Marine Platform Foundation Structure

Abstract

The invention relates to a marine platform foundation structure adapted to support an operating deck at an offshore site. The structure extends upwardly from the floor of a body of water to the surface, having the lower end operably connected to anchor means disposed at the ocean floor. Said anchor comprises one or more upstanding posts upon which the foundation structure is slidably registered. The connection therebetween permits the foundation structure to oscillate through a limited degree about the anchor in response to displacing forces, while maintaining said lower end horizontally stationary.

Description

BACKGROUND OF THE INVENTION

In the exploration and drilling for sources of crude oil at offshore locations, it has become necessary and will continue to be necessary to search and operate further from the shore. As a consequence, the depth of the water in which future drilling and producing operations will take place promises to be greater. While much of the basic offshore technology heretofore known by the industry can be applied to deep water operating sites as well as to shallow water sites, the former introduces many problems not previously dealt with.

There exist today basically two types of marine platforms in current use for offshore drilling. One consists of a rigidly positioned platform which embodies legs extending to the ocean floor and which are piled into the floor or bear against a foundation mat. The second form of platform embodies the use of either the floating ship, or semi-submersible space frame type floating structure which is either anchored or self-propelled to maintain a position over a desired site. The second type embodies the advantage of mobility in that it can be readily moved. However, there is considerable difficulty in maintaining said platform a constant predetermined desired position above a well head, particularly under adverse weather conditions. Another pertinent factor for consideration is that the floating type structures are unable to support a drilled well which is completed above the water's surface.

A further factor regarding the floating type ship or structure is that the drill string, together with any guide means utilized such as cables, chains and the like, which extend through the water, are subjected to lateral displacing forces in the form of ocean tides and currents.

Of the above mentioned platform types, by far the one achieving widest use consists of a working deck that is retained in place over a desired site by a rigid structure. It has been found however that in deep waters an elongated, rigid although buoyant structure, can be so anchored as to remain pivotally movable about the structure lower end while supporting a platform at the water's surface. Such a buoyant structure is thus fastened to an anchoring device at a single point whereby the surface located platform is laterally movable within a limited degree.

In this type of arrangement the rigid connecting member remains subject to extraordinary stresses due to the movement of the platform in response to surface phenomena such as wind and high waves prompted by stormy turbulent waters. Since the lower or base connection of the support structure withstands the entire displacing force, a considerable strain is introduced at the single point connection to an extent that the entire platform can become jeopardized in the event the connection weakens or fails.

It is therefore one of the objects of the invention to provide an offshore marine platform embodying a buoyant although rigid support structure which is operably connected to an anchoring member. A further object is to provide a device of the type described wherein the anchoring means includes facilities for permitting restrained vertical movement of the support structure with respect to the floor supported anchor member. Still another object is to provide a structure of the type described embodying an internal shock absorbing mechanism.

DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a side elevation view of a marine platform herein described submerged in a body of water.

FIG. 2 is a side elevation view illustrating the step in the method for assembling a platform.

FIG. 3 is an enlarged cross-sectional view taken through line 3--3 in FIG. 1.

FIG. 4 is an alternate embodiment of the view shown in FIG. 3.

FIG. 5 is an enlarged segmentary view in partial cross-section illustrating a single column of the platform shown in FIG. 1, and

FIG. 6 is a segmentary view similar to FIG. 5 illustrating the platform support column in an offset position.

In overcoming the foregoing mentioned problems, and toward achieving the suggested objectives, there is provided in brief a novel support foundation structure for an offshore platform adapted particularly to deep water operations. The unit includes an elongated superstructure or foundation member 10 that is buoyantly stabilized in the body of water to support a working deck 11 carrying equipment and materials necessary to a drilling, producing or storage operation.

Anchoring means 12, resting at or partially imbedded in the ocean floor, includes means for operably engaging the lower end of the support structure at a slidable joint 13. Said support structure is thereby free to oscillate within a limited arc about the slidable joint. Restraining means extending from the anchor means 12 to working deck 11, defines the degree of mobility within which the structure can move. Said restraining means includes a cable, chain, or similar non-elastic member provided at one end with a dampening mechanism.

Anchoring means 12 includes in essence a foundation pad 14, and may further include a ballastable barge 15 which supplements the downward force exerted by the anchor. The anchor is normally transported to an offshore drill site or installation, and lowered from barge 15 or a similar work vessel to the ocean floor.

FIG. 1 illustrates one embodiment of the apparatus presently contemplated which includes working deck 11 disposed at, and preferably above the water's surface. Deck 11 carries such equipment and materials as one or more derricks 9 together with draw-works, rotary table, and other features essential to the particular drilling operation.

