UNDERWATER FOUNDATION

The present invention relates to a method for installing underwater foundations particularly suited for supporting wave or tidal energy capture devices.

Underwater foundations (or anchors) are placed on the bed of a body of water to secure and provide support for a wide range of structures. These include wave or tidal energy capture devices and offshore wind energy capture devices. When used to support or anchor such structures foundations have to have sufficient strength and be sufficiently well secured to the bed of the body of water to resist the large turning, sliding, tension, compression and uplift movements and interaction of the structure with the high energy environment caused by tidal or wave movements. The devices may be tall, projecting a considerable height above the water bed and have substantial mass. Severe weather or sea state conditions increase the forces experienced on a foundation or anchor even more, leading to uprooting or dragging of foundations.

Using a large substantial mass as a foundation (a 'gravity anchor' ) has been found to provide insufficient resistance to the forces involved or to be prohibitively costly. The conventional approach is to use piling to secure a structure to the bed of the body of water. A typical foundation for a wave or tidal energy capture device may require single or multiple piles up to 4-5m diameter driven up to 20m or more into the bed of the water body. Whilst such arrangements may be effective they are expensive and difficult to install. The large excavation or piling operation required is carried out by using a ^λ jack-up'

drilling rig or ship which sails or is towed into position.

The rig or ship then lowers legs down to the water bed to provide a stable platform on or above the water. Alternatively a ship may use a dynamic positioning system to maintain its position above a selected location.

Piling, drilling or other excavation techniques are then carried out to provide the desired strong anchorage into the rock of the water bed onto which the rest of the structure is secured. Jack up rigs or vessels using dynamic positioning systems are very expensive to operate and can only be positioned and used in favourable weather conditions when wave height and sea swell and tidal current are relatively low. Therefore foundation installation costs can be prohibitive and subject to long delays.

Another difficulty encountered when installing foundations is the risk to divers, who are often required to carry out delicate engineering operations when foundations are installed or when the energy capture device is installed on or anchored to the foundation. Diver operations can only be carried out in favourable weather conditions.

Furthermore in the case of tidal energy capture devices the tidal flows in the areas best suited for energy capture may be so great that diver operations are simply not feasible, or only so at the brief moment of slack water.

It is an object of the present invention to provide methods for installing underwater foundations or anchors that avoids or at least reduces some of the abovementioned problems .

It is a further object of the present invention to provide foundation units or anchors that are economic in manufacture and installation and have sufficient strength for supporting or anchoring energy capture units.

The present invention provides a method of installing an underwater foundation, suitable for use in supporting or anchoring wave or tidal energy capture devices, comprising the steps of: providing a foundation unit equipped with a submersible drilling rig; placing said foundation unit on the bed of a body of water at a selected location; activating said submersible drilling rig to drill a plurality of pile holes in said bed of the body of water; and securing said foundation unit to the bed of the body of water by means of piles or tendons inserted in said pile holes and secured to said foundation unit.

The foundation units, which are installed using the method of the invention, constitute a second aspect of the invention. They are preferably towed into position before lowering to the bed of the water body. Towing can be achieved by attaching buoyancy to the foundation unit so that it floats. Preferably the foundation units employed in the method are themselves buoyant. Preferably the buoyant foundation units have air or gas filled chambers which are filled with water, or a denser filling material than water when the unit is being placed on the bed of the water body.

Using buoyant foundation units avoids the expense and complexity of attaching external buoyancy and then (on

installation) removing it. Preferably the air or gas filled chambers of a buoyant foundation unit are filled with a filler, such as a grout or a cement, for example, that is denser than water to provide a bond or key to resist lateral motion. The increased mass provided by the grout or cement increases the stability of the foundation unit. Grout or other fluid agents are readily supplied via suitable hoses from the vessel installing the foundation unit. Alternatively the foundation unit may be temporarily or permanently fitted with a grout tank which supplies the grout during installation of the foundation unit. The grout tank may conveniently be a flexible tank. The flexible tank is collapsible and expands as grout is loaded into it.

