US3783626A

US3783626A - Structure, and method and apparatus for founding a structure

Info

Publication number: US3783626A
Application number: US00223590A
Authority: US
Inventors: F Hansen
Original assignee: Redpath Dorman Long Ltd
Current assignee: Redpath Dorman Long Ltd
Priority date: 1971-02-08
Filing date: 1972-02-04
Publication date: 1974-01-08
Anticipated expiration: 1991-01-08
Also published as: NO136683C; DE2205704A1; NL7201658A; GB1380586A; NL7201657A; ZA72597B; FR2126778A5; FR2126777A5; NO136683B; BE779064A

Abstract

A method of founding a structure in a subaqueous bed comprising disposing the structure on the bed and excavating beneath the structure, the excavation being effected by a wheeled positively buoyant vehicle which moves upon an undersurface of the structure and which is provided with excavating equipment.

Description

Jan. 8, 1974 United States Patent [191 Hansen S T w T MM s E m T mm .MS e um n N U m u STRUCTURE, AND METHOD AND APPARATUS FOR FOUNDING A STRUCTURE 538,073 4/l895 Harris..................... [75 Inventor: Frode Johan Hansen, Kingswood, 3,683,632 8/1972 Van der England 3,218,739 11/1965 Kaufmann et a1. 3,629,963 12/1971 FOREIGN PATENTS OR APPLICATIONS [73] Assignee: Redpath Dorman Long (Contracting) Limited, Edinburgh, Scotland 2,021,441 3/1971 Germany...............................61/50 Primary Examiner-Jacob Shapiro Attorney-Bacon & Thomas 22 Filed: Feb. 4, 1972 21 Appl. No.: 223,590

[57] ABSTRACT A method of founding a structure in a subaqueous bed [30] Foreign Application Priority Data Feb. 8, 1971 Great Britain..................... 4,191/71 comprising disposing the Structure on the bed and cavating beneath the structure, the excavation being effected by a wheeled positively buoyant vehicle which moves upon an undersurface of the structure and which is provided with excavating equipment.

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10 Claims, 16 Drawing Figures PATENTEU N 8 74 SHEET 2 BF 7 PATENTED JAN 31974 SHEET 0F 7 mm Qmms mm 3 QR wE STRUCTURE, AND METHOD AND APPARATUS FOR FOUNDING A STRUCTURE This invention relates to a structure adapted to be founded on a subaqueous bed. It also relates to a method of founding a sinkable structure on a subaqueous bed, and to a vehicle for excavating a subaqueous bed under such structure.

In one aspect, to which the invention is not in general limited, there is provided a vehicle for excavating under a structure disposed on a subaqueous bed, the vehicle being adapted to have positive buoyancy and comprising means whereby the vehicle may contact andmove about on an undersurface of the structure, and means for supporting excavating equipment.

The vehicle may be provided with powered wheels or rollers to enable it to contact and move about on the said undersurface.

The wheels or rollers may be arranged to permit the vehicle to move in any direction on the said undersurface and to rotate about its own centre.

The vehicle may be provided with excavating equipment comprising a cutter or breaker head, and/or means for providing a high pressure water jet and/or a suction pipe, the suction pipe preferably also being adapted to deliver material to the excavated space for back filling.

The vehicle may comprise a buoyancy chamber and means for admitting and expelling water from the chamber to control the buoyancy of the vehicle.

The vehicle may be adapted to carry a crew, or alternatively the vehicle may be provided with remote control equipment so that no crew is necessary.

In another respect, to which the invention'is not in general limited, there is provided plant comprising a vehicle as set forth above, in conjunction with apparatus for moving the vehicle into a position from which it can move into contact with the said undersurface of a structure.

There may be means for measuring the position and heading of the vehicle relative to the apparatus.

The means for measuring may be adapted to measure the radial distance of the vehicle from the apparatus and the bearing of the apparatus relative to the vehicle.