Deck 11 is supported a desired distance above the water's surface by the buoyant foundation structure 10 which includes one or more, and preferably three relatively elongated column 16, 17 and 18 that columns a generally vertical disposition in the body of water. The lower end of each of the respective columns is slidably received at a connector post such as 19, 20 and 21 protruding from the upper face of the anchoring means 12. The respective columns 16, 17 and 18 provide lateral support for one or more casing guides 22, shown in FIGS. 3 and 4, which similarly extend upwardly through the water and enclose drill guide conduits extending longitudinally therethrough.

It is appreciated that the disclosed offshore platform together with its foundation structure, can be used readily in water of any depth and function in the desired manner. However, the device finds specific application in the instance of relatively deep water since the latter introduces the problems which preclude the practical use of rigid, ocean floor supported units

Referring to FIGS. 1 and 2, anchoring means 12 used for positioning the buoyant support structure 10, includes foundation pad 14. The latter is equipped with one or more upright posts 19, 20 and 21 usually formed of heavy walled steel or a similar high strength material which can be prefabricated into pad 14 at a shore location and transported to a drilling site. Foundation pad 14 includes any number of such post members appropriately disposed to restrict movement of superstructure 10 and to accommodate the respective columns thereof to prevent the latter from rotating when submerged in the water. A preferable disposition of posts 19, 20 and 21 includes an arrangement thereof in recognition of the prevailing direction of the water currents and storm winds at a particular site. Thus said posts are disposed in substantially horizontal alignment.

FIGS. 5 and 6 illustrate anchoring posts 19 and 20 depending upwardly from the surface of foundation pad 14. Each post such as 19, includes a heavy walled steel casing 24, having an open end 26 at one extremity, and a rounded closed end at the other. The casing open end is at least partially imbedded in concrete foundation pad 14 to form a firm connection with the latter.

Anchoring posts 19 and 20 can be provided internally with a rigid material such as a concrete filler 27, which may be further reinforced as needed. Under operating conditions, anchoring posts 19 and 20 are subjected to considerable bending stress while performing their function of anchoring the elongated support column. The filler material affords the respective posts with additional bending resistance as well as lateral support for the casing wall.

The respective posts 19, 20 and 21 are interconnected by bracing elements extending therebetween such as heavy gauge "I" beams 29 or the like similarly imbedded into the concrete base and so positioned to provide the respective posts with a desired lateral spacing arrangement. Said members are thus rigidly positioned whereby to facilitate being supported by barge and subsequently lowered to a drilling site at the ocean floor.

Support structure 10 includes elongated columns 16, 17 and 18 which, in their normal upright position, extend from the anchoring means 12 to and above the surface of the water. Each column, 16 for example, includes a central portion having internal tanks or other storage means adapted to hold water for buoyancy purposes and oil for storage purposes. While not presently shown, such buoyancy and storage tanks are well known in the art and do not constitute a specific feature of the invention.

The tanks at the upper end of each column 16, 17 and 18 are provided with a controllable buoyancy system so disposed to afford a desired uplift to the latter depending on the weight of the platform deck 11. The column lower ends are provided with shaped means forming an elongated cavity 32 adapted to receive the upper end of an anchoring post 19, 20 and 21 whereby to define a sliding yet operable engagement between the anchoring posts and the respective support column.

Said column lower end is formed with shell 33 having an annular bearing ring 34 defining the inlet to cavity 32. Ring 34 is welded or otherwise retained in place at the shell inner wall. The center opening of ring 34 is of a sufficient diameter to slidably register about the outer wall of the anchoring posts without binding as the column assumes a canted disposition. The connection thereby formed is both nonrigid, yet flexible.

Cavity 32 inlet defined by the rubbing ring 34 center opening, gradually widens along divergent walls of the shell 33, to a maximum diameter at the shell upper end. Thereafter the diameter of internal cavity 32 is reduced along a frustro-conical section 36 which terminates, and opens into the support column lower end.

Functionally, column 16 lower end is so arranged as to define a maximum angle of about 5.degree. intermediate anchor posts 19, 20 and 21, and the divergent wall cavity, whereby the entire column is permitted to oscillate about the lower guiding point in response to forces exerted against the column. Further, such movement is permitted regardless of the longitudinal relationship of the column with respect to the anchor post.

As presently shown, support structure 10 includes the three adjacently positioned columns 16, 17 and 18 of the type described. However, it is appreciated that the number of such columns utilized depends on the size and weight of the platform deck whereby to properly support the latter at a desired height during a drilling operation.

Also included in foundation structure 10 are one or more elongated tubular conduits extending substantially the length of structure 10. Said casings 23 and 24 are laterally supported by a plurality of casing guides 22. The latter comprise shaped cylindrical guide tubes of sufficient diameter to accommodate a drilling conductor casing whereby to guide the latter downwardly into the ocean floor. Cross members 38 and 39 extending between the respective columns and guides 22, serve to rigidly position the columns with respect to each other.

