US3528254A

US3528254A - Offshore platform structure and construction method

Info

Publication number: US3528254A
Application number: US805058*A
Authority: US
Inventors: John R Graham
Original assignee: Global Marine Inc
Current assignee: Global Marine Inc
Priority date: 1968-12-03
Filing date: 1968-12-03
Publication date: 1970-09-15
Anticipated expiration: 1987-09-15

Description

Sept. 15, 1970 J R. GRAHAM 3,528,254

OFFSHORE PLATFORM STRUCTURE AND CONSTRUCTION METHOD Filed Dec: 5, 1968 4 Sheets-Sheet 1 Sept. 15, 1970 JR. GRAHAM 3,528,254

OFFSHORE PLATFORM STRUCTURE AND CONSTRUCTION METHOD Filed Dec. 5, 1968 4 Sheets-Sheet 2 l v i V44 /7"- if v WW4 Z7 7U 64 E5 55 r se E5 E2 A, A1 45 lifil a; if; f; 46 F Q 1 INVENTOR. Jay/v Z 524/4444 BY 2% ya 4 5.; if M/ y Sept. 15, 1970- J. R. GRAHAM 3,528,254

OFFSHORE PLATFORM STRUCTURE AND CONSTRUCTIQN METHOD Filed Dec. 5, 1968 4 Sheets-Shet 5 INVENTOR. Lffl/m 6675M,

Sept. 15, 1970 J. R. GRAHAM 3,528,254

OFFSHORE PLATFORM STRUCTURE AND CONSTRUCTION METHOD Filed Dec. 5. 1968 AZZZ 4 Sheets-Sheet 4 INVENTOR. JfiA A/ P. 664M440 United States Patent 01 fice 3,528,254 Patented Sept. 1970 Int. Cl. E02b 17/00 U.S. Cl. 61-465 20 Claims ABSTRACT OF THE DISCLOSURE An offshore drilling structure which includes a drilledin and cemented, wholly submerged base structure defining a plurality of upwardly open receptacles disposed a selected distance below the water surface, and an upper platform which is floated into place over the base struc ture and secured to the receptacles of the base structure. The receptacles and the platform are arranged so that the platform is engageable with the receptacles by ballasting the platform to sink into mating contact with the base structure. The platform unit is removable from the base structure and transferable to another similar base structure at a different location in a different depth of water, for example.

CROSS-REFERENCE TO RELATED APPLICATION The application is a continuation-in-part of application Ser. No. 560,477, filed June 27, 1966, and now abandoned.

FIELD OF THE INVENTION This invention relates to an offshore platform for use in oil prospecting and production, for example, and which has a movable platform component and a fixed submerged foundation. The movable platform component may be of standardized configuration adapting it for use with a number of foundations placed in various depths of water. The invention also relates to a novel method of constructing offshore prospecting platforms and production facilities.

REVIEW OF THE PRIOR ART Mobile offshore drilling structures as presently developed and used are basically either of the submersible type or the jack-up type. Submersible structures are characterized by a rigid structure which mounts a drilling platform; the structure is fabricated as a unit in a shipyard or the like and is towed in a buoyant condition to the desired offshore location where the structure is rendered non-buoyant and sunk into position on the ocean floor. When the structure is in place, the drilling platform is supported a desired distance above the water surface. Any given submersible structure is best used only in a relatively narrow range of water depths. If a submersible structure is used in water of too great a depth, the platform is too close to the water surface and may be damaged unnecessarily by storms; if used in water much shallower than the design depth, the platform is unnecessarily high out of the water.

Jack-up structures have a buoyant platform component, equipped with a complete drilling rig facility, to which are mounted several jackable legs or piles. The structure is towed, with the legs retracted, to the desired offshore location where the legs are extended to engage the ocean bottom. Continued jacking causes the platform to be raised above the water surface. Jack-up structures can be used economically in a wider range of water depths than can submersible structures.

Both submersible and jack-up structures must be towed as a unit from the place of fabrication to the offshore site. As a result, the structure towed to the site should be as lightweight and mobile as possible. In the case of submersibles, topside weight is also limited by stability problems caused by the extreme height of such units when they are floating in towing condition; for optimum stability conditions during transit of the structure to its site of use, the center of gravity of the structure should be as low as possible. Jack-up structures are limited in weight during the critical jacking (up or down) operations because of limited capacity of jacks and uncertainty of sea floor foundation conditions. This means that, desirably, the least possible structure of the drilling rig should be in place on the platform at the time the structure arrives at its intended site. Accordingly, the construction of the drilling rig facility on both submersible and jack-up structures often must be completed at sea; a heavy duty crane barge must be used for this purpose if no heavy duty crane is incorporated into the initially fabricated structure. Also, the less complete the rig facility on the platform, the greater the chance that completion of the rig at sea will be interrupted by adverse weather conditions.

