OA10432A - Offshore platform with buoyant rig supplemental support - Google Patents

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

010432

OFFSHORE PLATFORM WITH BUOYANT RIG

SUPPLEMENTAL SUPPORT

The présent invention relates to a platform andSystem for conducting offshore hydrocarbon recoveryoperations. More particularly, the présent inventionrelates to a platform structure and System for allowing 5 the use of a jack-up rig in deeper water.

Jack-up rigs provide a derrick and associatedequipment for drilling, completing or working over awell. This equipment is mounted to a combined hull\deckwhich is capable of floating these facilities to site. A 10 plurality of rétractable legs are provided which renders the jack-up rig conveniently portable. Once floated intoposition for conventional operations, the legs arejacked-down until they engage the seafloor. Furtherjacking transfers the load from the buoyant hull to the 15 legs, then lifts the hull/deck out of the water and above the splash zone to produce a stable, bottom foundedoffshore platform for conducting well operations. A considération of this design is that to best takeadvantage of the mobile nature of the facilities provided 20 on the jack-up rig, the rig is removed after drilling is complété and does not remain deployed at the developmentduring the production phase except, possibly, fortemporary drilling and workover operations. Theconsidérable investment in drilling, completion and 25 workover equipment is best utilized by redeploying the jack-up rig to another location as soon as theseoperations are complété. Thus, surface complétions forproduction are not accommodated on the jack-up rigitself. A small structure called a "well jacket" can be 30 used with the jack-up rig to provide the benefits of a 010432 surface completion with the convenience of a jack-up rig.However, well jackets and jack-up rig combinations arelimited to shallow water deployment. Further, practicallimitations on the length of the rétractable legs more 5 directly restrict the depth in which jack-up rigs can be traditionally deployed.

The requirements of deeper water depths hâve mostoften been answered by the continued use of traditionalbottom founded platform structures. Topside facilities 10 provide convenient well access for production operations.

However, such structures must dedicate a significantamount of their structural strength to supportingdrilling facilities that are only required for arelatively short period of time in the life of the 15 overall operations from the platform in recovering oil and gas from a réservoir. Further, the structure must beable to withstand the maximum design environmentalconditions, the design hurricane criteria, with thesedrilling facilities in place. 20 Of course, recovery operations lead to déplétion of the hydrocarbon réservoir and, in time, the platformloses its usefulness at a site. Nevertheless, the welljacket that forms the tower supporting the deck of theplatform may be structural sound and capable of an 25 extended useful life. However, salvage operations are difficult and another constraint of traditional welljackets is that they are design spécifie for a givenwater depth. This tends to substantially limitredeployment opportunities. 30 Certain designs hâve been proposed for "piggyback" deployment of a jack-up rig onto a subsea structure, yetthese designs hâve carried forward many of the limitations of each structure producing a resuit that,although it increases water depth for the jack-up rig, 010432 otherwise remains the sum of the limitations of itsconstituent parts.

More recently a new platform concept has beenproposed combining the benefits of jack-up rigs and 5 traditional bottom founded platform structures, without carrying their drawbacks into the combination. Thus, the"Hyjack" platform has been proposed which combines asmall surface tower sufficient to support productionoperations with a substantial jacket base which supports 10 the surface tower and temporarily supports a jack-up rig for drilling operations. Following drilling, the jack-uprig is moved off and the small surface tower supportsproduction operations. This is described in greaterdetail in U.S. Patent Application 08/129,820, not pre- 15 published, filed September 30, 1993, by Dale M. Gallaher et al for an Offshore Platform Structure and System.Further features that facilitate salvage andredeployment, particularly in combination with theforegoing platform concept, are described more fully in 20 U.S. Patent Application 08/129,829, not pre-published, filed September 30, 1993, by George E. Sgouros et al fora Reusable Offshore Platform Jacket. The full disclosureof each of these patent applications are herebyincorporated by reference and made a part hereof. 25 The forgoing salvage and redeployment provisions allow a second use of pile sleeves without drydock orexpensive, complicated offshore operations. However, theoffshore platform structure remains rather limited in thedepths to which it can be redeployed. This is because 30 the envelope is restricted both by the depth of the subsea rig support interface and the height of thesurface tower over which the cantilever deck requiresclearance to position the derrick. The first of theseconstraints involves major structural components. But 35 the second constraint is only limiting due to the 010432 complexity of existing offshore operations and theexpense and inconvenience of towing the offshore platformstructure to dry dock or bringing a floating dry dock toit. 5 As platforms are used in progressively deeper water, their dynamic response may become a greater designconsidération as the traditional bottom-founded platformsbecome relatively less rigid in response to wind, waveand currents. However, dynamic response becomes of a 10 central concern for compilant towers where flexibility is a key design precept. Compilant towers are designed to"give" in a controlled manner in response to dynamicenvironmental loads rather than to nearly rigidly resistthose forces. 15 A basic requirement in controlling this response is to produce a structure having harmonie frequencies ornatural periods that avoid those encountered in nature.The total mass at the top to the jacket base is one ofthe controlling variables in defining the natural periods 20 of the structure. Adaptation of the hyjack platform concept to compilant towers represents a unique challengeis because one platform must accommodate such widelydifferent design States based upon the presence orabsence of the jack-up rig at the time in question. 25 FR-A-2362975 discloses an offshore platform structure according to the preamble of claim 1.