The said apparatus may consist of a pontoon adapted to be disposed above and in contact with the vehicle whereby the vehicle and pontoon are adapted to be submerged together and to move vertically whilst submerged.

There may be means for varying the buoyancy of the pontoon. i

The pontoon and the vehicle may be adapted to be disposed in a vertical shaft in the said structure, the pontoon being provided with means for sealing the space between its periphery and the wall of the shaft.

The pontoon may be provided with means for engaging the wall of the shaft whereby to vertically locate the pontoon relative to the shaft.

The vehicle and the apparatus each may include a pressure chamber, there then being means to permit personnel to pass between the pressure chambers whilst both chambers are submerged.

In another aspect to which it is not in general limited the invention provides a sinkable structure adapted to be founded on a subaqueous bed, and comprising an undersurface adapted to contact the subaqueous bed, the structure being in combination with apparatus for moving a vehicle as set forth above into a position in which it can move into contact with the undersurface and excavate the subaqueous bed beneath the structure.

The last mentioned apparatus for moving may be as set forth above.

The apparatus may be a moveable portion of that part of the structure which defines the undersurface.

The structure may have a shaft extending upwards from the undersurface and adapted to have the vehicle and the apparatus moveably disposed therein.

There may be means (e.g., the aforementioned means for sealing) for sealing the shaft so that water may be pumped out of the excavated bed beneath the undersurface.

There may be a diving bell for communicating between the pressure chamber of the apparatus and a part of the structure which is above the water surface when the structure has been sunk to the subaqueous bed.

The structure may be in two separate parts; a foundation part defining said undersurface and having a substructure which is adapted to project above the water surface when the structure has been sunk, and a superstructure part which is adapted to be supported above water level by the substructure.

In a further aspect, to which it is not in general limited, the invention provides a method of founding a structure in a subaqueous bed comprising disposing the structure on the bed and excavating beneath the structure, the excavation being effected by a positively buoyant vehicle which moves upon an undersurface of the structure and which is provided with excavating equipment.

The method may include excavating under the structure so that it is supported on a number of spaced apart regions (e.g., at four corners) and then increasing the effective load on the said regions so that the structure sinks into the bed.

When the structure has a recess (e.g., a shaft) extending upwardly from the undersurface, the vehicle preferably retires to the recess whilst the structure is sinking.

The effective load may be increased by sealing the excavated space beneath the undersurface and by pumping out water therefrom. t

The excavated space may be filled with solid material after the structure has been sunk into the bed to a desired extent.

An embodiment of the invention will be described merely by way of example with reference to the accompanying drawings wherein FIG. I shows asinkable structure according to the invention in a buoyant condition,

FIG. 2 shows the structure of FIG. 1 having been sunk,

FIG. 3 is a vertical section through FIG. 2 with some parts added,

FIG. 4 and 5 are vertical sections through part of the foundation raft of FIG. 3, showing the excavating vehicle in two different positions,

FIG. 6 is a section on line Vl-Vl of FIG. 4

FIGS. 7d and 7b are respectively top and bottom plan views of the excavating vehicle,

FIG. 8 is a vertical section through the foundation raft of FIG. 2, showing excavation proceeding,

FIG. 9 is a half-section on line IX-IX of FIG. 8,

FIGS. 10, 11 and 12 show successive stages in the founding of the foundation raft, the figures being vertical sections as FIG. 8,

FIG. 13 is a section similar to that of FIG. showing an alternative embodiment of the invention,

FIG. 14 shows the founded sinkable structure with its permanent superstructure positioned prior to being fitted, and

FIG. 15 shows the permanent superstructure in its final position attached to the sinkable structure.

When building a structure which when completed is founded in a subaqueous bed such as a seabed or lakebed, it is desirable to prefabricate as much as possible of the structure on-shore, and to do a minimum amount of work at sea. Furthermore, the work at sea should be of short duration and of a simple nature and it should not be dependent on weather conditions or require expensive floating equipment.