Submersible and jack-up drilling structures are also subject to problems in use. Both of these types of structures are footed on the ocean bottom, which bottom usually is defined by sedimentary deposits. If the structure is installed at a location where ocean or tidal currents are substantial or in water depths sufficiently shallow that storm and wave action (equivalent to current action) is sensible at the ocean bottom, the structure produces eddies in such currents with the result that the ocean bottom is eroded out from under the structure. Several structures of the submersible or jack-up type have been severely damaged or destroyed by tipping over after having been undermined by current and current-like action.

In addition to submersible and jack-up drilling structures, semi-submersible structures are also known. Semisubmersible structures are quite similar to submersible structures except that they are not submerged sufficiently in use to rest on the ocean bottom. Instead they are positively buoyant and are anchored over the location where drilling is to be performed. Semi-submersible structures usually are of great size, and are therefore expensive and cumbersome to move from place to place. Their great size is relied upon to provide a stable floating drilling platform. Because they are not bottom-footed, semi-submersible structures are not as subject to the adverse effects of currents and the like as are submersible and jackup structures, but the anchors of semi-submersible structures are subject to undermining by currents.

Because submersible, semi-submersible, and jack-up structures must be moved as a unit, subsequent workover of a submerged well from such structures is com plicated. Where the structure has been moved from a well site, and it is later desired that further work be carried out at the site, replacement of the structure over the well site must be accomplished with considerably accuracy. Such structures, however, are usually extremely large, and precise placement of such structures is accomplished only with great difliculty.

SUMMARY OF THE INVENTION This invention provides an improved offshore drilling and service structure which has the feature that the most costly portion thereof is readily movable from site to site, as desired. The movable part of the structure mates with a fixed submerged foundation. The foundation at any given site can easily be constructed to present a base which is uniform with the foundation provided at any other site. Accordingly, the movable portion of the structure can be of standardized configuration and still be tion conditions and bottom scouring or eroding by currents are appreciably reduced if not entirely eliminated. The movable portion of the structure is readily engagable with its foundation, and thus placement of the structure for subsequent work-over operations can be accomplished with ease and dispatch. Further, a drilling rig facility on the platform of the movable portion of the structure may be substantially complete when the movable portion leaves its place of fabrication on shore, and a minimum amount of time is required for completion of the rig at sea. Also, the rig as initially constructed may include a heavy duty crane, and a heavy duty crane barge is not needed for the at-sea completion procedures.

Generally speaking, in terms of structure, this invention provides an offshore drilling platform and the like. The structure includes a wholly submerged base structure which is essentially permanently secured to a geologic formation underlying a body of water. The base structure defines a plurality of laterally spaced, upwardly directed receptacles positioned substantially in a common plane a selected distance below the water surface and above the geologic formation. A plurality of pilings are connected between the base and the geologic formation to secure the base to the formation. The upper ends of the pilings are located in or below, but not significantly above, the plane of the receptacles so as to permit a platform structure to be engaged with the base by sinking the platform directly to the base from a floating condition on the surface of the body of water. The invention also includes a buoyant platform structure which is fioatable into position over the submerged base. The platform includes a plurality of hollow columns corresponding in number and arrangement to the number and arrangement of receptacles defined by the submerged base. A horizontal support structure is immovably mounted to and supported by the upper ends of the columns. The lower ends of the columns are configured to mate with the receptacles. Additionally, the invention includes means which cooperate between the base and the platform, when the base and platform are mated via the receptacles, to maintain engagement of the platform with the base.

In terms of method, the invention comprehends a procedure for erecting an offshore platform in an ocean. First, a base defining a plurality of upwardly open receptacles, is fully submerged in the ocean and disposed on the ocean bottom so that the receptacles are located a selected distance below the water surface. The base, as landed on the ocean bottom, provides lateral bracing for the receptacles above the ocean bottom. Second, the landed base is secured to the ocean bottom by a plurality of pilings connected between the base and the geology of the ocean bottom. A bouyant platform, having a draft less than the distance of the receptacles below the water surface and including a plurality of columns arranged at their lower ends to mate with the receptacles, is floated into position over the base. The platform is sunk directl from a floating state on the water surface into engagement with the base in such manner that the columns mate with the receptacles. Finally, the platform is secured in mating engagement to the base.

The term service facility is used hereinto refer to structure carried by the platform and which adapts the over all assemblage for its intended purpose. For example, where the over all assemblage is to be used to drill an oil well, the service facility is a drilling rig and its supporting equipment and structures. On the other hand, where the over all assemblage is to be used as an oceanographic research station, the service facility may be a laboratory facility and the like.

The above-mentioned and other features of the invention are more fully set forth in the following description of a presently preferred embodiment of the invention, the description being presented with reference to the accompanying drawings, wherein:

FIG. 1 is a side elevation view of an offshore drilling structure;

FIG. 2 is a side elevation view of an initial stage in the erection of the structure shown in FIG. 1;

FIG. 3 is a side elevation view of a subsequent stage in the erection of the structure shown in FIG. 1;

FIG. 4 is an enlarged, fragmentary elevation view, partly in cross-section, of a caisson-to-piling-to-column connection in the structure shown in FIG. 1;

FIG. 5 is a plan view taken along line 55 of FIG. 1;

FIG. 6 is an enlarged cross-sectional elevation view taken along line (-6 in FIG. 5 and FIG. 7 is an elevation view taken along line 7-7 of FIG. 4.