Nevertheless, there continues to be a need in somecircumstances for economically accommodating and evenenhancing the benefits of surface complétions and the 30 convenience and économies of jack-up rig operations in deeper water.

In accordance with the invention there is provided anoffshore platform structure for temporarily using ajack-up rig for well operations in deepwater 35 applications, comprising: 010432 a bottom founded jacket base, a surface tower supported by the jacket base and extending above the océan surface, a platform deck supported by the surface tower, and 5 a subsea rïg support interface presented at the top of the jacket base and adapted to support the jack-up rigfor well operations, characterized in that at least onerig support buoyancy tank is connected to the rig supportinterface, which buoyancy tank is provided with means for 10 adapting the buoyancy thereof to different loading conditions due to the presence or absence of the jack-uprig on the rig support interface.

An advantage of the présent invention is that itfurther minimizes the permanent structure that is 15 dedicated to serving the limited need for supporting drilling operations over the productive life of aplatform. Another advantage of some embodiments of theprésent invention is that it may afford an opportunity toadapt an installed hyjack platform to accept a broader 20 range jack-up rig footprints should the rigs of the initial design assumption prove unavailable.

The brief description above, as well as furtherobjects and advantages of the présent invention, will bemore fully appreciated by reference to the following 25 detailed description of the preferred embodiments which should be read in conjunction with the accompanyingdrawings in which: FIG. 1 is a side elevational view illustrating adeployed offshore platform structure,· 30 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 35 deployed in FIG. 1; 010432 5a FIG. 5 is a bottom perspective view of the rig mat ofFIG. 4; FIG. 6 is a side elevational view of an installationof a rig mat ; 010432 FIG. 7 is a side elevational view of a jack-up rigbeing deployed upon an offshore platform structure with arig mat; FIG. 8 is a partially cross sectioned view5 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 rigdeployed upon the offshore platform structure; FIG. 10 is a side elevational view of a compilant10 tower embodiment of the présent invention deploying a jack-up rig; FIGS. 11A-11D are side elevational views of thesalvage and redeployment of an offshore platformstructure into a different water depth; 15 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 rigsupport buoyancy tank of FIG. 12A.

In FIG. 1, rig support buoyancy tank 110 in the form20 of rig mat 110A is provided to compensate for the weight of jack-up rig 34 upon deployment onto bottom foundedjacket base 12. In this illustration, jack-up rig 34 isshown in its initial approach.

Offshore platform structure 10 provides a subsea rig25 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 océan floor 24 with piles 22which are secured to the jacket base at a plurality ofpile sleeves 20. 30 A surface tower 28 is supported by jacket base 12 to présent a platform deck 32 above océan surface 30.