The illustrated embodiments of the present invention are thought to meet these desiderata. Referring to FIGS. 2 and 15, there is shown an off-shore drilling rig having a foundation raft 20, a sub-structure 22 and a super-structure 24 (FIG. 2) or 26 (FIG. 15).

The foundation raft is shown resting on the sea-bed 27 in FIG. 2, and in its finally dug-in or founded condition in FIG. 15.

The super-structure 24, 26 is supported at a safe height above sea level and its exact nature depends on its purpose. Thus, in FIG. 15 the super-structure 26 is equipped with a drilling rig 28. In FIG. 2 the superstructure 24 is temporary only and is designed for use in founding the foundation raft in the sea-bed.

The sub-structure 22 supports the super-structure from the foundation raft 20 and comprises two rows of circular vertical steel columns 32, free at the top and suitably braced against each other at their lower parts which are submerged when the structure is on-site.

The upper part of the sub-structure has a minimum area exposed to wave action in order to reduce waveinduced forces to a minimum.

The lower part of the sub-structure is less subject to wave-induced forces and so is not designed to present a minimum surface area. Instead it is designed to have adequate buoyancy to ensure that the combined substructure and foundation raft has adequate stability for towing to the site.

The foundation raft 20 consists of a steel casing 34 (see FIG. 3), the bottom of which is reinforced with concrete 36 to prevent puncturing of the casing should it sink onto an uneven sea-bed and also to provide a low centre of gravity so that the foundation raft and sub structure are stable when floating, and during sinking. The casing 34 is divided into a number of water tight chambers 38 which can be wholly or partially flooded to control sinking of the structure. When the chambers 38 are not fully flooded, the foundation and substructure have sufficient buoyancy and are stable enough to be towed to the site without additional buoyancy aids being required.

The foundation raft 20 is provided with a flat undersurface 40 and a concrete-reinforced shaft 42 extending upwardly from the surface 40. These features are of importance during the founding of the raft 20 in the sea-bed, and will be discussed later.

The foundation raft 20, the sub-structure 22 and the temporary superstructure 24 are constructed into their complete state onshore. The substructure is provided on top of the columns 32 with lifting tackle (sheaves) 44, and the temporary superstructure 24 is provided with winches 46. Cables or chains 48 pass around the lifting tackle 44 and the winches 46.

When the structure is to be towed to the site where it is to operate, the superstructure 24 is temporarily supported as shown in FIG. 1 upon upper members 49 of the cross-bracing structure between the columns 32.

The structure is then towed in any suitable manner whilst floating as shown in FIG. 1.

When the structure arrives at the site, the temporary structure hoists itself upwards by winching-in the cables or chains 48, until it is in the position shown in FIG. 2. It will be noted that the hoisting operation is continuous rather than a discontinuous climbing operation. Also, the lifting tackle 44 and cables 48 do not transmit bending moments due to wave action from the columns to the super-structure 24.

The foundation raft 20 is then partially flooded so that it sinks in a controlled manner to the sea-bed 27, as shown in FIG. 2. The process of founding the raft 20 in the seabed then is effected, by means of equipment 'now to be described. This equipment permits the foundation raft 20 to be sunk into the sea-bed without using normal compressed air methods, which would limit the possible water and foundation depth.

The equipment comprises a crewed vehicle 50 adapted to have positive buoyance and which during sinking of the raft 20 is disposed in the shaft 42 beneath a pontoon 52. The pontoon 52 has a controllable positive and negative buoyancy.

Referring to FIGS. 4, 5 and 6 the pontoon 52 comprises a pressure chamber 54 adapted to contain a crew and ventilated via an air line 56. The chamber 54 may also be pressurised for use as a decompression chamber, should it be necessary for the crew of the vehicle 50 to be decompressed. Surrounding the chamber 54 is a buoyancy chamber 58 which may be flooded to a controlled extent to control thebuoyancy of the pontoon. An inflatable seal 60 is provided for sealing the space between the shaft 42 and the pontoon, for reasons discussed hereafter.

