US3896628A

US3896628A - Marine structures

Info

Publication number: US3896628A
Application number: US420294A
Authority: US
Inventors: Frode Johan Hansen
Original assignee: Redpath Dorman Long North Sea Ltd
Current assignee: Redpath Dorman Long North Sea Ltd
Priority date: 1972-12-01
Filing date: 1973-11-29
Publication date: 1975-07-29
Anticipated expiration: 1992-07-29
Also published as: IE39145B1; IE39145L; AU6307873A; JPS50158101A; CA1000958A; DE2359540A1; NL7316488A; FR2209376A5

Abstract

A maritime assembly having its foundation raft and tower structure constructed separately, and then joined by the engagement of spigot and socket portions. By this reduction of pressure between those portions, hydrostatic force can be used to secure the tower structure to the foundation raft. Features include the use of a semi submersible to lower the foundation raft to the seabed, and the use of sand jacks to found the raft thereon.

Description

United States Patent Hansen July 29, 1975 [541 MARINE STRUCTURES 3,528,254 9/1970 Graham 61/465 3,698,198 10/1972 Phelps 61/465 Inventor: Fmde 1011a" Hansen KmgSWOOd 3,736,756 6/1973 LlOyd 61 /46.5

England [73] Assignee: Redpath Dorman Long (North Sea) FOREIGN PATENTS APPLICATIONS Limmd, Bedford England 1,088.804 10/1967 Un ted K ngdom 61/465 551,449 2/1943 Umted Kmgdom 61/465 [22] Filed: Nov. 29, 1973 [21] Appl. No.: 420,294 Primary Examiner-Jacob Shapiro Attorney, Agent, or FirmBacon & Thomas [30] Foreign Application Priority Data Dec. 1, 1972 United Kingdom 55533/72 ABSTRACT July unltcd Kingdom 34679/73 A maritime assembly having its foundation raft and July unfted 34681/73 tower structure constructed separately, and then July 20, 1973 Umted Kmgdom 34682/73 joined by the engagement of Spigot and socket pop tions. By this reduction of pressure between those por- U-S- tions, hydrostatic force can be used to secure the [51] '3 E02B 17/00 E021) 23/16 tower structure to the foundation raft. Features in- [58] new Search 53'74 clude theuse of a semi submersible to lower the foundation raft to the seabed, and/the use of sand jacks to [56] References C'ted found the raft thereon.

UNITED STATES PATENTS 3.522.709 8/1970 Vilain 61/465 17 27 D'awmg guns SHEET PATENTEDJULZSISYS' F/ai PATENTEU JUL 2 9 I975 SHEET F/G. 770. A n

PATENTEI] JUL2 91975 SHEET Q GI MARINE STRUCTURES The invention relates to the construction and founding of maritime assemblies.

It is known to construct platforms founded in water. Such platforms have a foundation in a sub-aqueous bed, a tower structure extending upwardly therefrom, and a working deck and/or superstructure supported on top of the tower structure above the water surface.

The foundation may be formed by driving piles into the sub-aqueous bed. This method requires considerable time and effort on site. and may have to be interrupted in stormy weather.

To avoid the need for pile driving it has been proposed to found platforms deriving stability partly from their own weight. These last mentioned platforms may be termed gravity structures. One difficulty to be expected with gravity structures" is that when completed they are unwieldy to tow to their intended sites, and are relatively vulnerable to adverse weather.

The present invention is concerned with a maritime assembly constructed by first sinking a foundation raft to the sub-aqueous bed, and then fixing an upstanding tower structure thereto. The foundation raft and tower structure can be towed seperately to the site, and founded consecutively during relatively short periods of fair weather. Subsequently the deck or super structure can be added above water level to complete the platform.

The term maritime assembly as used herein means an assembly standing or intended to stand in a sea, lake, estuary or other expanse of water, without being structurally connected to land other than the sub-aqueous bed beneath the expanse of water; and includes the combination of a foundation raft and a tower structure.

The invention provides a maritime assembly for disposal on a subaqueous bed, comprising a foundation raft to be founded on the bed. and a tower structure to project upwardly therefrom when founded, in which one of the foundation raft and the tower structure has a socket portion and the other has a corresponding spigot portion for engagement therewith.

