The present invention stems from some work aimed at alleviating these problems by providing a form of marine foundation which can reduce the size of pile required for a given application and that can reduce the amount of time a jack-up platform or piling barge is required to remain on station at a given location.

Summaries of the Invention According to one aspect of the present invention we provide a transportable foundation unit adapted to form a composite structural

entity with a pile of pre-determined dimensions, and for use on ground under the sea or other body of water of a pre-determined depth, the foundation unit comprising a hollow pedestal comprising a top and depending side walls that are, in use, substantially vertical, the pedestal being substantially completely open at the bottom with the top rigidly connected and sealed to a hollow column which is open at both the top and the bottom, which is disposed below said pedestal top, and the bore of the column passing continuously through the top of the pedestal for receiving the pile, the length of the column above the top of the pedestal being greater than said depth sufficient to avoid overtopping of the column during installation and securing of the pile to the column, and a water pump connection to the interior of the pedestal, the arrangement being such that when the foundation unit has been lowered into the water with the pedestal resting on the ground, a net downwards pressure of water on the pedestal can be created by pumping water from the interior of the pedestal through the water pump connection to force or to assist in forcing the side wall of the pedestal and the bottom of the column into the ground and for holding the foundation unit stable whilst the pile is inserted into the ground through said bore.

Such a foundation unit can be fabricated at one site and then transported, by a vessel, to the location where it is to be installed.

The walls of the pedestal and the column are preferably substantially cylindrical and substantially coaxial, with their lower free ends lying in parallel planes, or in one plane, substantially perpendicular to the common axis.

The underside of said top of the pedestal preferably lies substantially in a plane that is perpendicular to the axis of the pedestal.

A load-bearing water-filter is preferably positioned against the underside of the closed top of the pedestal, and said water pump connection is so arranged as to permit water to be drawn upwards through the water filter on pumping by a pump connected to the connection.

The bottom surface of the water-filter preferably lies in a plane that is perpendicular to the axis of the foundation unit.

The composition and/or design of the water-filter is desirably chosen according to the nature of the soil in the ground at the installation location.

The bottom end of the column preferably projects below the bottom of the water-filter.

An additional outlet may be provided in the closed top of the pedestal for the free flow or pumped flow of water from within the pedestal without the water passing through the water-filter, thereby eliminating the resistance to flow offered by the water-filter.

The water-filter may be constructed of porous concrete and the pump connection may then comprise drainage channels formed in the upper surface thereof leading to an outlet extending through the top of the pedestal for connection to a pump inlet.

The water-filter may comprise pre-cast segments which are secured in position by attachment to ribs on the underside of the closed top of the pedestal.

The water-filter may also comprise wire mesh or a woven cloth of synthetic material secured over the bottom surface of the concrete.

Alternatively the water-filter may be constructed from a combination of one or more grids of steel bars and one or more sheets of wire mesh, the steel bars and wire mesh preferably being secured in position using a quick-setting epoxy resin.

Conveniently a platform may be rigidly fixed to the top end of the column to surround the top end of the column and provided with at least one lifting eye by which a cable can be attached to the platform to lower the unit on to the ground.

Said platform may also be used to support a larger, temporary working platform.

A close-fitting sliding collar may be fitted around the pedestal with a seal carried by the collar and so arranged as to resist flow of water between the pedestal and the collar, with retaining means being provided to retain the collar captive to the pedestal, and comprising a flexible impermeable membrane extending radially outwards from the collar covering the ground adjacent to the pedestal.

The membrane is preferably in the form of a flat sheet bounded by a radially inner margin and a radially outer margin, the radially inner margin being attached to and sealed to the sliding-collar, the membrane comprising a flexible tube incorporated around said outer margin, by means of which the membrane can be stretched out by pressurising the tube with water.

Desirably the impermeable membrane is made of synthetic rubber reinforced with synthetic fibre, and the collar is made of a moulded reinforced plastic with an adhesive used to seal the sheet to it.

The seal carried by the collar can be made of soft rubber with a coating of silicon grease.

The impermeable membrane can conveniently be supported by cords connected to rods attached radially to the collar by hinges circumferentially spaced apart around the collar, with a cable attached to the end of each rod by which means the membrane can be secured in a folded-up position while the foundation unit is lowered on to the ground and then when the foundation unit is resting on the ground the rods can be lowered and the tube round the outer perimeter of the membrane can be pressurised to stretch out the membrane on the ground.

In order to assist in deploying the membrane, flexible radial tubes that can be pressurised with water can be incorporated in the membrane.