A diving bell 62 is arranged on a pulley system 64 to provide for transport for men from the chamber 54 to the surface. An air-lock 66 permits men to move between the chamber 54 and the diving bell 62.

An an alternative to the diving bell 62 and the airlock 66, there may be provided a temporary access shaft between the chamber 54 and the superstructure 24. However, such a shaft would be subject to waveinduced forces and the diving bell is considered simpler and safer.

The pontoon 52 is provided with three hydraulically operated pins 68 (FIG. 6) which may be extended to engage in recesses 70 in the wall of the shaft 42 whereby-to fix the pontoon in the shaft 42. The recesses 70, are long slots permitting the pontoon to be fixed in any position between the position shown in FIG. 4 and the position shown in FIG. 5 and discussed hereafter.

Surrounding the chamber 72 is a buoyancy chamber 78 which in operation is subjected internally and externally to water pressure. This chamber may be flooded as necessary to control the buoyancy of the vehicle 50. When in operation the chamber 78 is not flooded so that the vehicle 50 has a positive buoyancy of several tons.

The vehicle 50 is provided in its upper part with wheels or rollers whereby it may move about on the under surface (see FIG. 7a) the foundation raft 20. In this embodiment (see FIG. 7a) there are one pair of oppositely-disposed powered wheels 80 and two spherical free-wheels 82. The wheels are arranged about a common centre coinciding with the axis of the vehicle 50, so that the vehicle can rotate about its own axis and move in any direction. The wheels 80 are powered by hydraulic pressure fluid motors shown diagrammatically at 84. The pressure fluid source for these motors is provided on the super-structure 24 and fed via lines (not shown) to the vehicle 50. Alternatively, the pressure fluid may be provided by an electrically driven pump in the chamber 78, Control of the motor is effected from within the chamber 72. Instead of having

wheels

80, 82 the vehicle may have tracks which pass around idler wheels and drive sprockets.

The vehicle 50 is-provided with a pivotally mounted tool holder 86 to which may be attached a variety of cutting or breaking tools, either rotary or percussive. A rotary tool is shown fitted at 88. It will be seen from FIG. 7 b that the tool holder 86 is pivotable in a vertical diametral plane of the vehicle 50. Hydraulic working fluid for the tool (provided from the superstructure 24) is fed down the interior of the tool holder.

Another similarly pivotably mounted tool holder 90 may support an interchangeable nozzle 92, to provide a high-pressure, soil-breaking waterjet. A downwardlydepending suction nozzle 94 also is provided for removing loosened soil. The suction is provided via a pipe 96 from a pump on the superstructure 24. The pump is reversible so that the pipe 96 and nozzle 94 may deliver suspended solid material for backfilling, as described later.

The vehicle has access hatches 98, 100 between the pressure chamber 72 and the buoyancy chamber 78, and between the buoyancy chamber 78 and the outside of the vehicle 50.

These hatches permit the crew to service or change the tool 88, the nozzle 92 and the suction nozzle 94 by pressurising the chamber 72 and then emerging into the buoyancy chamber 78. Alternatively the suction nozzle 94, which is flexible, may be pulled in to the chamber 78 for servicing (e.g., if it is blocked). The need for servicing can be seen by watching the discharge from the pipe 96.

When the foundation raft 20 is sunk, the pontoon 52 and the vehicle 50 are within the shaft 42 as shown in FIG. 3. To commence the founding operation, water is drained from the air-lock 75 into the chamber 54. This forces the top of the vehicle 50 into sealing contact with the bottom of the air-lock 75, permitting two men hicle 50 will sink. Resting on the sea-bed, the vehicle 50 digs itself in as shown in FIG. 4 until the pontoon 52 has sunk so deep that its underside is flush with the undersurface of the raft 20 as shown in FIG. 5. At that point the pins 68 are engaged with the recesses 70 in the shaft wall and the pontoon 52 cannot sink any deeper. The vehicle continues digging underneath itself until there is a clearance underneath it equal to the reach of its

digging equipment

86, 90 and the suction pipe 94.

