OFFSHORE PLATFORM WITH BUOYANT RIG SUPPLEMENTAL SUPPORT

Abstract: An offshore platform structure (10) is disclosed for temporarily using a jack-up rig (34), for well operations in deepwater applications having a bottom founded jacket base (12) which supports a surface tower (28) which extends above the ocean surface (30) and a subsea rig support interface (110) which is adapted to support the jack-up rig (34) for well operations. At least one selectively buoyant rig support buoyancy tank (112) is connected to the rig support interface whereby a portion of the temporary load on the jacket base in supporting the jack-up rig for drilling operations may be alleviated.

Description: OFFSHORE PLATFORM WITH BUOYANT RIG SUPPLEMENTAL SUPPORT The present invention relates to a platform and system for conducting offshore hydrocarbon recovery operations. More particularly, the present invention relates to a platform structure and system for allowing the use of a jack-up rig in deeper water. Jack-up rigs provide a derrick and associated equipment for drilling, completing or working over a well. This equipment is mounted to a combined hull\deck which is capable of floating these facilities to site. A plurality of retractable legs are provided which renders the jack-up rig conveniently portable. Once floated into position for conventional operations, the legs are jacked-down until they engage the seafloor. Further jacking transfers the load from the buoyant hull to the legs, then lifts the hull/deck out of the water and above the splash zone to produce a stable, bottom founded offshore platform for conducting well operations. A consideration of this design is that to best take advantage of the mobile nature of the facilities provided on the jack-up rig, the rig is removed after drilling is complete and does not remain deployed at the development during the production phase except, possibly, for temporary drilling and workover operations. The considerable investment in drilling, completion and workover equipment is best utilized by redeploying the jack-up rig to another location as soon as these operations are complete. Thus, surface completions for production are not accommodated on the jack-up rig itself. A small structure called a "well jacket" can be used with the jack-up rig to provide the benefits of a surface completion with the convenience of a jack-up rig. However, well jackets and jack-up rig combinations are limited to shallow water deployment. Further, practical limitations on the length of the retractable legs more directly restrict the depth in which jack-up rigs can be traditionally deployed. The requirements of deeper water depths have most often been answered by the continued use of traditional bottom founded platform structures. Topside facilities provide convenient well access for production operations. However, such structures must dedicate a significant amount of their structural strength to supporting drilling facilities that are only required for a relatively short period of time in the life of the overall operations from the platform in recovering oil and gas from a reservoir. Further, the structure must be able to withstand the maximum design environmental conditions, the design hurricane criteria, with these drilling facilities in place. Of course, recovery operations lead to depletion of the hydrocarbon reservoir and, in time, the platform loses its usefulness at a site. Nevertheless, the well jacket that forms the tower supporting the deck of the platform may be structural sound and capable of an extended useful life. However, salvage operations are difficult and another constraint of traditional well jackets is that they are design specific for a given water depth. This tends to substantially limit redeployment opportunities. Certain designs have been proposed for "piggyback" deployment of a jack-up rig onto a subsea structure, yet these designs have carried forward many of the limitations of each structure producing a result that, although it increases water depth for the jack-up rig, otherwise remains the sum of the limitations of its constituent parts. More recently a new platform concept has been proposed combining the benefits of jack-up rigs and traditional bottom founded platform structures, without carrying their drawbacks into the combination. Thus, the "Hyjack" platform has been proposed which combines a small surface tower sufficient to support production operations with a substantial jacket base which supports the surface tower and temporarily supports a jack-up rig for drilling operations. Following drilling, the jack-up rig is moved off and the small surface tower supports production operations. This is described in greater detail in U.S. Patent Application 08/129,820, filed September 30, 1993, by Dale M. Gallaher et al for an Offshore Platform Structure and System. Further features that facilitate salvage and redeployment, particularly