Surface tower 28 is positioned to allow unobstructedaccess to subsea rig support interface 26. Oneconvenient manner of providing this access for a three 35 leg jack-up rig 34 is to place the surface tower on one 010432 corner of the jack-up rig and to provide legs 14 of aquadrilatéral jacket base substantially aligned with thediscrète contact points such as spud buckets 38 thatgenerally correspond to the footprint of the jack-up rig. 5 Rig mat 110A is illustrated in greater detail in FIGS. 2, 4, 5 and 8. Figures 2 and 4 illustrate the topof the rig mat which présents secondary subsea supportinterface 138 on top of a tank member 112. The spudbuckets of secondary subsea support interface 138 are 10 positioned to receive feet 36 of jack-up rig 34. The bottom of tank member 112 présents jacket base interface114 (see FIG. 5) which correspond to spud buckets 38 ofthe subsea rig support interface presented at the top ofthe jacket base. See FIG. 3. 15 Rig mat 110A has a selectively buoyant and ballastable tank member 112 with jacket base interface114 on the lower surface (see FIG. 5) and secondarysubsea rig support interface 39 on the upper surface (seeFIG. 4). Internai structural members connect interfaces 20 114 and 39 in a load bearing relationship. Most conveniently, the load is transferred vertically betweendiscrète aligned contact points. However, if necessary,it may be possible to fabricate a rig mat with structuralframework suitable to distribute the load between the 25 jacket base and the jack-up rig in other than direct vertical alignment. Thus, it may be possible to use rigmat 110A as an adapter to allow use of a jack-up righaving a dissimilar footprint from that which was theoriginal design assumption when jacket base 12 was 30 fabricated.

Dissimilar footprints in jacket base interface 114and secondary subsea rig support interface 39 is one ofthe features illustrated in alternative embodiment HOCof the rig mat illustrated in FIGS. 12A and 12B. Here 35 discrète tank members 112 are interconnected by external 010432 structural members or framework 111. It may be désirableto compartmentalize in the interior of the tank members.These compartments can be connected with valves that willprovide greater control than merely providing an air line 5 in, a valve in the bottom for water to escape when air enters, and a valve on top for air to be released whenballast is allowed to enter through the bottom.

Providing extra control through valves and compartmentscan provide versatility in response to using a mixture of 10 compressible and incompressible fluids to control buoyancy across a range of pressure conditions. This canlimit the effective volume to which inserted gas canexpand, e.g., during platform raising operationsdiscussed below with FIGS. 11A-11D. Otherwise, the 15 volume of the gas in the tank member will increase as the tank member rises and pressure decreases. The expandedvolume of gas displaces more water, increasing thebuoyancy of the platform, causing it to rise faster, etc. FIGS. 6-9 illustrate installation of rig mat 110A and 20 deployment of jack-up rig 34. In FIG 6, rig mat 110A has been partially ballasted, filled with sufficient water tomake it less than neutrally buoyant. It is then loweredby crâne barge 116 to the top of jacket base 12 adjacentsurface tower 28, mating the jacket base interface with 25 the rug support interface, bringing feet 36A of jacket base interface 114 into spud buckets 38 provided with aplurality of mat locking connections 140. Since theseconnections will be below the wave zone, but within thedepth range for jack-up rigs, any number of positive 30 control locking devises are possible, including hydraulic control, ROV opérable, or even diver actuated. FIG. 8 illustrâtes one such mat locking connection tosecure rig mat 110A to jacket base 12. Here jacket baseinterface 114 présents a centering pin 37 extending from 35 a rimmed foot 36A. The spud bucket is provided in the 010432 form of a Steel lattice structure 38D which may be coatedwith a rubber or other elastomeric cushion 38B. A springloaded landing réceptacle 38E extends upwardly from thecenter of the lattice structure. Here this is 5 illustrated with springs 144/ the cathodic protection for which has been omitted for the sake of clarity. Otherspring Systems such as using elastomeric components ordampener Systems may be alternatively used. Uponinstallation, centering pins 37 of jacket base interface

10 114 are guided into recess 146 in landing réceptacle 38E which progressively loads and centers as the spring isdeflected and rimmed foot 36A seats upon latticestructure legs 34 of jack-up rig 34. Hydraulicallydriven gripping arms 41 are deployed to engage the edges 15 of foot 36A to secure the rig mat to the jacket base to enhance stability when the rig mat is buoyant and thejack-up rig is in place.