A completed offshore drilling platform structure 10, shown in FIG. 1, is supported on a geologic formation 11 which underlies a body of water 12, such as an ocean, having a surface 13. The structure includes a wholly submerged unitary foundation or base assembly 14 which is essentially permanently secured to the geologic formation, and a movable platform structure 15 which is releasably secured to the foundation. The novel and inventive features of structure 10 and the procedures by which it is erected will be most readily understood from the following description of the construction of the structure.

The first step in the erection of structure 10 is the placement of foundation 14 at the desired location on the bottom of the body of water 12. The invention may be practiced in water depths of from about 60 feet to substantially any greater depth desired. The exact size and configuration of the foundation actually used will vary with the depth of water over the location where the foundation is placed. Regardless of the depth of water involved, however, the foundation as initially landed on the ocean bottom is constructed so that its upper end lies a selected distance, say to feet, below water surface 13.

The foundation shown in the accompanying drawing has a substantially square planiform configuration (see FIG. 5); it will be understood, however, that the foundation may have a rectangular configuration, a triangular configuration, a pentagonal configuration, or any configuration desired without departing from the scope of the invention. The foundation used, however, includes a plurality of vertically oriented, hollow, open-ended guide caissons 17 which are positioned at desired laterally spaced locations in the foundation, preferably at the corners of the foundation. The caissons are rigidly interconnected along the vertical extent of the foundation by a plurality of major horizontal brace and spacer members 18; members 18 are further braced and interconnected by a plurality of truss members 19. Accordingly, the foundation as landed on formation 11 is an integrated unitary rigid structure having a vertical extent substantially equal to the difference between the depth of water over the location at which the foundation is landed and the selected distance mentioned above.

Foundation 14 may be transported to its intended site on a floating drilling vessel 20 (FIG. 2) which includes a crane or derrick 21 and a rotary table 22. Alternatively, the foundation may be rendered temporarily positively bouyant and towed to its intended site. Temporary positive buoyancy may be obtained by the use of auxiliary floats releasably connected to the foundation in a known manner, or by temporarily sealing caissons 17 and horizontal braces 18, which would then be fitted with flooding valves to allow progressive flooding for sinking the foundation to the bottom at a predetermined negative buoyancy and rate of descent. In any event, once at the site, the foundation is lowered into position on the geologic formation and levelled so that the upper ends of the caissons are in an essentially horizontal plane and the caissons are essentially vertical; divers may be required to level the foundation so that caissons 17 are essentially vertical.

After the foundation has been properly disposed on the ocean floor, it is used as a locating and guiding templet during the drilling-in and cementing to the geologic formation of a number of tubular pilings (see FIG. 4) formed by suitable lengths of oil well casing pipe, for example. Drilling vessel 20 is positioned above the foundation over one of caissons 17, as shown in FIG. 2, and a drill string 26 is landed on the formation through the caisson, the caisson serving as a guide and locator for the drill string. A hole 27 is drilled in the formation below the foundation to a desired depth determined by the nature of the formation, the depth of water over the formation, the anticipated weather conditions in the area, and the Weight of movable platform structure 15. When the hole has been drilled to the desired depth, the casing pipe piling is run into the hole through the caisson.

As shown in FIG. 4, the upper end of each caisson carties at number of projections 28 which extend radially outwardly from the caisson tube. The projections cooperate in inverted J-slots 34 formed in a collar sleeve 29 (FIG. 7) secured to the upper end of the piling disposed in the caisson so as to lie circumferentially of the exterior of the caisson. Engagement of the sleeve with the projections locks the piling to the caisson. Non-shrinking cement or grout 30 is then pumped from the vessel into the hole via the drill pipe 26 until the hole and interior of the piling is filled. An annular chamber 33 between the piling and caisson is then filled with non-shrinking cement or grout by pumping through conduit 31 into suitable fitting 32. The cement fills the hole, a portion of the interior of the piling, and at least a portion of chamber 33. This procedure is carried out at each caisson of the foundation.

As shown best in FIG. 4, an upwardly extending externally threaded stud 35 is disposed coaxially of each piling 25 below the upper end of the piling. The stud is rigidly secured to the piling by a plurality of transverse brace and strut members 36. Also, each piling at its upper end and above the upper end of the adjacent caisson carries an annular seat assembly 37 having an upwardly opening conically tapered inner seat surface 38. Accordingly, the upper end of each piling defines an upwardly open receptacle 39 for mating with the lower end of a vertical column member 40 of movable platform structure 15. The pilings have their upper ends disposed within or below the common plane Within which the receptacles are disposed; the upper ends of the pilings are not significantly above the receptacles so as to interfere with movement of platform structure 15 into mating engagement with the foundation. Each column member has a conically configured lower terminal portion 41 configured to mate with surface 38 of the adjacent seat assembly.