in combination with the foregoing platform concept, are described more fully in U. S. Patent Application 08/129,829, filed September 30, 1993, by George E. Sgouros et al for a Reusable Offshore Platform Jacket. The full disclosure of each of these patent applications are hereby incorporated by reference and made a part hereof. The forgoing salvage and redeployment provisions allow a second use of pile sleeves without drydock or expensive, complicated offshore operations. However, the offshore platform structure remains rather limited in the depths to which it can be redeployed. This is because the envelope is restricted both by the depth of the subsea rig support interface and the height of the surface tower over which the cantilever deck requires clearance to position the derrick. The first of these constraints involves major structural components. But the second constraint is only limiting due to the complexity of existing offshore operations and the expense and inconvenience of towing the offshore platform structure to dry dock or bringing a floating dry dock to it. As platforms are used in progressively deeper water, their dynamic response may become a greater design consideration as the traditional bottom-founded platforms become relatively less rigid in response to wind, wave and currents. However, dynamic response becomes of a central concern for compliant towers where flexibility is a key design precept. Compliant towers are designed to "give" in a controlled manner in response to dynamic environmental loads rather than to nearly rigidly resist those forces. A basic requirement in controlling this response is to produce a structure having harmonic frequencies or natural periods that avoid those encountered in nature. The total mass at the top to the jacket base is one of the controlling variables in defining the natural periods of the structure. Adaptation of the hyjack platform concept to compliant towers represents a unique challenge is because one platform must accommodate such widely different design states based upon the presence or absence of the jack-up rig at the time in question. Nevertheless, there continues to be a need in some circumstances for economically accommodating and even enhancing the benefits of surface completions and the convenience and economies of jack-up rig operations in deeper water. In accordance with the invention there is provided an offshore platform structure for temporarily using a jack-up rig for well operations in deepwater applications, comprising: a bottom founded jacket base. a surface tower supported by the jacket base and extending above the ocean surface, a platform deck supported by the surface tower, a subsea rig support interface presented at the top of the jacket base and adapted to support the jack-up rig for well operations, and at least one rig support buoyancy tank connected to the rig support interface. An advantage of the present invention is that it further minimizes the permanent structure that is dedicated to serving the limited need for supporting drilling operations over the productive life of a platform. Another advantage of some embodiments of the present invention is that it may afford an opportunity to adapt an installed hyjack platform to accept a broader range jack-up rig footprints should the rigs of the initial design assumption prove unavailable. The brief description above, as well as further objects and advantages of the present invention, will be more fully appreciated by reference to the following detailed description of the preferred embodiments which should be read in conjunction with the accompanying drawings in which: FIG. 1 is a side elevational view illustrating a deployed offshore platform structure; FIG. 2 is a top elevational view of a rig mat taken from line 2-2 in FIG. 1; FIG. 3 is a cross sectional view of the offshore platform structure of FIG. 1 taken at line 3-3 of FIG. 1; FIG. 4 is a top perspective view of a rig mat as deployed in FIG. 1; FIG. 5 is a bottom perspective view of the rig mat of FIG. 4; FIG. 6 is a side elevational view of an installation of a rig mat; FIG. 7 is a side elevational view of a jack-up rig being deployed upon an offshore platform structure with a rig mat; FIG. 8 is a partially cross sectioned view illustrative of one embodiment of a mat locking connection taken along line 8-8 in FIG. 9; FIG. 9 is a side elevational view of a jack-up rig deployed upon the offshore platform structure; FIG. 10 is a side elevational view of a compliant tower embodiment of the present invention deploying a jack-up rig; FIGS. 11A-11D are side elevational views of the salvage and redeployment of an offshore platform structure into a different water depth; FIG. 12A is a top elevational view of an alternative embodiment of a rig support buoyancy tank; and FIG. 12B is a side elevational view of the rig support buoyancy tank of FIG. 12A. In FIG. 1, rig support