more ballast distribution conduits in communication with the one or more ballast conduits. The ballast distribution conduit or conduits may extend around the perimeter of the base unit and may include apertures through which grout can be delivered to a plurality of locations around the extent of the skirt.

The base structure may further comprise a plurality of infill grout conduits communicating with the bearing surface of the base unit. The bearing surface of the base unit may have an array of grout ports arranged therein, each grout port having an associated infill grout conduit. In use grout or other cementitious material can be pumped, for example from a surface vessel, through the infill grout conduits into the void beneath the bearing surface to displace the water from the void volume. The grout can subsequently harden so that the base structure is then supported on the sea bed over the entire bearing surface, not only by those parts of the bearing surface in direct contact with the sea bed.

The base structure may include a superstructure fixed to the base unit. The superstructure may include a plurality of grout supply pipes.

One or more of the grout supply pipes may be infill grout supply pipes in

communication with the one or more infill grout conduits. The grout or other cementitious infill material used to fill the void can be delivered through the grout supply pipes to the void.

One or more of the grout supply pipes may be skirt grout supply pipes in

communication with the one or more ballast conduits. The ballast material used to urge the skirt into sealing contact with the sea bed can be delivered through the grout supply pipes to the skirt. The base structure may include one or more ballast chambers. The superstructure may include one or more ballast chambers. The ballast chambers may be controllably filled and/or emptied with water as ballast.

According to a second aspect of the invention there is provided a method of installing a base structure for supporting a structure on a sea bed, the method comprising the steps of:

providing a base unit having a bearing surface, a perimeter encompassing the bearing surface, and a flexible skirt attached to the perimeter of the base unit, lowering the base unit towards the sea bed until the bearing surface is in contact with the sea bed at one or more contact points,

deploying the skirt so that the skirt is attached at its upper edge to the base unit,

providing ballast material to cause the skirt to engage with the sea bed under the action of gravity, the skirt defining with the sea bed and the bearing surface a volume,

pumping cementitious infill material into the volume defined by the skirt, the sea bed and the bearing surface to displace water from the volume, and

allowing the cementitious infill material to harden.

The cementitious material may be grout.

The base structure may include a superstructure fixed to the base structure. The superstructure may include a plurality of grout supply pipes.

Preferably the skirt extends continuously around the perimeter of the base structure. The skirt may comprise a pocket portion which is substantially closed. The pocket portion may comprise an inner layer and an outer layer of the skirt joined to each other along the upper and lower edges of the pocket portion. The pocket portion may be provided at the lower perimeter edge of the skirt. Alternatively the pocket portion may extend substantially over the whole depth of the skirt.

The step of providing ballast material may include supplying ballast material through at least one ballast conduit communicating with the closed pocket portion. The ballast conduit may be a grout conduit. The ballast material may be a cementitious material. The ballast material may comprise grout. The ballast material may comprise metal aggregate. The step of providing ballast material may include pumping ballast material from a surface vessel. The ballast material may be pumped through one or more of the grout supply pipes in the superstructure.

The step of providing ballast material may include allowing the ballast material to harden inside the pocket portion so that the skirt forms a rigid or semi-rigid wall which substantially seals the volume defined by the skirt, the sea bed and the bearing surface.

The step of deploying the skirt may include operating a release mechanism to cause the lower perimeter edge of the skirt to become unattached from the base unit.

The step of pumping cementitious infill material into the volume defined by the skirt, the sea bed and the bearing surface may include pumping cementitious infill material through a plurality of infill grout conduits communicating with the bearing surface of the base unit. The bearing surface of the base unit may have an array of grout ports arranged therein, each grout port having an associated infill grout conduit.

The step of pumping cementitious infill material may include pumping cementitious infill material from a surface vessel, such as a ship or a fixed platform. The cementitious infill material may be pumped through one or more of the grout supply pipes in the superstructure. The step of pumping cementitious infill material may include a first stage in which cementitious infill material is placed at a perimeter portion of the volume adjacent to the skirt. The step of pumping cementitious infill material may include a second stage in which cementitious infill material is placed in an internal portion of the volume, inside the perimeter portion of the volume. The second stage may be carried out after the cementitious infill material placed in the first stage has hardened. Alternatively the second stage may be carried out immediately after or at the same time as the first stage.