In FIG. 7, jack-up rig 34 has been floated on hull 52into position adjacent surface tower 28 and legs 50 are 20 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 enablethis close maneuvering. An air compressor or othersource of high pressure gas is conveniently provided on 25 jack-up rig 34 and connected to rig mat 110A through conduit or air line 118. The interior of tank member 112has ballast chambers into which air or another gas may bepumped for buoyancy and a valve System 116 through whichgas may be pumped and displaced seawater released. 30 Tradeoffs between temporarily loading to jacket base 12, temporarily loading to rig mat locking connections 140,design criteria and failure scénarios will déterminewhether rig mat 110A is made buoyant before, during orafter installation of jack-up 34. 10 010432

Further jacking of legs 50 brings feet 36 intocontact with secondary subsea rig interface 39 and it maybe desired to releasably lock feet 36 of the jack-up rigto the interface through a rig locking connection 120 5 (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 thewater and to the desired platform height. At thisélévation, cantilever deck 58 will clear platform deck 32 10 of surface tower 28 and derrick 56 can be brought into position to commence drilling operations throughconductors 40.

After drilling operations are complété, jack-up rig34 may be removed by essentially reversing the 15 installation steps. Rig mat 110A may be ballasted to substantially neutral buoyancy by selectively allowingsea water to enter and the air to escape from tank member112. Unless useful for controlling dynamic response asdiscussed below, the rig mat can then be removed with a 20 crâne barge. FIGS. 10 and 11A-11D illustrate another embodiment ofa rig support buoyancy tank 110, here in the form of aplurality of vertically oriented, elongated cylindricaltank members 110B. The elongated tank members are 25 mounted to a plurality of levels of framework 16 in jacket base 12 in vertical alignment with discrètecontact points in subsea rig interface 38. FIG. 10 illustrâtes also illustrâtes a compilanttower embodiment. Although dynamic response is a 30 considération for traditional bottom-founded platforms having fixed or rigid tower structures to deepwater,dynamic response becomes of more central concern forcompilant towers. Compilant towers are designed to"give" in a controlled manner in response to dynamic 35 environmental loads rather than to nearly rigidly resist 010432 11 those forces. A basic requirement in controlling thisresponse is to produce a structure having harmoniefrequencies or natural periods that avoid thoseencountered in nature. Here, jacket base 12 has parallel 5 legs 14 to enhance its flexibility. For clarity sake, the middle régions of this long jacket base hâve beenomitted from FIG. 10.

The total mass at the top to the jacket base is oneof the controlling variables in defining the natural 10 periods of the structure. Thus, offshore platform structure 10, with jack-up rig 34 in place, is onecondition that must be accommodated. It may, however, bemore difficult to design an offshore platform having asuitably wide range to accommodate both having the mass 15 of the jack-up rig présent and having it absent. It may also be difficult to find two separate ranges avoidingnatural harmonies of the structure to accommodate theoffshore platform in both drilling operations with thejack-up rig in place and in production operations with 20 the jack-up rig removed.

Using ballastable tank member 110 to take on ballast when the jack-up rig is removed can substantially narrowthe range of masses that must be accommodated. This maybe conveniently provided by the same ballastable rig 25 support buoyancy tank 110 which alleviated the load of the weight of the jack-up rig. Although a rig mat 110Amay be deployed, the continued need for tank members, inboth the presence or absence of the jack-up rig, is hereaccommodated by elongated, cylindrical, vertically 30 oriented tank members 110B. If used to provide buoyancy support to offset the weight of jack-up rig 34 duringdrilling or other well operations, this buoyant reservecan be replaced with seawater with the removal of thejack-up rig, to substantially replace the mass of the 35 jack-up rig. Further, since the tanks are submerged this 010432 12 mass is added without introducing its correspondingweight in the System. This permits design for a morerealistic (narrow) window avoiding the natural harmonieresponses. 5 FIGS. 11A-11D illustrate a method for redeploying an offshore platform structure from a first site to a secondsite which has a different water depth. Selectivelybuoyant and ballastable tank members 110 at the top ofjacket base 12 are very useful for this purpose. 10 Application S.N. 08/129,829, discussed above, discloses the use of staged pile sleeves 20 having afirst stage 60 which projects above a second stage 62.