buoyancy tank 110 in the form of rig mat 110A is provided to compensate for the weight of jack-up rig 34 upon deployment onto bottom founded jacket base 12. In this illustration, jack-up rig 34 is shown in its initial approach. Offshore platform structure 10 provides a subsea rig support interface 26 at the top of bottom founded jacket base 12 having legs 14 and a framework 16 of braces 18. The jacket base is pinned to ocean floor 24 with piles 22 which are secured to the jacket base at a plurality of pile sleeves 20. A surface tower 28 is supported by jacket base 12 to present a platform deck 32 above ocean surface 30. Surface tower 28 is positioned to allow unobstructed access to subsea rig support interface 26. One convenient manner of providing this access for a three leg jack-up rig 34 is to place the surface tower on one corner of the jack-up rig and to provide legs 14 of a quadrilateral jacket base substantially aligned with the discrete contact points such as spud buckets 38 that generally correspond to the footprint of the jack-up rig. Rig mat 110A is illustrated in greater detail in FIGS. 2, 4, 5 and 8. Figures 2 and 4 illustrate the top of the rig mat which presents secondary subsea support interface 138 on top of a tank member 112. The spud buckets of secondary subsea support interface 138 are positioned to receive feet 36 of jack-up rig 34. The bottom of tank member 112 presents jacket base interface 114 (see FIG. 5) which correspond to spud buckets 38 of the subsea rig support interface presented at the top of the jacket base. See FIG. 3. Rig mat 110A has a selectively buoyant and ballastable tank member 112 with jacket base interface 114 on the lower surface (see FIG. 5) and secondary subsea rig support interface 39 on the upper surface (see FIG. 4) . Internal structural members connect interfaces 114 and 39 in a load bearing relationship. Most conveniently, the load is transferred vertically between discrete aligned contact points. However, if necessary, it may be possible to fabricate a rig mat with structural framework suitable to distribute the load between the jacket base and the jack-up rig in other than direct vertical alignment. Thus, it may be possible to use rig mat 110A as an adapter to allow use of a jack-up rig having a dissimilar footprint from that which was the original design assumption when jacket base 12 was fabricated. Dissimilar footprints in jacket base interface 114 and secondary subsea rig support interface 39 is one of the features illustrated in alternative embodiment HOC of the rig mat illustrated in FIGS. 12A and 12B. Here discrete tank members 112 are interconnected by external structural members or framework 111. It may be desirable to compartmentalize in the interior of the tank members. These compartments can be connected with valves that will provide greater control than merely providing an air line in, a valve in the bottom for water to escape when air enters, and a valve on top for air to be released when ballast is allowed to enter through the bottom. Providing extra control through valves and compartments can provide versatility in response to using a mixture of compressible and incompressible fluids to control buoyancy across a range of pressure conditions. This can limit the effective volume to which inserted gas can expand, e.g., during platform raising operations discussed below with FIGS. 11A-11D. Otherwise, the volume of the gas in the tank member will increase as the tank member rises and pressure decreases. The expanded volume of gas displaces more water, increasing the buoyancy of the platform, causing it to rise faster, etc. FIGS. 6-9 illustrate installation of rig mat 110A and deployment of jack-up rig 34. In FIG 6, rig mat 110A has been partially ballasted, filled with sufficient water to make it less than neutrally buoyant. It is then lowered by crane barge 116 to the top of jacket base 12 adjacent surface tower 28, mating the jacket base interface with the rug support interface, bringing feet 36A of jacket base interface 114 into spud buckets 38 provided with a plurality of mat locking connections 140. Since these connections will be below the wave zone, but within the depth range for jack-up rigs, any number of positive control locking devises are possible, including hydraulic control, ROV operable, or even diver actuated. FIG. 8 illustrates one such mat locking connection to secure rig mat 110A to jacket base 12. Here jacket base interface 114 presents a centering pin 37 extending from a rimmed foot 36A. The spud bucket is provided in the form of a steel lattice structure 38D which may be coated with a rubber or other elastomeric cushion 38B. A spring loaded landing receptacle 38E extends upwardly from the center of the lattice structure. Here this is illustrated with springs 144, the cathodic protection for which has been omitted for the sake of clarity. Other spring systems such as using