It is preferred that there is means to evacuate a space between the spigot and socket portions, whereby external hydrostatic pressure will keep the portions in engagement, so fixing the tower structure in position on the foundation raft.

It is preferred that a seal is carried on a shoulder surrounding the spigot to engage material surrounding the rim of the socket.

Preferably the space can be de-watered, and there is means to grout the space.

Preferably a hollow tubular member is incorporated in the tower structure. which member communicates with a valve leading to the space.

In one preferred form the tower structure carries the spigot portion. and the socket is incorporated in the foundation raft.

According to a preferred feature of the invention the foundation raft and the tower structure have shafts which are aligned when the spigot and socket are in engagement. whereby to provide a continuous shaft extending from near the water surface to the subaqueous bed beneath the foundation raft.

According to another preferred feature of the invention the tower structure has internal voids to render the tower structure buoyant for towing to a position above the founded foundation raft, and which may be selectively flooded. whereby the tower structure may be ballasted to sink with its axis substantially vertical.

According to yet another preferred feature of the invention there is a semi-submersible capable of being disposed upon the foundation raft, the foundation raft is capable of being deliberately flooded at will, and the semi-submersible has means for lowering the partially flooded foundation raft to the seabed.

Preferably the semi-submersible has buoyant legs extending from an upper level to a lower level, whereby the semi-submersible floats with a small waterplane area during the lowering of the foundation raft.

Advantageously there are on the base of the foundation raft three units, each having two vertically spaced chambers, and there are means to cause sand or the like to pass out of the lower chambers, whereby the raft can be lowered towards the seabed by displacement of the sand or the like.

It is preferred that each unit has a plate arranged at the foot of the lower chamber to bear on the seabed, which plate is connected to the unit by cables, and is slideable within the lower chamber.

It is also preferred that the units themselves are moveable as pistons within cylinders attached to or forming part of the base of the raft, and the height of the pistons can be adjusted by movement of sand from the aforesaid cylinders.

Preferably the pistons are suspended within the cylinders to prevent them from dropping out.

The invention also provides a method of founding a maritime assembly, which includes the steps of founding a foundation raft on a subaqueous bed, and sinking a tower structure onto the foundation raft, in which spigot and socket portions on the foundation raft and the tower structure are engaged to secure the tower structure to the raft.

It is preferred that the tower structure is held on the foundation raft by means of a sealed de-watered space between the foundation raft and the tower structure.

It is preferred that the tower structure is guided towards the foundation raft by means of cables which pass from the foundation raft through sheaves on the tower structure.

It is preferred that a semi-submersible is placed on the foundation raft, the foundation raft and semisubmersible are floated to a site, the foundation raft is at least partially filled with water, the foundation raft is lowered to the seabed by means of supporting elements suspended from the semi-submersible, the foundation raft is secured on the seabed. and the semisubmersible is removed.

It is preferred that three units are located on the base of the foundation raft. each of which units have two vertically spaced chambers, the lower of which is separated from the seabed; including the step of moving sand or the like from the lower chambers, whereby to lower the foundation raft towards the seabed.

Preferably the units themselves are moved vertically downwards from the foundation raft within cylinders attached to or forming part of the raft. prior to the founding of the raft.

Preferably further movement of the units within the cylinders is effected to level the foundation raft after the steps specified.

Two specific embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGS. 1 to 5 show successive stages in the founding and assembly of a first marine platform FIG. 6 is a view of a foundation raft (for a different maritime platform) in a drydock,

FIG. 7 shows the foundation raft afloat. with a semisubmersible supported thereon,

FIG. 8a shows the foundation submersible at sea.

FIG. 8b illustrates the floating stability of the combination.

FIG. 9 shows the foundation raft being lowered to the sea-bed,

FIGS. 10a and 1017 show the founding of the foundation raft on boulder clay and on a soft sea-bed respectively,

FIGS. 11a and 11 show the foundation raft back filled in position,

FIG. vl2 illustrates the foundation raft waiting for a tower structure.