Preferably the method also comprises the steps of aligning the foundation unit to a selected attitude. For most installations, where an energy capture device is supported by the unit, the foundation unit must be located on the bed of the water body at an appropriate attitude so that an energy capture device that is attached thereto is itself correctly aligned to allow efficient energy extraction from the waves and/or tide. For example, a turbine powered by tidal flow may be mounted on a vertical mast with the blades in a vertical plane. Aligning (or "levelling") the foundation unit which supports the mast is essential to ensure the correct orientation of the turbine blades to the tidal flow.

The foundation unit can be aligned on an uneven bed by providing the unit with at least three remotely controlled or automatic jacking devices. When the foundation unit is placed on the bed the jacking devices are operated to align or "level" the foundation unit.

Advantageously control of the operation can be achieved by providing at least one levelling device, which provides feedback on the attitude of the foundation unit to an operator or to an automatic system. The levelling device may be, a spirit level viewed by a camera transmitting pictures to a remote (above water) operator of the jacks or it may be an electronic or optoelectronic "spirit level" of the types well known in the art.

After the alignment operation has been carried out the jacking devices are locked into place. This may be achieved by mechanical locking devices. Preferably, where the jacking devices are hydraulically operated they are locked into place by using a fluid such as a grout in the hydraulic cylinder which will set after the levelling operation is complete. This locks each of the jacking device legs at the selected extension.

The foundation units are placed on the bed of the water body. Advantageously, to provide resistance to dragging caused by tidal and wave forces, a layer of a suitable grouting compound is interposed between the base of the foundation unit and the surface of the bed. This layer provides the benefit of filling in any unevenness between the bed and the foundation unit so that the unit is evenly supported across its entire base area. In addition the grout, when set, provides a degree of adhesion to the bed, resisting lateral movement i.e. dragging of the foundation. A further benefit is that the grout adds to the mass of the foundation unit, thereby increasing its effectiveness.

Advantageously the foundation unit is provided with deformable grout retention skirts around its periphery or

deformable grout retention tubes underneath the base. The skirts or tubes may be made of any suitable material that will deform (crumple) when the foundation unit is lowered into position. The skirts or tubes then provide a barrier, located between the bed and the underside of the foundation unit. The barrier defines a volume underneath the foundation, into which grout can be pumped to displace water and form the layer between the bed and the foundation unit as described above.

The deformable skirts or tubes may, for example be made of thin steel sheeting or a suitable textile material of sufficient strength to retain grout until it sets. Where grout retention skirts or tubes are used the foundation unit is lowered onto the bed with the levelling jacks at least partially extended. As the aligning or levelling operation is carried out the skirts or tubes conform to the bed surface, providing a barrier to retain grout pumped under the foundation unit following the levelling operation.

The foundation unit is provided with a drilling rig. Preferably the drilling rig is remotely activated, for example in response to a signal from an operator on the vessel that is carrying out the installation.

Advantageously the drilling rig is remotely controlled or automatic. This avoids the need for any diver intervention when starting the drilling operations. Once the foundation unit is located in position and any alignment operations are completed then piling operations using the drilling rig are commenced.

Preferably the drilling rig comprises a magazine of drill string lengths. Preferably the drilling rig includes a

conveyance mechanism for transfer of drill string lengths from the magazine to the drill drive. Preferably the drilling rig operation is automatic or semi-automatic with a sequence of pile holes being drilled according to a pre- programmed sequence. Automatic or semi-automatic operation has the benefit of reducing or removing the requirement for constant operator supervision during the installation of a foundation unit. The drilling rig operates in a generally conventional manner with drill string sections being connected sequentially as required to the drill string to drive the drill bit as the hole being drilled deepens.

Advantageously the drill is mounted so as to move about a pivot point on the foundation unit on an extendible mounting arm or arms. This arrangement allows a hole to be drilled at almost any location on the foundation unit, into the bed of the body of water underneath. Advantageously the drill is mounted for movement about a central point of the foundation unit. A typical foundation unit may include a "stub" or short upwardly directed projection (typically a tube section) to which the energy capture device will be connected in due course. Typically this is located on the centre of the foundation unit. Conveniently the extendible mounting arms of the drilling rig rotate about this stub. Conveniently the drill string magazine comprises an array of drill string sections arranged around the outside of the stub.