elastomeric components or dampener systems may be alternatively used. Upon installation, centering pins 37 of jacket base interface 114 are guided into recess 146 in landing receptacle 38E which progressively loads and centers as the spring is deflected and rimmed foot 36A seats upon lattice structure legs 34 of jack-up rig 34. Hydraulically driven gripping arms 41 are deployed to engage the edges of foot 36A to secure the rig mat to the jacket base to enhance stability when the rig mat is buoyant and the jack-up rig is in place. In FIG. 1 , jack-up rig 34 has been floated on hull 52 into position adjacent surface tower 28 and legs 50 are being lowered toward secondary rig support interface 39 presented on the upper surface of tank member 112. Derrick 56 is withdrawn on cantilever deck 58 to enable this close maneuvering. An air compressor or other source of high pressure gas is conveniently provided on jack-up rig 34 and connected to rig mat 110A through conduit or air line 118. The interior of tank member 112 has ballast chambers into which air or another gas may be pumped for buoyancy and a valve system 116 through which gas may be pumped and displaced seawater released. Tradeoffs between temporarily loading to jacket base 12, temporarily loading to rig mat locking connections 140, design criteria and failure scenarios will determine whether rig mat 110A is made buoyant before, during or after installation of jack-up 34. Further jacking of legs 50 brings feet 36 into contact with secondary subsea rig interface 39 and it may be desired to releasably lock feet 36 of the jack-up rig to the interface through a rig locking connection 120 (see FIG. 9) identical in construction and operation to the mat locking connection illustrated in FIG. 8. Further jacking of legs 50 raises hull 52 out of the water and to the desired platform height. At this elevation, cantilever deck 58 will clear platform deck 32 of surface tower 28 and derrick 56 can be brought into position to commence drilling operations through conductors 40. After drilling operations are complete, jack-up rig 34 may be removed by essentially reversing the installation steps. Rig mat 110A may be ballasted to substantially neutral buoyancy by selectively allowing sea water to enter and the air to escape from tank member 112. Unless useful for controlling dynamic response as discussed below, the rig mat can then be removed with a crane barge. FIGS. 10 and 11A-11D illustrate another embodiment of a rig support buoyancy tank 110, here in the form of a plurality of vertically oriented, elongated cylindrical tank members HOB. The elongated tank members are mounted to a plurality of levels of framework 16 in jacket base 12 in vertical alignment with discrete contact points in subsea rig interface 38. FIG. 10 illustrates also illustrates a compliant tower embodiment. Although dynamic response is a consideration for traditional bottom-founded platforms having fixed or rigid tower structures to deepwater, dynamic response becomes of more central concern for compliant towers. Compliant towers are designed to "give" in a controlled manner in response to dynamic environmental loads rather than to nearly rigidly resist those forces. A basic requirement in controlling this response is to produce a structure having harmonic frequencies or natural periods that avoid those encountered in nature. Here, jacket base 12 has parallel legs 14 to enhance its flexibility. For clarity sake, the middle regions of this long jacket base have been omitted from FIG. 10. The total mass at the top to the jacket base is one of the controlling variables in defining the natural periods of the structure. Thus, offshore platform structure 10, with jack-up rig 34 in place, is one condition that must be accommodated. It may, however, be more difficult to design an offshore platform having a suitably wide range to accommodate both having the mass of the jack-up rig present and having it absent. It may also be difficult to find two separate ranges avoiding natural harmonics of the structure to accommodate the offshore platform in both drilling operations with the jack-up rig in place and in production operations with the jack-up rig removed. Using ballastable tank member 110 to take on ballast when the jack-up rig is removed can substantially narrow the range of masses that must be accommodated. This may be conveniently provided by the same ballastable rig support buoyancy tank 110 which alleviated the load of the weight of the jack-up rig. Although a rig mat 110A may be deployed, the continued need for tank members, in both the presence or absence of the jack-up rig, is here accommodated by elongated, cylindrical, vertically oriented tank members HOB. If used to provide buoyancy support to offset the weight of jack-up rig 34 during drilling or other well operations, this buoyant reserve can be replaced with seawater with the removal of the jack-up