As shown in FIG. 1, the movable platform structure includes a plurality of corner column members 40 which extend vertically over the major portion of the height of the structure. The same number of columns are provided as there are receptacles defined by the drilled-in and cemented submerged foundation. Each column member preferably is hollow and defines within its length at least one fluid storage chamber 42 (see FIG. 4). The columns are spaced apart in a pattern which matches the pattern in which the foundation receptacles are arranged. The columns are secured rigidly in this relation within the platform structure by a plurality of horizontal structural brace and spacer members 43; a plurality of vertical and diagonal structural truss members 44 are interconnected between the brace and spacer members. Where the platform structure is to provide an abovewater drilling platform, as in FIG. 1, columns 40 are sufliciently greater in length than the distance between water surface 13 and receptacles 39 that a horizontal platform 45, carried by and fixed to the upper ends of the columns, is spaced a predetermined distance above water surface 13 when structure 15 is engaged with foundation 14 as shown in FIG. 1.

The movable platform structure is fabricated at some sheltered location on shore, such as in a shipyard, to as complete a state as possible. The structure is then towed into place over foundation 14. The platform structure, at the time of its arrival over the submerged foundation, is positively bouyant. To impart positive buoyancy to the structure, fluid storage chambers 42 within columns 40 are empty. In the event that emptying the chambers does not render the structure positively buoyant, positive buoyancy may be obtained by securing auxiliary flotation chambers 47 to columns 40 by quick-disconnect couplings 48, as shown in FIG. 3. When structure 15 arrives over the submerged foundation, the platform structure has a draft which is less than the distance of the foundation and the receptacles below water surface 13.

When the platform has been positioned roughly over the submerged foundation, a plurality of cables 49 are extended from the foundation through suitable guides 50 mounted to platform structure 15 to a corresponding number of constant tension winches 51 temporarily secured either to platform 45 or to the vessel which has towed the platform structure into place over the fundation. The cable guides are displaced laterally of the vertical axes of columns 40 the same distance as the lower ends of the corresponding cables are displaced laterally from the upper ends of the corresponding ones of receptacles 39. After the cables have been rigged to winches 51, the column buoyancy chambers are flooded to reduce the initial positive buoyancy of the floating platform structure. The columns are flooded by operating flooding valves 52 connected with chambers 42 through the col umns below the initial water line of the floating structure. The valves are operated from platform 45 by means of hand wheels 53 connected to the respective valves by valve stem extension members 54. As the positive buoyancy of the floating platform structure decreases, the structure sinks directly from its initial floating state toward the submerged foundation. As the structure sinks, cables 49 are reeled in on winches 51, thereby drawing the tapered lower ends of columns 40 into alignment with seat assemblies 37. The platform structure is readily movable from a floatnig position directly into engagement with the foundation because the foundation includes no structure projecting above receptacles 39 suflicient to interfere with movement of the platform structure into registry with the foundation. After the columns have registered with the seat assemblies, the flooding of chambers 42 is continued to render the platform structure negatively buoyant. The auxiliary buoyancy and flotation chambers 47, if provided, are then removed from structure 15 if they have not been removed previously.

After the movable platform structure has been engaged with piles 25, the structure is releasably secured to the foundation by elongate tie rods 56 which extend axially through columns 40 (see FIG. 4). Each tie rod preferably is fabricated from one or more lengths of oil well drill pipe. In a presently preferred embodiment of this invention, the tie rods are fabricated of 4 /2 inch drill pipe. Each tie rod is disposed'within a tubular guide 57 which extends axially within the column members; where the guide passes through a fluid storage chamber 42, the exterior of the guide is sealed to the end walls of the chamber. The lower end of each guide tube is open, as shown in FIG. 4. The lower end of the tie rod carries a sleeve '58 which is internally threaded to mate with externally threaded stud 35 disposed within the upper end of the adjacent piling. The upper end of each tie rod carries an externally threaded sleeve 59 which passes through a horizontal bulkhead 60 at the upper portion of the corresponding column member. Each column is releasably secured to its piling by threading sleeve 58 down snugly upon stud 35 and by threading a nut 61 over sleeve 59 against bulkhead 60. The nut is tightened down against the bulkhead until the tie rod is under a predetermined amount of tension along its length. The nut is then tack welded to bulkhead 60 to prevent the nut from backing off from sleeve 59.