At that stage power is supplied to the wheels 80 and the vehicle 50 moves away sideways from the pontoon onto the flat undersurface 40 of the raft 20 as shown in FIG. S'and starts digging whilst moving beneath the raft 20 around the pontoon 52 in bigger and bigger circles until ultimately the raft is resting on four spaced-apart triangular corner areas 101, as shown in FIGS. 8 and 9.

The digging is effected by breaking up soft soil with the high pressure water jet 92, and harder material by means of the tool 88. The broken-up material is removed via the suction pipe 94. Boulders may be broken up by means of a heavy drop-chisel (not shown in the drawings, but conventional in itself). Limited areas of rock may be dealt with by means of a rock-breaker (not shown) projecting from the side of the vehicle 50.

The most effective method of digging is to apply downward forces to the material to be removed. Then the undersurface 40 of the raft 20 provides a firm support for the vehicle 50 enabling the reaction force on the vehicle 50 to be absorbed without upward movement of the vehicle, which would reduce the effectiveness of the digging operation.

If the vehicle has to excavate against a vertical face, the lower part of the face is attacked with downward forces to produce local slips. The loose material falls beneath the vehicle and is removed via the suction pipe. There is no danger to the vehicle 50 because the slip tends to push it away from the face.

During the excavating process boulders that are too large to be removed by the suction pipe may collect on the floor of the excavation. These may have to be broken up by the drop-chisel if they impede movement of the vehicle.

As the excavation proceeds, the foundation raft 20 will sink, and the extent of the sinking is constantly checked by monitoring the clearance between the vehicle 50 and the floor of the excavation. A similar check is effected from the chamber 54 of the pontoon 52.

to pass via the

hatches

76, 77 from the chamber 54 to l the chamber 72. The water pressure in the airlock 75 is then re-established The buoyancy of the pontoon 52 is adjusted from the pontoon pressure chamber 54 by letting some air out of the buoyancy chamber 58 which automatically lets water in and at some point the pontoon 52 and the ve- A preferable method of combined excavation and sinking is for the vehicle to retire periodically to the shaft 42, and for the weight of the raft 20 to be increased by admitting more water to the chambers 38. The raft then sinks whilst the vehicle 50 is safely in the shaft 42, the pontoon 52 having raised itself to the FIG. 4 position so that the vehicle can be accommodated.

As the digging proceeds, there will ultimately be a stage in which the raft 20 is level and has sunk so deep that the excavated space is sealed from the external water around the perimeter of the raft 20. The vehicle 50 then moves onto the undersurface of the pontoon 52 and the pontoon and vehicle together move into their FIG. 4 position.

The chambers 38 of the foundation raft 20 are then completely flooded to increase the penetration into the sea-bed. The seal is inflated and the

suction pipe

94, 96 is used to pump water out of the excavated space 102 (FIG. 10), more quickly then it can permeate into the space 102 through the surrounding strata. The pressure in the space 102 is thus reduced and the downward force exerted by the raft 20 on the comer supports 101 is greatly increased, (perhaps by as much as four times) and the raft sinks deeper into the sea-bed if the corner supports 101 cannot withstand the increased downward force.

The operation can be repeated if necessary by deflating the seal 60, equalising the pressure inside the space 102 to the external water pressure and using the vehicle 50 to excavate more material from beneath the raft. Some of the water can be pumped out of the chamber 38 to further lighten the load.

Ultimately, the reduced water pressure in the space 102 and the corresponding temporary increase of the load on the supports 101 does not produce any further settlement of the structure, and it then follows that the four corner supports 101 and the friction on the side walls of the raft 20 can carry the structure with an ample factor of safety, because the temporarily increased load on the supports greatly exceeds the weight of the structure.