The second stage step of pumping cementitious infill material may include pumping cementitious infill material at a pressure sufficient to raise the base unit from the sea bed, preferably to a level position. The cementitious infill material may have a density sufficient to float the base unit on the cementitious infill material until it hardens. The ballast material in the skirt may cause the lower edge of the skirt to engage with the sea bed, and an upper portion of the skirt adjacent to the upper edge of the skirt may remain sufficiently flexible to allow the base unit to be at least partly raised away from the sea bed while the lower edge of the skirt remains engaged with the sea bed. The cementitious infill material placed in the first stage may assist the lower edge of the skirt to remain engaged with the sea bed.

The base structure may include one or more ballast chambers. The superstructure may include one or more ballast chambers. The step of lowering the base unit towards the sea bed may include controllably filling and/or emptying one or more ballast chambers. Water may be used as ballast.

The method may include the further step of filling one or more ballast chambers with ballast after the cementitious infill material has hardened. This increases the weight of the base structure, thereby increasing the stability of the base structure. Water may be used as ballast. Alternatively or in addition other materials may be used as ballast, for example cementitious materials such as grout or concrete, mineral or metal aggregate, sand, or a mixture thereof.

Brief description of the drawings

In this specification the term "grout" means any cementitious mixture which is capable of being pumped. The term "offshore structure" means any structure which is fixed to or supported on the sea bed. The term "sea bed" includes in its scope any bed under a body of water, including an estuary bed, a lake bed or a river bed.

The invention will now be described by way of example only with reference to the following drawings, in which:

Fig. 1 is a view of a base structure according to the present invention, which in the illustrated example supports a column of an offshore structure;

Figs. 2 and 3 are partial views of the base structure of Fig. 1 during installation of the base structure on the sea bed; Fig. 4 is a partial view of a skirt attached to a base structure according to the present invention;

Fig. 5 is an enlarged view of the attachment means used to attach the skirt of Fig. 4; Fig. 6 is a schematic drawing showing the deployment of the skirt during installation of a base structure on the sea bed;

Fig. 7 is a partial view of two skirt elements attached to a base structure according to the present invention; and

Fig. 8 is a cut away drawing of a base unit of another base structure according to the present invention. Detailed description of the invention

Referring to Fig. 1, there is shown a base structure 10 according to one embodiment of the present invention. The base structure 10 is installed on the sea bed 12 and supports a superstructure 14, in this case a column. The column 14 can be part of any offshore structure, for example the column of a tidal generating apparatus of the sort disclosed in WO 2013/054085. The base structure 10 can support any structure 14.

The base structure comprises a base unit 20 having a perimeter 22 and a

downwardly facing bearing surface 24, not seen in Fig. 1 but visible in Fig. 2. A skirt 26 depends from the perimeter 22 of the base unit. It is secured to the base unit 20 by fixing bolts 28, 30. A suitable sealant (not shown) may be used between the skirt 26 and the base unit 20, so that water or grout (as will be explained below) cannot pass between the skirt 26 and the base unit 20. The skirt 26 is continuous, and extends all the way around the perimeter 22. It comprises two layers of flexible, substantially impervious material, for example a plastic film, or a woven plastic web, or natural or synthetic rubber. At the lower perimeter edge 32 is a tubular pocket 34. The pocket 34 is connected by a number of ballast grout fill hoses 36 to a ballast grout distribution conduit 38, through which ballast material can be pumped. The ballast material fills the tubular pocket 34, so that the lower perimeter edge 32 is weighted down and caused to engage with the sea bed. On the underside of the base unit, not shown in Fig. 1 but shown in Fig. 2, is a series of infill grout ports 50 arranged in an array on the bearing surface 24. These infill grout ports 50 are arranged at the end of infill grout conduits 52 within the base unit 20 which communicate with the bearing surface 24 of the base unit 20. Infill grout can be pumped through the conduits 52 and ports 50 to the underside of the base unit 20, as will be explained below. The base unit 20 may be of reinforced or prestressed concrete, GRP, steel or other metal, or a combination thereof. In a preferred embodiment the base unit 20 is of reinforced concrete and includes ballast chambers, which can be empty to allow the base unit 20 to be floated to its installation location, and which can be flooded to allow the base unit to be sunk to the sea bed, or which can be filled with additional ballast which is heavier than water to increase the weight of the base unit 20 and to increase the stability of the completed base structure 10.