It will be understood that the pile holes may be drilled through the foundation unit, any associated grout and then into the bed. Advantageously the foundation unit may be provided with guide holes through which the drill string passes freely before entering the bed.

Advantageously the drill string magazine is located at or near the pivot point about which the drill moves. Drill string sections can then be conveyed outwards to the location of the drill by a suitable conveyor or transport device and connected to the end of the in use drill string, in the conventional manner.

The pile holes and the piles or tendons subsequently placed in them can be smaller in diameter and/or depth in comparison with typical piles employed for securing foundations for wave or tidal energy capture devices. This provides the advantage that making each pile hole in the bed of the water body and fitting of a pile or tendon is easier to achieve. Nevertheless by providing a suitable number of smaller ("mini") piles or tendons, which share the loads experienced by the foundation unit, a secure foundation or anchor for tidal or wave energy capture devices, or other structures secured to the bed of a body of water, can be realised.

The drill string sections may be solid or they may be hollow, to allow passage of drill cuttings. Typically 5 m sections of drill string of 100-150 mm diameter are used. Each pile hole may be drilled to a depth of about 15 m (i.e. three drill string sections are employed) .

α typical foundation unit may be of the order of 10 to 20 metres square in area and 50 - 100 pile holes may be drilled for the piles. It will be appreciated however that the depth of drilling and the number of pile holes drilled will vary depending on the local water conditions and water bed geology, when considered in the light of the structure to be supported.

It will also be understood that the pile holes may be drilled vertically into the bed or may be drilled at an angle from the vertical to suit the piling structure required to support the energy capture device or other structure fitted to the foundation unit.

The foundation unit is secured to the bed of the body of water by means of piles or tendons inserted in the pile holes. Separate piles may be provided for this purpose and can be inserted by use of the drilling rig or by a separate pile driver. Preferably the drill string sections, used when drilling the pile holes, are used to form the piles. The drill string, including the drill bit, used to form a pile hole is left in situ with a fresh drill bit and drill string being used for each subsequent hole. Forming of each pile is then completed by injection of a suitable grouting compound into the hole, filling spaces between the drill string and the walls of the hole and optionally filling the drill string sections (if hollow) .

If piles are used they are secured to the foundation unit. This may be by means of a grout or by fastenings such as bolts or clamps. Advantageously the piles are provided with nut runners or other suitable securing and tensioning devices. The nut runners are screwed down a portion of the pile projecting from the top surface of the foundation unit until they contact the foundation unit and tighten against it. This secures the pile to the foundation unit. The method then includes the additional step of activating the nut runners to secure and tension the piles to the foundation unit. This provides a particularly secure fixing of the foundation unit to the bed.

Alternatively tendons rather than piles may be used to secure the foundation. Tendons are cables, typically of steel provided with an anti-corrosion coating of a polymer, such as an epoxy polymer. A sacrificial anode may also be fitted to the cable to reduce corrosion. The tendons are installed in the pile holes, fixed in position and then secured to the foundation unit, under tension, to provide secure fixing.

After securing the foundation unit to the bed of this body of water the energy capture device or other underwater structure is then secured to the foundation unit. This can be achieved in a number of ways. Where a stub is supplied on the foundation unit, as described above, the structure or device may have a docking portion that fits into or onto the stub and can be secured thereto by means of grout, welding, bolts etc. Alternatively proprietory quick connecting systems such as Ballgrab® or Kwik-lok® may be employed as appropriate for the strength of join required.

Alternatively where a floating or buoyant structure is to be anchored to the foundation unit it may be secured by means of suitable cables attached to anchor points provided on the unit.

According to a third aspect the present invention provides a foundation for use in supporting or anchoring a wave or tidal energy capture device comprising: a foundation unit connected by a plurality of piles to the bed of a body of water.

According to a fourth aspect the present invention provides a remotely controllable or automatic submersible drilling

rig for use with a foundation unit, said drilling rig comprising: a drill head drive, mounted on at least one extendible mounting arm, said mounting arm turning about a pivot, in use on a said foundation unit; a drill string magazine, and a transfer device for transferring drill string units to the drill head drive, in use of the drilling rig.