Then there is no need to dig the raft 20 in any deeper and the vehicle 50 can now finalise the operation by backfilling the space 102 underneath the raft with sand and gravel 104 (FIG. 11) which further increases the safety factor of the foundation. The sand and gravel are obtained from elsewhere on the sea-bed and are deliv ered via the

suction pipe

94, 96. The extremities of the space 102 are filled by directing the sand and gravel by means of the high pressure jet 92. If necessary the vehicle 50 may move out onto the undersurface of the raft 20 to expedite the backfilling operation.

When the backfilling operation is complete and the shaft 42 has been filled as much as possible, (FIG. 12), the pins 68 of the pontoon are withdrawn from their slots 70 in the shaft wall. The buoyancy of the pontoon 52 and the vehicle 50 is adjusted to be slightly positive, and the pontoon and vehicle rise to the surface, their work completed. Alternatively, the buoyancy can be adjusted to be only slightly negative, the vehicle and the pontoon then being hauled to the surface by a winch on the superstructure 24.

The vehicle 50 can be made remotely controllable from the temporary superstructure 24, thus making it unnecessary for the vehicle to be crewed. When remotely controlled, the vehicle is provided with closedcircuit television cameras and sufficient external lighting for an operator on the superstructure 24 to control the digging operation. The equipment for controlling the movements of the vehicle and the operation of its

various excavating equipments

86, 90, 94 may be designed for each particular vehicle according to conventional hydraulics practice.

Since the vehicle is not crewed, the pressure chamber 72 need not be provided. The pressure chamber 54 in the pontoon 52, the diving bell 62 and the associated air-locks 75, 66 also can be dispensed with. The pontoon may merely be a concrete slab, effectively a moveable portion of the bottom of the foundation raft 20. In such a case, the inflatable seal 60 may be carried in a frame attached to the concrete slab.

By remotely controlling the vehicle, it becomes necessary for men to work under water only in an emergency or if the excavating

equipment

86, 90, 94 needs servicing.

A remotely controlled vehicle is shown at in FIG. 13. Parts shown in this figure and already described with reference to other figures carry the same reference numerals as in those figures.

The vehicle 150 operates in conjunction with a flatbottomed concrete slab 152 which serves as a pontoon. The slab 152 may be raised and lowered by cables 156. Downward movement of the slab 152 is limited by projection 157 arranged so that when the slab is in its lowest position its undersurface is flush with the undersurface 40 of the raft 20.

Compressed air is supplied via a line 158 to the interior of the vehicle 150. The air pressure is controlled by a float valve 159 operating in a stand-pipe 160 open to external water pressure so that the pressures inside and outside the vehicle 50 are roughly equal.

The vehicle 50 has a television camera 162. In order that the operator on the super-structure 24 may know the position of the vehicle, a gyrocompass 164 indicates the angular orientation or heading of the vehicle, and a distance meter 166 indicates the radial distance of the vehicle from the centre of the pontoon slab 152.

The distance meter consists of a wire 168 attached to the centre of the pontoon slab 152 at 170 and extending through a guide fuse 172, over a tensioning roller 174 to a drum 176. The guide tube is pivotally mounted about a vertical axis in the roof of the vehicle 50. The length of cable unwound from the drum 176 indicates the distance of the vehicle from the point 170, and the angular position of the guide tube 172 indicates the bearing of the point 170 relative to the vehicle 50. These two variables, together with the gyrocompass output, completely define the position and heading of the vehicle 50.

Similar position and heading finding equipment may be provided on the crewed vehicle 50 if required.

A telescopic hydraulic jack 178 is provided for sampling levelling or plate-testing the sea-bed.

It will be appreciated that with both the crewed and remotely controlled version of the vehicle, it is possible to establish the exact position and orientation of the vehicle, and to view the progress of the digging operation beneath the raft20. i

When the foundation raft 20 has been dug into the sea-bed to the required level and the chamber 38 fully flooded, the structure should be safe against any likely weather conditions. When the temporary superstructure 24 is removed the only exposed structure are the tops of the columns 32 with the lifting tackle 44 thereon; all the cross-bracing is now below the surface because the foundation raft 20 has sunk into the sea bed.