The superstructure 14 can be fixed to the base unit 20 by any suitable method. The fixing will normally take place on land, so that the base unit 20 and superstructure 14 are connected to each other and can be towed out together to the installation location. When they are in position, they can be lowered together onto the sea bed 12, so that the bearing surface 24 of the base unit 20 is in contact with one or more high points 12A, 12B on the sea bed. Water is still present in the void 16 between the bearing surface 24 and the sea bed 12 except at locations where the bearing surface 24 is in contact with the high points 12A, 12B.

The base unit 20 and the superstructure 14 structure may each include one or more ballast chambers, and they can be lowered by controllably filling and/or emptying one or more ballast chambers. Water may be used as ballast.

The base unit 20 includes on its outer perimeter 22 an attachment system 60 which attaches the lower perimeter edge 32 of the skirt 26 to the base unit 20 when the skirt 26 is in a stowed mode. Any suitable attachment system can be used, but Figs. 4 and 5 illustrate one embodiment of an attachment system in more detail. On each side of the base unit 20 there is provided a slidable control rod 80 which is slidably held by brackets 82. The control rod 80 is linked to the rod 84 of a hydraulic cylinder 86 which is fixed to the wall of the base unit. A number of eye plates 88 are secured to the lower perimeter edge 32 of the skirt 26, and each eye plate 88 engages with a locking pin 90 fixedly attached to the control rod 80. In the stowed mode, shown in Fig. 4, each locking pin 90 is engaged with a pin aperture 92 in the bracket 82 and with the eye plate 88, so that the eye plates 88 are held against the base unit 20.

While the base unit 20 and the superstructure 14 are lowered to the sea bed 12, the lower perimeter edge 32 of the skirt 26 remains attached to the base unit 20 by the attachment system 60, so that the skirt 26 remains in the stowed mode. When the base unit 20 has come to rest on the sea bed 12, a release mechanism is operated to detach the lower perimeter edge 32 of the skirt 26 remains attached to the base unit 20. The release mechanism can take any suitable form, but in the illustrated example of Figs. 4 and 5 the release mechanism comprises hydraulic control of the hydraulic cylinder 86 to extend the piston rod 84 and to slide the control rod 80. In this way each locking pin 90 is disengaged from the pin aperture 92 in the bracket 82 and from the eye plate 88, so that the eye plates 88 are free to fall away from the base unit 20. The hydraulic cylinder 86 can be connected to a hydraulic control system by suitable lines, so that the release mechanism can be operated remotely, for example from a surface vessel. The release mechanism can be operated locally by a diver or a ROV (remotely operated underwater vehicle) if preferred.

Operation of the release mechanism causes the lower perimeter edge 32 of the skirt 26 to become unattached from base unit 20. The skirt 26 then hangs freely in a deployed mode, supported at its upper perimeter edge by the fixing bolts 28, 30. In the deployed mode the lower perimeter edge 32 is free to conform to and engage with the sea bed. Ballast can then be introduced into the skirt to increase the weight of the skirt 26 and to urge it into firmer engagement with the sea bed 12.

When the skirt 26 engages with the sea bed 12, under the weight of the ballast therein, the skirt 26 effectively closes off the void, to form a closed volume bounded by the bearing surface 24, the sea bed 12 and the skirt 26. Although in the illustrated example the ballast material is introduced by pumping into the skirt 26 while the base structure 10 is in situ on the sea bed, alternative arrangements are possible in which the ballast material may be a permanent component of the skirt; for example the skirt may contain granular ballast in its stowed state, and upon deployment the ballast may be free to flow within the skirt to its lowest position, thereby sealing the skirt 26 against the sea bed 12.