FIGS. l3, l4 and 15 illustrate stages in the construction of the tower structure,

FIG. 16 shows the tower structure supported on pontoons,

FIG. 17 shows the tower structure floating on its own with an aid of a floation raft,

FIG. 18 illustrates the alignment of the tower structure on the foundation raft,

FIG. 19 shows the tower structure being winched into position on the foundation raft, and

FIG. 20 illustrates the completed gravity maritime platform,

FIG. 21 shows a central section of the marine platform nine legs, and

FIGS. 22, 23, and 24 show successive stages in the lowering of the foundation raft.

In the manufacture of maritime platforms. for example off-shore platforms for use in the production of hydrocarbons, it is desirable that as little as possible of the work be done at sea. This is particularly so when there is a considerable depth of water at the site. The illustrated embodiments of the present invention have been designed with this desideratum in mind.

Referring first to FIG. 5 a platform 10 (in this example an off-shore oil drilling platform) is shown founded on a sub-aqueous bed 12. The platform comprises a foundation raft or caisson 14, a tower structure I6 and a superstructure 32.

The caisson I4 is of reinforced concrete construction and has an upwardly open conical recess 18 (best seen in FIG. I) and a shaft 20 coaxial with the recess. The caisson is hollow and is subdivided into a number of selectively floodable watertight compartments (not shown).

The tower structure 16 may be a steel fabrication or a composite steel and concrete construction comprising a lower cylindrical section 22. in this instance circular in cross-section and having a lower part 24 (FIG. 4) which is tapered to be a close fit in the upwardly open recess 18 of the caisson I4. A projecting peg 25 on the tower structure engages a slot 26 in the side of the recess 18. to ensure correct angular orientation of the tower structure relative to the recess.

The main body of the tower structure I6 is subdivided internally into a number of water-tight compartraft and semiments 27 which can be flooded selectively. These are employed in the operation of sinking the tower structure.

The upper end 28 of the tower structure is of wavepenetrable construction. That is to say. it presents to the waves a section consisting largely of apertures so that the waves can pass around and through the structure, rather than impinge heavily upon it. The stresses in the structure thereby are reduced. In this example, the upper part 28 of the tower structure is of lattice construction.

A water-tight central shaft or tube 29 extends throughout the length of the tower structure. along the longitudinal axis thereof. This shaft communicates with the shaft 20 in the caisson I4, thus providing a watertight shaft giving direct access to the sub-aqueous bed 12 from above water level.

A superstructure 32 is disposed on top of the wave penetrable and 28 above wave level. The superstructure houses the necessary equipment and services for the drilling operation.

The method by which the platform is assembled on the sub-aqueous bed will now be described.

Firstly. with reference to FIG. I, the caisson 14, with its internal compartments empty (in which condition it is buoyant) is towed to the site. carrying on its upper surface a semi-submersible 34 which is equipped with power winches. Cables 36 from the winches are attached to the top of the caisson I4.

On arrival at the site. the internal compartments of the foundation raft are flooded under control so that it sinks in a horizontal attitude. Semi-submersible 34 floats as the caisson l4 sinks, and controls the sinking of the caisson by means of the cables 36.

When the caisson has reached the sub-aqueous bed it is founded in the bed (FIG. 2). The founding operation may be effected conventionally. or by means of the method described and claimed in our copending British Patent application 4l9l/7I. to which the reader is referred for further details. Briefly this method consists of employing a positively buoyant remotely controlled excavating vehicle serviced from the semi-submersible 34 and which passes down the shaft 20 of the caisson l4 and then excavates beneath it whilst moving on the undersurface 38. This undersurface is sufficiently flat for the vehicle to move about on wheels or tracks on its roof. The excavated material is removed from beneath the caisson 14 by suction pipes. As the material is excavated and removed, leaving a void 40, the caisson 14 slowly sinks until it is founded sufficiently deeply in the sub-aqueous bed. The void 40 can then be back-filled, and the vehicle is finally recovered via the shaft. The cables 36 are then suitably buoyed for future use in guiding the tower section 22 into the recess 18 of the caisson.