Further preferred features and advantages of the present invention will appear from the following detailed description given by way of example of some preferred embodiments illustrated with reference to the accompanying drawings in which:

Figure 1 shows in schematic perspective view a foundation unit ready for installation according to the method of the invention;

Figure 2a shows the foundation unit of Figure 1 in schematic plan view;

Figure 2b shows the unit of Figure 1 in schematic cross section;

Figure 2c shows a detail from Figure 2b showing operation of a jacking device;

Figure 2d shows the device of Figure 1 in elevation;

Figure 3 shows shows in a cross section detail the foundation unit of Figure 1 installed on a water bed;

Figure 4a shows in schematic perspective view another foundation unit ready for installation according to the method of the invention; and

Figure 4b shows the foundation unit of figure 4a in schematic plan view.

Figure 1 shows a foundation unit 1 which has a base portion 2. The base portion 2 is a concrete slab 4 of the order of 10m square. The base portion 2 is buoyant by the provision

of cavities 6 (see figures 2) which are air filled. Access holes 8 are provided into the cavities 6 but are kept sealed until the foundation unit 1 is lowered underwater.

The concrete slab 4 has a deformable skirt 10, of thin sheet steel, placed around its lower edge 12. A stub 14, a cylindrical projection from the centre top 15 of the slab 4, has a remotely controllable drilling rig, generally indicated by the reference numeral 16 fitted to it.

The drilling rig 16 includes a magazine 18, of drill string sections 20 disposed around the outside of the stub 14. The drilling rig 16 includes a frame 22 which rotates around the stub 14. The frame 22 has a pair of extendible arm sections 24,26 each of which has a drill vertical drive 28 and drill clamps 30 of generally conventional design mounted to a vertical support 31. The extendible arms 24,26 also include a feed conveyor mechanism 32 for delivering drill sections 20 from the magazine 18 to the vertical support 31. Towing eyes 34 are supplied on the slab 4 to allow the foundation unit to be towed along on the water surface. The towing eyes 34 can also be used as anchor attachment points for securing a floating or buoyant energy capture device (not shown) in use of the foundation unit.

Further features and details of the foundation unit can be seen from the plan and cross sectional views of figures 2.

The plan view of Figure 2a shows the location of three jacking devices 36 which are used for levelling the foundation unit 1 (see also the cross section of Figure 2b taken along the line BB shown on figure 2a) . One of the

jacking devices 36 is shown in more detail in the partial cross section view in Figure 2c (magnification of the circled portion C of figure 2b) . The jacking device consists of a piston 38 in a cylinder 40. The piston 38 can be moved hydraulically by the injection of a grout compound 42 through the grout passage 44 in the slab 4 of the foundation unit 1. A seal 46 prevents grout 42 passing out of the cylinder 40. An electronic levelling device 47 (figure 2a) is fitted on top of the stub 14 for use in conjunction with the jacking devices 36.

Figure 2a also shows, in dashed lines, the positions of cavities 6 (see also Figure 2d) each of which has an access hole 8, in the top of the slab 4. Also shown in Figure 2a are guide holes 48 which are sized to allow free passage of drill bits and associated drill string sections 20 through the slab 4. Only one row of guide holes is shown on the drawing, for clarity. An array of 50-100 guide holes 48 disposed more or less evenly across the slab 4 will normally be used. The number of guide holes provided and the actual number of piles used to secure a foundation unit to the bed of a body of water can be varied as required, including outside this typical range. The number of piles is chosen to be adequate to support the anticipated load and depending on the geology of the location.

Figure 3 shows in cross section a portion of a foundation unit 1 installed on a water bed 50. The foundation unit 1 is supported by jacking devices 36 (not shown, see Figure 2d) and by a layer 52 of grout 42 interposed under the slab 4 and retained by the deformable skirt 10. A pile 54 comprising drill string sections 20 passes through a guide hole 48 into a pile hole 56. The pile 54 is secured in place by grout 42. The pile 54 is further secured to the

foundation unit by means of a nut runner 58, a nut turned onto a threaded section 60 of the pile 54 to tighten against the top of the slab 4. As an alternative to the pile 54 a tendon (not shown) can be placed in the pile hole 56 and secured and tensioned to the slab 4 in a similar fashion.