In the embodiment of the skirt 26 illustrated in Fig. 1, the skirt 26 includes a tubular pocket portion 34 comprising a substantially closed chamber and extending along the perimeter length of the skirt 26, and extending along the lower edge of the skirt 26. Alternatively the pocket portion may extend substantially over the whole depth of the skirt 26, as shown in the embodiment of Fig. 4. The pocket portion comprises an inner layer and an outer layer joined along the upper and lower edges thereof to form a closed pocket. In the embodiment of Fig. 1 a number of radial ballast conduits 36 communicate directly with the tubular pocket portion 34 at one end and a ballast distribution conduit 38 at the other end. However in the embodiment of Fig. 4, where the whole skirt 26 forms a single pocket portion, the radial ballast conduits 36 may be omitted since the ballast distribution conduit 38 can communicate directly, via ports (not shown), with the interior volume of the skirt. The ballast distribution conduit 38 is connected to one or more skirt grout supply pipes 40, which deliver the ballast material from a pumping source (not shown). Alternatively the skirt grout supply pipes 40 can communicate directly with the interior of the skirt 26 and the ballast distribution conduit 38 can be omitted. The skirt grout supply pipes 40 are shown in Fig. 1 extending from the superstructure 14. The pumping source may be provided in the superstructure 14 or may be positioned on a separate surface vessel or the like.

The infill grout ports 50, through which the infill grout conduits 52 communicate with the bearing surface 24 of the base unit 20 are best seen in Fig. 2. Once the skirt 26 has been deployed and has sealed the void 16, grout or other cementitious infill material is pumped, for example from a surface vessel, through the base grout supply pipes 42 into the void 16 beneath the bearing surface 24 to displace the water from the void volume. The grout subsequently hardens so that the base structure 10 is then supported on the sea bed 12 over the entire bearing surface 24, not only by those parts of the bearing surface in direct contact with the raised portions 12A, 12B of the sea bed, such as rocks.

Vent pipes (not shown) are provided at the underside of the base unit 20 so that water can escape from the void 16 beneath the bearing surface 24 as the grout is pumped into the void 16. These vent pipes can either extend to the sea surface, where they can be monitored and controlled, or to the top of the base unit 20. An ROV, a remote monitoring system or visual inspection can be used to determine when all the water is expelled. The vent pipes can then be shut off, for example by remotely actuated valves, to allow the grout to be pumped into the void at increased pressure. If required the cementitious infill material can be pumped into the void 16 at a pressure sufficient to raise at least part of the base unit 20 from the sea bed, for example to level the base unit or to ensure that the whole of the underside of the base unit 20 is evenly supported. The base unit 20 can be provided with level sensors, and the cementitious infill material can selectively be pumped at selected pressures to different grout ports 50 on the bearing surface 24. The grout supply pipes 42 can each be in communication with one or more grout ports 50 in a particular location on the bearing surface 24. Levelling of the base unit 20 is shown schematically in Fig. 3. In the solid outline the base unit 20 is at an angle supported on the outcrops of rock 12A, 12B. The skirt 26 has been deployed and ballasted to seal the void 16. In the dotted outline the base unit 20 has been lifted by the pressure of the infill grout beneath the bearing surface 24, so that the base unit 20 is now horizontal. The skirt 26 is free to rest against the sea bed 12 and so continues to maintain the sealed void 16. Ideally the level sensors communicate with a support vessel, and the levelling procedure can be controlled from the support vessel by controlling the pumping of infill material. The infill grout may be placed in two stages. In a first stage, it is placed through infill grout conduits 52 on the bearing surface 24 of the base unit 20 adjacent to the perimeter 22 of the base unit 20, so that the grout, being denser than water, falls to the sea bed at a perimeter portion of the void volume and helps to seal the lower perimeter edge 32 of the skirt 26 to the sea bed 12. In a second stage, after the grout placed in the first stage has hardened, grout displaces the remainder of the water in the void 16, i.e. the water in an internal portion of the void volume, inside the perimeter portion of the void volume.