On the initial deployment, the piles are locked to thepile sleeves in the first stage. Then, at time for 15 retrieval and reuse, the first stage sleeve is accessible for cutting, e.g., through ROV operations. See ROV 122in FIG. 11A. Severing the first stage sleeve 60 with thepile to sleeve connection inside and the top of the pilewithin releases the platform from its pinned connection 20 at sea floor 24. Battered piles may require severing below the pile sleeve as well for releasing the jacketbase.

Turning to FIG. 11B, water is then displaced with airpumped into selectively buoyant and ballastable tank 25 members 110B. A suitable air pump may be supplied on crâne barge 116. Similarly, air may also be pumped intoone or more of legs 14 of jacket base 12 which aregenerally formed of hollow tubular goods. Jacket baseshaving a quadrilatéral cross section may be helped by 30 providing such buoyancy to the corner supporting surface tower 28. Other jacket bases may benefit from theadditional buoyancy generally, in the jacket legs orthrough auxiliary provisions. However, the bulk of thebuoyancy is provided at the top of jacket base and the 35 jacket base is lifted off the sea floor and toward 010432 - 13 - surface 30 where the vertically floating jacket base hassufficient stability to conduct offshore fabricationoperations supported by crâne barge 116. Ail or part ofsurface tower 28 is removed, see FIG. 11C, and a resized 5 surface tower 28A is installed. See FIG. 11D. Thus, significant différences in water depth "Ad" may beaccommodated in offshore operations involving only thesurface tower. Such operations provide the jacket basewith convenient versatility that substantially enhances 10 its reuse by facilitating resizing of the surface tower to correctly accommodate the water depth and cooperatewith a cantilever deck mounted derrick on a jack-up rig.

The reworked jacket base is then towed to a new siteand redeployed, ballasting the tank members 110 and legs 15 16. The base is then pinned to océan floor 24 though piles 22 securely locked within pile sleeves 20 at secondstage locking profile 62. For longer tow distances, itmay be désirable to provide auxiliary buoyance to upendthe platform for horizontal relocation. At site, it 20 would be rotated to vertical and set down.

Other modifications, changes, and substitutions are also intended in the forgoing disclosure. Further, insome instances, some features of the présent inventionwill be employed without a corresponding use of other 25 features described in these illustrative embodiments.

Accordingly, it is appropriate that the appended daimsbe construed broadly and in a manner consistent with thespirit and scope of the invention herein.

Claims (14)

14 01 0432 £_,L,A IMS

1. An offshore platform structure for temporarily usinga jack-up rig for well operations in deepwaterapplications, comprising a bottom founded jacket base(12), a surface tower (28) supported by the jacket baseand extending above the océan surface (30), a platformdeck supported by the surface tower, and a subsea rigsupport interface (26) presented at the top of the jacketbase and adapted to support the jack-up rig (34) for welloperations, characterized in that at least one rigsupport buoyancy tank (110) is connected to the rigsupport interface, which buoyancy tank is provided withmeans for adapting the buoyancy thereof to differentloading conditions due to the presence or absence of thejack-up rig on the rig support interface.

2. An offshore platform structure in accordance withclaim 1 wherein the rig support buoyancy tank (110) is arig mat (110A), comprising a selectively buoyant andballastable tank member (112), a jacket base interface(114) presented on the bottom of the tank member whichattaches on top of the jacket base on the rig supportinterface, and a secondary subsea rig support interface(39) presented on the top of the tank member, interconnected by internai structural members in a loadbearing relationship with the jacket base interface, andadapted to receive the jack-up rig.

3. An offshore platform structure in accordance withclaim 1 or 2, further comprising a mat locking connection(140) between the subsea rig support and the jacket baseinterface to releasably secure the rig mat (110A) to thejacket base (12) , and a rig locking connection (120)between the jack-up rig and the secondary subsea rig 15 01 0432 support to releasably secure the jack-up rig to the rigmat and therethrough to the jacket base.