As noted, the herein described connection 13 between the respective columns and anchor posts, permits relative vertical movement between said members as well as oscillatory motion of the column and deck about anchor 12. The column assembly however is restrained to movement within a limited degree of arcuate motion to preclude its becoming disconnected from the anchor means.

Such restraining means includes an elongated cable 41 or similar relatively non-elastic member, having the lower end firmly held in a retainer 42 disposed at the upper, inner wall of the respective anchor posts 19, 20 and 21. Cable 41 extends through an opening 43 in the casing upper end and is carried to the top of column 16 and thence over pulley 56 where the end is retained in a dampening mechanism 46.

Exemplary of the latter, and as shown in FIG. 5, a reservoir is provided holding a quantity of transfer fluid within a compression cylinder 51. The latter guidably encloses a piston 52. Cable 41 is connected to the piston end 53 whereby to exert tension on the latter at such time as connection 13 at the column lower end tends to separate. Thus when foundation structure 10 is subject to wind or water forces, the structure will tend to displace laterally as well as vertically. Cable 41 however will adjust whereby to pay out or increase tensional pull in response to adjustment of dampening mechanism 46.

Referring to FIG. 2, in a practical application of the disclosed apparatus and its method of use, anchor means 12 is initially transported to an offshore drill site. This is achieved most readily by carrying anchor 14 on a barge or, if overly large, supported in a submerged condition thus lessening the installation problem.

Anchor 14 is then controllably lowered to its desired ocean floor location by winches or other means such as a derrick barge 25, during which time the ends of guide cables 41 connected to anchor posts 19, 20 and 21 are maintained at the water's surface by buoys 31 or the like.

To facilitate movement of foundation structure 10 to the site, the unit is made buoyant to the extent that it lies substantially horizontal at the water's surface. In such condition it is readily towed or barged as the circumstances require. Prior to the submerging of said foundation structure at the drill site, the ends of guide cables 41 are threaded through ring 34, the length of the respective columns 16, 17 and 18, to the upper end thereof. The buoyancy and attitude of the structure are then adjusted such that the unit will assume a generally upright disposition in the water with the upper end protruding above the surface. With the foundation structure 10 buoyantly positioned above anchor 14, buoyancy tanks in the respective columns are partially filled such that the unit will slowly and controllably descend through the water guided by cables 41. The descent is further regulated through upward pull of a crane barge 25 or similar mechanism at the water's surface.

As the lower end of the structure reaches the anchor means 12, the respective column ends will register with the upstanding anchor posts 19, 20 and 21 through guide rings 34 to complete the nonrigid connection. Thereafter, the upper ends of the guide cables 41 are secured to dampening mechanism 46. To properly adjust the length of guide cables 41, consideration must be given to the differential in water depth at the drilling site, and also the proposed variation in water depth which will be reflected in the longitudinal travel of the ring 34 along the anchor posts 19, 20 and 21.

Obviously, many modifications and variations of the invention, as hereinafter set forth, may be made without departing from the spirit and scope thereof, and therefore, only such limitations should be imposed as are indicated in the appended claims.

Claims

I claim:

1. A foundation structure for a marine platform having a work deck, and being disposed in an offshore body of water, said structure including;

a plurality of laterally joined elongated columns having opposed upper and lower ends, and a plurality of vertically spaced bracing members interconnecting said respective elongated columns, said respective columns being floatably positioned in said body of water in a generally upright disposition, the said column upper ends extending beyond the water's surface and being adapted to supportably engage said work deck,

anchor means fixedly positioned at the floor of said body of water, and including a plurality of upstanding posts disposed in vertical alignment with said respective elongated columns, said posts having a substantially uniform diameter along the outer surface thereof,

a shell forming a cavity at the column lower end thereof, said shell embodying an annular bearing ring having a center opening of a sufficient diameter to slidably register about the said anchoring post and to maintain a sliding relationship therewith when said column assumes a position canted from the vertical,

the lower of said column bracing members being disposed adjacent to said shell forming said cavity,

whereby to permit vertical and pivotal movement of said respective columns about their respective anchor posts in response to lateral displacement of the column upper ends.

2. In a foundation structure as defined in claim 1 wherein said upstanding anchor posts are formed with a cylindrical outer surface, and the center opening in said bearing ring includes a circular opening having a diameter greater than that of said cylindrical outer surface.

3. In a foundation structure as defined in claim 1, including a frustro-conical section carried on said shell to define the upper end thereof, said conical section including a constricted portion connected to and depending from said column lower end.