Figure 4a shows a foundation unit 1 of generally similar form to that of figure 1, with like parts numbered the same. The foundation unit of figure 4a includes, as additional structural support for the stub 14, a tripod support structure 62, comprising three legs 64 extending from the stub 14 to the top of the slab 4. The tripod legs 64, in this example are spaced at 120° angles around stub 14 (see figure 4b) . The tripod structure 62 provides additional structural stability and strength for the drilling rig 16 and for a wave, tidal or wind energy capture device (not shown) when fitted to the stub 14 following removal of the drilling rig 16. Alternative support structures, for example a four-legged arrangement may be employed (tetrapod) .

The foundation unit 1 of figure 4a also features an integral grout tank 66 located on the stub 14 inside the magazine 18 of the drilling rig 16. Also shown on the unit 1 of this figure is an umbilical connection 68 to the surface and a convenient additional towing eye/anchor point 70 on top of the grout tank 66. The tank 66 supplies grout as required for the various operations described below in deploying the foundation 1. The grout tank 66 may be empty when the foundation 1 is being towed to a location and then filled with grout to assist in sinking the foundation to the bed of the water body. The grout tank 66 may be

detachable so that it can be removed from the foundation unit 1 at the same time as the drilling rig 16.

The operation of the method of the invention using the foundation unit 1 of the Figures will now be described.

The location for the foundation unit on the bed of the body of water is first prepared by 'cleaning' i.e. removal of loose material such as small rocks and/or levelling of the surface. The buoyant foundation unit 1 is towed into position above the selected location and the buoyancy removed by flooding the cavities 6 with water or filling them with grout 42. If the foundation includes the integral grout tank 66 as shown in figure 4a then the grout tank may be filled at the same time or just prior to flooding the cavities 6. When the foundation unit 1 reaches the surface of the water bed the deformable skirt 10 conforms to any irregularities in the water bed surface 50 (see Figure 3) and the pistons 38 of the jacking devices 36 rest on the water bed surface 50. The pistons 38 are then activated by injection of grout into the cylinders 40 to level the foundation unit 1 in accordance with the output signals from the electronic levelling device 47.

Once the unit 1 has been levelled the space between the unit 1 and the water bed surface, bounded by the skirt 10, is filled with a grout layer 52. After the grout 42 has set the drilling operation is started. Pile holes 56 are drilled in the water bed via the guide holes 48 in the slab 4 of the foundation unit 1. As each hole 56 lengthens drill string sections 20 are added from the magazine 18 as required.

Once the pile hole 56 is the desired depth the drill string is disconnected from the drilling rig 16, which is then removed by means of rotation about the stub 14, and movement of the extendible arm section 24 or 26 to a new location above a new guide hole 48. This process is repeated to provide a pile 54, of drill string sections 20, in each pile hole 56.

The piles 54 are then secured into their respective pile holes 56 by means of grout injections. Conveniently the drilling rig 16 may carry a grouting umbilical which communicates with a grout supply on the water surface (not shown) for supply of grout as required. Alternatively grout may be supplied from the integral grout tank 66 shown in figure 4a, for some or all grouting operations. Each pile (of drill string sections) can then be grouted into place while still connected to the drilling rig 16. The grout injected is also used to fill the space between the guide hole 48 and the drill string 20 sections, securing the piles 54 to the foundation unit 1. Nut runners 58 are then used to further secure the piles, by screwing down on the projecting pile ends onto the top of the slab 4. The drilling rig 16 may be fitted with appropriate tools for carrying out this operation.

After the foundation unit 1 is fully secured to the water bed 50 the drilling rig 16 (including grout tank 66, if fitted) is removed to the surface for reuse and a wave, tidal or wind energy capture device (not shown) can be attached to the stub 14. All of the operations of the drilling rig 16 (for example grouting, drilling and fixing nut runners), and of any other ancillary equipment fitted to the foundation 1, may be remotely controlled from the water surface, by means of signals supplied via an

umbilical 68 such as that shown in figure 4a. The umbilical 68 or other umbilicals where fitted may also be used to supply services such as electricity, compressed air, drilling mud or grout as may be required for foundation fixing operations.