It was noted above that movable platform structure 15 is fabricated in as complete a condition as possible prior to the time the structure is towed to submerged foundation 14. Accordingly, at the time structure 15 arrives over the foundation, when the platform is to be used as an above-water oil or gas drilling installation, there is present on platform 45, as permanent structure, an essentially complete drilling rig facility 63 which includes a derrick skid and track framework 64, a drilling platform 65 mounted on the skid and track framework and including a rotary table 66. A derrick 67 is located on the platfrom, and a drawworks shed 68 is also mounted on the platform for housing a suitable drawworks windless mechanism (not shown) of the type commonly used in drilling oil wells. The derrick skid and track framework extends along one side of platform 45 and is elevated above platform 45. To the other side of the platform along the skid and track framework is located a complete building 69 housing pumps, generators, and facilities for storing dry and liquid mud, cement for use in spudding in oil wells, and other stores and supplies required in the drilling of submarine oil wells. Also, at the time the structure arrives over the fundation, a heavy duty rotary crane 70 is mounted on the platform, preferably on a supporting pylon 71 located above building 69. The platform structure may also include an additional crane 72. Suitable hoppers and accumulators 73 for the various fluids used in drilling operations are also installed on the platform. The platform at the time of its arrival at the foundation may also include a living quarters structure 74 and a heliport landing pad 75; these components of the facility may be fabricated and lifted into place on the platform from a barge by crane 70 after structure 15 has been tied down to foundation 14. The platform is also prefabricated complete with fenders 76 on each of column members 40, and replaceable fender piles 77 are installed between platform 45 and spacer and brace member 43 at desired locations around the periphery of the platform structure. A suitable catwalk 78 is provided below platform 45. More or less of the above structure, as desired, may be installed on platform 45 before structure 15 is towed to its foundation, but as a minimum the derrick skid and track framework, drilling platform 65, derrick 67, crane 70 and house 69 are installed on the platform before the structure is moved from the shore-based construction site. It is apparent, therefore, that final outfitting of the drilling rig facility may be accomplished without the use of heavy duty barge cranes after structure 15 has been secured to the submerged foundation; crane 70 and crane 72, if provided, provide adequate lifting capacity for transferring drill pipe, stores and supplies to platform 45 from smaller supply barges brought alongside completed platform 10.

The total cost of completed structure may be kept to a minimum since a maximum amount of the construction work required is done at shore where labor and transporation costs are considerably lower than at sea. The only significant construction work which must be done at sea is the drilling-in and cementing of pilings 25 through foundation 14. This work, however, if readily accom plished, and the costs of such work are relatively minor. Further, the construction procedures which must be performed at sea are such that, in the vent of adverse weather conditions, they may be interrupted and resumed readily and without appreciable economic loss or harm to the partially completed submerged structure. The task of securing platform structure to the submerged foundation may be accomplished rapidly, well within the duration of fair weather conditions at the site.

Movable platform structure 15 is constructed so that when it is secured to pilings 25, braces 43 and trusses 44 are located below water surface 13. As a result, minimum lateral forces are imparted to the completed structure by wave action. Chambers 42 are used during operations from platform to store drilling mud and other fluids used during the drilling process.

Foundation 14 functions essentially entirely as a templet during the drilling-in of pilings 25. After the pilings have been cemented in place, the foundation carries substantially none of the vertical load of the superadjacent structure. Instead, this load is carried by pilings 25 and is transferred effectively to geologic formation 11 by the pilings and the cement securing the pilings to the formation. Foundation 14, however, does provide lateral support for the pilings in view of the presence of cement 30 in annulus 33 between each. caisson 17 and its piling. For this reason, the column capacity of the pilings is substantially increased over the load bearing capacity of like pilings unsupported between the bottom of the ocean and the lower ends of column members 40. The resulting structure, therefore, is more than adequate to withstand the lateral shear loads imposed upon structure 10 by Wind, wave and current action. Also, because of the lateral support given the pilings by a foundation constructed as described above, the invention may be used to advantage in water of great depth, even though the pilings extend to within 40 or feet of the water surface.

As illustrated in FIGS. 5 and 6 where platform 15 provides a well drilling facility, foundation 14 is constructed to provide a plurality of guides 80 for use in drilling a number of oil wells from platform 45. An elongate plate 81 is disposed across the upper extent of foundation 14 below drilling platform 65. The plate is reinforced along its upper surface by a plurality of parallel I-beams 82, or the like, welded to the plate. At spaced locations between adjacent pairs of I-beams, a plurality of holes 83 are formed through the plate. The holes are lined by upwardly flaring guide collars 84. During a drilling operation, a conductor pipe 85 and the necessary drill pipes are passed to the corresponding well through the guide collar.

In the event it is desired to use the movable platform structure at another location, in deeper or shallower water, it is a simple matter to remove structure 15 from foundation 14 and to move the structure to the new location. The removal procedure requires that auxiliary buoyancy chambers 47, if originally provided, be reengaged with columns 40. If tanks 42 are dry after removal of the desired equipment from platform 45, the tanks are at least partially flooded to give the structure negative buoyancy. Nuts 61 are then loosened and the tie rods are disconnected from studs 35. The water in tanks 42 is then pumped out to cause the structure to become positively buoyant. After the structure has cleared receptacles 39, it may be towed to the new location. Preferably, cables 49 are reconnected to foundation 14 after structure 15 is removed, and are secured to a buoy over the foundation for ready access in the event that structure 15 is subsequently returned to the foundation.