During the second stage the grout may be pumped at a pressure sufficient to raise the base unit 20 from the sea bed 10, preferably to a level position. The grout has a density sufficient to float the base unit 20 on the grout until it hardens. As can be seen in Fig. 3 the skirt ballast grout does not completely fill the skirt 26, so that while the lower edge 32 of the skirt 26 engages with the sea bed 10, the upper portion of the skirt 26, which extends to the upper perimeter edge, is sufficiently flexible to allow the base unit 20 to be at least partly raised away from the sea bed 10, while the skirt 26 continues to seal the perimeter of the void 16. Once the cementitious infill material has hardened and the base unit is horizontal and evenly supported by the infill material, one or more ballast chambers in either or both of the base unit 20 and superstructure 14 can be filled with ballast. This increases the weight of the base structure 10, thereby increasing the stability of the base structure 10. Water may be used as ballast. Alternatively or in addition other materials may be used as ballast, for example cementitious materials such as grout or concrete, mineral or metal aggregate, sand, or a mixture thereof.

Although the invention has been described with separate skirt grout supply pipes 40 and base grout supply pipes 42, at least part of the grout supply pipe system can be shared. The initial grouting operation is the supply of grout to the skirt 26. Once grouting of the skirt is complete, a three way valve (not shown) can be operated, by remote actuation or by a ROV, to divert the flow of grout from the skirt grout supply pipes 40 to the base grout supply pipes 42, and to seal off the radial ballast conduits 36. The shared part of the grout supply system is then flushed with water, so that the grout in the base grout supply pipes 42 does not set. Once the grout in the skirt has set, grout can be pumped through the base grout supply pipes 42 to fill the void 16.

Fig. 6 shows schematically the deployment of the skirt 26. The reference numerals correspond to the description of the deployment set out previously. Fig. 6A shows the base unit 20 after its soft touch down on the uneven sea bed 12, with the skirt 26 still stowed. Fig. 6B is an enlarged view of the skirt 26 in its stowed mode.

Fig. 6C shows the base unit 20 after deployment of the skirt 26 and after the pocket of the skirt has been partially filled with grout. In this embodiment the skirt 26 includes a separate tubular pocket 34 which is fed by separate skirt ballast conduits 36A routed through the interior structure of the base unit 20. The hem of the skirt 26 adopts the profile of the sea bed 12 and forms a seal around the base unit 20. Fig. 6D is an enlarged view of the skirt 26 in its deployed mode of Fig. 6C.

Fig. 6E shows the base unit 20 after deployment of the skirt 26 and after the pocket of the skirt has been completely filled with grout and after the grout has cured. The skirt 26 forms a solid wall around the void 16 beneath base unit 20. Fig. 6F is an enlarged view of the skirt 26 in its deployed mode of Fig. 6E.

In this embodiment the remaining volume of the skirt 26 itself forms a volume separate from the tubular pocket 34, and is fed by separate skirt ballast conduits 36B which are located at the outer surface of the base unit 20. However in other embodiments the separate internal tubular pocket 34 may be omitted, and grout may be fed directly to the interior volume of the skirt 26. If required the stages of Fig. 6C and Fig. 6E may be combined, so that the filling of the skirt 26 with grout takes place in a single operation through a single set of skirt ballast conduits. Fig. 6G shows the base unit 20 after grout 18 has been pumped to displace the water in the void 16 between the base unit 20 and the sea bed 12, with the base structure in its complete mode. The volume is filled with hardened grout 18, so that the base unit 20 is uniformly supported over the entire bearing surface 24 on the underside of the base unit 20. The cured grout 18 provides additional mechanical adhesion between the base unit 20 and the sea bed. Fig. 6H is an enlarged view of the skirt 26 in the complete mode of Fig. 6G. After the grout has cured, ballast chambers in the base unit 20 (as shown in Fig. 8 for example) or in the superstructure 14 may be flooded with water or grout, to provide additional weight and increase the stability of the base structure 10.

Fig. 7 shows an embodiment of a base structure 10 in which the skirt 26 is provided as four separate skirt elements 26, one on each side of the base unit 20 of the base structure. A corner skirt seal 126 is provided at each corner, and when deployed the ends of the two adjacent skirts 26 partially cover and seal against the corner skirt seal 126. In this embodiment the corner of the base unit 20 is provided with a plurality of connection ports 100. These ports 100 may be fluid or electrical connection ports 100 or a mixture of both. Typically one corner may be provided with import connection ports and another corner may be provided with export connection ports, and the base unit may have built in connections between the ports, for example a pipe connecting a fluid import port with a fluid export port, or an electrical connector cable connecting an electrical import port with an electrical export port.