The section 22 is towed to the site with its compartments 27 empty so that it is buoyant and stable in a horizontal attitude. On arrival at the site. the compartments 27 of the tower structure are successively flooded. one after the other. starting with the compartment in the tapered portion 24, until the tower structure adopts a vertical attitude. Winches 41 are installed on the top of the section 22. The cables 36 already attached to the caisson I4 are passed through sheaves 42 on the lower part of the tower structure and thence to the winches 4!. Further controlled flooding of the tower structure now is effected, so that it slowly sinks towards the caisson. The guide cables 36 are winchedin by the winches 4] so to position correctly the tapered portion 24 of the tower structure over the recess 18. Sinking of the tower structure is continued and finally it comes to rest firmly engaged in the recess 18. FIG. 5. The correct angular orientation of the tower portion is achieved initially by means of the guide cables 36, and finally by the peg and

recess

25 and 26.

As an alternative to the winches 41. the cables 36 may be passed through the sheaves 42 to winches on tugs (not shown).

The rigidity of the connection between the tapered portion 24 of the tower structure and the recess 18 is improved if necessary by pressure grouting to fill any spaces between the two parts.

The

central shaft

20, 29 is at least partially dewatered, so that the water pressure beneath the caisson 14 is reduced. Then the external water pressure further improves the stability of the structure, and it may be utilised to reduce future settlement by preconsolidating the sub-aqueous bed beneath the structure before it has been commissioned for its final working purpose.

Finally, the superstructure 32 is towed to the site and winched onto the top of the assemblycomprising the tower structure and the foundation raft, for example in the manner described in our co-pending British patent application No. 5609/72, to which the reader is referred for further details.

The remaining figures and description relate to the founding ofa second, and slightly more elaborate maritime platform, which is founded on the same principals which have been described above.

FIG. 6 illustrates a submersible concrete foundation raft 101 being constructed in a drydock. If required a soft skirt 103 or a more rigid but downwardly hinged skirt can be clamped round the bottom edge of the foundation raft before it leaves the drydock. FIG. 7 shows the foundation raft afloat, with a draft of about 8 metres. The bottom portion of the raft is advantageously launched from the drydock with its bottom slab and 40% of the walls completed, such that it only draws half of this amount. The foundation raft is then completed while it is afloat. Temporary support columns 102 may be integral with, or fixed to the under-surface of the bottom portion when it is afloat in sufficiently deep water. Should it be envisaged that the foundation raft will be founded on boulder clay or rock the soft skirt 103 may be weighted down with sand or concrete.

A purpose made semi-submersible 104 can be constructed on top of the foundation raft. This semi submersible has a flat deck and a plurality of hollow pillars 110 leading down to its base.

FIGS. 8a and 8h illustrate the stability of the foundation raft and semi-submersible when towed in a rough sea. Provided the semi-submersible is securely lashed to the foundation raft, the combination should be stable and structurally sound in any wave conditions.

FIG. 9 illustrates the founding of the foundation raft on the sea-bed. A compresser 105 and a pump 106 (exemplary of many such compressors and pumps) are supplied on the deck of the semi-submersible to adjust the buoyancy and the internal air pressure of the foundation raft during the sinking operation. The compressor and pump are connected to the foundation raft by tubes 105a and 106a respectively. Power winches 107 are also provided to give fine adjustment to the lower ing procedure, and are connected to the foundation.

raft by lines designated 107a. The semi-submersible I04 is designed to have a small water plane area when suitably ballasted so that forces in the lowering equipment are comparatively indifferent to wave movements.

FIGS. 10a and 10!) show the foundation raft on the sea-bed. In the case of FIG. ]0a the bed is of extremely hard boulder clay with big boulders and there is no scour or settlement risk, and accordingly the skirts I03 are filled with sand or concrete to protect the backfilling underneath the raft. Three temporary support columns 102 can be used to set up the foundation raft in a level condition when operated as sand jacks. As shown in FIG. lla sand or concrete is then admitted to the space beneath the foundation raft down a central tube to back till the gap between the sea bed and the foundation raft, and thereafter the foundation raft itself can be partly ballasted with sand, for instance in case the raft is later to be used for oil storage. Displaced seawater escapes through peripheral venting holes. Optionally a weak sand-cement mixture can be used for grouting purposes.

In FIG. 1017 the foundation raft rests on a soft sea bed, and material from the sea bed is withdrawn from beneath the foundation raft by any convenient means. Thus the foundation raft is allowed to settle into a trough in the sea bed formed thereunder. As shown in FIG. ill) the space beneath the foundation raft can be back filled with sand; or raft may be lowered right down to the excavated level without requiring back filling. Thereafter the foundation raft can be fully flooded.