At its new location, the movable platform structure is readily engaged with another drilled-in and cemented foundation which defines a receptacle arrangement like that defined by foundation 14 and which receptacles are at substantially the same distance below the water surface as those of foundation 14. Foundation 14 might be in 75 feet of water. whereas the new foundation might be in 200 feet of water. The movable structure, however, is insensitive to the change in the depth of water. It is apparent, therefore, that this invention provides an economical platform assembly for use at a number of drill sites located in widely varying depths of water. The economic benefits inherent in the invention increase in significance as the number of drill sites increases since standardization in a number of movable platform structures is pos sible.

If it is desired that the wells drilled below foundation 14 be worked over at a time after structure 15 has been moved to another location, structure 15 (or one essentially identical to it in terms of mating with foundation 14) can readily be reengaged with receptacles 39 as described above. The reengagement procedure is facilitated by use of cables 49 secured to the buoy over the foundation. Existing movable drilling rigs, either of the jack-up type, the submersible type or the semi-submersible type, cannot readily be used in subsequent workover operations on a submarine well; such rigs are extremely difiicult to position with the accuracy required in workover operations. Structure 15, because its foundation is already in place over the well, is readily positioned exactly where it was when the well was first drilled. As a result, the use of this invention substantially reduces the expense now incurred in working over a submerged oil well by means of a bottom-footed movable drilling platform. Because the foundations provided by this invention are drilled into the ocean bottom, they cannot be undermined by current action.

The invention has been described above with reference to a movable drilling platform to illustrate the utility of the invention. The invention may also be used to advantage as a meteorological research or oceanographic research station, or it could be used as a part of an undersea agricultural or mining facility. In such event, the service facility mounted to platform 45 would be altered appropriately, such alterations being within the spirit of this invention. Other modifications and alterations within the spirit of the invention will also be apparent to others skilled in the art of marine structures, and for this reason the foregoing description should not be regarded as limiting the scope of this invention.

What is claimed is:

1. An offshore structure for use in the location in and the production of oil and gas from a geologic formation underlying an ocean or the like, for example, comprising a wholly submerged unitary base disposed on the ocean bottom and defining a plurality of horizontally spaced upwardly open receptacles located substantially in a common plane disposed a selected distance below the ocean surface and above the ocean bottom, a plurality of pilings connected between the base and the geology of the ocean bottom for securely holding the base in place on the ocean bottom, a buoyant platform structure fioatalble into position over the base and submersible from the ocean surface directly to engagement with the base, the platform comprising a plurality of hollow columns corresponding in number and arrangement to the number and arrangement of the receptacles and a horizontal support structure immovably mounted to and supported by the columns at the upper ends thereof, the columns having lower ends configured for mating with the receptacles, the pilings having upper ends disposed effectively in or below said receptacle common plane so that the buoyant platform may be landed upon the base and engaged in mating relation with the receptacles directly from a floating position of the platform on the ocean surface, and means cooperating between the base and the platform when the platform is engaged with the base and the columns are mated with the recepacles for maintaining the engagement of the platform with the base.

2. An offshore structure for use in the location in and the production of oil and gas from a geologic formation underlying an ocean or the like, for example, comprising a wholly submerged unitary base disposed on the ocean bottom and defining a plurality of horizontally spaced upwardly open receptacles located substantially in a common plane disposed a selected distance below the ocean surface and above the ocean bottom, a plurality of pilings connected between the base and the geology of the ocean bottom for securely holding the base in place on the ocean bottom, a buoyant platform structure fioatable into position over the base and submerisible from the ocean surface directly to engagement with the base, the platform comprising a plurality of hollow columns corresponding in number and arrangement to the number and arrangement of the receptacles, the columns having lower ends configured for mating with the receptacles, means cooperating between the base and the platform when the platform is engaged with the base and the columns are mated with the receptacles for maintaining the engagement of the platform with the base, a plurality of cables connected between the base and the platform, the connections of the cables to the base and the platform being cooperatively arranged to guide the platform into column-to-receptacle mating engagement with the base, the pilings having upper ends disposed effectively in or below said receptacle common plane so that the buoyant platform may be landed upon the base and engaged in mating relation with the receptacles directly from a floating position of the platform on the ocean surface, and winch means located above the ocean surface to which the cables are connected and operable for maintaining tension on the cables during submersion of the platform into engagement with the base.