The ports 100 enable a number of base structures to be linked together. For example, if the base structure 10 is used with a tidal or wind power generator, then an array of base structures could be daisy chained together, so that electrical power generated by each generator is fed from its own base unit to the next base unit and so on. The ports may include wet connectors, for example standard wet mate cable connectors in the oil and gas industry such as "Tronic™" or "Diamould™" connectors. Connecting a plurality of generators in this way avoids the need for expensive pre- laid cabling, which may lie dormant for some time.

If the base structure 10 is used with oil or gas production apparatus, then an array of base structures could be daisy chained together to provide an oil or gas pipeline. Other variations are envisaged. For example, a base structure 10 might include a booster pump installation or "in line" separation or other hydraulic process function. Fig. 7 illustrates an alternative way of attaching the curtain 26 to the attachment means 60 in the stowed state. Instead of eye plates 88, cables or cords 102 are provided which loop around the folded skirt. The ends of the cables 102 have loops 104 which engage with the locking pins 90, so that when the locking pins 90 are moved sideways the loops 104 disengage and the cables 102 and skirt 26 are free to fall way from the base unit 20.

Fig. 8 shows another embodiment of a base unit 20, manufactured from reinforced concrete. The unit 20 includes a collar 70 to safely transfer bending moments from the superstructure 14 to the base unit 20. The unit is made of reinforced concrete and includes ballast chambers 72, 74 so that the floating and/or sinking of the base structure 10 can be controlled by pumps (not shown) communicating with the ballast chambers.

The invention provides a simply and relatively quick method of forming a stable base for an offshore structure. The method can be controlled remotely and does not require extensive sub-sea operations. If the combined base structure and superstructure, including any payload on the superstructure, is sufficiently stable, then the combined structure can be towed with the base unit lowermost and the top of the superstructure above the sea surface. This method could be used for such structures as support piles for wind turbines, satellite oil and gas production platforms, riser platforms, lighter structures which may have telescopically erectable components, or heavier structures such as oil and gas PLEM, PLET or manifold structures where conventional piling may be difficult or impossible.

Alternatively the combined structure can be towed in a submerged partially buoyant state, with stability achieved by the buoyancy of the payload. Such a towing method may be applicable for structures whose mass and centre of gravity would not permit stable towing in a non-submerged state. This method could be used for such structures as the tidal turbine described in WO 2013/054085, taller structures incorporating either permanent or removable buoyancy to provide tow stability such as metrology masts, assembled wind turbines, oil & gas flare towers and pipeline vents, or starter structures for supporting heavier payloads which can be installed later by crane vessel or floated over and installed using controlled buoyancy methods. The invention can be used to provide a plurality of foundation supports for a single larger offshore structure, for example to provide a non-piled stable foundation for a multi legged jacket or tower structure, supporting payloads such as: power collection and transformer stations, oil and gas production systems, offshore strategic facilities, launching facilities, offshore mooring facilities etc.

The invention is applicable wherever conventional percussive piling would be difficult or drilled piling methods prohibitively expensive, or in conditions of large tidal range, or in depths of water greater than those that "Jack Up" installation vessels can operate.

Reference numerals

10 base structure

12 sea bed

12A, 12B high points on the sea bed

14 superstructure or column

16 void beneath base structure grout

base unit

perimeter of base unit

downwardly facing bearing surface of base unit skirt

, 30 skirt fixing bolts

lower perimeter edge of skirt

tubular pocket of skirt

, 36A, 36B radial skirt ballast grout fill hoses

skirt ballast grout distribution conduit skirt grout supply pipes

infill grout supply pipes infill grout ports

infill grout conduits

attachment system collar

, 74 ballast chambers slidable control rod of attachment system brackets

rod

hydraulic cylinder

eye plates

locking pin

pin aperture 2 cables or cords

4 loops of cables or cords

6 corner skirt seal