Sand may be deposited on the membrane before pumping of water from the interior of the pedestal is started, to bed down the membrane on the ground, and cement grout or underwater concrete is preferably deposited on top of the sand to provide additional protection to the membrane.

According to a second aspect of the invention we provide a method of installing a foundation unit in accordance with the first aspect of the invention at a location in a body of water, comprising the steps of: (a) estimating the expected maximum depth of water at said location during the installation procedure and during securing a pile to the foundation unit, and selecting the height of the column of the foundation unit above said pedestal to be greater than said maximum depth, so as to minimise the risk of overtopping of the column during said installation and securing of the pile,

(b) transporting the foundation unit to the location, (c) lowering the foundation unit whilst suspended in a substantially vertical orientation onto the ground, and (d) pumping water from the pedestal to cause the bottom of the pedestal side wall and bottom of the column to enter the ground.

Following installation of the foundation unit, a pile is preferably driven or sunk through the bore of the column.

During driving or sinking of the pile into the ground, pumping of water from the interior of the pedestal preferably takes place, either continuously or intermittently, to assist in holding the foundation unit stable.

When a water-filter is provided in the pedestal, the soil characteristics at said installation location are desirably first assessed, and the characteristics of the water-filter are selected accordingly, so as substantially to prevent flow of soil particles to the pump.

Preferably water is allowed to flow freely through the water pump connection, or through an outlet which by-passes the water-filter, as the unit sinks into the ground, and the rate of transfer of load to the ground is controlled by means of a cable attached to the lifting eye to prevent a blow-out under the bottom of the pedestal from a sudden surge in water pressure within the pedestal.

Submerged buoyancy bags may be attached to the unit with means for controlling the amount of their buoyancy during transfer of the dead weight of the unit on to the ground.

The pedestal may be vibrated to assist in forcing it into the ground during the action of pumping, or the unit may be subjected to hammer blows to assist in forcing the pedestal into the ground during the action of pumping.

The column may be kept vertical while the pedestal is forced into the ground by means of ground anchors deployed around the foundation unit.

The ground anchors are preferably connected by cable to individual winches mounted on the platform, the winches being automatically controlled by transducers sensitive to tilt and mounted on the platform.

A temporary working-platform may be attached to the top end of the column after the pedestal side walls have been fully embedded in the ground and from the platform a pile may be driven or sunk within the column into the ground when the unit has sufficient stability from continuous or intermittent pumping of water from the ground within and beneath the pedestal.

Conveniently some or all of the equipment for driving or sinking the pile can be installed on the working-platform before the working platform is installed on the foundation unit.

The pile may be centralised within the column by piling leaders that have been installed within the column during the construction of the foundation unit and remain in place after installation of the foundation unit.

Alternatively, piling leaders connected together in the form of a cage are lowered into the column at a preliminary stage of the piling work, and

the cage is replaced by a cage of steel reinforcement or other reinforcing material after the pile has been driven or sunk to its final position.

The length of pile is preferably chosen such that when the pile has been installed in the ground a sufficient length remains within the column for a composite structural entity to be formed between the pile and the column by filling the space between them and any space within the pile with a bonding material, which is preferably concrete.

According to a third aspect of the invention we provide the combination of a pile and a foundation unit in accordance with the first aspect of the invention, the pile having been bonded to the foundation unit after the pile has been driven or sunk into the ground.

According to a fourth aspect of the present invention there is provided a transportable foundation unit for use on ground under the sea or other body of water; it is comprised of a hollow pedestal which is completely open at the bottom with the top rigidly connected and sealed to a hollow column which is open at both the top and the bottom and which passes continuously through the top of the pedestal, the length of column above the top of the pedestal being greater than the depth of water including waves at the location where the unit is to be used; provision for pumping water from within the pedestal is incorporated in such a manner that when the foundation unit has been lowered into the water with the pedestal resting on the ground, a net downwards pressure of water on the pedestal is created by pumping to force or to assist in forcing the wall of the pedestal and the bottom of the column into the ground; pumping is continued to provide the foundation unit with sufficient stability while a pile is driven or sunk within the column into the ground and a composite structural entity formed of the foundation unit and the pile.