FIG. 12 illustrates the foundation raft in a founded condition with two floating buoys 109 indicating its position beneath the surface of the sea.

The construction of the tower structure for the platform is shown in FIGS. 13 to 15. Four units 111 (each unit shown being three deep) are erected vertically and supported by guide ropes 112, to form horizontal tubular members in the completed tower structure. Four units 113 (each unit shown being three deep) are welded into position between the units 111, to form columns (131) in the completed tower structure, and a deck 114 is constructed parallel with the units 111. In FIG. 15 the steelwork is completed by fitting diagonals I15 and stressing these to make the tower structure rigid.

In FIG. 16 a completed tower structure, possibly weighing 10,000 tonnes, is launched on two

pontoons

116 and 117, and this tower structure can be towed to the site. The pontoon ll6 supports the tower structure through blocks 119, and between the pontoon 117 and the tower structure there is a floatation raft 121.

As shown in FIG. 17 the pontoons can be sunk from beneath the tower structure, whereon the tower structure will float under its own buoyancy.

As shown in FIG. 18 firstly the floatation raft 121 has been allowed to lift the upper end of the tower structure. and secondly power winches 122 on the deck 114 can be attached to the lines 123 which lead up to the buoys 109 in FIG. 12.

In FIG. 19 the tower structure is shown just above the foundation raft 101 and FIG. 20 shows an assembly comprising the tower structure with its feet securely anchored to sockets in the foundation raft. At this stage the super-structure (not shown) for the platform can be lifted into position by a crane 125 located on the deck I14.

It will be appreciated that the construction of the foundation raft 101 and the tower structure 118 as separate units has fabrication advantages. Further the founding of the foundation raft at an early stage in the construction period permits the tower structure to be adjusted in case the local sea bed conditions have caused a variation in foundation level.

The tower structure has eight external legs 131 and a central leg 132. The legs of the tower structure are connected to the foundation raft by means ofjoints 135 hereinafter described. The distance between the sea bed and the mean sea level may typically be 150 metres and to enable the pressure beneath the foundation raft to respond to the differential pressures created by the passage of waves passing the deck 114 the central leg 132 is hollow, and has a water duct passing up there through. The water duct is open to the under sea strata beneath the foundation raft. and is connected to

flap valves

136 and 137 mounted on the central leg 132 beneath lowest wave trough level. The valves are so arranged that water can escape from the central column whenever the trough of a wave passes the flaps. Typically the

flap valves

136 and 137 are metres below the surface of the sea. In this way any build up of pressure beneath the foundation raft may be reduced.

As shown in FIG. the legs are braced together by bracing members 138 and to give stability and additional strength to the platform as a whole it may be convenient for parts of the legs 131 to be filled with con crete.

As may be seen from FIG. 21, the base of the illustrated leg 131 has a spigot 141 and a foot plate 147 which is supported by

brackets

142 and 143. A cross member 144 and diagonal bracing member 138 connect this foot to the other feet.

The foundation raft 101 has a socket 145 and there is an annular space 146 between the socket 145 and the spigot 141. A rubber seal 148 fits between the edge of the plate 147 (forming a shoulder) and the part of the raft 101 surrounding the socket 145.

It will be apprecaited that each of the legs may have a spigot arrangement. and that the foundation raft has a socket for each spigot. Alternatively the spigots may be formed on the raft and the sockets may extend upwardly into the legs.

In use the spigot 141 beneath the tower structure is lowered into the socket 145 as the tower structure is sunk onto the foundation raft. When the tower structure feet are firmly founded in the foundation raft, the rubber seal 148 backed by the edge shoulder is gripped between the plate 147, and the top slab of the raft. and effectively seals the space underneath the foot plate from external water pressure. The annulus can be pumped out. and air at atmospheric pressure can be admitted to the space through a drain valve 149. In this way the pressure beneath the plate 147 is greatly reduced as compared to the sea water pressure on the upper surface of that plate. and the tower structure may be held in contact with the foundation raft 101 by the full external hydrostatic pressure on the foot plate 147.

Subsequently the space between the spigot and the socket can be grouted from within the spigot. providing an additional tension connection between tower and raft.