3. An offshore drilling platform and the like comprismg (a) a wholly submerged base structure secured to a geologic formation underlying a body of water and including -(1) a plurality of caissons extending from the bottom of the body of water to upper ends thereof located a selected distance below the surface of the body of water,

(2) means interconnecting the caissons along their length for laterally bracing the same,

(3) means defining an upwardly open receptacle at the upper end of each caisson,

(4) a piling disposed within and secured to each caisson, each piling extending from an upper end disposed below the upper extent of the corresponding receptacle to a lower end disposed 'within and secured to the geologic formation, and

-(b) an essentially rigid platform unit fioatable in an upright state into and out of registry with the base structure and supported by the base structure, the platform unit including (1) a corresponding plurality of hollow column members each of which has its lower end registered in a corresponding receptacle,

('2) a platform irnmovably mounted to and supported by the columns at their upper ends, and

(3) connector means for releasably securing the platform unit to the base structure with the lower ends of the column received in the receptacles.

4. An offshore platform according to claim 3 wherein the upper end of each piling carries an annular seat assembly adapted to mate with the lower end of a corresponding column member, said seat assemblies compris ing said receptacles.

5. An offshore platform according to claim 4 wherein each column member has a downwardly tapering lower terminal portion, and the corresponding seat assembly defines an inner annular tapered seat surface configured to mate with the terminal portion of the column member.

'6. An offshore drilling platform according to claim 3 wherein the connector means comprises an elongate tie rod disposed in each column member and extending through the lower end thereof, means cooperating between the lower end of each tie rod and the corresponding piling for releasably securing the tie rod to the piling, and means cooperating between the upper end of the tie rod and the column member within the column member for tensioning the tie rod along its length.

7. An offshore platform according to claim 3 including means for imparting suflicient positive buoyancy to the platform unit that the unit is floatable into position over the base structure, and controllable means for decreasing said buoyancy so that the unit is controllably lowered into engagement with the base structure.

8. An offshore platform according to claim 7 wherein the column members define a plurality of liquid storage chambers internally thereof, and valve means connected to the chambers operable for flooding said chambers to impart negative buoyancy to the platform unit.

9. An offshore platform according to claim 3 wherein the columns support the platform above the water surface and including, as permanent structure on the platform supported by said columns, a drilling platform including a rotary table, a foundation for the drilling platform, a building for housing drilling machinery and stores, and a crane.

10. An offshore drilling platform and the like comprising a submerged base structure secured to a geologic formation underlying a body of water, the base structure including a plurality of vertical piling members secured to the geologic formation at spaced locations and extending to upper ends disposed substantially in a common horizontal plane spaced a predetermined distance below the water surface, framework means cooperating with the piling members between the upper ends thereof and the geologic formation for laterally bracing said piling members, an essentially rigid removable platform unit supported by the piling members and including a corresponding plurality of hollow column members arranged for registry of their lower ends with the upper ends of the piling members, the column members having a length sufiicient to extend a selected distance above the water surface from said piling members, a platform supported by the column members said selected distance above the water surface, means for rendering the platform unit including the platform sufiiciently positively buoyant that the unit is floatable into and out of position over the base structure, means for controllably decreasing the buoyancy of the platform unit for lowering the platform unit into registry of its column members with the piling members, and means for releasably securing the lower ends of the column members to said piling members to secure the platform unit to the base structure.

11. A method for erecting an offshore oil well drilling and production platform or the like in an ocean or the like comprising the steps of (a) disposing on the ocean bottom a base defining a plurality of upwardly open submerged receptacles located a selected distance below the ocean surface and laterally braced among themselves above the ocean bottom,

(b) securing the base to the ocean bottom by pilings connected between the base and the geology of the ocean bottom,

(c) floating into position over the base a buoyant platform having a draft less than said selected distance and including a corresponding plurality of columns adapted at their lower ends to register with the receptacles,

(d) sinking the platform directly from the ocean surface bodily toward the base and into engagement of the column lower ends with the receptacles, and

(e) securing the platform to the base following engagement of the platform with the base.

12. A method according to claim 11 including the further steps of (a) connecting a plurality of cables between spaced locations on the base and correspondingly spaced locations on the platform following positioning of the platform over the base and before sinking of the platform toward the base, and

(b) applying tension to the cables during the sinking sequence to cause the lOWer ends of the columns to be guided into registry with the receptacles.

13. A method for erecting an offshore platform and the like including the steps of (a) constructing on the bottom of a body of water a wholly submerged foundation defining a plurality of submerged receptacles located a selected distance below the water surface and laterally braced among themselves above said bottom,

(b) floating into position over the foundation in an erect state a buoyant platform unit which includes a corresponding plurality of hollow columns having lengths greater than said selected distance and a platform immovably mounted to and supported by the columns above the water surface,

(c) sinking the platform unit bodily toward the foundation and into engagement of the lower ends of the columns with the receptacles, and

(d) releasably securing the platform unit to the foundation.

14. The method according to claim 13 wherein the securing of the platform unit to the foundation includes (a) disposing in each column an elongate tie rod,

(b) releasably connecting the lower end of each tie rod to the foundation at the corresponding receptacle through the lower end of the corresponding column, and

(c) tensioning each tie rod along its length against the corresponding column.