Description of Preferred Embodiments Specific embodiments of the invention will now be described by way of example only with reference to the accompanying drawings: Brief Description of the Drawings In the drawings: Figure 1 shows a vertical section through the centre of a foundation unit lowered on to a sea-bed, Figure 2 shows a cross-section on AB in Figure 1, Figure 3 shows a cross-section on CD in Figure 1, Figure 4 shows the foundation unit of Figure 1 embedded in the sea-bed with a pile installed to form a composite foundation, Figure 5 shows the foundation unit of Figure 1 embedded in the sea-bed and supporting a working platform for the purpose of sinking a pile within the column, Figure 6 shows a vertical section through an alternative form of pedestal, Figure 7 shows a view of the pedestal of Figure 6 looking in the direction of arrow E with successive elements only partially shown for clarity, Figure 8 shows a vertical section of part of the foundation unit of Figure 1 with an impermeable blanket sealed to it being lowered on to the surrounding surface of the sea-bed by means of hinged rods,

Figure 9 shows an enlarged view of the seal between membrane and pedestal shown in Figure 8, Figure 10 shows the part of the foundation unit of Figure 8 fully embedded in the sea-bed with the impermeable membrane stretched out and covering the surface of the surrounding sea-bed.

In this description the words sea-bed and ground are interchangeable, as are the words sea-water and water. And the word steel can be taken to include any material possessing rigidity and strength.

Referring to the drawing, the foundation unit shown in Figures 1,2 and 3 is constructed of steel and is comprised of a hollow cylindrical pedestal 2, open at the bottom with a tapered edge 3, and a closed top 4, rigidly connected and sealed to a concentric hollow cylindrical column 1 which passes completely through the closed top 4 with gusset plates 5 strengthening the junction between 1 and 4. Steel piling leaders 12 are fixed at intervals within the length of the column and are equally spaced around the inside of the column.

Steel ribs 14 seen in Figure 3 but not in Figure 1 are of the same depth as segments of a load-bearing water-filter 13 made of a porous material such as porous concrete located between and held in place by the ribs which also serve to strengthen the top of the pedestal 4, and the junctions between 1 and 4 and between 2 and 4.

The water-filter 13 may be cast in-situ with the pedestal inverted before the upper part of the column 1 has been constructed, or the segments may be pre-cast or pre-formed and held in position by flanges secured to the bottom of the ribs 14 so that in cross-section each rib and flange forms an inverted'T'. A fine wire mesh made from a metal that will not

corrode, or a cloth woven with synthetic fibre may be attached to the underside of the segments but whether or not there is a need for such an attachment is dependent on the particular composition of the sea-bed and the porosity of the segments.

The top of each segment of the water-filter contains a radial drainage channel 15 leading into a drainage channel 16 which forms part of an annular drainage channel: the ribs 14 each have an aperture where the rib is joined to the top of the pedestal 4 so that with all of the segments in position a complete annular drainage channel is formed. An outlet pipe 17 connected to a suction pump (not shown) is sealed into the top of the pedestal 4 at a point above the annular drainage channel. By this means water within the pedestal can be drawn upwards through the whole of the water-filter through the drainage channels and outlet 17 to the pump.

A by-pass outlet pipe 23 with remotely controlled stopcocks 18 and 19 is sealed into the top of the pedestal and passes completely through one of the segments so that water can flow or be pumped from within the pedestal without it passing through the water-filter: stop-cock 18 controls direct flow to the surrounding water 25 and stop-cock 19 controls flow to a suction pump (not shown). Outlet 17 does not have a remotely controlled stop-cock to ensure that flow cannot be inadvertently stopped during construction, the flow being controlled solely by the pump.

A platform 7 above the surrounding water 25 is rigidly attached to the top of the column, the junction being strengthened by steel gusset plates 8. A crane cable 11 is attached to the platform 7 through cables 10 connected with hooks (not shown) to removable lifting eyes 9 secured to the platform. This platform 7 and gusset plates 8 are incorporated for transportation and construction purposes, and they may

be removable attachments to the column if not required in the final construction.

Referring to Figure 1, as the dead weight of the foundation unit is transferred from the crane cable 11 to the sea-bed 20, the pedestal sinks into the sea-bed and water displaced from within the pedestal flows directly to the surrounding water through stop-cock 18 equalising the internal and external water pressures.

The transfer of dead weight should be carried out in a controlled manner to avoid a sudden surge of water pressure within the pedestal which could cause a blow-out under the bottom of the pedestal and a detrimental disturbance of the integrity of the sea-bed. A controlled transfer is readily achieved if the foundation unit is lowered by a crane on a jack-up platform. But if the unit is lowered by a crane on a ship or from a winch on a helicopter, buoyancy bags need to be attached to the foundation unit and the cable released from the unit when the pedestal is well clear of the sea-bed. The dead weight of the unit is then transferred to the sea-bed by reducing the volume of the buoyancy bags, with the buoyancy bags fully submerged. In such cases it is not possible to eliminate the effect of waves or swell completely due to the projection of the column above the surface, but provided this method of depositing the foundation unit is used in reasonably calm conditions the pedestal will act as a damper to reduce fluctuation in vertical movement to an acceptable level.