rig, to substantially replace the mass of the jack-up rig. Further, since the tanks are submerged this mass is added without introducing its corresponding weight in the system. This permits design for a more realistic (narrow) window avoiding the natural harmonic responses. FIGS. 11A-11D illustrate a method for redeploying an offshore platform structure from a first site to a second site which has a different water depth. Selectively buoyant and ballastable tank members 110 at the top of jacket base 12 are very useful for this purpose. Application S.N. 08/129,829, discussed above, discloses the use of staged pile sleeves 20 having a first stage 60 which projects above a second stage 62. On the initial deployment, the piles are locked to the pile sleeves in the first stage. Then, at time for retrieval and reuse, the first stage sleeve is accessible for cutting, e.g., through ROV operations. See ROV 122 in FIG. HA. Severing the first stage sleeve 60 with the pile to sleeve connection inside and the top of the pile within releases the platform from its pinned connection at sea floor 24. Battered piles may require severing below the pile sleeve as well for releasing the jacket base. Turning to FIG. HB, water is then displaced with air pumped into selectively buoyant and ballastable tank members HOB. A suitable air pump may be supplied on crane barge 116. Similarly, air may also be pumped into one or more of legs 14 of jacket base 12 which are generally formed of hollow tubular goods. Jacket bases having a quadrilateral cross section may be helped by providing such buoyancy to the corner supporting surface tower 28. Other jacket bases may benefit from the additional buoyancy generally, in the jacket legs or through auxiliary provisions. However, the bulk of the buoyancy is provided at the top of jacket base and the jacket base is lifted off the sea floor and toward surface 30 where the vertically floating jacket base has sufficient stability to conduct offshore fabrication operations supported by crane barge 116. All or part of surface tower 28 is removed, see FIG. HC, and a resized surface tower 28A is installed. See FIG. HD. Thus, significant differences in water depth "Δd" may be accommodated in offshore operations involving only the surface tower. Such operations provide the jacket base with convenient versatility that substantially enhances its reuse by facilitating resizing of the surface tower to correctly accommodate the water depth and cooperate with a cantilever deck mounted derrick on a jack-up rig. The reworked jacket base is then towed to a new site and redeployed, ballasting the tank members 110 and legs 16. The base is then pinned to ocean floor 24 though piles 22 securely locked within pile sleeves 20 at second stage locking profile 62. For longer tow distances, it may be desirable to provide auxiliary buoyance to upend the platform for horizontal relocation. At site, it would be rotated to vertical and set down. Other modifications, changes, and substitutions are also intended in the forgoing disclosure. Further, in some instances, some features of the present invention will be employed without a corresponding use of other features described in these illustrative embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

Claims: C L A I M S 1. An offshore platform structure for temporarily using a jack-up rig for well operations in deepwater applications, comprising a bottom founded jacket base, a surface tower supported by the jacket base and extending above the ocean surface, a platform deck supported by the surface tower, a subsea rig support interface presented at the top of the jacket base and adapted to support the jack-up rig for well operations, and at least one rig support buoyancy tank connected to the rig support interface. 2. An offshore platform structure in accordance with claim 1 wherein the rig support buoyancy tank is a rig mat, comprising a selectively buoyant and ballastable tank member, a jacket base interface presented on the bottom of the tank member which attaches on top of the jacket base on the rig support interface, and a secondary subsea rig support interface presented on the top of the tank member, interconnected in a load bearing relationship with the jacket base interface, and adapted to receive the jack-up rig. 3. An offshore platform structure in accordance with claim 1 or 2, further comprising a mat locking connection between the subsea rig support and the jacket base interface to releasably secure the rig mat to the jacket base, and a rig locking connection between the jack-up rig and the secondary subsea rig support to releasably secure the jack-up rig to the rig mat and therethrough to the jacket base. 4. An offshore platform structure in accordance with claim 3 wherein the connection between the subsea rig support and the jacket base interface comprises a plurality of downwardly disposed, outwardly extending rimmed feet forming the jacket base interface, a guide pin extending downwardly from the rimmed feet, a plurality of steel lattice structures forming the subsea rig support disposed to receive the rimmed feet in load bearing relation, a plurality of central recesses in the steel lattice structures disposed to receive the guide pin extending from the rimmed feet, and a plurality of hydraulically driven gripping arms mounted on the subsea rig interface and disposed to releasably secure the rimmed feet of the jacket base interface. 