If at some future date it is desirable to dismantle the platform again it would be natural to bolt the tower to the raft by means of bolts 150 after the seal 148 had been engaged and the socket dewatered. To dismantle the platforms the bolts can be released and the legs released from the spigots which remain fixed to this foundation raft.

Details of the founding of this foundation raft as shown in FIGS. 10a and 110 will now be described in more detail.

As shown in FIG. 22. the foundation raft 101 is floating in water. The foundation raft has generally horizontal upper and lower slabs 212 and 212A respectively. and generally vertical webs 214 between the slabs.

Near the periphery of the raft there are three cylinders 215 each containing a piston 216. For convenience only one such piston cylinder arrangement is shown. The piston 216 is hollow and has vertically spaced upper and

lower chambers

217 and 218 respectively. At the base of the chamber 218 there is a generally horizontal plate 219 supported on cables 221. The plate 219 closes the base of, and is movable vertically within. the lower chamber 218.

As shown in FIG. 23, the piston 216. which was earlier completely retracted into the recess 215. has now been lowered downwardly and is supported by cables 222. This is the situation when the foundation raft has reached its intended site.

A mixture of sand and water is pumped into the space above the piston 216 and also into the lower chamber 218. In this configuration the foundation raft is lowered towards the sea bed.

Initially the piston 216 penetrates a distance a' into the strata of the sea bed. This penetration can be effected by suitable ballasting of the foundation raft or by high pressure water jetting underneath the lower plate 219.

If further penetration into the sea bed is required this can be achieved by shifting sand from chamber 218 to chamber 217 by means of high pressure water jets. The walls of the piston will penetrate the distance h into the sea bed in accordance to the amount of sand shifted. leaving plate 219 at its original level.

By excavation under the foundation raft as a whole. and shifting sand as required from chambers 218 to 217 (arrow X) it is possible to lower the raft to a level where plate 219 is resting against the dividing floor 220 and all the sand has been removed from chamber 218.

The lowering process can thereafter be continued by jetting sand out of the space (225) above the piston. When the space 225 has been reduced to the same depth as the working space underneath the raft the ex cavation may be stopped. If thereafter further sand is jetted from the space 225 (arrow Y) the raft will settle down to a lower level.

We claim:

1. A maritime assembly for disposal on a sub-aqueous bed. comprising a foundation raft to be founded on the bed. and a tower structure to project upwardly therefrom when founded. in which one of the foundation raft and the tower structure has a socket portion and the other has a corresponding spigot portion for engagememt therewith, and there are on the base of the foundation raft three units. each having two vertically spaced chambers. the lower of which chambers contains particulate material and there are means to remove particulate material from the lower chambers. whereby the raft can be lowered towards the seabed by displacement of the particulate material.

2. An assembly as claimed in claim 1, in which each unit has a plate arranged at the foot of the lower chamber to bear on the seabed, which plate is connected to the unit by cables, and is slideable within the lower chamber.

3. An assembly as claimed in claim 2, in which the units themselves are moveable as pistons within cylinders attached to or forming part of the base of the raft. and the height of the pistons can be adjusted by movement of particulate material from the aforesaid cylinders.

4. An assembly as claimed in claim 3 wherein the pistons are suspended within the cylinders to prevent them from dropping out,

5. A method of founding a maritime assembly, which includes the steps of founding a foundation raft on a sub-aqueous bed, and sinking a tower structure onto the foundation raft, in which spigot and socket portions on the foundation raft and the tower structure are engaged to secure the tower structure to the raft, and in which three units are located on the base of the foundation raft, each of which units have two vertically spaced chambers, the lower of which chambers contains particulate material and is separated from the seabed; including the step of removing particulate material from the lower chambers, whereby to lower the foundation raft towards the seabed.

6. A method as claimed in claim 5, in which the units themselves are moved vertically downwards from the foundation raft within cylinders attached to or forming part of the raft prior to the founding of the raft.

7. A method as claimed in claim 5, in which further movement of the units within the cylinders is effected to level the foundation raft after the steps specified.