When the dead weight of the foundation unit has been transferred to the sea-bed, stop-cock 18 is closed, stop-cock 19 is opened, and a net downwards pressure of water on the pedestal is created by pumping to force the bottom of the pedestal 3 and the bottom of the column 6 into the sea-bed.

It will be appreciated that the rigidity and impermeability of the pedestal and column create an impermeable barrier between the interior of the pedestal and the water surrounding the unit, which facilitates the creation of a pressure differential across the top of the pedestal for producing a downwards force on the pedestal.

When the underside of the water-filter 13 is just above the surface of the sea-bed, stop-cock 19 is closed and pumping continued through outlet 17 to bring the underside of the water-filter into firm contact with the sea- bed surface. If necessary vibrators (not shown) may be attached to the pedestal and the column to assist penetration, or driving may be applied to the column or pedestal.

Pumped flow that by-passes the water-filter provides for the case where due to the nature of the ground a water-filter of low porosity is used and the resistance to flow offered by the water filter will hinder the installation of the foundation unit into the ground. If such a consideration does not apply, the outlet through stop-cock 19 may be excluded and all pumped flow taken through outlet 17.

Pumping is continued through stop-cock 17 either continuously or intermittently, depending on the particular composition of the sea-bed, to provide the foundation unit with sufficient stability for the remaining work on the foundation to be completed. For some applications, pumping may continue throughout the life of the completed structure.

Transducers with remote read-out to measure water pressure and load may be installed within the pedestal to monitor stability during construction and during subsequent use of the composite foundation.

An important consideration throughout the installation of the foundation unit into the sea-bed is that the column is kept vertical. If the unit is

lowered from a jack-up platform or piling barge, non-vertical movement can be restrained by means of a frame attached to the jack-up platform or barge.

Alternatively, and in cases where a jack-up platform or piling barge is not used, ground anchors may be deployed around the foundation unit and connected by cable to individual winches mounted on the platform 7 which are automatically controlled by transducers sensitive to tilt mounted on the platform.

In considering the length of the column 1 in the manufacture of a foundation unit, the length above the top of the pedestal 4 is determined by the depth of water and possible wave height where it is to be installed, whereas the length below the top of the pedestal will be influenced by the type of sea bed. The greater this length the greater will be the strength of the foundation, but resistance to penetration will increase and not only may it be more difficult to embed the pedestal in the sea-bed but if the column projects below the bottom of the pedestal the amount of penetration of the pedestal due to the dead weight will be reduced.

During installation, if the bottom of the pedestal 3 penetrates only a short distance into the sea-bed under the dead weight then to avoid a reversed blow-out only a small reduction of pressure within the pedestal can be created initially by pumping, and this can only be increased gradually as the pedestal sinks into the sea-bed under the action of the pumping.

Consequently although a long length of column below the pedestal is desirable it may be found that the optimum position for the bottom of the column 6 is within the pedestal. In this case the bottom of the column should be sufficiently clear of the surface of the sea-bed before pumping starts for the surface not to be disturbed by water flowing from within the column. Also it is advisable that the bottom of the column should

finish below the bottom of the water-filter to avoid the possibility of a cavity forming in the ground under the pedestal.

Figure 4 shows the foundation unit fully installed in the sea-bed 20 with a pile 22 sunk into the ground and centralised within the column 1 by piling leaders 12. The pile shown in the drawing is a steel tube pile but it represents any type of cylindrical pile sunk by any piling method.

Before the pile is installed, soil within the column is removed to the bottom of the column if possible but not so far as to cause the underlying soil to become unstable. After the pile has been installed the space between the pile and the column is filled with a strong bonding material such as freshly mixed concrete 21 to form a structural entity between the foundation unit and the pile. If the pile is hollow, as in Figure 4 some of the soil within it may be removed and the space within the pile filled with concrete.

As an alternative to having the piling leaders fixed inside the column as shown they may be constructed in the form of a cage which is lowered into the column immediately before the pile is installed. Such a cage can then be withdrawn for re-use and, if required, a cage of reinforcement lowered into the space before introducing the freshly mixed concrete.

The composite foundation so formed requires a smaller pile than would otherwise be required using a monopile alone. This gives the advantages of requiring lighter equipment and less time to install the pile.

The pile 22 may be driven or sunk in position from a jack-up platform.

Alternatively as shown in Figure 5 a temporary working platform 24 may be lowered on to and secured to the smaller platform 7, and the piling work carried out from this larger platform. If possible, the equipment required or at least some of it is permanently installed on the working

platform saving time in moving the equipment from one column to another.