The permanent super-structure 26 leaves the fabrication yard as a completely self-contained buoyant seaworthy unit with all the required equipment and facilities installed'and in working order.

It is towed to the site and when the tide and weather conditions are suitable it is moved into position between the columns 32, as shown in FIG. 14.

Heavy winches 108 on the super-structure 26 are utilised with the light lifting tackle 44 to hoist heavy lifting tackle to the tops of the columns 32, the heavy tackle being shown installed at 110 in FIG. 14.

Heavy chains and cables 112 are passed around the lifting tackle 110 to the winches 108, and by means of these the super-structure 26 lifts itself from the water. When it reaches the required level, permanent mountings for the super-structure 26 are substituted for the lifting tackle 110. These permanent mountings consist essentially of brackets 114 (FIG. which contact the tops of the columns 32 via rollers 116, so that bending moments due to wave action are not transmitted from the columns to the superstructure.

It will be appreciated that the full carrying capacity of the supporting columns is established before any permanent super-structure load is applied, and the lifting starts immediately when load is applied to the columns.

The lifting operation is one continuous hoisting operation and not a discontinuous climbing up the supporting columns, as witha jack-up platform.

The lifting operation is completely independent of any wave or swell action as soon as the superstructure is clear of the water, and neither during the lifting nor in its final position will the superstructure be subject to additional bending stresses arising from wave action on the substructure.

I claim:

1. A sinkable structure adapted to be founded on a subaqueous bed, and comprising an undersurface adapted to contact the subaqueous bed, in combination with a vehicle forexcavating under the structure being founded on the subaqueous bed, the vehicle comprising means for providing positive buoyancy, means enabling the vehicle to contact and move about on the undersurface of the structure, and means for supporting excavating equipment; and an apparatus for moving the vehicle into a position in which it can move into contact with the undersurface, and excavate the subaqueous bed beneath the structure.

2. A structure as claimed in claim 1 wherein the apparatus comprises a variable-buoyancy pontoon.

3. A structure as claimed in claim 1 wherein the apparatus is a moveable portion of that part of the structure which defines the undersurface. Y

4. A structure as claimed in claim 1 wherein the structure has a shaft extending upwards from the undersurface and adapted to have the vehicle and the apparatus moveably disposed therein.

5. A structure as claimed in claim 4 provided with means for sealing the shaft so that water may be pumped out of the excavated bed beneath the undersurface.

6. A method of founding a structure having an undersurface in a subaqueous bed, comprising the steps of disposing the structure on the bed moving a positively buoyant vehicle provided with excavating equipment into contact upon the undersurface of the structure, moving said vehicle about on the undersurface of the structure, and excavating the subaqueous bed beneath the structure with said excavating equipment.

7. A method as claimed in claim 6, comprising excavating under the structure so that it is supported on a number of spaced apart nonexcavated regions and then increasing the effective load on the said regions so that the structure sinks into the bed.

8. A method as claimed in claim 7, wherein the vehicle retires to a recess in the structure extending upwardly from the undersurface whilst the structure is sinking.

9. A method as claimed in claim 7 wherein the effective load is increased by sealing the excavated space beneath the undersurface and by pumping out water therefrom.

10. A method as claimed in claim 6 wherein the excavated space is filled with solid material after the structure has been sunk into the bed to a desired extent.

Claims

1. A sinkable structure adapted to be founded on a subaqueous bed, and comprising an undersurface adapted to contact the subaqueous bed, in combination with a vehicle for excavating under the structure being founded on the subaqueous bed, the vehicle comprising means for providing positive buoyancy, means enabling the vehicle to contact and move about on the undersurface of the structure, and means for supporting excavating equipment; and an apparatus for moving the vehicle into a position in which it can move into contact with the undersurface, and excavate the subaqueous bed beneath the structure.

3. A structure as claimed in claim 1 wherein the apparatus is a moveable portion of that part of the structure which defines the undersurface.