15. The method according to claim 13 includes (a) connecting a plurality of cables between spaced locations on the foundation and corresponding spaced locations on the platform unit following positioning of the platform unit over the foundation and before commencement of the sinking sequence, and

16. The method according to claim 13 including rendering the platform unit negatively buoyant following engagement of the columns with the receptacles.

17. The method according to claim13 wherein the construction of the foundation includes the steps of (a) landing on the bottom of the body of water a drilling templet including a plurality of laterally spaced, interconnected, vertically arranged caissons,

(b) drilling a hole in the geologic formation below each caisson,

(c) disposing in each caisson and the hole therebelow a pile the upper end of which defines one of the receptacles, and

(d) cementing the piles within the respective holes and caissons.

18. A method for erecting an offshore platform and the like including the steps of (a) constructing on the bottom of a body of water over a geologic formation a drilled-in and cemented fundation including a plurality of spaced pilings having their upper ends located a selected distance below the water surface and transverse bracing for said pilings by (1) landing on the bottom of the body of water a drilling templet comprising a corresponding plurality of vertical open-ended caissons and strut means spacing the caissons apart and rigidly bracing the same,

(2) drilling a hole in the formation below each caisson using the caisson as a guide for the drilling means,

(3) passing througheach caisson a piling of selected length greater than the length of the caisson and of smaller diameter than the inner di ameter of the caisson and disposing the piling in the hole so that the upper end of the piling is adjacent the upper end of the caisson, and

(4) disposing cement in each hole around the pil-. ing, in the piling along a major portion of its length, and between the piling and the caisson so that the piling is laterally supported along substantially its entire length,

(b) positioning over the foundation a buoyant platform structure which includes a corresponding plurality of hollow columns having lengths greater than said selected distance, a platform supported by the columns above the water surface, and a partially complete service facility including a crane on the platform,

(c) registering the lower ends of the columns with the upper ends of the pilings,

(d) releasably securing the platform structure to the foundation, and

(e) completing the construction of the service facility.

19. A method for erecting an offshore platform and the like including the steps of (a) construction on the bottom of a body of water over a geologic formation a drilled-in and cemented foundation including a plurality of spaced pilings having their upper ends located a selected distance below the water surface and transverse bracing for said pilings,

(b) prefabricating at a shore-based location in a substantially complete condition, a buoyant platform structure having a draft less than said selected distance and which includes a corresponding plurality of hollow columns having lengths greater than said selected distance, the platform supported by the columns above the waterline of the platform structure, a partially complete service facility including a crane on the platform, and constant tension winch means,

(0) towing the buoyant platform structure to over the foundation,

(d) extending a plurality of cables from the foundation through guide means carried by the platform structure to the winch means, the guide means being located so that tensioning of the cables aligns the columns with the pilings,

(e) controllably decreasing the buoyancy of the platform structure to lower the structure to registry with the pilings,

(f) operating the winch means to reel in the cables during decreasing of the buoyancy of the platform structure,

(g) rendering the platform structure negatively buoyant after registry of the columns with the pilings,

(h) releasably securing the platform structure to the foundation, and

(i) completing the construction of the service facility 50 via use of said crane.

20. An offshore structure for use in the location and production of underwater minerals such as oil and gas and comprising a wholly submerged unitary base disposed on an ocean bottom and extending upwardly therefrom to an upper end disposed a selected distance below the ocean surface, the upper end of the base defining an upwardly open and laterally extending receiver means for a removable platform structure, means located effectively below the receiver means in association with the base for essentially permanently securing the base to the ocean bottom, a buoyant platform structure fioatable into position over the base and submersible from the ocean surface directly into engagement with the receiver means, the platform structure including controllable buoyancy means adequate to float the platform structure in a vertical attitude with a draft less than said selected distance, the platform structure having a vertical dimension substantially greater than said selected distance whereby the platform structure when engaged with the base has an upper end disposed above the water surface, a horizontal support structure immovably mounted to and supported by the platform structure at the upper end thereof, the platform structure having a lower end corresponding in hoizontal extent to that of the receiver means for cooperation with the receiver means upon engagement of the platform structure and the base, the receiver means and the lower end of the platform structure defining a plurality of laterally spaced and vertically extending projection and receptacle means for aligning the platform structure in a predetermined angular relation to the base during said engagement and for preventing lateral relative movement between the platform structure and the base during said engagement, and means countereffective to positive buoyancy of the platform structure for maintaining engagement of the platform structure with the base.

References Cited UNITED STATES PATENTS 2,574,140 11/1951 Boschen 6146.5 2,865,179 12/1958 McNeill 61-46.5 2,433,898 4/1960 Upson 61-465 2,940,266 6/1960 Smith 61-465 3,036,438 5/1962 Sims 61-46.5 X 3,138,432 6/1964 Kofahl et al. 6146.5 3,355,899 12/1967 Koonce et al. 61-465 FOREIGN PATENTS 1,389,216 1/1965 France.

JACOB SHAPIRO, Primary Examiner