Figures 6 and 7 show a pedestal containing an alternative form of water- filter for a foundation unit installed in a sea-bed predominantly of sand and gravel where a water-filter of porous concrete would present too much restriction to flow.

Essentially the body of the pedestal is the same as that shown in Figures 1 to 5 except that ribs 14 are excluded, and strengthening ribs 26 are incorporated on the top of the pedestal 4 between an upward extension of the cylindrical wall of the pedestal 27 and the column 1. An enlarged pumping outlet 28 corresponding to outlet 17 in Figure 1 is provided to take a greater rate of flow, no by-pass to the water filter is required, and stopcock 29 has the same function as stopcock 18 in Figure 1.

The construction of the water-filter is seen more clearly in Figure 7 which shows the pedestal of Figure 6 looking in the direction of arrow E.

Attached to the underside of the top of the pedestal 4 are steel bearers 30, 31, and 32, all of square or rectangular cross-section and all of the same depth. Bearers 30 and 31 are located round the outside and inside perimeters respectively of the water-filter, and 32 are straight parallel bars evenly spaced across the pedestal of such length as to allow free flow of water around them to the pump outlet 28 and stopcock 29.

Attached to these bearers are steel cross-bearers 33 of square or rectangular cross-section all of the same depth, equally spaced across the whole width of the pedestal but not all shown for clarity in the drawing.

Likewise a woven wire mesh 34 is attached to the cross-bearers and only partly shown for clarity. If required, depending on the composition of the sea-bed, one or more layers of wire mesh of successively smaller aperture are attached to the wire mesh 34.

None of the bearers or any of the wire mesh is subjected to direct operating load until the pedestal is firmly embedded in the sea-bed and the attachments of bearers to pedestal, cross-bearers to bearers, and wire mesh to cross-bearers are only subjected to light loading during transportation and while the pedestal is being installed in the sea-bed.

Consequently for speed of assembly the attachments are made preferably with a quick-setting epoxy resin.

Referring to Figures 8 and 9 a flexible impermeable blanket 40, made for example of synthetic rubber reinforced with synthetic fibre is sealed to the bottom flange 43 of a sliding collar 35 fitted round the cylindrical wall 2 of the foundation unit of Figure 1. Preferably the collar is made of a moulded reinforced plastic with an adhesive used to seal the blanket to it. A soft rubber seal 45 is located around the inside of the collar with a coating of silicon grease between the seal 45 and the wall of the pedestal 2.

Retention cables 41 spaced at equal intervals round the pedestal are connected between the top of the collar and retention plates 42 attached to the top edge of the pedestal: they ensure that the collar is retained on the pedestal as it is lowered to the sea-bed while allowing free vertical movement between the two.

Rods 36 connected to the collar 35 by hinges 44 spaced at equal intervals round the collar support the blanket 40 by cords 38 made of a synthetic material attached to the rod. The lengths of the cords are such that the

rods can be lowered by means of cables 37 from a vertical position down to the sea-bed 20 as shown in Figure 10 without imposing undue stress on the blanket.

The rods are in a vertical position until the dead weight of the foundation unit has been transferred to the sea-bed and then they are lowered as shown in Figure 8, the collapsed inflatable tube 39 round the circular perimeter of the blanket being inflated with sea-water to stretch out the blanket before the rods are finally lowered to rest on the blanket. A feeder line (not shown) connected to the inflatable tube 37 is attached to one of the cables 37.

Sand 46 is deposited on the blanket to bed down the blanket on to the surface of the sea-bed and to afford some protection to the blanket.

Pumping is then started to embed the foundation unit into the sea-bed as previously explained, the pedestal sliding down inside the collar 35 until the unit is fully embedded as shown in Figure 10. The inflatable tube can then be collapsed and a layer of underwater concrete or cement grout laid on top of the sand 46 to provide additional protection to the blanket.

As an alternative to employing hinged rods, the cables 37 can be connected directly to the blanket, with radial inflatable tubes incorporated into the blanket and used to spread out the blanket as it is being lowered on to the sea-bed.

The impermeable blanket enables a greater net downwards pressure of water on the pedestal to be created during the installation of the unit without the risk of a reverse blow-out into the pedestal. And when attached to a foundation unit installed in a sea-bed of granular soil it increases the shear strength of the sea-bed around and under the foundation unit by creating greater and more extensive reduction of pore

pressure from pumping, and it reduces the rate of pumping needed for stability. On any type of sea-bed its purpose is to afford some protection against scour of the sea-bed around the pedestal, and if pumping is maintained when the structure is completed and operating, to protect against liquefaction of the sea-bed around the pedestal under vibration or cyclic loading.