8. A method of founding a maritime assembly on a seabed, comprising the steps of locating a unit on the base of the assembly which unit has downwardly extending sidewalls forming an open ended downwardly facing chamber having sand therein which chamber is separated from the seabed by movable closure means arranged across the open end of the chamber and movable thereinto to reduce the capacity of the chamber, sinking the assembly onto the seabed torest on the unit, and removing sand from the chamber to reduce its capacity and to lower the assembly toward the seabed as sand supporting the assembly is removed from the chamber thereby increasing penetration of the sidewalls of the unit into the seabed.

9. A unit for attachment to the base of a foundation raft to be founded on a subaqueous bed, and having downwardly extending sidewalls forming an open ended downwardly facing chamber adapted to contain particulate material, movable closure means therefor disposed across the open end of the chamber constructed and arranged to be movable thereinto to reduce the capacity of the chamber and to prevent particulate material contained therein from falling out therefrom, means movably supporting said movable closure means, and water jetting means to remove particulate material therefrom.

10. A unit as claimed in claim 9 containing particulate material.

ll. A unit as claimed in claim 9 in which the chamber has a uniform horizontal cross section, and the closure means is a horizontal plate retained within the chamber and arranged to move up or down therein.

12. A unit as claimed in claim 9 in which the aforesaid chamber is a lower chamber, and there is an upper chamber arranged to receive particulate material removed from the lower chamber.

13. The unit as claimed in claim 9 in which the chamber is itself movable as a piston within a downwardly facing cylinder for attachment to the base of the raft. and the height of the piston can be adjusted upwardly by removal of particulate material from the cylinder.

14. The unit as claimed in claim 13 including means for suspending the piston within the cylinder to prevent it from dropping out.

15. A foundation raft for a maritime structure having at least three units of the kind set forth in claim 10 attached to its base, whereby the raft can be lowered towards the sea bed. by the displacement of particulate material chambers.

16. A method of founding a foundation raft for a maritime structure in which at least three units of the kind set forth in claim 10 are located on the base of the foundation raft, and including the step of removing particulate material from the chambers to reduce their capacity and to lower the foundation raft towards the seabed as particulate material supporting the foundation raft is removed from the chambers, thereby increasing penetration of the sidewalls of the units into the subaqueous bed.

17. The method as claimed in claim 16 including the step of controlling removal of particulate material from said units to level the foundation raft after its initial founding.

Claims

1. A maritime assembly for disposal on a sub-aqueous bed, comprising a foundation raft to be founded on the bed, and a tower structure to project upwardly therefrom when founded, in which one of the foundation raft and the tower structure has a socket portion and the other has a corresponding spigot portion for engagememt therewith, and there are on the base of the foundation raft three units, each having two vertically spaced chambers, the lower of which chambers contains particulate material and there are means to remove particulate material from the lower chambers, whereby the raft can be lowered towards the seabed by displacement of the particulate material.

3. An assembly as claimed in claim 2, in which the units themselves are moveable as pistons within cylinders attached to or forming part of the base of the raft, and the height of the pistons can be adjusted by movement of particulate material from the aforesaid cylinders.

4. An assembly as claimed in claim 3 wherein the pistons are suspended within the cylinders to prevent them from dropping out.

8. A method of founding a maritime assembly on a seabed, comprising the steps of locating a unit on the base of the assembly which unit has downwardly extending sidewalls forming an open ended downwardly facing chamber having sand therein which chamber is separated from the seabed by movable closure means arranged across the open end of the chamber and movable thereinto to reduce the capacity of the chamber, sinking the assembly onto the seabed to rest on the unit, and removing sand from the chamber to reduce its capacity and to lower the assembly toward the seabed as sand supporting the assembly is removed from the chamber thereby increasing penetration of the sidewalls of the unit into the seabed.

10. A unit as claimed in claim 9 containing particulate material.

11. A unit as claimed in claim 9 in which the chamber has a uniform horizontal cross section, and the closure means is a horizontal plate retained within the chamber and arranged to move up or down therein.

13. The unit as claimed in claim 9 in which the chamber is itself movable as a piston within a downwardly facing cylinder for attachment to the base of the raft, and the height of the piston can be adjusted upwardly by removal of particulate material from the cylinder.

15. A foundation raft for a maritime structure having at least three units of the kind set forth in claim 10 attached to its base, whereby the raft can be lowered towards the sea bed, by the displacement of particulate material chambers.