5. An offshore platform structure in accordance with claim 4 wherein the rig mat is removable. 6. An offshore platform structure in accordance with claim 2 wherein the subsea rig support and secondary subsea rig support interfaces each comprises a plurality of discrete contact points and these respective sets of discrete contact points do not fully correlate in vertical alignment. 7. An offshore platform structure in accordance with any of claims 1-6 wherein a plurality of the rig support buoyancy tanks are provided and wherein the subsea rig interface comprises a plurality of discrete contact points corresponding to the footprint of the jack-up rig, each tank forming a vertically oriented elongated tank member directly under one of the discrete contact points of the subsea rig interface in a load bearing relationship. 8. An offshore platform structure in accordance with claim 1, wherein said subsea rig support interface comprises a plurality of discrete contact points corresponding to the footprint of the jack-up rig, and wherein the rig support buoyancy tank comprises a plurality of vertically oriented elongated cylindrical tank members, each positioned substantially under one of the discrete contact points of the subsea rig interface in a load bearing relationship and connected to the jacket base at a plurality of framework levels. 9. An offshore platform structure in accordance with any of claims 1-8, wherein the bottom founded jacket base forms a compliant jacket base designed for dynamic response with the mass of the jack-up engaged, and whereby the weight of the jack-up rig is substantially offset by buoyant forces supplied by the rig support buoyancy tank when the jack-up rig is deployed on the jacket base and the mass of the jack-up rig is substantially replaced in the offshore platform structure by adding water as ballast in the rig support buoyancy tank when the jack-up rig is removed to contribute toward avoiding harmonic periods for the compliant tower during production operations which do not require the presence of the jack-up rig. 10. A method for redeploying an offshore platform structure from as claimed in claim 8 from a first site to a second site, comprising releasing a connection between the jacket base and a plurality of piles which are anchored in the ocean floor at the first site, said releasing comprising cutting through a plurality of pile sleeves and the piles locked therein to remove pile-to-pile sleeve connections in an extended first stage of the pile sleeves, vertically raising the offshore platform structure by pumping air into the tank members, towing the offshore platform structure to the second site, vertically lowering the offshore structure by ballasting the tank members, installing the platform at the second site with the surface tower above the ocean surface and a subsea rig support interface presented within the depth capability of a jack-up rig. 11. A method for redeploying an offshore platform structure in accordance with claim 10, wherein the subsea jacket base has a quadrilateral cross section having four legs, three of said legs each providing direct support to one of the discrete contact points, the forth leg providing principle support for the surface tower, and the method further comprising providing supplementary buoyancy under the surface tower. 12. A method for redeploying an offshore platform structure in accordance with claim 10 or 11 wherein vertically raising the offshore platform structure further comprises raising the subsea jacket base until the base of the surface tower is above the ocean surface. 13. A method for redeploying an offshore platform structure in accordance with any of claims 10-12 further comprising resizing the surface tower by removing the old surface tower from the subsea jacket base and installing a new surface tower to the subsea jacket base. 14. A method for redeploying an offshore platform structure in accordance with claim 13 wherein resizing the surface tower comprises removing a deck from the surface tower, shortening the surface tower and installing another deck on the top of the shortened surface tower.

Inventors: GALLAHER DALE MARION, SGOUROS GEORGE EMANUEL, SMITH MARK ANDREW, GALLAHER, DALE, MARION, SGOUROS, GEORGE, EMANUEL, SMITH, MARK, ANDREW
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1995
Main classification: E02B17
IPC codes: E02B17