4. An offshore platform structure in accordance withclaim 3 wherein the connection between the subsea rigsupport and the jacket base interface comprises aplurality of downwardly disposed, outwardly extendingrimmed feet (36A) forming the jacket base interface, aguide pin (37) extending downwardly from the rimmed feet,a plurality of Steel lattice structures forming thesubsea rig support disposed to receive the rimmed feet inload bearing relation, a plurality of central recesses(146) in the Steel lattice structures disposed to receivethe guide pin (37) extending from the rimmed feet, and aplurality of hydraulically driven gripping arms (41)mounted on the subsea rig interface and disposed toreleasably secure the rimmed feet of the jacket baseinterface.

5. An offshore platform structure in accordance withclaim 4 wherein the rig mat (110A) is removable.

6. An offshore platform structure in accordance withclaim 2 wherein the subsea rig support (114) andsecondary subsea rig support interfaces (39) eachcomprises a plurality of discrète contact points andthese respective sets of discrète contact points do notfully correlate in vertical alignment.

7. An offshore platform structure in accordance with anyof daims 1-6 wherein a plurality of the rig supportbuoyancy tanks (110B) are provided and wherein the subsearig interface (26) comprises a plurality of discrètecontact points (38) corresponding to the footprint of thejack-up rig, each tank (110B) forming a verticallyoriented elongated tank member directly under one of thediscrète contact points (38) of the subsea rig interfacein a load bearing relationship. 010432

8. An offshore platform structure in accordance withclaim 1, wherein said subsea rig support interface (26)comprises a plurality of discrète contact points (38)corresponding to the footprint of the jack-up rig, and 5 wherein the rig support buoyancy tank (110) comprises a plurality of vertically oriented elongated cylindricaltank members (110B), each positioned substantially underone of the discrète contact points (38) of the subsea riginterface in a load bearing relationship and connected to 10 the jacket base at a plurality of framework levels.

9. An offshore platform structure in accordance with anyof daims 1-8, wherein the bottom founded jacket base(12) forms a compilant jacket base designed for dynamicresponse with the mass of the jack-up engaged, and 15 whereby the weight of the jack-up rig (34) is substantially offset by buoyant forces supplied by therig support buoyancy tank (110) when the jack-up rig isdeployed on the jacket base and the mass of the jack-uprig is substantially replaced in the offshore platform 20 structure by adding water as ballast in the rig support buoyancy tank (110) when the jack-up rig (34) is removedto contribute toward avoiding harmonie periods for thecompilant tower during production operations which do notrequire the presence of the jack-up rig. 25

10. A method for redeploying an offshore platform structure from as claimed in claim 8 from a first site toa second site, comprising releasing a connection betweenthe jacket base and a plurality of piles (22) which areanchored in the océan floor at the first site, said 30 releasing comprising cutting through a plurality of pile sleeves (20) and the piles locked therein to removepile-to-pile sleeve connections in an extended firststage of the pile sleeves (20), vertically raising theoffshore platform structure (12) by pumping air into the 35 tank members (110B), towing the offshore platform 010432 - 17 - structure to the second site, vertically lowering theoffshore structure by ballasting the tank members (110B),installing the platform at the second site with thesurface tower above the océan surface and a subsea rig 5 support interface presented within the depth capability of a jack-up rig.

11. A method for redeploying an offshore platformstructure in accordance with claim 10, wherein the subseajacket base (12) has a quadrilatéral cross section having 10 four legs (14), three of said legs each providing direct support to one of the discrète contact points, the fourthleg providing principle support for the surface tower(28), and the method further comprising providingsupplémentary buoyancy under the surface tower (28). 15

12. A method for redeploying an offshore platform structure in accordance with claim 10 or 11 whereinvertically raising the offshore platform structurefurther comprises raising the subsea jacket base (12)until the base of the surface tower is above the océan 20 surface.

13. A method for redeploying an offshore platformstructure in accordance with any of daims 10-12 furthercomprising resizing the surface tower by removing the oldsurface tower (28) from the subsea jacket base and 25 installing a new surface tower to the subsea jacket base.

14. A method for redeploying an offshore platformstructure in accordance with claim 13 wherein resizingthe surface tower (28) comprises removing a deck from thesurface tower, shortening the surface tower and 30 installing another deck on the top of the shortened surface tower.